A comparative study of and phosphorylation of murine mammary tumour virus, a type B RNA virus, is reported. The protein kinase activity associated with murine mammary tumour virus catalysed the phosphorylation of endogenous virus polypeptides. This kinase activity required a divalent metal cation, a non-ionic detergent, and was stimulated in the presence of dithiothreitol. Exogenous cyclic AMP was not required. The P-labelled products of the reaction were completely sensitive to pronase digestion and the phosphate was attached mainly by phosphomonoester linkage to serine residues. As determined by SDS-polyacrylamide gel electrophoresis, heterogeneous labelling of major and minor virus polypeptides was observed under conditions.

In contrast, the labelling of type B virus produced by a continuous cell line (MuMT-73), established from pooled mammary adenocarcinomas of Balb/cfC3H mice, demonstrated specific phosphoproteins associated with murine mammary tumour virus. The major phosphorylated proteins were found to have mol. wt. of 18000 and 12000 (p18 and p12) after isolation by molecular sieving chromatography and analysis by gel electrophoresis.


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