Calcium-dependent interaction of N-type calcium channels with the synaptic core complex

Abstract

NEUROTRANSMITTER release is initiated by influx of Ca²⁺ through voltage-gated Ca²⁺ channels^1,2, within 200 μs of the action potential arriving at the synaptic terminal³, as the Ca²⁺ concentration increases from 100 nM to >200μM⁴. Exocytosis requires high Ca²⁺ concentration, with a threshold of 20–50 μM and half-maximal activation at 190 μM^5,6. The synaptic membrane proteins syntaxin^7,8, 25K synaptosome-associated protein (SNAP25)⁹, and vesicle-associated membrane protein (VAMP)/ synaptobrevin^10–12, are thought to form a synaptic core complex which mediates vesicle docking and membrane fusion^13–19. Synaptotagmin may be the low-affinity Ca²⁺-sensor^20–24, but other Ca²⁺-sensors are involved^25–27 as residual neurotransmission persists in synaptotagmin-null mutants. Syntaxin binds to N-type Ca²⁺ channels^7,8,28,29 at a site in the intracellular loop connecting domains II and III³⁰. Here we describe Ca²⁺-dependent interaction of this site with syntaxin and SNAP25 which has a biphasic dependence on Ca²⁺, with maximal binding at 20 μM free Ca²⁺, near the threshold for transmitter release. Ca²⁺-dependent interaction of Ca²⁺ channels with the synaptic core complex may be important for Ca²⁺-dependent docking and fusion of synaptic vesicles.