SV40 large T antigen binds preferentially to an underphosphorylated member of the retinoblastoma susceptibility gene product family.

Extracts of monkey cells (CV-1P) synthesizing SV40 large T antigen (T) were immunoprecipitated with monoclonal antibodies to T or p110-114Rb, the product of the retinoblastoma susceptibility gene (Rb). While a family of p110-114Rb proteins can be detected in anti-Rb immunoprecipitates, only one member of this family, p110Rb, was found in anti-T precipitates of these extracts. Identical results were obtained with extracts of CV-1P cells which had been previously mixed in vitro with highly purified T. The p110-114Rb family is composed of two sets--p110Rb, an un- or under-phosphorylated species, and pp112-114Rb, a group of overtly phosphorylated proteins. Thus, T bound preferentially to the un- or underphosphorylated member of the family. In addition, T failed to alter the relative abundances of these species. These results suggest a model in which the growth suppression function of Rb is down modulated either by phosphorylation or T antigen binding.