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J Biol Chem, Vol. 274, Issue 14, 9463-9471, April 2, 1999
From the Divisions of Hematology/Oncology, Ophthalmology,
Pathology, and Research Immunology/Bone Marrow Transplant, Childrens
Hospital Los Angeles, and the Departments of Pediatrics, Microbiology,
Ophthalmology, Pathology, and Immunology, University of Southern
California, School of Medicine, Los Angeles, California 90027
The conformation and activity of pRb,
the product of the retinoblastoma susceptibility gene, is dependent on
the phosphorylation status of one or more of its 16 potential
cyclin-dependent kinase (cdk) sites. However, it is not
clear whether the phosphorylation status of one or more of these sites
contributes to the determination of the various conformations and
activity of pRb. Moreover, whether and how the conformation of pRb may
regulate the phosphorylation of the cdk sites is also unclear. In the
process of analyzing the function and regulation of pRb, we uncovered
the existence of an unusual structural motif, m89 (amino acids
880-900), the mutation of which confers upon pRb a hypophosphorylated
conformation. Mutation of this structural domain activates, rather than
inactivates, the growth suppressor function of pRb. In order to
understand the effect of the mutation of m89 on the phosphorylation of
cdk sites, we identified all the cdk sites (Thr-356, Ser-807/Ser-811, and Thr821) the phosphorylation of which drastically modify the conformation of pRb. Mutation of each of these four sites alone or in
combinations results in the different conformations of pRb, the
migration pattern of which, on SDS-polyacrylamide gel electrophoresis, resembles various in vivo hypophosphorylated forms. Each of
these hypophosphorylated forms of pRb has enhanced growth suppressing activity relative to the wild type. Our data revealed that the m89
structural motif controls the exposure of the cdk sites Ser-807/Ser-811 in vitro and in vivo. Moreover, the m89 mutant
has enhanced growth suppressing activity, similar to a mutant with
alanine substitutions at Ser-807/Ser-811. Our recent finding, that the
m89 region is part of a structural domain, p5, conserved antigenically
and functionally between pRb and p53, suggests that the evolutionarily
conserved p5 domain may play a role in the coordinated regulation of
the activity of these two tumor suppressors, under certain growth conditions.
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