The retinoblastoma gene product (Rb) induces binding of a conformationally inactive nuclear factor-kappaB

Nuclear factor-kappaB (NF-kappaB) regulates expression of several viral and cellular genes including the human immunodeficiency virus long terminal repeat, major histocompatibility complex class I, and interleukin 2Ralpha cytokine genes. Here we report that the retinoblastoma gene product (Rb) stimulates binding of the NF-kappaB p50 homodimer. The addition of Rb protein to an in vitro gel shift binding assay stimulated p50 binding greater than 10-fold. Interestingly, by analyzing NF-kappaB-dependent transcription activity in vitro, we demonstrate that Rb suppresses transcriptional activity of p50. Chymotrypsin analysis suggests that Rb induces a conformational change in the NF-kappaB-DNA complex, resulting in binding of a transcriptionally inactive complex. Finally, we demonstrate by coimmunoprecipitation analysis that the Rb-p50 complex is present in Jurkat cell extracts. Our results suggest that Rb may play an important role in regulation of NF-kappaB transcriptional activity.