Figure 8

Proposed model of conformation of VAMP2 transmembrane domain during exocytosis. The different steps of exocytosis are viewed from general scale with the presence of the SNARE cytosolic domain (a) and more focused on the TMD structure from VAMP2 WT (b) and VAMP2 VV (G₁₀₀V/C₁₀₃V) (c). Before vesicle docking, VAMP2 is uniformly distributed over the vesicle, corresponding to a low local concentration of VAMP2, and the TMDs are mainly in α-helical conformation (a i,b i, c i). Following docking, VAMP2 proteins concentrate at the fusion site (a ii). The increased local TMD concentration alters the peptide/lipid ratio and induces the switch of its conformation from an α-helical to a β-sheet conformation. The structural change is accompanied by a tilt of angle of the transmembrane domain from 30° to 54°²¹, promoting the lipid reorganisation and an increase of membrane viscosity (b ii,c ii). Once fusion has occurred and following lipid rearrangement, the SNAREs are localised on the same membrane (a iii). The decrease of the local concentration of VAMP2 and membrane tension results in the return to an α-helical conformation in the case of VAMP2 WT. This is accompanied by an increase of fluidity of the membrane that promotes the opening of the fusion pore for VAMP2 WT (b iii) and its subsequent expansion (a iv and b iv). The VV mutant has reduced capacity for reversible structural changes and is either lacks flexibility around G₁₀₀ or is locked in the β-sheet conformation. This causes a delay in the fusion pore opening in the case of VAMP2 VV (c iib, c iii). The persisting increase in membrane viscosity does not permit further expansion of the pore and induces its premature closure in most cases (c iic). In some cases full fusion may occur as documented by recording of membrane capacitance.