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 pocket binds E2F, the A/B pocket binds the LXCXE peptide motif, and the C pocket binds the nuclear c-Abl tyrosine kinase. Substitution of Trp for Arg 661 in the B region of RB(mutant 661) inactivates both LXCXE binding. The tumor-suppression function of mutant 661 is not abolished because this allele predisposes its carriers to retinoblastoma development with a low penetrance. In cell-based assayss, 661 is shown to inhibit G₁/S progression. This low- penetrance mutant also induces terminal growth arrest with reduced but detectable activity. We have constructed mutations that disrupt C pocket activity. When overproduced, the RB C-terminal fragment did not induce terminal growth arrest but could inhibit G₁/S progression, and this activity was abolished by the C-pocket mutations. In full-length RB, the C-pocket mutations reduced but did not abolish RB function. Interestingly, combination of the C-pocket and 661 mutations completely abolished RB's ability to cause an increase in the percentage of cells in G₁ and to induce terminal growth arrest. These results suggest that the A/B or C region can induce a prolongation of G₁ through mechanisms that are independent of each other. In contrast, long-term growth arrest requires combined activities from both regions of RB. In addition, E2F and LXCXE binding are not the only mechanisms through which RB inhibits cell growth. The C pocket also contributes to RB- mediated growth suppression.