GROWTH SUPPRESSION BY AN E2F-BINDING-DEFECTIVE RETINOBLASTOMA PROTEIN(RB) - CONTRIBUTION FROM THE RB-C POCKET

Growth suppression by the retinoblastoma protein (RB) is dependent on its ability to form complexes with transcription regulators. At least three distinct protein-binding activities have been identified in RB: the large A/B pocket binds E2F, the A/B pocket binds the LXCXE peptide motif, and the C pocket binds the nuclear c-Abl tyrosine kinase, Subs titution of Trp for Arg 661 in the B region of RB (mutant 661) inactiv ates both E2F and LXCXE binding. The tumor suppression function of mut ant 661 is not abolished, because this allele predisposes its carriers to retinoblastoma development with a low penetrance. In cell-based as says, 661 is shown to inhibit G(1)/S progression. This low-penetrance mutant also induces terminal growth arrest with reduced but detectable activity. We have constructed mutations that disrupt C pocket activit y. When overproduced, the RB C-terminal fragment did not induce termin al growth arrest but could inhibit G(1)/S progression, and this activi ty was abolished by the C-pocket mutations. In full-length RB, the C-p ocket mutations reduced but did not abolish RB function. Interestingly , combination of the C-pocket and 661 mutation's completely abolished RB's ability to cause an increase in the percentage of cells in G(1) a nd to induce terminal growth arrest.These results suggest that the A/B or C region can induce a prolongation of G(1) through mechanisms that are independent of each other. In contrast, long-term growth arrest r equires combined activities from both regions of RB. In addition, E2F and LXCXE binding are not the only mechanisms through which RB inhibit s cell growth. The C pocket also contributes to RB-mediated growth sup pression.