SNARE proteins--why so many, why so few?

Both trafficking and secretion critically depend on accurate and specific membrane recognition and fusion. A key step in these processes is the assembly of a complex consisting of a small number of proteins, i.e., the exocytic core complex. In nerve terminals, this set consists of VAMP and synaptotagmin, which reside at membranes of synaptic vesicles, and syntaxin and SNAP-25 at the plasma membrane. In this survey, different secretory systems that depend on the exocytic core proteins are considered. The possibility that specificity in membrane recognition and fusion is achieved by the numerous variants of proteins of the exocytic core is discussed. Variability of the core complex proteins is determined by the complexity of gene families, isoform-specific localization, and posttranslational modifications. Basic biochemical properties depend on specific isoforms, and the possible protein-protein interactions are determined, in turn, by the compatibility of different isoforms. A correlation between specific variants and distinct biochemical or cellular properties is shown. The outcome of this survey is that heterogeneity in secretion may be dictated by the large number of possible combinations of variants of only a few proteins.