Negatively regulates the formation of synaptic vesicle clustering at active zone to the presynaptic membrane in postmitotic neurons. Positively regulates a late step in exocytosis of various cytoplasmic vesicles, such as synaptic vesicles and other secretory vesicles. Also involved in mast cell exocytosis (PubMed:15870114). Binds to the SNARE core complex containing SNAP25, VAMP2 and STX1A. Translocated from the perikaryon to the presynaptic terminals during maturation of neuronal cells (PubMed:7654227). In mast cells, cytosol and nucleus. Becomes enriched near plasma membrane following stimulation. Nervous system. Strongly expressed in brain, where it is predominant in neurons from cerebral cortex and hippocampus. Also present in mast cells (at protein level). Ser-93 is dephosphorylated by PP2A. Belongs to the complexin/synaphin family.