Modification of simian virus 40 protein A

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Abstract
The A protein SV-40 is phosphorylated in productive and transforming infection. The phosphorylated amino acid was identified as serine and was localized in a single tryptic peptide of the protein. Because the A protein synthesized in infection by A mutants is phosphorylated to the same extent and in the same peptide as in infection by wild-type virus, the functional defect of the A mutants is apparently unrelated to phosphorylation. At least 3 distinct forms of the A protein with apparent MW of 85,000, 88,000 and 100,000 can be identified in extracts of [monkey kidney] cells infected by wild-type virus. After exposure of cells to Nonidet P-40, the 85,000- and 88,000-dalton proteins were found in varying amounts in extracts of permissive cells but not in extracts of transformed cells. This finding raised the question of the possible functional importance of the smaller proteins in productive infection. The virtual absence of the 85,000- and 88,000-dalton proteins in some extracts of the fully permissive [African green monkey kidney] CV-1 cell line indicates that a conversion of the larger to the smaller forms of the A protein is not required in significant quantity for productive infection. A study of extraction conditions shows that the smaller proteins are easily generated during extraction and provides an explanation for the appearance of these proteins in some cells after extraction under unfavorable conditions.