A retinoblastoma-binding protein related to a negative regulator of Ras in yeast

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Abstract
THE growth suppression function of the retinoblastoma protein (Rb) is thought to be mediated by Rb binding to cellular proteins1. p48 is one of the major proteins that binds to a putative functional domain at the carboxy terminus of the Rb protein2. Here we report the isolation of a full-length complementary DNA (RbAp48) encoding p48. Complex formation between p48 and Rb occurs in vitro and in vivo, and apparently involves direct interaction between the proteins. Like RbAp48 is a ubiquitously expressed nuclear protein. Rb, p48 share sequence homology with MS11, a negative regulator of the Ras-cyclic AMP pathway in the yeast Saccharomyces cerevisiae3. Furthermore, like MS11, human RbAp48 suppresses the heat-shock sensitivity of the yeast ira1 strains and RAS2val19 strains. Interaction with p48 may be one of the mechanisms for suppression of growth mediated by Rb.