pRB-dependent, J domain-independent function of simian virus 40 large T antigen in override of p53 growth suppression

Simian virus 40 (SV40) large T antigen (LT) can immortalize and transform m any cell types. These activities are attributed in large part to the bindin g and functional inactivation by LT of two major tumor suppressors: p53 and the retinoblastoma protein, pRB. Most effects of LT on pRB have been shown to additionally require an intact J domain, which mediates binding to Hsc7 0. We show here that the J domain is not required for p53 override in full- length LT. Although LT binds p53, it was shown previously that overcoming a p53-induced cell cycle arrest requires binding to pRB family members (R. S . Quartin et al., J. Virol. 68:1334-1341). We demonstrate that an LT mutant defective for pRB family member binding (K1) can be complemented for effic ient override of p53 arrest by a construct encoding the first 135 amino aci ds of LT with a J domain-inactivating mutation, H42Q. Hence, complementatio n does not require the J domain, and pRB binding by LT is important for mor e than dissociating pRB-E2F complexes, which is J dependent. In accordance with this notion, LT alleviates pRB small-pocket-mediated transcriptional r epression independently of the J domain. The LT K1 mutant can also be compl emented for p53 override by small t antigen (st) in a manner independent of its J domain. Our observations underscore the importance of multiple SV40 functions, two in LT and one in st, that act cooperatively to counteract p5 3 growth suppression.