The synthesis of Sendai virus polypeptides in infected cells. III. Phosphorylation of polypeptides.

Phosphorylated and unphosphorylated forms of the membrane polypeptide (M) and the nucleocapsid polypeptide (NP) of Sendai virus have been identified in both infected cells and virions. Polypeptide B, found previously in infected cells, has been shown to be a phosphorylated form of M by peptide mapping, by conversion of M to B by phosphorylation in both pulse-chase experiments in infected cells and in vitro by a virion-associated protein kinase and, conversely, by conversion of B to M through the loss of phosphate in cell lysates. Although very little of the phosphorylated form was found in virions, B and M were found in similar proportions in infected cells after a 30-min pulse, suggesting that the nonphosphorylated form is preferentially incorporated into virions or that phosphate is removed during the maturation process. The finding of phosphorylation of M and NP in cells suggests that this may play a role in virus replication or assembly. Two phosphorylated forms of the nucleocapsid polypeptide (NP_P1 and NP_P2) have been found, but the unphosphorylated NP is the predominant form in both cells and virions. The similarity of these three polypeptides, except for their phosphate content, has been shown by peptide mapping. The membrane polypeptides B and M were separated by electrophoresis on polyacrylamide gels in the absence of urea, but in the presence of 4 M urea they comigrated. In contrast to these results with polypeptides B and M, polypeptides NP, NP_P1 and NP_P2 were resolved in gels in the presence of 4 M urea but comigrated in its absence. Thus, the behavior of viral phosphopolypeptides in different polyacrylamide-gel systems varies depending on the polypeptides. Possible biological roles of phosphorylation of the virus polypeptides are discussed.