Binding model for SNAREs in cytokine secretion. Ligand/receptor binding elicits signaling that initiates trafficking in classic secretory pathways. At each step of transport (eg, a secretory granule fusing with the plasma membrane), a specific R-SNARE (VAMP) in the secretory organelle membrane partners with a Q-SNARE complex (eg, composed of SNAP-23 and syntaxin-4) in the target membrane. SNARE binding involves the formation of a 4-helix coiled-coil structure that is modeled to winch membranes together to allow membrane fusion and extrusion of cytokines from the granule interior.