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 b rings about vesicle fusion with the plasma membrane. This rapid process (<1 00 <mu>s) involves multiple proteins located at the plasma and vesicular me mbranes. Because of their homology to proteins participating in constitutiv e secretion and protein trafficking, they have been characterized extensive ly. The sequential events that lead these proteins to vesicle docking and f usion are still unclear. We will review recent studies that demonstrate the operative role played by voltage-sensitive Ca2+ channels and discuss the r elevance for the process of evoked transmitter release. The regulation of C a2+ 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 s ynaptic proteins expressed in Xenopus oocytes demonstrate a strong function al interaction, which could be pertinent to the mechanism of secretion. Fir st, the voltage-sensitive Ca2+ channels are negatively modulated by syntaxi n: 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 syna ptotagmin, Finally, these interactions provide evidence for the assembly of the voltage-sensitive Ca2+ channel with syntaxin 1A, SNAP-25 and synaptota gmin into an excitosome complex: a putative fusion complex with a potential role in the final stages of secretion. Studies suggest that cross-talk bet ween the synaptic proteins and the channel in a tightly organized complex m ay enable a rapid secretory response to an incoming signal such as membrane depolarization.