Role of pRb-related proteins in simian virus 40 large-T-antigen-mediated transformation.

Simian virus 40 large T-antigen (TAg) transformation is thought to be mediated, at least in part, by binding to and modulating the function of certain cellular proteins, including the retinoblastoma tumor suppressor gene product, pRb. TAg can disrupt the inhibitory complexes formed by pRb with the oncogenic transcription factor E2F, and this mechanism has been suggested to be important for TAg-mediated transformation. Residues 102 to 114 of TAg (including the LXCXE motif) are required for binding to pRb. Mutations within this LXCXE motif abolish the ability of TAg to bind to pRb as well as to transform certain cell types. TAg can also bind to at least two other cellular proteins, p107 and p130, that are related to pRb by sequence homology and share the ability to bind E2F. However, whether p107 and p130 are also targets in TAg-mediated transformation is less clear. To assess the relative contribution of the inactivation of pRb, p107, and p130 to transformation by TAg, fibroblasts were prepared from embryos derived from matings of mice heterozygous for an Rb knockout allele. The ability of TAg to transform fibroblasts homozygous for either wild-type or knockout Rb alleles was evaluated. It is demonstrated that the integrity of the LXCXE motif provides a growth advantage in Rb+/+ and Rb-/- cells. Furthermore, wild-type TAg, but not the LXCXE mutants, could bind to p107 and p130 and disrupt p107-E2F and p130-E2F binding complexes. These results suggest that p107 and p130 participate in TAg-mediated transformation and that they may behave as tumor suppressors.