Membrane fusion activity of Semliki Forest virus in a liposomal model system: specific inhibition by Zn2+ ions

Semliki Forest virus (SFV) has been shown previously to fuse efficiently with cholesterol- and sphingolipid-containing liposomal model membranes in a low-pH-dependent manner. Several steps can be distinguished in this process, including low-pH-induced irreversible binding of the virus to the liposomes, facilitated by target membrane cholesterol, and subsequent fusion of the viral membrane with the liposomal bilayer, specifically catalyzed by target membrane sphingolipid. Binding and fusion are mediated by the heterodimeric viral envelope glycoprotein E2/E1. At low pH the heterodimer dissociates, and the E1 monomers convert to a homotrimeric structure, the presumed fusion-active conformation of the viral spike. In this paper, we demonstrate that SFV-liposome fusion is specifically inhibited by Zn2+ ions. The inhibition is at the level of the fusion reaction itself, since virus-liposome binding was found to be unaffected. Zn2+ did not inhibit E2/E1 dissociation, but severely inhibited exposure of an acid-specific epitope on E1, E1 homotrimer formation, and acquisition of trypsin-resistance. It is concluded that virus--liposome binding solely requires low-pH-induced E2/E1 heterodimer dissociation, while fusion depends on further rearrangements in the E1 spike protein. As these rearrangements occur subsequent to the binding step, their precise course, including the formation of a fusion complex, may be influenced by interaction of E1 with target membrane lipids.