MEMBRANE-FUSION OF SEMLIKI FOREST VIRUS INVOLVES HOMOTRIMERS OF THE FUSION PROTEIN

Infection of cells with enveloped viruses is accomplished through membrane fusion. The binding and fusion Processes are mediated by the spike proteins in the envelope of the virus particle and usually involve a series of conformational changes in these proteins. We have studied the low-pH-mediated fusion process of the alphavirus Semliki Forest virus (SFV). The spike protein of SFV is composed of three copies of the protein heterodimer E2E1. This structure is resistant to solubilization in mild detergents such as Nonidet P-40 (NP40). We have recently shown that the spike structure is reorganized during virus entry into acidic endosomes (J. M. Wahlberg and H. Garoff, J. Cell Biol. 116:339-348, 1992). The original NP40-resistant heterodimer is dissociated, and the El subunits form new NP40-resistant protein oligomers. Here, we show that the new oligomer is represented by an El trimer. From studies that use an in vitro assay for fusion of SFV with liposomes, we show that the El trimer is efficiently expressed during virus-mediated membrane fusion. Time course studies show that both El trimer formation and fusion are fast processes, occurring in seconds. It was also possible to inhibit virus binding and fusion with a monoclonal antibody directed toward the trimeric El. These results give support for a model in which the El trimeric structure is involved in the SFV-mediated fusion reaction.