Positively regulates a late step in exocytosis of various cytoplasmic vesicles, such as synaptic vesicles and other secretory vesicles. Organizes the SNAREs into a cross-linked zigzag topology that, when interposed between the vesicle and plasma membranes, is incompatible with fusion, thereby preventing SNAREs from releasing neurotransmitters until an action potential arrives at the synapse. Also involved in glucose-induced secretion of insulin by pancreatic beta-cells. Essential for motor behavior. Binds to the SNARE core complex containing SNAP25, VAMP2 and STX1A. Enriched at synaptic-releasing sites in mature neurons. Nervous system. Strongly expressed in brain, where it is predominant in neurons from cerebral cortex and thalamus (at protein level). Belongs to the complexin/synaphin family.