Complexins: cytosolic proteins that regulate SNAP receptor function.

A family of proteins called complexins was discovered that compete with alpha-SNAP, but not synaptotagmin, for SNAP receptor binding. Complexins I and II are highly homologous hydrophilic proteins that are tightly conserved, with 100% identity among mouse, rat, and human complexin II. They are enriched in neurons where they colocalize with syntaxin and SNAP-25; in addition, complexin II is expressed ubiquitously at low levels. Complexins bind weakly to syntaxin alone and not at all to synaptobrevin and SNAP-25, but strongly to the SNAP receptor-core complex composed of these three molecules. They compete with alpha-SNAP for binding to the core complex but not with other interacting molecules, including synaptotagmin I, suggesting that the complexins regulate the sequential interactions of alpha-SNAP and synaptotagmins with the SNAP receptor during exocytosis.