What is "complexin"

n. A type of protein found in nerve tissue.

Complexin (also known as synaphin) refers to a one of a small set of eukaryotic cytoplasmic neuronal proteins which binds to the SNARE protein complex (SNAREpin) with a high affinity. These are called synaphin 1 and 2. In the presence of Ca, the transport vesicle protein synaptotagmin displaces complexin, allowing the SNARE protein complex to bind the transport vesicle to the presynaptic membrane.

Complexin acts as both an inhibitor and a facilitator of synaptic vesicle fusion and neurotransmitter release. In one conformation, it clamps SNAREpin complexes, preventing vesicle fusion, while in a different conformation it releases the SNAREpins, allowing synaptotagmin to trigger fusion. Whereas complexin is not necessary for synaptic vesicle exocytosis, it does increase neurotransmitter release by 60–70% as demonstrated by complexin gene knockout in mice. A number of human neurological diseases have been linked to a deficiency of complexin.

Synaphin can promote exocytosis by promoting interaction between the complementary syntaxin and synaptobrevin transmembrane regions that reside in opposing membranes prior to fusion.