Characterization of the Complex between SV40 Large T Antigen and the 53K Host Protein in Transformed Mouse Cells

Cells transformed by SV40 express the same func-tions that appear early after lytic infection. The first polypeptide characterized as both a product of the early region of the viral DNA and characteristic of virus-transformed cells was large T antigen. This pro-tein is the product of the A cistron and has an apparent molecular weight of about 94,000 (Rundell et ai. 1977). The second polypeptide, small T antigen, was initially detected by in vitro protein synthesis (Greenblatt et al. 1976; Prives et al. 1977). It has a molecular weight of about 20,000 after alkylation (Crawford and O'Farrell 1979), consistent with the estimate of 20,503 for its molecular weight based on the DNA sequence (Fiers et al. 1978; Reddy et al. 1978). Large and small T antigens have common N-terminal sequences (Paucha et al. 1978) and share antigenic determinants (Lane and Robbins 1978). Large T antigen is clearly implicated in transfor-mation, since cells transformed by temperature-sen-sitive mutants in the A cistron lose some of their transformed properties when grown at the high, non-permissive temperature (Brugge and Butel 1975;