Different functional states of fusion protein gB revealed on human cytomegalovirus by cryo electron tomography with Volta phase plate
Fig 7
Schematic illustration of conformation changes of gB during membrane fusion.
(A, B) Subtomographic averages of prefusion gB (A) with domains illustrated as in Fig 3 and “postfusion” gB (B) with domains colored as in [16]. (C) A working model of gB conformational change during membrane fusion. In step 1, destabilization of the endodomain of prefusion gB either by cytotail conformational changes following gH/gL receptor-binding or by other means (e.g., mechanical stress such as high-speed centrifugation during viral purification) triggers DI and DII to reorient, exposing the fusion loops on DI. Subsequently, the exposed fusion loops could make contact either with cell membrane in close proximity (in the case of receptor binding) (step 2) or with viral membrane. Finally (step 3), DV refolds into an extended form, transforming gB into its “postfusion” conformation: in the presence of cell membrane, the C-terminal part and the fusion loops come together and the membranes fuse; in the absence of cell membrane, the exposed fusion loops insert into the viral membrane.