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Molecular and Cellular Biology, December 1998, p. 7487-7498, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Raf-1 Physically Interacts with Rb and Regulates Its Function: a Link between Mitogenic Signaling and Cell Cycle Regulation

Sheng Wang, Richik N. Ghosh,dagger and Srikumar P. Chellappan*

Department of Pathology, College of Physicians and Surgeons, Columbia University, New York, New York 10032

Received 22 April 1998/Returned for modification 1 June 1998/Accepted 26 August 1998

Cells initiate proliferation in response to growth factor stimulation, but the biochemical mechanisms linking signals received at the cell surface receptors to the cell cycle regulatory molecules are not yet clear. In this study, we show that the signaling molecule Raf-1 can physically interact with Rb and p130 proteins in vitro and in vivo and that this interaction can be detected in mammalian cells without overexpressing any component. The binding of Raf-1 to Rb occurs subsequent to mitogen stimulation, and this interaction can be detected only in proliferating cells. Raf-1 can inactivate Rb function and can reverse Rb-mediated repression of E2F1 transcription and cell proliferation efficiently. The region of Raf-1 involved in Rb binding spanned residues 1 to 28 at the N terminus, and functional inactivation of Rb required a direct interaction. Serum stimulation of quiescent human fibroblast HSF8 cells led to a partial translocation of Raf-1 into the nucleus, where it colocalized with Rb. Further, Raf-1 was able to phosphorylate Rb in vitro quite efficiently. We believe that the physical interaction of Raf-1 with Rb is a vital step in the growth factor-mediated induction of cell proliferation and that Raf-1 acts as a direct link between cell surface signaling cascades and the cell cycle machinery.

* Corresponding author. Mailing address: Department of Pathology, College of Physicians and Surgeons, Columbia University, 630 W. 168th St., New York, NY 10032. Phone: (212) 305-3736. Fax: (212) 305-5498. E-mail: spc10{at}columbia.edu.

dagger Present address: Cellomics, Inc., Pittsburgh, PA 15238.

Molecular and Cellular Biology, December 1998, p. 7487-7498, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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