PHOSPHORYLATION OF P53: A NOVEL PATHWAY FOR P53 INACTIVATION IN HTLV-I-TRANSFORMED CELLS

Abstract

O-14 Inhibition of p53 function, either through mutation or interaction with viral transforming proteins, correlates strongly with oncogenic potential. Only a small percentage of human T-lymphotropic virus type-I-(HTLV-I) transformed cells carry p53 mutations and mutated p53 genes have been found in only one-fourth of ATL cases. We have demonstrated that wild-type p53 is stabilized and transcriptionally inactive in HTLV-transformed cells. Further, the viral transcriptional activator Tax inactivates the N-terminal activation domain. Using phospho-amino acid-specific antibodies, we have shown that Ser15 and Ser392 are hyper-phosphorylated in HTLV-I-transformed cells. Since HTLV-I p53 binds DNA in a sequence-specific manner but fails to interact with TFIID, we tested whether phosphorylation of the N-terminus of p53 affected p53-TFIID interaction. Using biotinylated peptides we show that phosphorylation of Ser15 alone inhibits p53-TFIID interaction. In contrast, phosphorylation at Ser15 and 37 restores TFIID binding and blocks MDM2 binding. Our studies provide evidence that HTLV-I utilizes the post-translational modification of p53 in vivo to inactivate the tumor suppressor protein. The importance of ATM and DNA-PK kinases are being addressed using kinase deficient cell lines and GAL4-p53 fusion proteins.