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Molecular and Cellular Biology, October 1999, p. 6585-6597, Vol. 19, No. 10
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

The Borgs, a New Family of Cdc42 and TC10 GTPase-Interacting Proteins

Gérard Joberty,* Richard R. Perlungher, and Ian G. Macara

Markey Center for Cell Signaling and Department of Pharmacology, University of Virginia, Charlottesville, Virginia 22908

Received 21 April 1999/Returned for modification 9 June 1999/Accepted 30 June 1999

The Rho family of GTPases plays key roles in the regulation of cell motility and morphogenesis. They also regulate protein kinase cascades, gene expression, and cell cycle progression. This multiplicity of roles requires that the Rho GTPases interact with a wide variety of downstream effector proteins. An understanding of their functions at a molecular level therefore requires the identification of the entire set of such effectors. Towards this end, we performed a two-hybrid screen using the TC10 GTPase as bait and identified a family of putative effector proteins related to MSE55, a murine stromal and epithelial cell protein of 55 kDa. We have named this family the Borg (binder of Rho GTPases) proteins. Complete open reading frames have been obtained for Borg1 through Borg3. We renamed MSE55 as Borg5. Borg1, Borg2, Borg4, and Borg5 bind both TC10 and Cdc42 in a GTP-dependent manner. Surprisingly, Borg3 bound only to Cdc42. An intact CRIB (Cdc42, Rac interactive binding) domain was required for binding. No interaction of the Borgs with Rac1 or RhoA was detectable. Three-hemagglutinin epitope (HA3)-tagged Borg3 protein was mostly cytosolic when expressed ectopically in NIH 3T3 cells, with some accumulation in membrane ruffles. The phenotype induced by Borg3 was reminiscent of that caused by an inhibition of Rho function and was reversed by overexpression of Rho. Surprisingly, it was independent of the ability to bind Cdc42. Borg3 also inhibited Jun kinase activity by a mechanism that was independent of Cdc42 binding. HA3-Borg3 expression caused substantial delays in the spreading of cells on fibronectin surfaces after replating, and the spread cells lacked stress fibers. We propose that the Borg proteins function as negative regulators of Rho GTPase signaling.

* Corresponding author. Mailing address: Room 7191 Hospital West, Box 577, HSC, University of Virginia School of Medicine, Charlottesville, VA 22908. Phone: (804) 982-0083. Fax: (804) 924-1236. E-mail: gmj4h{at}virginia.edu.

Molecular and Cellular Biology, October 1999, p. 6585-6597, Vol. 19, No. 10
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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