The phosphorylation at Thr 124 of simian virus 40 large T antigen is crucial for its oligomerization

SV40 large T antigen is phosphorylated at up to ten different amino acids clustered in an N-terminal and a C-terminal part of the polypeptide chain. The N-terminal phosphorylated residues include Ser 123 and Thr 124. We have analyzed the oligomerization, the complex formation with the cellular oncoprotein p53 and the DNA-binding properties of T antigen from two different SV40 transformed cell lines which have either an amino acid exchange at Ser 123 to Phe (W7) or Thr 124 to Ile (D29). In comparison to wild-type T antigen both mutant T antigens have a slightly reduced binding affinity for both binding sites, I and II, of SV40 DNA. Phosphorylation at both residues of T antigen is not essential for formation of the complex with p53. Only the phosphorylation at Thr 124 seems to be critical for the formation of high molecular mass oligomers. Our data support the hypothesis that the oligomerization of T antigen seems to be implicated in viral DNA replication.