English: Schematic of membrane fusion mediated by paramyxovirus fusion proteins. After the prefusion form of the fusion protein (A) is triggered, the coiled-coil HRB domain melts leading to the formation of the open stalk form (B). The FP is then projected towards the target membrane and HRA refolds to a trimeric coiled coil (pre-hairpin intermediate, C). HRB then translocates toward HRA, forcing the viral membrane and the target membrane into a hemifusion state (D). The complete refolding of HRB around HRA forms the stable 6HB and allows for the formation of the fusion pore (E). Blue: FP, Orange: HRA, Green: HRB, Red: TM.