Ancient GTPase Gate Controls Secretion in Parasites and Ciliates

Every eukaryotic cell must decide, moment by moment, when to release the contents of its secretory vesicles. In neurons, this decision is governed by an elaborate cast of proteins—SNAREs, Munc18, Munc13, synaptotagmin and small GTPases—that hold docked vesicles in a primed state and allow them to fuse with the plasma membrane only when calcium signals arrive. But how do single-celled organisms that branched from our lineage more than a billion years ago manage the same feat? A new study published in Nature Microbiology answers that question for one of the most species-rich branches of eukaryotic life, revealing a molecular gate that has been conserved across an entire kingdom of microbes and repurposed, in the case of dangerous parasites, to time the delivery of their invasion machinery.

The research, led by teams at the University of Montpellier and the University of Pennsylvania, focused on the Alveolata, a vast clade of unicellular eukaryotes that includes free-living ciliates and dinoflagellates as well as the apicomplexan parasites responsible for malaria, toxoplasmosis and cryptosporidiosis. These organisms share a distinctive secretory architecture. Ciliates such as Tetrahymena thermophila and Paramecium carry organelles called mucocysts and trichocysts that sit pre-docked at the cell cortex, ready to discharge synchronously when the cell is threatened. Apicomplexan parasites such as Toxoplasma gondii deploy related organelles called micronemes and rhoptries, whose sequential secretion coordinates motility, host attachment and the injection of effector proteins into host cells. The evolutionary kinship of these organelles has long been suspected, but the regulatory logic that controls their release remained obscure.

Clues came from an unusual structure first glimpsed in the 1970s by freeze-fracture electron microscopy: a rosette of intramembranous particles embedded in the plasma membrane at every secretory docking site. Recent cryo-electron tomography work showed that in apicomplexans this rosette is part of a larger assembly, the rhoptry secretory apparatus, which displays eightfold rotational symmetry and extends beneath the parasite membrane to connect with the rhoptry tip through an intermediate apical vesicle. Previous genetic studies had identified an Alveolata-restricted protein complex—Nd6, Nd9, NdP1 and NdP2—as essential for building the rosette and for exocytosis itself. What was missing was the switch: the component that decides when the apparatus fires.

That switch, the new study reports, is an atypical small GTPase. By mining proteomic pulldown data from Toxoplasma, the researchers identified a protein, TGGT1_277840, that interacts with Nd9. Sequence analysis and AlphaFold modelling confirmed that it carries the canonical G domain of the Ras superfamily, complete with the conserved P-loop motif that positions nucleotides for hydrolysis. The team named it Nd-GTPase. Fluorescence microscopy and ultrastructure expansion microscopy showed that the protein accumulates at the apical tip of the parasite, precisely overlapping with Nd6 at the rhoptry exocytic site, and that it is recruited there late in cell division, as daughter parasites assemble their own secretion machinery.

Functionally, the GTPase proved indispensable. Parasites engineered to lack the gene were viable but grew poorly on host-cell monolayers, and their invasion efficiency dropped by roughly seventy percent. The defect was specific: microneme secretion, replication, egress and attachment to host cells all proceeded normally, but rhoptry discharge collapsed. The researchers measured this using a reporter assay in which a rhoptry kinase, once injected into host cells, drives the nuclear translocation of phosphorylated STAT6; in the knockout parasites only about a quarter of host cells received the rhoptry payload. Electron microscopy confirmed that the rhoptries themselves remained properly docked, indicating that the GTPase acts at the moment of fusion rather than during organelle biogenesis.

The story became far more compelling when the team searched for related proteins in ciliates. Among dozens of candidate small GTPases in Tetrahymena, expression profiling pointed to one gene, TTHERM_00449050, whose product co-purified with Nd9 and NdP1 and localized to the tips of docked mucocysts. Phylogenetic analysis across a broad range of alveolates—including dinoflagellates and chromerids—showed that these Nd-GTPases form their own branch, distinct from the classic Ras, Rho, Arf, Rab and Ran subfamilies. In other words, an entire eukaryotic lineage has evolved its own dedicated GTPase switch for regulated secretion, one that animals and fungi simply do not possess.

Small GTPases act as binary switches, toggling between an inactive GDP-bound state and an active GTP-bound state. By introducing well-established mutations that lock the protein into either conformation, the researchers could ask which state drives secretion. The answer was unambiguous. In both Toxoplasma and Tetrahymena, only the GTP-locked form rescued the secretion defect of the knockout, while the GDP-locked form failed entirely—in Toxoplasma it was even intrinsically unstable. Yeast two-hybrid screens and co-immunoprecipitation showed that Nd9 binds the GTPase preferentially in its GTP-bound state, and that Nd9 also contacts NdP1 directly, forming a tripartite Nd-GTPase–Nd9–NdP1 complex.

To see where this complex sits within the secretion machinery, the team combined AlphaFold-based structural prediction with integrative fitting into refined cryo-electron tomography maps of the rhoptry secretory apparatus. The tripartite complex unambiguously occupied an anchor density near the apparatus’s central channel. Nd9, it turned out, contains a pleckstrin homology-like domain that faces the apical vesicle membrane, likely tethering the complex, and a MOG1-like domain in its GTPase-binding region—reminiscent of a nuclear protein that stimulates nucleotide release from the Ran GTPase. When the researchers deleted the GTPase in Toxoplasma and re-examined the apparatus by cryo-electron tomography, they observed two striking mutant conformations: one in which the interface between the apical vesicle and the secretory apparatus opened, and another in which the entire structure collapsed and the interface locked shut. These opposing outcomes neatly explain why knockout parasites show only partial loss of secretion—some cells retain a leaky, permissive state while others fail completely.

Perhaps the most vivid evidence for a gating function came from Tetrahymena. Cells lacking Nd-GTPase, Nd9 or NdP1 did not simply fail to secrete when stimulated—they leaked. Mucocyst proteins accumulated in the growth medium without any trigger, and the mutant cells clumped together, apparently because sticky mucocyst contents were being released spontaneously and then re-ingested through phagocytosis. Freeze-fracture microscopy revealed deformed membrane regions where the regular rosettes should have been, resembling the fusion scars left behind after discharge. Expressing the GTP-locked form restored restraint, while the GDP-locked form only partially did so. The authors conclude that the GTP-bound GTPase acts as a gatekeeper, holding the docked organelle in a fusion-incompetent but primed state until an external signal—host contact for the parasite, a chemical stimulus for the ciliate—flips the gate open.

The comparative analysis also revealed an elegant evolutionary refinement. In ciliates, secretory organelles touch the plasma membrane directly, and losing the gate causes immediate leakage. In apicomplexans, an intermediate apical vesicle sits between the rhoptry and the secretory apparatus, creating an additional sealed interface that must be remodelled before cargo can escape. Even when the Toxoplasma gate was removed, rhoptry proteins did not spill into the medium, suggesting the vesicle imposes a second checkpoint. The authors propose that this extra valve evolved to meet the parasite’s stringent need to restrict rhoptry injection to the precise moment of host contact, integrating signals from microneme-derived sensor complexes that link host recognition to exocytic activation. Beyond its value for understanding parasite virulence—and potentially for identifying new drug targets that block invasion—the work offers a broader lesson in how evolution overlays new regulatory modules onto an ancient fusion machine, tailoring one conserved secretory core to the wildly different demands of predator evasion and host parasitism.

Subject of Research: A conserved alveolate-specific GTPase complex that gates regulated exocytosis in ciliates and apicomplexan parasites

Article Title: A conserved GTPase complex gates exocytosis for defence and parasitism across alveolates

Article References: Cova, M. M., Morley, L., Maynadier, M., Delabre, J., Graindorge, A., Berry-Sterkers, L., Mageswaran, S. K., Rofidal, V., Powell, C. J., Boulanger, M. J., Chang, Y.-W., Sparvoli, D., & Lebrun, M. (2026). A conserved GTPase complex gates exocytosis for defence and parasitism across alveolates. Nature Microbiology. https://doi.org/10.1038/s41564-026-02501-5

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02501-5

Keywords: exocytosis, GTPase, Alveolata, Toxoplasma gondii, Tetrahymena thermophila, rhoptries, mucocysts, cryo-electron tomography, Nd9, apicomplexan parasites, secretory apparatus, host invasion