Exocytosis relating proteins in the nervous system.

Hypothetical models of the molecular mechanism underlying presynaptic exocytosis were reviewed and the exocytosis relating proteins were categorized into four groups: docking, anchoring, fusion and inhibiting proteins. HPC-1/syntaxin, an axonal membrane protein, was classified as an anchoring protein, not as the vesicle docking protein, because electron microscopic study using cryoimmunogold technique revealed that HPC-1 distributed over the entire axonal membrane, where the synaptic vesicles were not 'docked' to the membrane. Since selective toxin or antibody against HPC-1 affected exocytosis, HPC-1 might be a necessary component for the exocytosis, but HPC-1 by itself seemed to have no ability to bind synaptic vesicles to the membrane in vivo. The molecular mechanism for Ca-dependent, rapid exocytosis and possible roles of the exocytosis relating proteins in the neurite morphogenesis are discussed.