C-Terminal Complexin Sequence Is Selectively Required for Clamping and Priming But Not for Ca2+ Triggering of Synaptic Exocytosis

Complexins are small soluble proteins that bind to assembling SNARE complexes during synaptic vesicle exocytosis, which in turn mediates neurotransmitter release. Complexins are required for clamping of spontaneous mini release and for the priming and synaptotagmin-dependent Ca2+ triggering of evoked release. Mammalian genomes encode four complexins that are composed of an N-terminal unstructured sequence that activates synaptic exocytosis, an accessory alpha-helix that clamps exocytosis, an essential central alpha-helix that binds to assembling SNARE complexes and is required for all of its functions, and a long, apparently unstructured C-terminal sequence whose function remains unclear. Here, we used cultured mouse neurons to show that the C-terminal sequence of complexin-1 is not required for its synaptotagmin-activating function but is essential for its priming and clamping functions. Wild-type complexin-3 did not clamp exocytosis but nevertheless fully primed and activated exocytosis. Strikingly, exchanging the complexin-1 C terminus for the complexin-3 C terminus abrogated clamping, whereas exchanging the complexin-3 C terminus for the complexin-1 C terminus enabled clamping. Analysis of point mutations in the complexin-1 C terminus identified two single amino-acid substitutions that impaired clamping without altering the activation function of complexin-1. Examination of release induced by stimulus trains revealed that clamping-deficient C-terminal complexin mutants produced a modest relative increase in delayed release. Overall, our results show that the relatively large C-terminal complexin-1 sequence acts in priming and clamping synaptic exocytosis and demonstrate that the clamping function is not conserved in complexin-3, presumably because of its distinct C-terminal sequences.