This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The ARF-p53 tumor suppressor pathway is one of the cell?s major defenses against the stimulation of uncontrolled cell division induced by activated cellular and viral oncogene. ARF and/or p53 are mutated in over 70% of human cancers. The inappropriate activation of growth promoting cellular signaling pathways by oncogenes can result in the induction of ARF. The expression of ARF can activate p53 leading to apoptotic cell death or cell cycle arrest. The mechanisms by which the ARF-p53 pathway is regulated remains to be precisely elucidated. The expression of ARF can activate p53 leading to apoptotic cell death or cell cycle arrest. We have shown that the polyoma virus oncogene, PYMT, activates an ARF-induced p53 mediated block. We find that the polyoma virus small T-antigen, PYST, via its ability to bind to cellular protein phosphatase 2A (PP2A), can negate the ARF-induced block to cell division induced by PYMT. We intend to use the PY induction and inhibition of ARF signaling to p53 to better define this important tumor suppressor pathway. Our hypothesis is that the polyoma virus proteins are revealing an important new aspect of the ARF-p53 tumor suppressor signaling circuit, and we plan to use these viral proteins as tools to study its molecular basis. To better define the role of PYMT in activating ARF and as an oncogene, and to define the role of PYST in blocking ARF signaling to p53 we plan to characterize the proteins complexed to PYMT, PYST and members of the ARF-p53 signaling pathway. In the first instance proteins bound to TAP fusion constructs would be identified by Mass Spectrometry.