K+ ionophores stimulate retrovirus-induced fusion-from-within

Fusion-from-without (FFWO) and fusion-from-within (FFWI) for Moloney MLV in SC-1 cells were selectively stimulated by the polycation polybrene and the ionophore amphotericin B, which can be used to discriminate between the two fusion types. FFWI was stimulated by a number of different K+ ionophores. The stimulation occurred within few hours, did not require protein synthesis, and depended on the uninfected cell type, which suggests that FFWI is stimulated by a permutation of the K+ gradient of the uninfected cell. Plasma membrane vesicles from infected cells could also stimulate fusion with the same stimulation pattern as FFWI.