Rab proteins are low-molecular-weight GTP-binding proteins that form the largest branch of the Ras superfamily of GTPases. Located on the cytoplasmic face of organelles and vesicles, rab proteins are involved in intracellular membrane fusion reactions. Three membrane proteins, synaptosomal associated protein of 25 kDa (SNAP-25), synaptobrevin, and syntaxin, form the core of a ubiquitous membrane fusion machine that interacts with the soluble proteins N-ethylmaleimide-sensitive factor (NSF) ａ