A SINGLE-POINT MUTATION CONTROLS THE CHOLESTEROL DEPENDENCE OF SEMLIKI-FOREST-VIRUS ENTRY AND EXIT

Membrane fusion and budding are key steps in the life cycle of all env eloped viruses. Semliki Forest virus (SFV) is an enveloped alphavirus that requires cellular membrane cholesterol for both membrane fusion a nd efficient exit of progeny virus from infected cells. We selected an SFV mutant, srf-3, that was strikingly independent of cholesterol for growth. This phenotype was conferred by a single amino acid change in the El spike protein subunit, proline 226 to serine, that increased t he cholesterol independence of both srf-3 fusion and exit. The srf-3 m utant emphasizes the relationship between the role of cholesterol in m embrane fusion and virus exit, and most significantly, identifies a no vel spike protein region involved in the virus cholesterol requirement .