Interactions of Semliki Forest virus spike glycoprotein rosettes and vesicles with cultured cells.

Abstract

Semliki Forest virus (SFV)-derived spike glycoprotein rosettes (soluble octameric complexes), virosomes (lipid vesicles with viral spike glycoproteins), and liposomes (proteinfree lipid vesicles) have been used to investigate the interaction of subviral particles with BHK-21 cells. Cell surface binding, internalization, degradation, and low pH-dependent membrane fusion were quantitatively determined. Electron microscopy was used to visualize the interactions.Virosomes and rosettes, but not liposomes, bound to cells. Binding occurred preferentially to microvilli and was inhibited by added SFV; it increased with decreasing pH but was, in all cases, less efficient than intact virus. At 37°C the cell surface-bound rosettes and virosomes were internalized via coated pits and coated vesicles. After a lag period of 45 min the protein components of the internalized ligands were degraded and appeared, as acid-soluble activity, in the medium. The uptake of rosettes and virosomes was found to be similar to the adsorptive endocytosis of SFV except that their average residence times on the cell surface were longer. The rosettes and the liposomes did not show low pH-induced membrane fusion activity. The virosomes, however, irrespective of the lipid compositions used, displayed hemolytic activity at mildly acidic pH and were able to fuse with the plasma membrane of cells with an efficiency of 0.25 that observed with intact viruses. Cell-cell fusion activity was not observed with any of the subviral components.The results indicated that subviral components possess some of the entry properties of the intact virus.

Cholesterol is required for infection by Semliki Forest virus.

“…Thus, although purified virus particles contain cholesterol in the virus membrane, they still require cholesterol in the target membrane in order to fuse . Conversely, SFV spike proteins reconstituted into phospholipid liposomes without sterol can still fuse with cholesterol-containing membranes (Marsh et al, 1983). Our studies have shown that irreversible conformational changes occur in both the E1 and E2 subunits of the SFV spike protein upon exposure to low pH Kielian and Heleuius, 1985).…”

Chapter 9 Fusion of Viral Envelopes with Cellular Membranes

“…Isolated E glycoproteins form octameric micelles which, however, do not have hemolytic activity (Vaananen and Kaariainen, 1979). The glycoproteins reconstituted into liposomes showed low pH-induced hemolytic and fusion activities (Marsh et al, 1983a). The fusion efficiency was lower (25%) than that of the parent virus, possibly due to lower spike density on the reconstituted membranes.…”

The entry of enveloped viruses into cells by endocytosis

“…SFV spike glycoprotein rosettes and reconstituted lipid vesicles (virosomes) have binding properties on BHK cells similar to those of the intact virus. The particles bind preferentially to the microvilli, the binding is pH-dependent and the particles compete for binding sites with intact viruses (Fries & Helenius, 1979;Marsh et al, 1983a). The efficiency with which the particles bind to cells is, however, reduced when compared with intact SFV.…”