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

Large T-antigen, the major regulatory protein encoded by polyomaviruses, including Simian Virus 40 (SV40) and JC virus (JCV), is a multifunctional phosphoprotein that is involved in many viral and cellular events. In addition to its integral role in viral replication and cellular transformation, T-antigen also regulates transcription of both viral and cellular genes. In particular, the viral late promoter has been used as a model for the analysis of T-antigen-mediated transcriptional activation. Earlier studies have demonstrated that the cellular protein Pur alpha is able to attenuate the transcriptional activity of JCV T-antigen, We investigated the mechanism whereby Pur alpha affects T-antigen function. Co-immunoprecipitation studies demonstrated that Pur alpha and JCV T-antigen associate in vivo, and glutathione S-transferase affinity binding assays revealed that these two proteins interact in vitro. Moreover, we localized the sequences of Pur alpha that are important for the interaction between Pur alpha and JCV T-antigen, In addition, we demonstrated that Pur alpha interacts with the SV40 T-antigen, Transient transfection studies demonstrated that Pur alpha and JCV T-antigen interact functionally as well. More specifically, Pur alpha and a deletion mutant that interacts with T-antigen attenuated T-antigen-mediated transcriptional activation. A Pur alpha deletion mutant that is unable to interact with JCV T-antigen, however, was found to be incapable of abrogating JCV T-antigen transactivation. Taken together, these data demonstrate that Pur alpha and T-antigen interact both physically and functionally and that this interaction modulates T-antigen-mediated transcriptional activation, The implication of these findings with respect to the cellular role of Pur alpha is discussed.