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Mol. Cell. Biol., 10 1995, 5542-5551, Vol 15, No. 10
PJ Welch and JY Wang
The decision to enter the cell division cycle is governed by the interplay
between growth activators and growth inhibitors. The retinoblastoma protein
(RB) is an example of a growth inhibitor whose main function appears to be
the binding and inactivation of key cell cycle activators. One target of RB
is a proto-oncoprotein, the c-Abl tyrosine kinase. RB binds to the
ATP-binding lobe in the kinase domain and inhibits the nuclear pool of
c-Abl in quiescent and G1 cells. Phosphorylation of RB at G1/S releases
c-Abl, leading to the activation of this nuclear tyrosine kinase. In this
report, we describe the construction of a mutant Abl, replacing the
ATP-binding lobe of c-Abl with that of c-Src. The mutant protein AS2 is
active as a tyrosine kinase and can phosphorylate Abl substrates, such as
the C-terminal repeated domain of RNA polymerase II. AS2, however, does not
bind to RB, and its activity is not inhibited by RB. As a result, the
nuclear pool of AS2 is no longer cell cycle regulated. Excess AS2, but not
its kinase-defective counterpart, can overcome RB-induced growth arrest in
Saos-2 cells. Interestingly, wild-type c-Abl, in both its kinase-active and
-inactive forms, can also overcome RB. Furthermore, overexpression of a
kinase-defective c-Abl in rodent fibroblasts accelerates the transition
from quiescence to S phase and cooperates with c-Myc to induce
transformation. These effects, however, do not occur with the
kinase-defective form of AS2. Thus, the growth-stimulating function of the
kinase-defective c-Abl is dependent on the binding and the abrogation of RB
function. That RB function can be abolished by the overproduction of one of
its binding proteins is consistent with the hypothesis that RB induces cell
cycle arrest by acting as a "molecular matchmaker" to assemble protein
complexes. Exclusive engagement of RB by one of its many targets is
incompatible with the biological function of this growth suppressor
protein.
Copyright © 1995, American Society for Microbiology
Abrogation of retinoblastoma protein function by c-Abl through tyrosine kinase-dependent and -independent mechanisms
Department of Biology, University of California, San Diego, La Jolla 92093-0347, USA.
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