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Mol Cell Biol, July 1998, p. 4032-4042, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Growth Suppression by an E2F-Binding-Defective Retinoblastoma Protein (RB): Contribution from the RB C Pocketdagger

Laura L. Whitaker, Heyun Su, Rajasekaran Baskaran, Erik S. Knudsen, and Jean Y. J. Wang*

Department of Biology, Center for Molecular Genetics, and Cancer Center, University of California, San Diego, La Jolla, California 92093-0322

Received 14 January 1998/Returned for modification 24 March 1998/Accepted 22 April 1998

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. Substitution of Trp for Arg 661 in the B region of RB (mutant 661) inactivates both E2F and 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 assays, 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.

* Corresponding author. Mailing address: Department of Biology, Bonner Hall 3326, UCSD, 9500 Gilman Dr., La Jolla, CA 92093-0322. Phone: (619) 534-6253. Fax: (619) 534-2821. E-mail: jywang{at}ucsd.edu.

dagger In memory of Laura L. Whitaker, 25 February 1998.

Mol Cell Biol, July 1998, p. 4032-4042, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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