Complexin is a cytoplasmic nerve tissue protein which binds to the SNARE protein complex (SNAREpin) with a high affinity. The transport vesicle protein synaptotagmin

Synaptotagmin

Synaptotagmins constitute a family of membrane-trafficking proteins that are characterized by an N-terminal transmembrane region , a variable linker, and two C-terminal C2 domains - C2A and C2B. There are 15 members in the mammalian synaptotagmin family...

, among others, in the presence of Ca²⁺, displaces complexin allowing the SNARE protein complex to bind the transport vesicle to the presynaptic membrane, although the precise mechanism remains unknown. Complexin acts both as an inhibitor and a facilitator of vesicle fusion and neurotransmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 release at the synapse

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

. In one conformation, it clamps SNAREpin complexes, preventing vesicle fusion, while in a different conformation it releases the SNAREpins, allowing synaptotagmin to trigger fusion.
This makes complexin a critical gatekeeper in the synaptic vesicle

Synaptic vesicle

In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell...

fusion process.