Evolutionary tuning of complexin-synaptotagmin fusion clamp governs regulated neurotransmitter release

Synaptic vesicles must be held in a fusion-ready but restrained state until calcium influx triggers release. Synaptotagmin-1 (Syt1) and Complexin (CPX) both contribute to this fusion clamp, yet removing either protein produces opposite effects in different species. Whether this reflects genuine divergence in their molecular roles, or species-specific tuning of a conserved mechanism, has remained unresolved. Here we address this by comparing mammalian and C. elegans orthologs in a reconstituted single-vesicle assay that isolates clamping from upstream docking and priming. We find that clamping in both species requires the combined action of CPX and Syt1, with neither sufficient on its own. CPX clamps only with its cognate SNARE complex, a specificity conferred entirely by its accessory helix. The Syt1 orthologs differ markedly in intrinsic clamping strength, which maps to C2B mediated higher-order oligomerization and sets the functional dependence on CPX in each system. These results support a dual-regulator model of exocytosis in which Syt1 and CPX act sequentially and cooperatively through discrete molecular states to control vesicle clamping and calcium-triggered release. This framework reconciles long-standing functional discrepancies across species and reveals how evolutionary divergence modulates conserved mechanisms governing synaptic fidelity.