Acts as inhibitor of neurotransmitter release. Overexpression leads to a decrease in the amplitude of the excitatory postsynaptic potential in dissected cholinergic and glutaminergic neurons while depletion with antisense oligonucleotides leads to an increase. Overexpression of isoform 1 blocks the reversal of synaptic depression by serotonin in sensory neurons. Binds 3 Ca(2+) ions per subunit. The ions are bound to the C2 domains. Binds SNAP25. Isoform 3 binds SNAP25 with higher affinity. And vesicle-like structures. Belongs to the synaptotagmin family.