Interaction of the single-stranded DNA-binding protein Pur alpha with the human polyomavirus JC virus early protein T-antigen

Large T-antigen, the major regulatory protein encoded by polyomaviruses, in cluding Simian Virus 40 (SV40) and JC virus (JCV), is a multifunctional pho sphoprotein that is involved in many viral and cellular events. In addition to its integral role in viral replication and cellular transformation, T-a ntigen also regulates transcription of both viral and cellular genes. In pa rticular, the viral late promoter has been used as a model for the analysis of T-antigen-mediated transcriptional activation. Earlier studies have dem onstrated that the cellular protein Pur alpha is able to attenuate the tran scriptional activity of JCV T-antigen, We investigated the mechanism whereb y Pur alpha affects T-antigen function. Co-immunoprecipitation studies demo nstrated that Pur alpha and JCV T-antigen associate in vivo, and glutathion e S-transferase affinity binding assays revealed that these two proteins in teract in vitro. Moreover, we localized the sequences of Pur alpha that are important for the interaction between Pur alpha and JCV T-antigen, In addi tion, we demonstrated that Pur alpha interacts with the SV40 T-antigen, Tra nsient transfection studies demonstrated that Pur alpha and JCV T-antigen i nteract functionally as well. More specifically, Pur alpha and a deletion m utant that interacts with T-antigen attenuated T-antigen-mediated transcrip tional activation. A Pur alpha deletion mutant that is unable to interact w ith JCV T-antigen, however, was found to be incapable of abrogating JCV T-a ntigen transactivation. Taken together, these data demonstrate that Pur alp ha and T-antigen interact both physically and functionally and that this in teraction modulates T-antigen-mediated transcriptional activation, The impl ication of these findings with respect to the cellular role of Pur alpha is discussed.