The soluble NSF attachment proteins (SNAPs) enable N-ethyl-maleimide-sensitive fusion protein (NSF) to bind to target membranes. Here we report the cloning and sequencing of complementary DNAs encoding alpha-, beta- and gamma-SNAPs. Two of these proteins, alpha and gamma, are found in a wide range of tissues, and act synergistically in intra-Golgi transport. The third, beta, is a brain-specific isoform of alpha-SNAP. Thus, NSF and SNAPs appear to be general components of the intracellular membrane fusion apparatus, and their action at specific sites of fusion must be controlled by SNAP receptors particular to the membranes being fused, as described in the accompanying article.