MEMBRANE-FUSION ACTIVITY OF SEMLIKI-FOREST-VIRUS IN A LIPOSOMAL MODELSYSTEM - SPECIFIC-INHIBITION BY ZN2+ IONS

Semliki Forest virus (SFV) has been shown previously to fuse efficient ly with cholesterol-and sphingolipid-containing liposomal model membra nes in a low-pH-dependent manner. Several steps can be distinguished i n this process, including low-pH-induced irreversible binding of the v irus to the liposomes, facilitated by target membrane cholesterol, and subsequent fusion of the viral membrane with the liposomal bilayer, s pecifically catalyzed by target membrane sphingolipid. Binding and fus ion are mediated by the heterodimeric viral envelope glycoprotein E2/E 1. At low pH the heterodimer dissociates, and the El monomers convert to a homotrimeric structure, the presumed fusion-active conformation o f the viral spike. In this paper, we demonstrate that SFV-liposome fus ion is specifically inhibited by Zn2+ ions. The inhibition is al the l evel of the fusion reaction itself, since virus-liposome binding was f ound to be unaffected. Zn2+ did not inhibit E2/E1 dissociation, but se verely inhibited exposure of an acid-specific epitope on El, El homotr imer formation, and acquisition of trypsin-resistance. It is concluded that virus-liposome binding solely requires low-pH-induced E2/E1 hete rodimer dissociation, while fusion depends on further rearrangements i n the El spike protein. As these rearrangements occur subsequent to th e binding step, their precise course, including the formation of a fus ion complex, may be influenced by interaction of El with target membra ne lipids. (C) 1997 Academic Press.