THE MAJOR SITE OF PHOSPHORYLATION WITHIN THE ROUS-SARCOMA VIRUS MA PROTEIN IS NOT REQUIRED FOR REPLICATION

About one-third of the MA protein in Rous sarcoma virus (RSV) is phosp horylated, Previous analyses of this fraction have suggested that seri ne residues 68 and 106 are the major sites of phosphorylation. As a fo llow-up to that study, we have characterized mutants which have these putative phosphorylation sites changed to alanine, either separately o r together, None of the substitutions (S68A S106A, or S68/106A) had an effect on the budding efficiency or infectivity of the virus, Upon ex amination of the P-32-labeled viral proteins, we found that the S68A s ubstitution did not affect phosphorylation in vivo at all, In;contrast , the S106A substitution prevented all detectable phosphorylation of M A, suggesting that there is only one major site of phosphorylation in MA, We also found that the RSV MA protein is phosphorylated on tyrosin e, but the amount was low and detectable only with large numbers of vi rions and an antibody specific for phosphotyrosine.