A Kinase Activity Associated with Simian Virus 40 Large T Antigen Phosphorylates Upstream Binding Factor (UBF) and Promotes Formation of a Stable Initiation Complex between UBF and SL1

Abstract

Simian virus 40 large T antigen is a multifunctional protein which has been shown to modulate the expression of genes transcribed by RNA polymerase I (Pol I), II, and III. In all three transcription systems, a key step in the activation process is the recruitment of large T antigen to the promoter by direct protein-protein interaction with the TATA binding protein (TBP)-TAF complexes, namely, SL1, TFIID, and TFIIIB. However, our previous studies on large T antigen stimulation of Pol I transcription also revealed that the binding to the TBP-TAF_I complex SL1 is not sufficient to activate transcription. To further define the molecular mechanism involved in large T antigen-mediated Pol I activation, we examined whether the high-mobility group box-containing upstream binding factor (UBF) plays any role in this process. Here, using cell labeling experiments, we showed that large T antigen expression induces an increase in UBF phosphorylation. Further biochemical analysis demonstrated that UBF is phosphorylated by a kinase activity that is strongly associated with large T antigen, and that the carboxy-terminal activation domain of UBF is required for the phosphorylation to occur. Using in vitro reconstituted transcription assays, we demonstrated that the inability of alkaline phosphatase treated UBF to efficiently activate transcription can be rescued by large T antigen. Moreover, we showed that large T antigen-induced UBF phosphorylation promotes the formation of a stable UBF-SL1 complex. Together, these results provide strong evidence for an important role for the large T antigen-associated kinase in mediating the stimulation of RNA Pol I transcription.

ACKNOWLEDGMENTS

We are grateful to the members of the Gene Expression Group at the University of Southern California for advice and discussions throughout the course of this work. We thank H.-M. Jantzen for constructs, A. Schönthal for Hs68 and Hs68-LT cell lines, R. Voit and I. Grummt for the pUBF-at construct, and N. Bernt for valuable advice on 2D phosphopeptide mapping. We are also thankful to Joann Tuan and Tiffany Bui for their technical support.

W.Z. is partially supported by the Heidelberger Predoctoral Scholarship Award in Cancer Research. This work was supported by a grant to L.C. (R01 GM53949) from the National Institutes of Health.