Identification of simian virus 40 protein A

Abstract

A large SV-40-specific protein can be efficiently immunoprecipitated from infected [African green monkey kidney CV-1] cell extracts with antisera obtained from hamsters bearing SV-40-induced tumors. The protein has an apparent MW of 88,000-100,000 with respect to markers with known molecular weights, but behaves anomalously on sodium dodecyl sulfate (SDS)-polyacrylamide gels. Cell lines infected by 2 different strains of SV-40 synthesize immunoreactive proteins that differ slightly in mobility during SDS-polyacrylamide gel electrophoresis, indicating 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. A constructed deletion mutant that lacks part of the SV-40 A gene, dl-1001, 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 indicate that the proteins are products of the SV-40 A gene. The deletion fragment reacts with antitumor sera and binds to double-stranded DNA in the presence of the complete A protein.