SNAP-29 is a promiscuous syntaxin-binding SNARE

SNARE proteins are key regulators of membrane fusion and are proposed to di ctate the specificity with which particular vesicles fuse with particular t arget organelles. On intracellular organelles that serve as targets for tra nsport vesicles, organelle-specific syntaxins form heterodimers with either SNAP-23 or its recently described homolog SNAP-29. We have performed a var iety of in vitro and in vivo binding assays in an attempt to determine whet her SNAP-23 and SNAP-29 differ in their ability to form binary SNARE comple xes with different intracellular syntaxins. While SNAP-23 preferentially bi nds to plasma membrane-localized syntaxins, SNAP-29 binds Ito both plasma m embrane and intracellular syntaxins equally well. Furthermore, binding to S NAP-29 augments the ability of syntaxin to bind to vesicle-associated SNARE s and the presence of vesicle SNAREs dramatically increases SNAP-29 binding to syntaxin. These data suggest that SNAP-23 preferentially regulates plas ma membrane-vesicle fusion events while SNAP-29 plays a role in the mainten ance of various intracellular protein trafficking pathways.