Figure 5.

The fusion machinery in mast cell secretion. In resting cells membrane fusion is blocked likely by molecules such as STX3-bound Munc18-2. Upon activation through FcεRI secretory granules translocate to and dock at the plasma membrane where the SNARE proteins assemble into tetrameric complexes. Multiple accessory proteins such as Munc18-2, Munc13-4, Rab27, Doc2α, complexin II, synaptotagmin II, or SCAMPs, are involved as “gatekeepers” of the fusion reaction holding effectors in an inactive state, as tethering factors controlling docking and priming, or as regulators of the SNARE complexes triggering the calcium-dependent membrane fusion.