THE RETINOBLASTOMA PROTEIN ASSOCIATES WITH THE PROTEIN PHOSPHATASE TYPE-1 CATALYTIC SUBUNIT

The retinoblastoma protein (p110RB) interacts with many cellular proteins in complexes potentially important for its growth-suppressing function. We have developed and used an improved version of the yeast two-hybrid system to isolate human cDNAs encoding proteins able to bind p110RB. One clone encodes a novel type 1 protein phosphatase catalytic subunit (PP-1alpha2), which differs from the originally defined PP-1alpha by an amino-terminal 11-amino-acid insert. In vitro-binding assays demonstrated that PP-1alpha isoforms preferentially bind the hypophosphorylated form of p110RB. Moreover, similar p110RB sequences are required for binding PP-1alpha2 and SV40 large T antigen. Cell cycle synchrony experiments revealed that this association occurs from mitosis to early G1. The implications of these findings on the regulation of both proteins are discussed.