Fig. 3. Membrane fusion. (A) Putative model of SNARE-mediated membrane fusion. At stage I, Q- and R-SNAREs are separate from each other and the Q-SNARE syntaxin exists in its closed conformation. SNARE complex formation releases the Q-SNARE H_ABC N-terminal domain and is associated with vesicle docking (stage II). Several SNARE complexes may associate at the fusion site via their TMSs. Docking may result in hemifusion (III), which is followed by formation of lipidic (IV) or proteinaceous (V) fusion pores prior to full fusion (V). Note that proteins and membranes are not drawn to scale. (B) Lipid topology in hemifusion.