SEVERAL proteins have been implicated in the rapid (millisecond) calcium-controlled release of transmitters at nerve endings(1,2), including soluble N-ethylmaleimide-sensitive fusion protein (NSF3-5) and soluble NSF attachment protein (alpha-SNAP(3,6)), the synaptic SNAP receptor (SNARE)(3,7) and the calcium-binding protein synaptotagmin(2), which may function as a calcium sensor in exocytosis(8). A second SNAP isoform (beta-SNAP), which is 83% identical to alpha-SNAP, is highly expressed in brain(9), but its role is still unclear. Here we show that these proteins assemble cooperatively to form a docking and fusion complex. beta-SNAP (but not alpha-SNAP) binds synaptotagmin and recruits NSF, indicating that the complex may link the process of membrane fusion to calcium entry by attaching a specialized fusion protein (beta-SNAP) to a calcium sensor (synaptotagmin). Polyphosphoinositols that block transmitter release, inositol 1,3,4,5-tetrakisphosphate (InsP(4)), inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol 1,2,3,4,5,6-hexakisphosphate (InsP(6)), also block the assembly of the particle by preventing beta-SNAP from binding to synaptotagmin.