COMMENT

Fig. 1. Scheme of the SNARE model. Along with NSF and SNAP, SNAP receptors on the intracellular vesicle (vSNARE) and on the target membrane (t-SNARE) form a SNARE complex, which triggers membrane docking/fusion for exocytosis. Docking and fusion of secretory vesicles with the plasma membrane are critical processes for the secretion of extracellular molecules. This molecular mechanism has been intensively investigated, especially synaptic vesicle docking and fusion at the active site of the neuronal cell membrane. So far, the SNARE docking/fusion model is the most convincing. The action of the ATPase NSF, and SNAP (soluble NSF attachment protein) cause v-SNARE and t-SNARE, receptors for SNAP on the cytoplasmic surface of the vesicles and the target plasma membrane respectively, to bind to each other. This in turn triggers membrane fusion (Ungermann et al., 1998; Weber et al., 1998; Fig. 1). The primary structure of a neuronal cellspecific membrane protein, HPC-1 (later renamed as syntaxin), determined in 1992 (Inoue et al., 1992; Bennett et al., 1992), contains a C-terminal hydrophobic domain for membrane anchoring, but no N-terminal signal peptide for secretion. Because syntaxin/HPC-1 has cytoplasmic orientation and binding capacity with proteins involved in intravesicular targeting (including v-SNARE proteins) this protein is proposed to be a core molecule in the t-SNARE complex (Fujita et al., 1995). At the same time, membrane proteins similar to syntaxin/