Central to this research is the endoplasmic reticulum-resident chaperone protein GRP94, a member of the HSP90 family, known for its critical involvement in protein folding and quality control. GRP94’s client proteins include a spectrum of receptor molecules and secretory proteins crucial for cell signaling and homeostasis. This study uncovers that GRP94 engages in a direct and functional interaction with the proprotein convertase furin, a pivotal protease responsible for processing numerous substrates, including growth factors, receptors, and viral proteins. The intimate crosstalk between GRP94 and furin uncovered here suggests a carefully orchestrated mechanism whereby GRP94 regulates the bioactivation of key signaling molecules.

M2 macrophages, alternatively activated macrophages characterized by anti-inflammatory and tissue repair properties, are central effectors in immune modulation. The study reveals that in these cells, GRP94’s interaction with furin is instrumental in controlling the maturation of latent TGF-beta precursors. TGF-beta, a multifunctional cytokine, requires precise proteolytic cleavage to release its active form, crucial for mediating cellular differentiation, proliferation, and extracellular matrix production. The findings indicate that GRP94 ensures the correct conformational and functional presentation of furin, thereby fine-tuning the cleavage and activation of TGF-beta within the cellular milieu.

Delving deeper, the researchers demonstrate that disrupting GRP94 function leads to a significant reduction in furin’s proteolytic activity, culminating in a downstream inhibition of TGF-beta maturation. This effect highlights a previously unrecognized regulatory axis whereby chaperone-mediated control at the level of protein folding and enzyme activation directly influences cytokine availability and function. Such mechanistic insights provide a molecular framework explaining how macrophages calibrate their immune responses through modulating extracellular signaling cascades.

Methodologically, the study harnesses an impressive repertoire of cutting-edge techniques, including co-immunoprecipitation assays to elucidate protein-protein interactions, enzyme activity assays to quantify furin function, and flow cytometry for assessing macrophage phenotypes. Confocal microscopy further establishes the subcellular colocalization of GRP94 and furin within the endoplasmic reticulum, reinforcing the spatial context of their interaction. These complementary approaches combine to create an in-depth and multi-dimensional perspective on how molecular chaperones orchestrate proprotein convertase activity in immune cells.

From a broader perspective, the implications of this regulatory mechanism extend beyond basic biology, intersecting with pathophysiological conditions marked by aberrant TGF-beta signaling. Fibrotic diseases, cancer progression, and chronic inflammatory states often hinge upon dysregulated activation of growth factors like TGF-beta. By delineating how GRP94 modulates furin-dependent TGF-beta maturation, this study paves the way for novel therapeutic strategies aimed at targeting the molecular chaperone machinery to mitigate excessive or inappropriate TGF-beta activation.

Moreover, these findings challenge existing paradigms, which typically consider proprotein convertases as autonomous enzymes with intrinsic regulatory controls. The newly identified dependency on GRP94 adds a layer of complexity and nuance to our understanding of protease regulation, suggesting that chaperone systems can exert upstream control over proteolytic cascades. This could have significant ramifications for the development of pharmacological agents targeting chaperones, traditionally pursued in oncology and neurodegeneration.

Intriguingly, the study also hints at potential crosstalk between stress pathways and immune function. Given that GRP94 is a stress-inducible chaperone elevated during endoplasmic reticulum stress, its role in modulating furin and consequently TGF-beta maturation may link cellular stress responses with immune modulation. This intersection offers fertile ground for exploring stress-associated diseases and devising interventions that address both protein homeostasis and immune regulation concurrently.

Importantly, the focus on human primary M2 macrophages underscores the physiological relevance of these findings. Unlike immortalized cell lines, primary cells preserve native differentiation states and functional profiles, enhancing the translational value of the research. This also reflects the heterogeneity status of macrophage populations in vivo, where the balance between pro-inflammatory M1 and anti-inflammatory M2 subsets critically affects disease outcomes.

Another fascinating aspect is the potential feedback loops implied by this interaction. Activated TGF-beta itself modulates immune cell differentiation and function, possibly affecting GRP94 expression or activity, which in turn impacts furin efficiency. Such feedback could establish regulatory circuits ensuring homeostasis or fueling pathological states when disrupted. Future research elucidating these dynamic relationships might uncover additional layers of immune regulation.

This pioneering study exemplifies the power of integrating molecular biology with immunology to unravel complex cellular processes. Understanding how chaperone proteins like GRP94 exert precise control over essential enzymatic functions reveals the exquisite regulatory networks that underpin immune responses. Such insights not only deepen our grasp of cell biology but also sharpen the tools for designing targeted interventions that can fine-tune immune activity in health and disease.

The potential for GRP94 as a therapeutic target is particularly exciting in light of this research. Pharmacological modulation of chaperone activity to influence furin function and TGF-beta maturation could offer innovative treatments for fibrotic disorders, autoimmune diseases, and cancer. Such strategies might harness small-molecule inhibitors or stabilizers of GRP94, providing nuanced control over cytokine signaling without complete ablation of essential protease functions.

Furthermore, these findings underscore the importance of proteostasis networks in immune cell functionality. By linking chaperone systems directly to cytokine maturation pathways, this study expands our appreciation of how protein folding, enzyme activation, and receptor signaling converge to sculpt immune landscapes. Understanding these intersections is crucial for developing comprehensive models of immune regulation and dysfunction.

In conclusion, the discovery that GRP94 interacts with and regulates the proprotein convertase furin, thereby controlling TGF-beta maturation within M2 macrophages, represents a significant advancement in cellular immunology. This research elucidates a novel chaperone-protease axis that fine-tunes cytokine availability and immune phenotype modulation. As we continue to decode these intricate mechanisms, the opportunities for translating such knowledge into clinical applications promise to transform therapeutic approaches to a host of inflammatory and fibrotic diseases, marking an exciting frontier in biomedical science.

Subject of Research: The interaction between the chaperone protein GRP94 and the proprotein convertase furin, with an emphasis on the regulation of TGF-beta maturation in human primary M2 macrophages.

Article Title: The chaperone GRP94 interacts with the proprotein convertase furin and regulates TGF-beta maturation in human primary M2 macrophages.

Article References:
Baverel, V., Wang, F., Garrido, C. et al. The chaperone GRP94 interacts with the proprotein convertase furin and regulates TGF-beta maturation in human primary M2 macrophages. Cell Death Discov. 11, 558 (2025). https://doi.org/10.1038/s41420-025-02866-2

Image Credits: AI Generated

DOI: 15 December 2025

The ER, a membrane-bound organelle often described as the cell’s protein factory, is where roughly one-third of all human proteins, including nearly 7,000 unique molecules, begin their complex folding journey. This environment is chaotic and crowded, filled with nascent polypeptides, folding enzymes, and molecular chaperones. Among these, chaperones function as specialized molecular guardians that assist proteins in reaching their native, functional conformations or, failing that, direct irreparably misfolded proteins toward degradation pathways. Defects in this quality control system can trigger a cascade of cellular malfunctions, implicated in diseases such as cystic fibrosis, emphysema, and Alzheimer’s.

Despite the vital role chaperones play, a fundamental question has persisted: how do these molecular caretakers discriminate between properly folded and misfolded proteins amidst the entropic sea of the ER lumen? The answer, as detailed by Hebert and his team, lies in a sophisticated carbohydrate-based “glyco-code” inscribed on proteins themselves via attachment of specialized sugar structures called N-glycans. These N-glycans act as molecular zip codes, precisely positioned on the protein’s surface, encoding vital information that directs the chaperone machinery’s interactions and decisions.

The concept of a glyco-code marks a paradigm shift from traditional views that have primarily focused on the polypeptide chain as the sole bearer of folding information. Instead, the review artfully illustrates how carbohydrate moieties, especially N-glycans, serve as dynamic modulators of protein destiny within the ER. This code utilizes sugar patterns, sugar-processing enzymes, and lectin chaperones—carbohydrate-binding proteins that can “read” the sugar code—to guide substrate folding, sorting, and degradation. The interplay between these elements ensures only correctly folded proteins proceed toward secretion or membrane integration, while aberrant proteins are sequestered or targeted for destruction.

Central to this glyco-code is the enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT), described in prior work involving Hebert’s research group and highlighted in the current review. UGGT functions as a folding sensor by recognizing misfolded regions and selectively reglucosylating N-glycans, thereby generating a recognizable signal for ER-resident lectin chaperones such as calnexin and calreticulin. These chaperones engage in cycles of binding and release, giving proteins multiple opportunities to achieve their native fold, a process critical to maintaining cellular proteostasis.

Moreover, the review elucidates the intricate biochemical pathways that regulate how N-glycans are attached co-translationally and post-translationally to asparagine residues within consensus sequences on nascent polypeptides. The precise positioning and structural diversity of these glycans significantly influence the affinity and specificity of chaperone interactions. This spatially encoded information facilitates compartmentalized maturation processes and ensures the fidelity of sorting mechanisms that traffic proteins from the ER to the Golgi apparatus and beyond.

A remarkable aspect highlighted is the dual role of the glyco-code: not only does it assist in protein folding quality control, but it also functions as an addressing system that dictates intracellular trafficking pathways. Lectin chaperones interpret the glyco-code to direct folded proteins toward their ultimate cellular destinations, whereas misfolded or unassembled proteins are recognized by ER-associated degradation (ERAD) machinery, which retrotranslocates them for cytosolic proteasomal degradation.

The work by Hebert, and elucidated by his last graduate student Kevin Guay, also stresses the implications for human health. Many protein conformational diseases are rooted in failures of this glyco-code-based quality control, where either the recognition or processing of N-glycans is impaired, leading to accumulation of toxic protein aggregates or loss of essential functional proteins. Therapeutic strategies targeting components of this glycosylation-dependent chaperone network are increasingly attractive for treating diseases linked to protein misfolding.

In essence, the review synthesizes decades of biochemical, structural, and cellular biology research into a comprehensive framework that redefines our understanding of how protein folding and quality control are intricately regulated by N-glycans. This new vision fosters a broader appreciation that genetic information encoded in DNA extends beyond sequence alone, encompassing a multilayered molecular code integrated within protein glycosylation patterns.

Hebert’s magnum opus not only honors his lifetime contributions to the field but also establishes a foundation upon which future studies will build, aiming to fully decipher the glyco-code. This, in turn, promises to unlock novel therapeutic avenues and advance our ability to manipulate protein folding processes in disease and biotechnology.

The collaborative nature of this work, involving UMass Amherst researchers and their deep expertise in enzymology, structural biology, and cellular machinery, stands as a testament to the enduring quest to unravel the complexities of life at the molecular level. As chaperone biology evolves, embracing the glyco-code paradigm will be pivotal in transforming molecular medicine and our grasp of cellular homeostasis.

With the publication of this comprehensive review in Nature Reviews Molecular Cell Biology, the scientific community gains critical insight into an elegant, carbohydrate-guided proofreading system. Ultimately, this work reinvents the classic narrative of genetic coding and protein folding by placing glycosylation—not just amino acid sequence—at the forefront of post-translational quality control.

Subject of Research: Protein folding quality control mediated by N-glycan-dependent chaperone systems within the endoplasmic reticulum.

Article Title: N-glycan-dependent protein maturation and quality control in the ER

Web References:
https://www.nature.com/articles/s41580-025-00855-y
http://dx.doi.org/10.1038/s41580-025-00855-y

Image Credits: UMass Amherst

Keywords: Protein folding, N-glycans, glyco-code, endoplasmic reticulum, molecular chaperones, UGGT, calnexin, calreticulin, ER-associated degradation, protein quality control, secretome, biochemistry, molecular biology

At the helm of this research is Professor David Drew from Stockholm University, whose team, in collaboration with researchers across Europe and Japan, has marveled at the intricate molecular choreography governing ATP translocation into the ER. By employing cutting-edge cryo-electron microscopy (cryo-EM), they elucidated high-resolution structures of SLC35B1, visualizing the transporter in multiple conformational states. These structural snapshots have unraveled the mechanistic underpinnings of how SLC35B1 effectively recognizes and facilitates the passage of ATP molecules from the cytosol into the ER interior, a compartment critical for cellular homeostasis.

The ER functions as a cellular nexus, orchestrating the synthesis, folding, and trafficking of proteins and lipids essential for cell survival. These energetically demanding processes rely heavily on ATP, whose precise delivery into the ER has remained an unresolved mystery due to the organelle’s isolation from direct ATP synthesis and cytosolic ATP pools. The confirmation of SLC35B1 as the ATP transporter fills this vital knowledge gap, fundamentally advancing our comprehension of intracellular energy logistics.

Intriguingly, the data show that SLC35B1 operates through a step-wise translocation mechanism, involving specific binding sites that selectively recognize ATP’s physiochemical properties. The cryo-EM structures detail key amino acid residues integral to ATP binding and conveyance, highlighting prospective molecular targets for drug design. By modulating these critical residues, future therapies could influence ATP transport efficiency, offering novel interventions for managing ER stress-related pathologies.

Diseases such as type 2 diabetes, various cancers, and neurodegenerative disorders like Alzheimer’s disease have all been linked to impaired ER function characterized by energy imbalance and protein misfolding. The ability to alter ATP supply within the ER through pharmacological agents targeting SLC35B1 is poised to revolutionize treatment paradigms. Enhanced ATP delivery could restore ER homeostasis in conditions marked by energy deficits, while downregulating transport might suppress aberrant activities in pathological states where ER stress fuels disease progression.

Particularly notable is the interdisciplinary approach behind this advance. Early attempts at identifying the ATP transporter candidate were confounded by conflicting reports and scant biochemical validation. To resolve this, the team leveraged a large-scale CRISPR/Cas9 knockout screening conducted collaboratively with the Giulio Superti-Furga Lab at Austria’s CeMM. This functional genomics approach ranked SLC35B1 among the top five crucial transporters for cellular viability, consolidating its role in ATP transport.

Further experimental validation came from the generation of a highly specific antibody against human SLC35B1 by Norimichi Nomura’s group at Kyoto Medical School. This antibody proved indispensable for stabilizing the transporter protein, effectively increasing its molecular size to a threshold amenable for cryo-EM imaging. Without this step, capturing detailed structural information of such a relatively small membrane protein would have remained elusive, demonstrating the ingenuity behind the methodological advancements.

Professor Drew emphasizes that the uncovered molecular blueprint extends beyond fundamental biology into translational medicine. By revealing SLC35B1’s conformational dynamics and ATP-binding motifs, the study provides a scaffold for the rational design of small molecules capable of fine-tuning transporter activity. Therapeutic modulation could either safeguard ER function by enhancing ATP import in disease states or inhibit it where pathological ER hyperactivity contributes to disease.

Currently, the research consortium is actively screening compound libraries for small molecules that can specifically interact with SLC35B1. These efforts aim to identify candidate molecules capable of modulating ATP transport, thereby paving the way for targeted therapies that rectify ER-related metabolic imbalances. Such drug candidates could usher in a new class of treatments addressing the root causes of ER-associated disorders.

The ramifications of this discovery also resonate with broader cellular physiology, as it sheds light on energy distribution mechanisms within organelles. Understanding how ATP is selectively delivered and consumed within intracellular compartments is a fundamental biological problem with implications across metabolism, signaling, and cell survival pathways. This study places SLC35B1 at the center of this intricate web, providing a tangible target for further exploration.

From a technical standpoint, the study showcases how modern structural biology techniques such as cryo-EM have transformed our ability to visualize membrane proteins in action. By capturing SLC35B1 in multiple functional states, the researchers not only confirm its role but also reveal the dynamic conformational landscape that underpins transporter function. This insight is essential for any future endeavors seeking to manipulate transporter behavior pharmacologically.

Ultimately, this seminal research on SLC35B1 catalyzes a paradigm shift in how we perceive organellar bioenergetics and its linkage to disease. As we deepen our grasp on molecular transport mechanisms, we open avenues to innovative therapeutic strategies that target intracellular energy pathways. The promise of controlling ATP flow within the ER is poised to impact a wide spectrum of diseases where cellular energy dysregulation is pathogenic.

In summary, the identification and detailed characterization of SLC35B1 as the human ER ATP transporter resolves a pivotal question in cell biology and medicine. This work exemplifies the power of multidisciplinary collaboration combining structural biology, biochemistry, genetics, and chemical biology to untangle complex cellular phenomena. With ongoing drug discovery efforts, the path from fundamental discovery to clinical application looks increasingly attainable, heralding a new frontier in combating ER-associated human diseases.

Subject of Research: Cells

Article Title: Step-wise ATP translocation into the ER by human SLC35B1

News Publication Date: 21-May-2025

Web References: https://www.nature.com/articles/s41586-025-09069-w

References: DOI: 10.1038/s41586-025-09069-w

Image Credits: Made by Surabhi Kokane using Biorender.com

Keywords: ATP transport, SLC35B1, endoplasmic reticulum, cryo-electron microscopy, membrane transporter, ER stress, protein folding, cellular bioenergetics, targeted therapy, molecular structure, CRISPR/Cas9 screening, drug discovery