Functional architecture of an intracellular membrane t-SNARE

Lipid bilayer fusion is mediated by SNAREs (soluble N-ethyl-maleimide-sensitive factor attachment protein receptors) located on the vesicle membrane (v-SNAREs) and the target membrane (t-SNAREs)(1,2). The assembled v-SNARE/t-SNARE complex consists of a bundle of four helices, of which one is supplied by the v-SNARE and the other three by the t-SNARE(3). For t-SNAREs on the plasma membrane, the protein syntaxin(4) supplies one helix and a SNAP-25 protein(5) contributes the other two. Although there are numerous homologues of syntaxin on intracellular membranes(6), there are only two SNAP-25-related proteins in yeast, Sec9 and Spo20, both of which are localized to the plasma membrane and function in secretion(7) and sporulation(8), respectively. What replaces SNAP-25 in t-SNAREs of intracellular membranes? Here we show that an intracellular t-SNARE is built from a `heavy chain' homologous to syntaxin and two separate nonsyntaxin `light chains'. SNAP-25 may thus be the exception rather than the rule, having been derived from genes that encoded separate light chains that fused during evolution to produce a single gene encoding one protein with two helices.