Functional and physical coupling of voltage-sensitive calcium channels with exocytotic proteins: ramifications for the secretion mechanism

The secretion of neurotransmitters is a rapid Ca2+-regulated process that brings about vesicle fusion with the plasma membrane. This rapid process (<100 <mu>s) involves multiple proteins located at the plasma and vesicular membranes. Because of their homology to proteins participating in constitutive secretion and protein trafficking, they have been characterized extensively. The sequential events that lead these proteins to vesicle docking and fusion are still unclear. We will review recent studies that demonstrate the operative role played by voltage-sensitive Ca2+ channels and discuss the relevance for the process of evoked transmitter release. The regulation of Ca2+ influx by syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and synaptotagmin, and the reciprocity of these proteins in controlling the kinetic properties of the channel will be discussed. Calcium channel and synaptic proteins expressed in Xenopus oocytes demonstrate a strong functional interaction, which could be pertinent to the mechanism of secretion. First, the voltage-sensitive Ca2+ channels are negatively modulated by syntaxin: this inhibition is reversed by synaptotagmin. Second, the modulation of N-type Ca2+ channel activation kinetics strongly suggests that the vesicle could be docked at the plasma membrane through direct interaction with synaptotagmin, Finally, these interactions provide evidence for the assembly of the voltage-sensitive Ca2+ channel with syntaxin 1A, SNAP-25 and synaptotagmin into an excitosome complex: a putative fusion complex with a potential role in the final stages of secretion. Studies suggest that cross-talk between the synaptic proteins and the channel in a tightly organized complex may enable a rapid secretory response to an incoming signal such as membrane depolarization.