DUAL RETINOBLASTOMA-BINDING PROTEINS WITH PROPERTIES RELATED TO A NEGATIVE REGULATOR OF RAS IN YEAST

The retinoblastoma protein (Rb) interacts with multiple cellular proteins that mediate its cellular function. We have identified nine polypeptides that bind to the T-binding domains of Rb using an Rb affinity resin. RbAp48 and RbAp46 are quantitatively the major Rb-associated proteins purified by this approach. RbAp48 was characterized previously and was found to be related to MSI1, a negative regulator of has in the yeast Saccharomyces cerevisiae. Here we report the cloning and characterization of RbAp46. RbAp46 shares 89.4% amino acid identity with RbAp48. The internal WD repeats, which are found in a growing number of eukaryotic proteins, are conserved between RbAp46 and RbAp48. Like RbAp48, RbAp46 forms a complex with Rb both in vitro and in vivo and suppresses the heat-shock sensitivity of the yeast RAS2(val-19) strains. We have also isolated the murine cDNA homologs of RbAp48 and RbAp46. Although both mRNA can be detected in all mouse tissues, their mRNA levels vary dramatically between different tissues. No significant differences were observed in the expression patterns of these genes in most tissues except thymus, testis, and ovary/uterus, in which 2-fold differences were observed. Interestingly, the mouse and human RbAp48 amino acid sequences are completely identical, and the mouse and human RbAp46 differ only by one conserved amino acid substitution. These results suggest that RbAp48 and RbAp46 may have shared as well as unique functions in the regulation of cell proliferation and differentiation.