Semliki Forest virus mRNA capping enzyme requires association with anionicmembrane phospholipids for activity

The replication complexes of all positive strand RNA viruses of eukaryotes are associated with membranes, In the case of Semliki Forest virus (SFV), t he main determinant of membrane attachment seems to be the virus-encoded no n-structural protein NSP1, the capping enzyme of the viral mRNAs, which has guanine-7-methyltransferase and guanylyltransferase activities. We show he re that both enzymatic activities of SFV NSP1 are inactivated by detergents and reactivated by anionic phospholipids, especially phosphatidylserine. T he region of NSP1 responsible for binding to membranes as well as to liposo mes was mapped to a short segment, which is conserved in the large alphavir us-like superfamily of viruses. A synthetic peptide of 20 amino acids from the putative binding site competed with in vitro synthesized NSP1 for bindi ng to liposomes containing phosphatidylserine, These findings suggest a mol ecular mechanism by which RNA virus replicases attach to intracellular memb ranes and why they depend on the membranous environment.