SIMIAN-VIRUS-40 LARGE T-ANTIGEN ALTERS THE PHOSPHORYLATION STATE OF THE RB-RELATED PROTEINS P130 AND P107

p130 and p107 are nuclear phosphoproteins related to the retinoblastom a gene product (pRb). pRb, p107, and p130 each undergo cell cycle-depe ndent phosphorylation, form complexes with the E2F family of transcrip tion factors, and associate with oncoproteins of DNA tumor viruses, in cluding simian virus 40 (SV40) large T antigen (TAg) and adenovirus E1 A protein. The results of recent studies with mouse embryo fibroblasts (MEFs) lacking the retinoblastoma gene (Rb-1) have suggested that p13 0 and p107 may be important targets for SV40 large TAg-mediated transf ormation (J. B. Christensen and M. J. Imperiale, J. Virol. 65:39453948 , 1995; J. Zalvide and J. A. DeCaprio, Mel. Cell. Biol. 15:58005810, 1 995). In this report, we demonstrate that the expression of TAg affect s the phosphorylation state of p130 and p107. In cells expressing wild -type TAg, only un(der)phosphorylated p130 and p107 were detected. To determine the domains within TAg that contribute to this effect on the phosphorylation of p130, we performed transient expression assays. Wh ile transiently expressed p130 was apparently phosphorylated normally, only un(der)phosphorylated p130 was detected when p130 was coexpresse d with TAg. Using this assay, we found that the first 147 amino acids of TAg were sufficient to alter the phosphorylation state of p130. Wit hin this region, the LXCXE domain of TAg, required for binding to the retinoblastoma family of proteins, was necessary but not sufficient to affect p130 phosphorylation. Residues within the first 82 amino acids of TAg were also required. TAg with mutations in the N terminus retai ned the ability to efficiently associate with p130 but did not affect its phosphorylation state. This demonstrates that the effect of SV40 T Ag on p130 is not simply the result of binding and suggests that TAg h as a novel effect on p130 and p107 that differs from its effect on pRb .