SNAP-25 is targeted to the plasma membrane through a novel membrane-binding domain

Abstract

SNAP-25, syntaxin, and synaptobrevin are SNARE proteins that mediate fusion of synaptic vesicles with the plasma membrane. Membrane attachment of syntaxin and synaptobrevin is achieved through a C-terminal hydrophobic tail, whereas SNAP-25 association with membranes appears to depend upon palmitoylation of cysteine residues located in the center of the molecule. This process requires an intact secretory pathway and is inhibited by brefeldin A. Here we show that the minimal plasma membrane-targeting domain of SNAP-25 maps to residues 85-120. This sequence is both necessary and sufficient to target a heterologous protein to the plasma membrane. Palmitoylation of this domain is sensitive to brefeldin A, suggesting that it uses the same membrane-targeting mechanism as the full-length protein. As expected, the palmitoylated cysteine cluster is present within this domain, but surprisingly, membrane anchoring requires an additional five-amino acid sequence that is highly conserved among SNAP-25 family members. Significantly, the membrane-targeting module coincides with the protease-sensitive stretch (residues 83-120) that connects the two alpha-helices that SNAP-25 contributes to the four-helix bundle of the synaptic SNARE complex. Our results demonstrate that residues 85-120 of SNAP-25 represent a protein module that is physically and functionally separable from the SNARE complex-forming domains.