Doc2b is a high-affinity Ca2+ sensor for spontaneous neurotransmitter release.

Synaptic vesicle fusion in brain synapses occurs in phases that are either tightly coupled to action potentials (synchronous), immediately following action potentials (asynchronous), or as stochastic events in the absence of action potentials (spontaneous). Synaptotagmin-1, -2, and -9 are vesicle-associated Ca2+ sensors for synchronous release. Here we found that double C2 domain (Doc2) proteins act as Ca2+ sensors to trigger spontaneous release. Although Doc2 proteins are cytosolic, they function analogously to synaptotagmin-1 but with a higher Ca2+ sensitivity. Doc2 proteins bound to N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complexes in competition with synaptotagmin-1. Thus, different classes of multiple C2 domain-containing molecules trigger synchronous versus spontaneous fusion, which suggests a general mechanism for synaptic vesicle fusion triggered by the combined actions of SNAREs and multiple C2 domain-containing proteins.