Novel Mechanisms of E2F Induction by BK Virus Large-T Antigen: Requirement of Both the pRb-Binding and the J Domains

ABSTRACT

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

ACKNOWLEDGMENTS

We thank the members of our laboratory for useful discussions and comments about this work, J. Nevins for the pE2F-CAT and GST constructs, E. Moran for the E1A construct, J. Chamberlain for the pAdCMVβ construct, N. Dyson for anti-p107 antibodies, and E. Harlow for various hybridomas.

This work was supported in part by American Cancer Society grant VM-11A, the Elsa U. Pardee Foundation, and a student development award from NIH Prostate SPORE grant P50 CA69568. K.F.H. was supported in part by the Nancy Newton-Loeb Foundation and a Rackham Predoctoral Fellowship from the University of Michigan.

ADDENDUM IN PROOF

Since the submission of this paper, Schaffhausen and colleagues have published data indicating that the J domain of mouse polyomavirus TAg also regulates pRb family function (J. Virol.71:9410–9416, 1997).