Kinesin is a molecular motor that carries cargo through cells by moving in discrete steps along filamentous tracks called microtubules. Its two “feet,” or motor domains, must coordinate their movement so that the protein advances efficiently without leaving the track. Previous work established that a connecting segment known as the neck region is important for coordinating the feet, but its three-dimensional structure and its direct interaction with microtubules had remained unresolved because current imaging methods could not capture the relevant atomic dynamics. The new findings, published in Biophysical Journal, address that gap through all-atom molecular dynamics modeling rather than direct observation in cells.

To overcome the limitations of imaging, the team constructed an atomic model of a kinesin motor attached to a microtubule, a system containing approximately three million atoms. The simulations were performed on the Fugaku supercomputer. Using an enhanced sampling method called generalized replica exchange with solute tempering, abbreviated gREST, the researchers examined the neck region across two independently constructed computational physics models, or force fields. This approach allowed them to sample the region’s flexible conformations and determine a high-confidence structure for its most stable state. The resulting model showed that the neck contains a twisted, two-strand coiled coil positioned perpendicular to the microtubule, with part of the neck remaining in contact with the track surface.

The researchers next simulated the motor’s initial step using a simplified detachment model in which some underlying microtubule subunits were removed to trigger movement. Under these conditions, the trailing foot did not move directly over the leading foot. Instead, interactions between the neck region and the microtubule surface biased its trajectory around the right side of the forward foot, producing a counterclockwise stepping path. Control simulations supported the importance of this interaction: alternative neck configurations with weak contact against the track failed to generate forward movement. The work therefore identifies a local atomic mechanism that can connect the conformation of kinesin’s neck region with the direction of its motion. It does not demonstrate a corresponding behavior in living cells, however, because the evidence comes from computational models of a simplified motor-track system.

The study also highlights three important simplifications that constrain the model. It uses a truncated kinesin construct rather than the complete protein, artificially removes microtubule subunits to initiate stepping, and omits the flexible E-hook elements extending from the microtubule surface. The authors note that future simulations incorporating full-length kinesin and more realistic track elements could refine the proposed structural framework. Even with those limitations, the simulations provide a testable physical explanation for one of the motor’s essential functions: steering its steps. By showing how contact between a flexible molecular structure and a cellular track can bias movement, the research adds a detailed structural basis to the mechanics of intracellular cargo transport.

Subject of Research: Atomic simulations of neck–microtubule interactions that bias kinesin stepping direction

Article Title: Supercomputer simulations reveal how the molecular motor kinesin steers itself along tracks

Article References: Supercomputer simulations reveal how the molecular motor kinesin steers itself along tracks. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: kinesin, microtubules, molecular motors, supercomputing, molecular dynamics, Fugaku, intracellular transport, coiled coil, cell biology, Supercomputer, simulations, reveal

Rather than working with each cellular profile in isolation, the system represents cells in a form that makes similarities and differences across tissues, species and disease states measurable. Stephen Quake, a Stanford professor of bioengineering and co-lead author of the two articles, said the project aims to open a new way to think about cell biology. The central challenge is scale: modern cell atlases characterize cells and tissues using measurements across tens of thousands of genes, and some atlases contain profiles from hundreds of millions of cells.

Gene expression indicates which genes a cell is using, allowing researchers to distinguish cell types and identify differences among healthy cells, diseased cells and organisms from different species. Human analysts, however, cannot readily consider all of those variables at once. Quake said the models are algorithms for understanding data too complex to wrap around in the human mind, while describing their intended role as a tool for skilled biologists investigating difficult questions.

The first model, universal cell embedding, established a foundation for the second-generation TranscriptFormer by learning representations from large collections of gene-expression data. In the resulting framework, an embedding is a mathematical representation that captures a cell’s expression profile while allowing comparisons with other cells. Quake described the combined output as a “universal space,” emphasizing that it is not a physical location but a coordinate system in which cells can be positioned according to their relationships.

TranscriptFormer was trained on cell atlases covering humans, yeast, mice, rabbits, chickens, zebrafish, fruit flies, African clawed frogs, malaria-causing parasites, sea urchins, sponges and the roundworm Caenorhabditis elegans. The training strategy was analogous to the masked-word method used in large language models such as ChatGPT. In a language model, portions of text are withheld and the system adjusts its parameters to predict the missing words.

TranscriptFormer used an analogous reconstruction task, but its inputs and prediction targets were gene-expression values rather than words. The resulting model was therefore trained to learn structure in cellular expression data at a scale encompassing many cell types and 12 species. One application was evolutionary comparison across species.

Scientists studying sponges had long wondered which sponge cell type was most closely related to neurons in other animals, a question connected to the evolution of nervous systems. TranscriptFormer found that choanocytes, a type of sponge cell, had gene-expression profiles quite similar to neurons in roundworms and frogs. In the universal space, this similarity provides a way to connect cellular profiles from organisms with very different body plans.

The analysis also challenged the interpretation of a sponge cell called the neuroid cell. Although its name had been assigned because it possessed some neuron-like features, the model found that its expression profile more closely resembled a frog gland cell. The researchers said this result suggests that neuroid cells may be involved in digestion rather than serving as direct neuronal counterparts, while choanocytes may provide a more relevant comparison for studying the emergence of neurons.

The model was also tested on a species that was not among its 12 training organisms. Given gene-expression data from a species outside the training set, TranscriptFormer was able to identify its cell types. The team additionally demonstrated that TranscriptFormer could distinguish healthy cells from diseased cells.

Together, these experiments showed two distinct capabilities: transferring cell-type identification to a previously unseen species and separating cells according to disease state. The release does not provide performance values, name particular diseases or evaluate the system prospectively in patients, so these findings are computational demonstrations rather than evidence of diagnostic readiness or clinical benefit. The model’s broad training set also provides a means of asking whether cellular similarities extend beyond the species used to train it.

By learning from yeast, insects, vertebrates, parasites and simple multicellular organisms, TranscriptFormer can represent relationships that would be difficult to inspect directly in raw expression tables. Researchers can compare the position of a newly profiled cell with reference profiles, place a cell type in an evolutionary context or examine how disease-associated cells depart from a healthy pattern. Researchers are also exploring whether a model of this kind could help design functional cells that do not currently exist.

The release presents this as a future possibility rather than a tested treatment. Such a system might eventually support efforts to design cell-based treatments, but the current work did not generate a therapeutic cell, test a treatment in animals or humans, or demonstrate a clinical outcome. Its value lies in making relationships among large expression datasets more accessible and comparable.

The work is a computational modeling study grounded in a large, cross-species collection of cell-atlas data. Its technical contribution is the combination of a masked-expression training strategy with a shared embedding that can support comparisons across evolutionary distance, unfamiliar species and health states. The biological examples provide concrete tests: choanocytes were compared with roundworm and frog neurons, neuroid cells were compared with frog gland cells, cell types were identified in an unseen species, and healthy and diseased cells were distinguished.

These analyses produce model-based relationships from gene-expression profiles, while the interpretation of cell roles remains a biological inference that can guide further investigation. By placing diverse cells into one mathematical map, TranscriptFormer offers a scalable route for examining cell identity, evolutionary similarity and disease-associated differences across a collection of 112 million profiles.

Subject of Research: Cross-species AI modeling of cellular gene-expression profiles

Article Title: Stanford Medicine-developed AI model opens new horizons for cell biology

Article References: Stanford Medicine-developed AI model opens new horizons for cell biology. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: TranscriptFormer, cell biology, artificial intelligence, gene expression, cell atlases, cross-species modeling, evolutionary biology, computational biology, disease-state classification, Stanford, Medicine-developed, model

Photoreceptor outer segments are described as the light-sensitive antennae of photoreceptor cells. They are continuously renewed throughout a person’s life: each day, photoreceptors add dozens of new disc-shaped membranes containing photopigment to the outer segment while discarding old ones. These membranes must be folded into flat layers and packed precisely, a process Spencer compared to intricate origami. The arrangement maximizes the efficiency of light capture. Earlier work from Spencer’s laboratory found that the proteins produced by PRCD and KIAA1549 participate in the first steps of constructing the membrane material packed inside outer segments. Without these proteins, the membranes are folded and packed in a highly disorganized fashion, leading to cell dysfunction, cell death and blindness.

The five-year award has two specific aims. The first will examine the molecular mechanism through which PRCD keeps the outer segment’s membrane layers tightly adhered during the earliest stages of their morphogenesis. The second will examine how KIAA1549 regulates disc formation through the connecting cilium, a tubular conduit that physically links the outer segment to the photoreceptor cell body. Together, the aims address two molecular processes associated with outer-segment formation: membrane assembly and the cellular structure involved in disc formation. Spencer said the project seeks to unravel the molecular mechanisms underlying the function of PRCD and KIAA1549 in photoreceptor cells, with the stated hope that the work could contribute to the development of novel therapies for blinding retinal disease.

Spencer described the project as addressing a long-standing molecular mystery about vision: how outer segments are built and maintained. He said there is a complete lack of therapies for retinal diseases caused by defects in outer-segment formation and that a major barrier to developing treatments is an incomplete understanding of how the light-sensing structures are built. The R01 award therefore funds mechanistic investigation into a knowledge gap associated with inherited retinal disease. The announcement identifies no drug candidate, clinical trial or immediate therapeutic application resulting from the grant. Instead, its stated purpose is to define how the two genes’ proteins operate in photoreceptors, with potential therapeutic development presented as a hoped-for consequence of that work rather than an established outcome.

Subject of Research: Molecular mechanisms of photoreceptor outer-segment formation in inherited retinal disease

Article Title: Upstate researcher receives $2.3 million grant to study causes of blindness

Article References: Upstate researcher receives $2.3 million grant to study causes of blindness. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: photoreceptor outer segments, PRCD gene, KIAA1549 gene, retinitis pigmentosa, retinal degeneration, photoreceptor renewal, National Eye Institute, inherited blindness, cell biology, Upstate, researcher, receives

The switch in question is ARF-1, a small regulatory protein belonging to a family of molecular timers known as small GTPases. These proteins act like toggle switches, alternating between an active state and an inactive state, and in doing so they control when and where other cellular machines can engage with their targets. According to the new study, published in the Journal of Cell Biology, ARF-1 performs exactly this role during the loading of recyclable cargo into transport vesicles, the small membrane bubbles that ferry molecules between cellular compartments. When ARF-1 is active, the adaptor complex AP-1 can bind the cargo destined for recycling. When ARF-1 switches off, AP-1 releases its grip, and the cargo is free to enter the recycling vesicle, where it is captured once again for the journey back to the cell surface.

Dr. Jachen Solinger, first author of the study, described the behavior of ARF-1 as that of a true molecular switch during vesicle loading. Crucially, the protein must not stay in either state for long. If ARF-1 remains permanently switched on, or permanently switched off, recycling is disrupted. The cell therefore depends not on the presence of the protein alone, but on its continuous cycling between active and inactive forms. This cycling is directly coupled to the transfer of cargo: each on-and-off transition gives AP-1 a chance to let go of one molecular passenger and prepare for the next, turning a simple biochemical oscillation into a sorting mechanism.

The new findings build on earlier work from the Spang laboratory that revealed the protein machinery FERARI, which plays a central role in endosomal recycling. Recycling vesicles dock onto sorting endosomes and partially fuse with them in a transient encounter that researchers have dubbed kiss-and-run. During this brief contact, the vesicle remains intact and can detach again, and material is exchanged in both directions between the two compartments. What remained unclear after the discovery of FERARI was how the cell decides which proteins actually make their way into the recycling vesicle during these fleeting meetings. The new study provides the missing piece: FERARI organizes the kiss-and-run contact site itself, while ARF-1 and AP-1 coordinate the actual transfer of cargo across it.

One of the most striking aspects of the mechanism is that it does not demand perfection in a single pass. Recycling vesicles can dock repeatedly onto endosomes and exchange cargo several times, giving the cell repeated opportunities to correct sorting errors. Much like a recycling facility in which items are checked and re-sorted over several rounds rather than sorted once and for all, cellular components are progressively refined toward their correct destination. This iterative quality may explain how cells achieve high fidelity in a process that operates at enormous throughput and in the crowded, noisy environment of the cytoplasm.

The experimental evidence for the mechanism spans several model organisms. The researchers observed key aspects of ARF-1-dependent sorting in the roundworm C. elegans, as well as in yeast and in mammalian cells. This breadth suggests that the principle of cellular recycling described in the study has been conserved throughout evolution, a hallmark of mechanisms that are fundamental to life rather than incidental to one lineage. When the researchers interfered with ARF-1 activity, cellular recycling slowed down, and both too little and too much ARF-1 activity impaired cargo transport, underscoring that the timing of the switch, not merely its presence, is what matters.

For Anne Spang, the evolutionary dimension of the work is among its most intriguing aspects. It is fascinating, she noted, to see how cells take basic principles they have developed and reuse them in very different contexts, rather than reinventing the wheel every time. Similar switching mechanisms involving small GTPases and adaptor proteins are already known from other cellular transport processes, including the well-characterized trafficking pathways that move material between the Golgi apparatus and other compartments. The new study now shows that such a principle is also deployed for sorting during kiss-and-run recycling, extending a proven biochemical strategy into a context where it had not previously been demonstrated.

The broader significance of the work lies in what efficient recycling means for the cell’s economy. By reusing receptors and transport proteins instead of constantly producing them anew, cells save energy and maintain rapid control over their surface composition, which is essential for signaling, nutrient uptake, and communication with the environment. Disruptions to such transport pathways have been linked to various diseases, including neurological disorders, where the precise trafficking of receptors at synapses is critical for normal brain function. Understanding the molecular switches that keep recycling on track may therefore illuminate not only basic cell biology but also the origins of disease states in which trafficking goes awry.

The findings provide another piece of the puzzle in understanding the cell’s complex transport system. They reveal how FERARI, ARF-1 and AP-1 work together to ensure that the right cargo reaches the right destination, coupling the physical architecture of endosomal contact sites to the biochemical rhythm of a GTPase switch. As researchers continue to map the cell’s internal logistics, the picture that emerges is one of elegant reuse: a handful of molecular principles, cycled and combined in different contexts, generating the extraordinary diversity of trafficking behaviors that keep cells alive. In the case of ARF-1, the humble act of switching on and off turns out to be the very signal that directs the cargo home.

Subject of Research: The role of the ARF-1 protein and the AP-1 adaptor complex in sorting recyclable cargo during kiss-and-run endosomal recycling in cells.

Article Title: Cellular recycling: A molecular switch directs the cargo

Article References: Cellular recycling: A molecular switch directs the cargo. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: ARF-1, AP-1 adaptor complex, endosomal recycling, kiss-and-run, FERARI, vesicle transport, cell biology, C. elegans, Journal of Cell Biology, University of Basel, cargo sorting, small GTPases

CTR1, encoded by the SLC31A1 gene, is best known for importing copper across the plasma membrane with high affinity. Copper is an essential trace metal that serves as a catalytic cofactor for enzymes involved in respiration, antioxidant defense, iron metabolism, and neurotransmitter synthesis. Yet the review highlights a puzzle: several CTR1-dependent signaling outputs cannot be readily explained by bulk intracellular copper abundance alone. CTR1 has been linked to growth-factor signaling, kinase regulation, redox control, intracellular trafficking, and copper-sensitive disease states. If CTR1 were simply a passive importer, its effects would track total cellular copper levels. Instead, the evidence points to a protein whose conformational state and interaction network actively shape signaling outcomes.

Two experimental observations provide the strongest foundation for the transporter–organizer framework. First, elevated intracellular copper rapidly remodels the oligomeric state of CTR1 before any detectable internalization of the protein occurs. This remodeling is associated with acute suppression of copper uptake, suggesting that the transporter’s quaternary structure acts as a fast molecular switch. Rather than waiting for the slower process of endocytosis to remove CTR1 from the membrane, the cell appears to tune the transporter’s activity by changing how CTR1 subunits associate with one another. This rapid structural plasticity implies that CTR1’s oligomeric state is a dynamic variable that the cell can adjust on timescales relevant to signaling, not just metal homeostasis.

Second, CTR1 engages vascular endothelial growth factor receptor 2, or VEGFR2, through a redox-sensitive interaction that supports receptor signaling and co-internalization. VEGFR2 is the primary receptor driving angiogenesis, the formation of new blood vessels, making this partnership potentially significant for vascular biology and disease. Crucially, transport-defective and redox-defective CTR1 mutants functionally separate proximal VEGF signaling from canonical copper transport. In other words, a CTR1 protein that cannot move copper can still support VEGFR2 signaling, and a CTR1 protein that cannot engage in redox-sensitive interactions loses signaling support even if its transport function remains intact. This genetic dissection demonstrates that CTR1’s transport role and its signaling-organizer role are mechanistically separable, a finding with major implications for how researchers design experiments targeting this protein.

Building on these observations, Wen and Chen propose two testable hypotheses. The first concerns local copper delivery: transport-competent CTR1 may generate local copper-delivery windows that favor copper capture and exchange in the immediate vicinity of the transporter. Rather than copper simply diffusing evenly throughout the cytoplasm, the model suggests that CTR1 creates microdomains of copper availability that proximal metallochaperones and target proteins can exploit. This concept of local copper handling echoes a broader theme in cell biology, where spatial organization within the cytoplasm often matters as much as bulk concentration. The second hypothesis concerns trafficking: Rab-directed trafficking may tune the persistence and resetting of CTR1-associated signaling states. Rab GTPases are the master regulators of vesicular transport, directing cargo between membrane compartments. If CTR1’s signaling outputs depend on which endosomal compartments it visits, then its trafficking itinerary becomes a variable that cells can manipulate to control signaling duration and reset.

The review emphasizes the importance of separating established observations from testable models, a discipline that distinguishes rigorous framework-building from speculation. The authors outline four variables for dissecting CTR1-dependent outputs: transport competence, local copper handling, specific protein interactions, and trafficking history. Each variable is experimentally addressable. Transport competence can be assessed with transport-defective mutants. Local copper handling can be probed with copper-sensitive probes and metallochaperone interactions. Protein interactions can be mapped with co-immunoprecipitation and proximity labeling. Trafficking history can be tracked with live-cell imaging and Rab-specific perturbations. By defining these variables clearly, the review provides a roadmap for researchers seeking to understand how CTR1 coordinates its multiple cellular roles.

The implications extend beyond copper biology. CTR1 provides a model for understanding how membrane transporters can coordinate metabolism, membrane traffic, and signal transduction within a single molecular entity. This concept challenges the traditional division of labor in cell biology, where transporters are often treated as separate from signaling proteins. If CTR1’s conformational state, interaction network, and trafficking itinerary all feed into its signaling outputs, then other transporters may operate similarly. The framework invites researchers to reconsider transporters across the genome as potential signaling organizers whose metal flux, protein partnerships, and membrane itineraries are coupled rather than independent.

The review also touches on disease relevance. Copper dysregulation has been implicated in a range of conditions, including neurodegenerative diseases, cancer, and cardiovascular disorders. The recent recognition of cuproptosis, a form of copper-dependent cell death, has intensified interest in how cells manage copper flux and signaling. If CTR1’s transporter–organizer framework holds up experimentally, it could offer new angles for therapeutic intervention. Rather than simply blocking copper uptake, future approaches might target CTR1’s interaction with VEGFR2, its oligomeric remodeling, or its trafficking itinerary. Such strategies could modulate signaling outputs without wholesale disruption of copper homeostasis, potentially reducing side effects associated with broad copper chelation.

Methodologically, the framework draws on membrane biophysics, live-cell imaging, and genetic perturbation. The rapid oligomeric remodeling of CTR1 in response to copper suggests that single-molecule or single-transporter measurements could reveal how individual CTR1 complexes transition between states. The redox-sensitive nature of the CTR1–VEGFR2 interaction points to the importance of the cellular redox environment in tuning signaling. The involvement of Rab GTPases in CTR1 trafficking suggests that endosomal sorting decisions are not just housekeeping but active determinants of signaling persistence. Together, these methodological threads weave a picture of CTR1 as a protein whose behavior emerges from the interplay of structural dynamics, chemical environment, and membrane traffic.

As research on CTR1 continues, the transporter–organizer framework offers a lens through which to view the protein’s many facets. The four variables identified by Wen and Chen provide a structured approach to future experiments, and the two testable hypotheses give researchers concrete directions to pursue. Whether CTR1’s local copper-delivery windows and Rab-directed trafficking tuning hold up under experimental scrutiny remains to be seen, but the framework itself represents a significant conceptual advance. By treating CTR1 as a coupled system rather than a collection of separate functions, the review opens new avenues for understanding how membrane proteins integrate metal transport, signaling, and cellular organization. The coming years of CTR1 research promise to reveal whether this framework generalizes to other transporters and how cells exploit the coupling between metal flux and signaling state.

Subject of Research: CTR1 as a transporter–organizer in copper signaling: molecular state, trafficking, and protein interactions

Article Title: CTR1 as a transporter–organizer in copper signaling: molecular state, trafficking, and protein interactions

Article References: Wen, M.-H., & Chen, T.-Y. (2026). CTR1 as a transporter–organizer in copper signaling: molecular state, trafficking, and protein interactions. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06491-9

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06491-9

Keywords: copper, CTR1, cell signaling, membrane transport, VEGFR2, cell biology, trafficking, metallochaperones, transporter, organizer, signaling, molecular

The AP-1 complex recognizes its cargo through two well-characterized sorting signals embedded in the cytoplasmic tails of membrane proteins. The first is a tyrosine-based motif, written Yxxφ, where φ stands for a bulky hydrophobic residue; this motif docks onto the µ1 subunit of AP-1. The second is a dileucine-based motif of the sequence [D/E]xxxL[L/I], which binds to the σ1 subunit. Because these two binding sites sit on different subunits of the same adaptor, researchers had generally assumed that a single cargo tail engages one AP-1 complex at a time, with oligomerization of adaptors driven by ARF1 and by clathrin’s self-association.

Deng and colleagues took a reductionist approach to test this assumption. They reconstituted coat assembly in a test tube using purified human AP-1, a GTP-locked version of ARF1 that lacked its N-terminal amphipathic helix, and a synthetic cargo peptide carrying both sorting motifs. When all three ingredients were combined, robust clathrin cages assembled spontaneously. Strikingly, when the cargo peptide was omitted, ARF1 and AP-1 failed to form any higher-order structures at all. This simple but powerful experiment demonstrated that cargo is not a passive client but a necessary ingredient for building the adaptor lattice.

To understand how the peptide achieves this, the team turned to cryoelectron microscopy. The structures revealed a previously unrecognized interface between AP-1 complexes, formed by the µ1 subunit of one tetramer and the σ1 subunit of a neighboring one. The cargo peptide acts as a physical bridge: its tyrosine-based motif is gripped by the µ1 subunit of one AP-1 complex while its dileucine-based motif, located downstream on the same tail, is simultaneously bound by the σ1 subunit of a second complex. A single cargo molecule can therefore tether two adaptors together, and multiple cargo molecules can stitch many adaptors into a two-dimensional lattice on the membrane surface.

The functional importance of this µ1–σ1 interface was tested by mutational analysis. Disrupting the interface reduced clathrin cage assembly in vitro to roughly the same extent as disrupting the ARF1-mediated contacts between AP-1 complexes. In other words, the cargo bridge contributes as much to lattice stability as the GTPase-driven scaffold that scientists have studied for years. This is a remarkable result, because it implies that the classical ARF1-centric model of coat assembly captures only half of the stabilizing interactions.

To confirm that the mechanism operates in living systems rather than only in purified protein mixtures, the researchers performed rescue experiments in HeLa cells and in zebrafish lacking either the µ1 or the σ1 subunit of AP-1. Re-expression of the wild-type subunits restored normal function and rescued the knockout phenotypes. In contrast, mutant versions of the subunits engineered to disrupt the µ1–σ1 interface failed to rescue the defects in both the cultured cells and the fish. The cargo-bridging interface is therefore not an artifact of in vitro reconstitution but a physiologically essential element of AP-1 function.

The implications reach well beyond AP-1 itself. The same tyrosine-based and dileucine-based sorting motifs are recognized by the related adaptor complexes AP-2, which mediates endocytosis at the plasma membrane, and AP-3, which sorts cargo to lysosome-related organelles. If cargo bridging drives AP-1 oligomerization, it is plausible that a similar mechanism operates across the adaptor protein family, making cargo-driven multimerization a general principle of protein transport. This would also help explain how AP-3 can concentrate cargo independently of clathrin, as previous work has suggested, since a cargo-bridged adaptor lattice could concentrate cargo even without an outer clathrin shell.

The study also reframes the architecture of coated vesicle formation. Clathrin multimerization has long been considered the driving force behind adaptor lattice formation, but the new work raises the question of whether clathrin is required for adaptor oligomerization at all, or whether cargo- and ARF1-mediated stabilization is sufficient to nucleate a lattice. It further suggests that coat assembly is driven not only by interactions among adaptors, coat proteins, and GTPases, but also by the cargo molecules that the coat is built to carry. Cargo concentration at the budding site, in this view, is a direct consequence of the assembly mechanism itself rather than a byproduct of it.

Several questions remain open. The cryo-EM structures were obtained in the absence of membranes, so it will be important to recapitulate the findings on cargo-loaded liposomes and ultimately in cells, where membrane curvature and lipid composition could modulate the bridging interaction. It is also unclear whether the µ1–σ1 interface observed between AP-1 tetramers can form between other adaptor complexes, given that different adaptors show distinct patterns of interaction with ARF1. Deng and colleagues note that their AP-1 assembly may represent an intermediate conformational state between previously described open and closed AP-1 trimers, implying that additional interactions beyond cargo binding are needed for AP-1 to reach the open conformation in which it can engage the membrane and bind cargo and clathrin simultaneously.

What is already clear is that the cargo bridge concept fundamentally changes how scientists view the construction of transport vesicles. Rather than being passive clients that stabilize a particular adaptor conformation, cargo proteins actively participate in the assembly of the molecular machine that carries them. This cargo-driven mechanism provides an elegant explanation for how adaptors achieve site-specific multimerization on membranes and how the right proteins are concentrated into the right vesicles. As researchers test these ideas across the adaptor family and in membrane-reconstituted systems, the humble cargo molecule may turn out to be one of the principal architects of its own journey through the cell.

Subject of Research: Cargo-bridged oligomerization of the AP-1 adaptor complex in clathrin-coated vesicle transport

Article Title: The cargo bridge — how cargo binding triggers AP-1 oligomerization

Article References: Stockhammer, A., & Ungermann, C. (2026). The cargo bridge — how cargo binding triggers AP-1 oligomerization. Cell Research. https://doi.org/10.1038/s41422-026-01316-7

Image Credits: AI Generated

DOI: 10.1038/s41422-026-01316-7

Keywords: AP-1 adaptor complex, clathrin-coated vesicles, cargo sorting motifs, ARF1 GTPase, cryo-EM, oligomerization, trans-Golgi network, endosomal transport, µ1–σ1 interface, vesicle trafficking, cell biology, adaptor protein complexes

Sperm dimorphism, the production of two different sperm types by a single male, is considered to play a role in reproductive strategies such as sperm competition, in which the gametes of different males vie for access to eggs. In the strawberry conch, parasperm are equipped with numerous globular bodies, membrane-bound spheres packed within the cell, as well as cilia-based undulating membranes that ripple along the cell surface. For years, researchers have wondered what the globular bodies contain and what function they serve, and how the undulating membranes propel these unusual cells. The new work addresses the first of these questions by describing, step by step, the process and regulation of globular body exocytosis, the cellular mechanism by which internal vesicles fuse with the cell’s outer membrane and release their cargo to the exterior.

The researchers found that when parasperm are diluted in artificial seawater, a trigger that mimics the conditions the cells encounter after ejaculation, exocytosis begins. The process occurs through membrane fusion between the membrane surrounding each globular body and the plasma membrane of the cell itself. Once the two membranes merge, the contents of the globular bodies are released outside the cell. This is a classic exocytotic pathway, the same fundamental mechanism that neurons use to release neurotransmitters and that many secretory cells use to export hormones, enzymes and other molecules. Its presence in parasperm suggests that these non-fertilizing cells are not passive passengers in the reproductive tract but active participants capable of delivering specific substances into their environment.

Central to the discovery is the role of calcium. The team demonstrated that exocytosis was markedly suppressed under calcium-deficient conditions, meaning that when extracellular calcium was removed or reduced, the globular bodies largely failed to fuse with the plasma membrane and release their contents. Conversely, when the researchers artificially raised the level of calcium inside the cells using an ionophore called A23187, a chemical that shuttles calcium ions across membranes, exocytosis was enhanced. This pattern establishes calcium as a key regulatory signal for the discharge process, consistent with the way calcium triggers membrane fusion in many other secretory systems across the animal kingdom. In parasperm, then, an influx of calcium from the surrounding seawater appears to be the switch that converts the cells from a storage state into a secretory one.

Perhaps the most surprising finding concerns potassium, another ion abundant in seawater. The researchers observed that both the complete absence of extracellular potassium and high extracellular potassium concentrations enhanced exocytosis. This bidirectional effect, in which two very different potassium conditions produce the same outcome, hints at a more complex regulatory circuitry than a simple calcium trigger alone. It suggests that potassium dynamics across the parasperm plasma membrane may modulate the exocytotic machinery, possibly through effects on membrane potential or on the activity of ion channels that indirectly influence intracellular calcium levels. Untangling exactly how potassium exerts this dual effect will be a task for future work, but the observation alone broadens the picture of how these cells are controlled after they leave the male.

The study did not stop at exocytosis. The team also examined the behavior of the undulating membranes, the cilia-driven structures that generate the parasperm’s characteristic swimming motion. Here they found a striking dissociation between the two cellular activities. The direction of wave propagation along the undulating membrane was reversed by high extracellular calcium concentrations or by treatment with the calcium ionophore A23187, whereas the wave propagation was suppressed under high extracellular potassium conditions. In other words, calcium and potassium influence the undulating membrane, but in ways that do not simply mirror their effects on globular body release.

This divergence is the study’s most important conceptual contribution. Because calcium enhances exocytosis while reversing the direction of membrane waves, and because potassium manipulations enhance exocytosis while suppressing wave propagation, the two processes must be governed by different regulatory mechanisms within the same cell. The globular bodies and the undulating membranes, though housed together in a single parasperm cell, respond to ionic cues through separable pathways. This kind of intracellular division of labor allows the cell to fine-tune its secretory output and its swimming behavior independently, a level of control that could be crucial for whatever role parasperm play inside the female reproductive tract.

What might that role be? The authors suggest that their findings provide novel insight into the post-ejaculatory behavior and potential function of parasperm within the female reproductive tract of gastropods. One plausible scenario, consistent with theories of sperm dimorphism and sperm competition, is that parasperm act as helpers or manipulators of the environment through which eusperm must travel. If globular bodies release their contents upon exposure to the ionic conditions of the female tract, those contents could alter the viscosity of reproductive fluids, nourish fertilizing sperm, or influence female physiology in ways that favor the male’s gametes. The new demonstration that release is an active, calcium-gated event makes such hypotheses far more testable than before, because researchers can now manipulate the trigger experimentally and observe the consequences.

The technical achievement of the study lies in connecting a visible morphological feature, the globular bodies, to a defined molecular mechanism, calcium-dependent membrane fusion, and then showing that this mechanism is regulated differently from the cell’s motility machinery. By using artificial seawater dilution as a physiological trigger, calcium deprivation to block the process, the ionophore A23187 to force intracellular calcium upward, and potassium manipulations to probe a second regulatory axis, the team built a coherent pharmacological map of parasperm behavior. The parallel analysis of undulating membrane wave direction adds an unexpected layer, revealing that the same ions that promote secretion can redirect or suppress movement, depending on concentration and context.

Beyond the strawberry conch, the findings resonate with broader questions in evolutionary biology and cell physiology. Sperm dimorphism has evolved independently in several animal lineages, and understanding how parasperm function after ejaculation may illuminate why males invest so heavily in cells that cannot fertilize eggs. At the same time, the work offers a tractable experimental system for studying exocytosis and cilia-driven motility in a single cell with two separable outputs. As researchers begin to identify the contents of the globular bodies and trace their effects inside the female reproductive tract, the humble parasperm of the strawberry conch may turn out to be far more than an evolutionary curiosity, and instead a sophisticated agent in the competitive drama of marine reproduction.

Subject of Research: Calcium-dependent exocytosis of parasperm globular bodies in the strawberry conch gastropod Conomurex luhuanus

Article Title: Ca 2+ -dependent exocytosis of parasperm globular bodies in the strawberry conch gastropod Conomurex luhuanus

Article References: Yokoya, R., Shiba, K., Morita, M., & Inaba, K. (2026). Ca2+-dependent exocytosis of parasperm globular bodies in the strawberry conch gastropod Conomurex luhuanus. PLOS One, 21(10), e0360627. https://doi.org/10.1371/journal.pone.0360627

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360627

Keywords: parasperm, sperm dimorphism, exocytosis, calcium signaling, gastropod, Conomurex luhuanus, reproductive biology, undulating membrane, marine invertebrates, sperm competition, cell biology, ionophore A23187

The new studies, published in the journal Molecular Cell, come from the Institute of Biochemistry at ETH Zurich, led by professors Ulrike Kutay, Karsten Weis and Madhav Jagannathan, together with colleagues from the University of Fribourg, the Paul Scherrer Institute, the European Molecular Biology Laboratory and RWTH Aachen University. Their work demonstrates for the first time that two molecular tools are required for the two membranes of the nuclear envelope to fuse correctly: an enzyme called Torsin-1A and a complex built from multiple copies of a protein known as CLCC1 in human cells. In fungi, the same job is carried out by the related proteins Brl1 and Brr6, a detail that underscores how deeply conserved this fusion machinery is across the tree of life, from yeast to flies to mammals.

Until now, scientists knew that a defect in Torsin-1A brings nuclear pore assembly to a halt, with fatal consequences for the cell, but the mechanics of the partnership remained obscure. The ETH-led teams have now shown how the cooperation works at a molecular level. Torsin-1A binds to CLCC1 and helps several of these building blocks join together into a larger assembly. Once the complex is formed, Torsin-1A escorts it to the site of pore assembly on the nuclear envelope, positioning it exactly where the two membranes must merge. In other words, the enzyme acts both as a chaperone that assembles the fusion machinery and as a guide that delivers it to its workplace.

The fusion mechanism itself caught the researchers off guard. Using computer simulations, the teams hypothesised that CLCC1 proteins, or their fungal relatives Brl1 and Brr6, forge a temporary channel inside the nuclear envelope. This channel allows lipid molecules to flow across the bilayers, a movement that is normally restricted. The resulting lipid exchange destabilises the local membrane structure and forces the inner and outer nuclear membranes to fuse with one another. Daria Maslennikova, the first author of one of the studies, who completed her doctorate in Kutay’s group, said the teams had not expected such a mechanism. The idea that a protein complex could catalyse membrane merger by opening a lipid conduit, rather than by directly pulling the membranes together, adds a striking new entry to the catalogue of ways cells remodel their internal boundaries.

The stakes of this microscopic operation become clear when it fails. In higher organisms such as mammals, flies and fungi, the genome is enclosed in a nucleus bounded by a nuclear envelope of two stacked membranes. Nuclear pores are the only conduits through the envelope, and each one must be assembled by fusing the inner and outer membranes at a single point. If fusion stalls, pore assembly stalls with it, and the vital traffic of proteins and RNA between nucleus and cytoplasm is compromised. Cells that cannot build their pores cannot survive, which is why the machinery described in the new papers is so critical, and why mutations that impair it produce disease.

That disease is primary dystonia, specifically DYT1 dystonia, a rare and currently incurable neurological movement disorder that affects roughly six in every 100,000 people worldwide. The condition usually begins in childhood and manifests as painful muscle spasms and abnormal postures that can be severely disabling. Treatment options today are limited to alleviating symptoms, chiefly through physiotherapy or injections of botulinum toxin, and no therapy addresses the underlying cause. The new findings give that cause a molecular identity: in people with primary dystonia, a mutation in the Torsin1A gene prevents the Torsin1A molecule from functioning correctly, leaving the CLCC1 complex without sufficient support to carry out its membrane-fusion duties during nuclear pore assembly.

Crucially, the research also points toward a possible intervention. In experiments with fruit flies, the researchers showed that artificially increasing CLCC1 levels can correct defects that arise, for example, during germline cell development, even when Torsin-1A function is impaired. This suggests that the CLCC1 complex is the limiting factor in the fusion reaction, and that boosting its availability can compensate for a weakened partner enzyme. The result transforms a fundamental cell-biological discovery into a concrete therapeutic hypothesis: if the balance between Torsin1A and CLCC1 can be shifted in patients, the cellular defects that drive dystonia might be partially or fully corrected.

One lingering mystery is why the same Torsin1A mutation produces symptoms in only about a third of the people who carry it. The researchers speculate that person-specific differences in CLCC1 protein levels may contribute to this variable penetrance, meaning that individuals with naturally higher amounts of the fusion protein might be better buffered against the effects of the faulty enzyme. If that hypothesis holds up, measuring or modulating CLCC1 levels could become not only a treatment strategy but also a way to predict who is at risk. Ulrike Kutay noted that the finding helps the team envision a possible therapy, with one potential strategy being to boost CLCC1 production or to prevent the protein from being broken down inside cells. This approach has already shown success in human cell cultures and in fruit flies, although whether it can be translated to patients must first be evaluated in further clinical studies and drug screens.

Beyond the immediate therapeutic horizon, the work opens several new lines of inquiry. The teams plan to investigate additional relatives of Torsin1A; human cells contain other torsins whose precise functions remain unknown, and the tools and concepts developed in this study should now make those functions accessible to experiment. In collaboration with plant scientists, Kutay, Weis, Jagannathan and their colleagues also aim to determine whether similar molecules and fusion mechanisms operate in plants, which would indicate that the pathway is a universal feature of eukaryotic life. For the researchers, the journey from a basic question about how cells punch holes in their own nucleus to a candidate therapy for a devastating childhood disease is a powerful illustration of why curiosity-driven research matters. Fundamental discoveries about the smallest building blocks of life, they argue, remain the most reliable starting point for the medicines of tomorrow.

Subject of Research: Membrane fusion mechanism of nuclear pore complex assembly and its link to DYT1 dystonia

Article Title: How cells make their command center accessible

Article References: How cells make their command center accessible. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nuclear pore complex, nuclear envelope, membrane fusion, Torsin-1A, CLCC1, DYT1 dystonia, ETH Zurich, Molecular Cell, cell biology, neurological disorder, gene mutation, basic research

The material at the center of the study belongs to a family of compounds known as Metal-Organic Frameworks, or MOFs. These are crystalline structures built from metal ions or clusters connected by organic linker molecules, arranged in a repeating three-dimensional lattice. The result is a material with an extraordinarily high internal surface area and a modular architecture: by swapping the metal nodes or the organic linkers, chemists can tune the framework’s porosity, stability, reactivity, and interactions with biological systems. This design flexibility has made MOFs attractive for applications ranging from gas storage and separation to catalysis, sensing, and drug delivery. Their significance to chemistry was underscored in 2025, when the Nobel Prize in Chemistry was awarded to Susumu Kitagawa, Richard Robson, and Omar Yaghi for the development of these reticular materials.

Applying such materials to mitochondria, however, is far from straightforward. Mitochondria are delicate, double-membraned organelles whose function depends on the precise organization of protein complexes embedded in their inner membrane. Harsh chemical conditions, osmotic stress, or physical disruption can quickly collapse the membrane potential that drives energy production, rendering the organelles useless. Any coating strategy must therefore be gentle enough to preserve the structural integrity and bioenergetic activity of the mitochondria while still forming a stable layer on their outer surface. This tension between chemical modification and biological preservation is precisely the challenge the Milanese teams set out to solve.

The collaboration itself is a deliberate marriage of disciplines. It was conducted within the BraiNs joint laboratory, inaugurated in 2023, which brings together the mitochondrial biology expertise of the group led by Dr. Valeria Tiranti of the Medical Genetics and Neurogenetics Unit at Fondazione IRCCS Istituto Neurologico Carlo Besta, who also heads the Fondazione Mariani Centre for Paediatric Mitochondrial Diseases, and the chemical and materials engineering expertise of the group led by Professor Pierangelo Metrangolo at Politecnico di Milano. The pairing is apt: understanding what mitochondria need in order to survive manipulation is as important as knowing how to synthesize and functionalize a framework material around them.

The researchers developed a process that allows the surface of mitochondria to be coated directly with MOFs, using the materials in very small quantities. This minimization of dose is a key technical detail. At micromolar concentrations, as the study’s title indicates, the engineered frameworks interact with the organelles without overwhelming them or triggering the kind of chemical stress that would compromise their membranes. The most important finding of the work is what happened after the coating was applied: the mitochondria retained their ability to produce energy. In other words, the bioenergetic machinery inside the wrapped organelles continued to operate, suggesting that the coating does not interfere with the transport processes and membrane functions on which ATP synthesis depends.

For the field of mitochondrial medicine, the implications are considerable. Mitochondrial diseases are a group of often rare genetic disorders caused by defects in the organelles’ energy-producing machinery, which can reside either in the cell’s nuclear DNA or in the small circular genome that mitochondria carry themselves. Because mitochondria supply power to every tissue, these diseases can affect multiple organ systems, but they tend to strike hardest at the brain, muscles, heart, and other high-energy tissues. Treatment options today remain limited, consisting largely of supportive care, symptom management, and attempts to slow disease progression rather than correct the underlying defect. Any technology that allows mitochondria to be safely modified, protected, or delivered to cells would therefore represent a meaningful addition to a sparse therapeutic toolbox.

One application that the researchers highlight, still at an experimental stage, is mitochondrial transplantation. The idea behind this strategy is conceptually simple: transfer healthy mitochondria into cells whose own organelles are not functioning properly, thereby supplementing or replacing the defective energy supply. The practical obstacles, however, are substantial. Mitochondria isolated from donor cells are fragile, and getting them to survive the transfer process and then integrate productively into recipient cells is difficult. A specifically designed coating could, in principle, act as a protective and functional shell during this journey, stabilizing the organelles outside their native environment and facilitating their interaction with the recipient cell once they arrive.

Professor Metrangolo emphasized that the ability to modify the coating opens new research prospects. In the future, it should be possible to develop MOFs with different characteristics depending on the objective, creating coatings capable of performing specific functions. Because the chemical composition of a Metal-Organic Framework can be engineered with great precision, a coating could be designed to carry targeting molecules, to release protective agents, to respond to particular biological signals, or to alter how the mitochondria interact with their surroundings. In this sense, the coated organelle becomes something closer to a programmable biological unit: a living power plant wrapped in a tunable synthetic shell whose properties can be adjusted for the task at hand.

Dr. Tiranti echoed this vision, noting that a specifically designed coating could in the future facilitate both the transfer of mitochondria and the interaction between the transplanted organelles and the cells that receive them. Yet the researchers are careful to stress that this prospect has not yet been demonstrated. Further studies will be needed to determine whether a MOF coating actually offers a tangible advantage in transplantation compared with uncoated mitochondria. The current work establishes feasibility and safety of the coating process itself; whether the approach translates into improved outcomes for cell models, and eventually for patients with mitochondrial disease, remains an open question that the team and others in the field will now pursue.

The research was carried out as part of the doctoral project of Marco D’Amato, funded by the Calabria HUB for Innovative and Advanced Research (Cal.Hub.Ria) through funding under the Italian Ministry of Health’s Health Operational Plan. That a proof-of-concept of this kind emerged from a PhD candidate’s project reflects the way targeted national funding can support interdisciplinary work at the interface of materials chemistry and biomedicine. For now, the study stands as a striking demonstration that two of chemistry’s most celebrated synthetic materials can be brought into intimate contact with one of biology’s most delicate structures without breaking it. If subsequent experiments confirm that these engineered shells can genuinely enhance mitochondrial survival, delivery, and function, the humble power plants of the cell may one day be rebuilt, wrapped, and transplanted with the same precision that chemists apply to any other molecular architecture.

Subject of Research: Coating mitochondria with metal-organic framework materials while preserving their energy-producing function

Article Title: Coating mitochondria without damaging them: research opens a possible avenue for addressing their dysfunction

Article References: Coating mitochondria without damaging them: research opens a possible avenue for addressing their dysfunction. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: mitochondria, metal-organic frameworks, MOF, mitochondrial diseases, mitochondrial transplantation, bioenergetics, nanomaterials, cell biology, materials chemistry, Nobel Prize in Chemistry, ATP production, neurogenetics

The technical challenge was considerable. Clathrin-coated pits are tiny, dome-shaped invaginations of the plasma membrane that mature into vesicles roughly 60 to 120 nanometres in diameter, involving more than 50 proteins recruited in a tightly choreographed sequence of initiation, maturation and membrane fission. Earlier attempts to detect lipid sorting relied on indirect evidence, such as tracking fluorescent lipid analogues or lipid-binding toxins like cholera toxin to endosomes. Both approaches carry serious caveats. Fluorescent analogues carry bulky chromophores that alter the chemical behaviour of the parent lipid, while toxins such as cholera toxin actively cluster their lipid targets, distorting the very sorting mechanisms they are meant to reveal. Moreover, measuring lipid uptake at the level of endosomes conflates sorting at the plasma membrane with sorting inside the endosomal system itself.

To sidestep these problems, the team turned to bifunctional lipid probes, a class of near-native lipids modified with a diazirine group that can be photochemically cross-linked to neighbouring biomolecules, and an alkyne handle used for fluorescent labelling by click chemistry. The combined effect of these modifications on lipid behaviour is approximately equivalent to adding a single extra double bond, meaning the probes closely mimic the metabolism and biophysical properties of their natural counterparts. The researchers loaded the probes into the outer leaflet of the plasma membrane of living cells, then captured snapshots by firing a high-power 365-nanometre LED pulse that cross-linked the lipids in place, followed immediately by chemical fixation. The entire cross-linking and fixation sequence was completed in under ten seconds, fast enough to freeze clathrin-coated pits at different stages of maturation without disturbing major membrane trafficking pathways.

Imaging was performed in a well-characterized SK-MEL2 melanoma cell line expressing dynamin-2 fused to green fluorescent protein, with clathrin and adaptor protein 2 (AP2) revealed by immunostaining. Because clathrin and the lipid channel were imaged at super-resolution while AP2 and dynamin-2 were acquired in confocal mode, the team could classify each pit by its maturation stage: clathrin-only structures, early-stage pits positive for AP2 and clathrin, and late-stage pits that additionally recruit dynamin-2, the protein that catalyses the final membrane scission. For each pit, the mean lipid fluorescence inside the segmented clathrin mask was divided by the mean signal of the surrounding membrane, yielding an apparent partitioning value. For the phosphatidylcholine probe PC(16:0|Y), this apparent partitioning rose from 1.64 in early pits to 2.09 in late pits, a trend that at first glance suggested lipids accumulate strongly inside forming vesicles.

That apparent enrichment, however, turned out to be largely an optical illusion. As a pit invaginates, the curved membrane folds a large surface area into a small volume, and even with STED resolution the microscope’s point spread function blurs this geometry, artificially concentrating the signal at the site of the invagination. To correct for this, the team built three-dimensional models of pit shapes based on previously published electron microscopy data from the same cell line, combining partial orbs for the pit dome with catenoids for the neck. Assuming an even distribution of point emitters across these modelled membranes, they simulated STED images using the experimentally measured point spread function. The model predicted apparent partitioning values of 1.82 for early pits and 2.10 for late pits, almost identical to the experimental measurements for PC(16:0|Y). By normalizing the experimental distributions to the model, the researchers converted convoluted apparent values into true absolute partitioning coefficients, a key methodological advance for studying lipids in curved membrane compartments.

Armed with this corrected framework, the team screened a library of ten bifunctional lipids spanning a broad chemical space, including glucosylceramide, sphingomyelin, phosphatidylethanolamine, a plasmalogen phosphatidylcholine, and phosphatidylcholine species differing in chain length, saturation and fatty acid positioning. The true partitioning values for late-stage pits ranged from 1.07-fold enrichment to 0.84-fold exclusion, differences of up to 21 percent between species. Although subtle, these differences were highly significant in two-sided randomized permutation tests. Saturated, long-chain lipids such as PC(18:0) tended to be modestly enriched in pits, while polyunsaturated species such as PC(20:4) were slightly excluded. The ordering placed PC(18:0|Y) at the top and PC(20:4|Y) at the bottom, with sphingomyelin and the unsaturated PC species in between.

The researchers then tested which of three candidate mechanisms could explain the pattern. Lipid shape, or resting curvature, predicts that cone- and inverse-cone-shaped lipids should prefer curved membranes, but resting curvature did not correlate with pit partitioning, ruling this mechanism out. Phase separation predicts that curved membranes should accumulate liquid-disordered, low-melting-temperature lipids because they bend more easily, yet the team observed the opposite: a strong positive correlation between melting temperature and pit partitioning, with high-melting-temperature lipids enriched in pits. The explanation that fit best was lipid asymmetry. The plasma membrane maintains an asymmetric distribution of lipids between its outer and inner leaflets, and because the curved pit changes the relative surface areas of the two leaflets, lipids enriched in the cytoplasmic leaflet should be partially enriched in the pit while exoplasmic-leaflet lipids should be partially excluded. This prediction held for nearly all the glycerophospholipids tested, with sphingomyelin the notable outlier, hinting at additional mechanisms such as specific lipid-protein interactions.

The team also probed whether the density of transmembrane cargo receptors could drive lipid sorting by measuring lipid and receptor partitioning simultaneously in three-colour STED images. Correlations between lipid and receptor enrichment across individual late-stage pits were negligible to moderate, ranging from 0.041 to 0.343 by Spearman’s rank correlation, indicating that cargo content largely does not explain the observed lipid differences. Attempts to perturb lipid asymmetry directly revealed just how tightly it is woven into the endocytic machinery. When the researchers triggered calcium-dependent lipid scrambling via the scramblase TMEM16F with ionomycin in SK-MEL2 cells, clathrin was rapidly released from the plasma membrane, dynamin-2 dissociated within three minutes, and transferrin uptake was fully blocked. Pits disassembled roughly four times faster than phosphatidylserine was externalized, and because HEK293 cells with low scramblase activity showed no such response, the effects could be attributed to scrambling rather than calcium itself. Lipid asymmetry, in other words, is not merely a correlate of endocytosis but a prerequisite for it.

Finally, pulse-chase experiments tracking lipid delivery to early endosomes, marked by EEA1, showed that uptake rate constants were essentially identical across all ten lipid species, averaging 1.09 per minute, with similar amplitudes for all but glucosylceramide and phosphatidylethanolamine, whose deviations likely reflect competing non-vesicular transport routes. The overall picture that emerges is strikingly simple: clathrin-mediated endocytosis is a selective protein transporter but a largely non-selective lipid transporter, with vesicle lipid composition essentially a reflection of the plasma membrane and its inherent transbilayer asymmetry. Since previous work from the same groups showed that more than 80 percent of retrograde lipid traffic occurs through non-vesicular routes, the modest selectivity of endocytosis is unlikely to meaningfully shape plasma membrane composition, though it may gently bias organelles toward plasma-membrane-like lipid profiles. The authors note that their analysis deliberately excluded PI(4,5)P2 and phosphatidylserine, established regulators of the endocytic machinery, so selective handling of these lipids cannot be excluded. The quantitative toolkit they have built, pairing near-native probes, super-resolution imaging and geometry-aware modelling, now offers cell biologists a way to measure lipid partitioning in any curved membrane compartment, promising new insight into how the endomembrane system maintains its remarkable compositional identity.

Subject of Research: Quantification of lipid sorting into clathrin-coated pits during clathrin-mediated endocytosis

Article Title: Quantification of lipid sorting during clathrin-mediated endocytosis

Article References: Lennartz, H. M., da Costa Nunes, S., Böhlig, K., Chhatre, A., Šachl, R., Kuhn, S. M., Moneta, L., Barahtjan, P., Yee Yau, W., Iglesias-Artola, J. M., Hof, M., Opálka, L., Modes, C. D., Honigmann, A., & Nadler, A. (2026). Quantification of lipid sorting during clathrin-mediated endocytosis. Nature Cell Biology. https://doi.org/10.1038/s41556-026-02068-7

Image Credits: AI Generated

DOI: 10.1038/s41556-026-02068-7

Keywords: clathrin-mediated endocytosis, lipid sorting, plasma membrane, STED microscopy, bifunctional lipid probes, lipid asymmetry, clathrin-coated pits, membrane trafficking, phosphatidylcholine, sphingomyelin, super-resolution imaging, cell biology