An N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) has recently been purified on the basis of its ability to restore transport to NEM-inactivated Golgi membranes in a cell-free transport system. NSF is a peripheral membrane protein required for the fusion of transport vesicles. We now report the existence of two novel components that together bind NSF to Golgi membranes in a saturable manner. These components were detected by examining the requirements for reassociation of purified NSF with Golgi membranes in vitro. One component is an integral membrane receptor that is heat sensitive, but resistant to Na2CO3 extraction and to all proteases tested. The second component is a cytosolic factor that is sensitive to both proteases and heat. This soluble NSF attachment protein (SNAP) is largely resistant to NEM and is further distinguished from NSF by chromatography. SNAP appears to act stoichiometrically in promoting a high-affinity interaction between NSF and the membrane receptor. Because NSF promotes vesicle fusion, it seems likely that these two new factors that allow NSF to bind to the membrane are also part of the fusion machinery.

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