A primordial synaptotagmin-independent function of complexin in regulated exocytosis

In Ca²⁺-triggered exocytosis such as synaptic neurotransmitter release, vesicle fusion is tightly regulated by synaptotagmin (Syt) and complexin (Cpx), which together clamp partially assembled SNARE complexes to prevent premature fusion. However, Cpx is evolutionarily more ancient than Syt, suggesting that it may also regulate exocytosis independently of Syt. To test this possibility, we sought to identify an extant exocytic pathway that requires Cpx but naturally lacks Syt. Here, we uncovered such a pathway - hormone-triggered exocytosis of glucose transporters in adipocytes. In this pathway, Cpx acts exclusively as a positive regulator, accelerating the evoked phase of exocytosis without affecting basal fusion. Mechanistically, this Syt-independent activity depends on the central helix of Cpx for SNARE binding and on its C-terminal membrane-binding peptide, which remodels the lipid bilayer to promote exocytosis. Our findings support a model in which Cpx originally evolved to accelerate exocytosis independently of Ca²⁺, enabling rapid mobilization of exocytic cargoes in response to environmental cues. With the later emergence of Syt, Cpx acquired an additional role - acting in concert with Syt to regulate Ca²⁺-triggered exocytosis.