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Identification of simian virus 40 protein A

A large simian virus 40 (SV40)-specific protein can be efficiently immunoprecipitated from infected cell extracts with antisera obtained from hamsters bearing SV40-induced tumors. The protein has an apparent molecular weight of 88,000 to 100,000 with respect to markers with known molecular weights, but behaves anomalously on sodium dodecyl sulfate (SDS)-polyacrylamide gels. Cell lines infected by two different strains of SV40 synthesize immunoreactive proteins that differ slightly in mobility during SDS-polyacrylamide gel electrophoresis, evidence that the protein is coded for by the virus. These differences in protein size correlate with differences in the electrophoretic mobility of viral DNA fragments obtained by digestion with HindII and III restriction enzymes. The size of the viral capsid proteins VP2 and VP3 also varies with the strain of virus. dl-1001, a constructed deletion mutant that lacks part of the SV40A gene, directs the synthesis of a 33,000-dalton polypeptide that is not detected in cells infected with wild-type virus. The deletion fragment, like the larger protein, is phosphorylated. Maps of tryptic peptides from the 88,000- to 100,000-dalton protein and the 33,000-dalton fragment show common peptides and provide strong direct evidence that the proteins are products of the SV40 A gene. The deletion fragment reacts with antitumor sera and binds to double-stranded DNA in the presence of the complete A protein.