The Borgs, a new family of Cdc42 and TC10 GTPase-interacting proteins

Mol Cell Biol. 1999 Oct;19(10):6585-97. doi: 10.1128/MCB.19.10.6585.


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 (HA(3))-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. HA(3)-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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding Sites
  • Blood Proteins
  • Cell Size
  • Cytoskeletal Proteins
  • GTP Phosphohydrolase Activators*
  • GTP Phosphohydrolases
  • GTP-Binding Protein Regulators*
  • Gene Expression
  • Gene Library
  • JNK Mitogen-Activated Protein Kinases
  • Mice
  • Mitogen-Activated Protein Kinases / antagonists & inhibitors
  • Molecular Sequence Data
  • Protein Binding
  • Protein Structure, Tertiary
  • RNA-Binding Proteins
  • Recombinant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Signal Transduction
  • Tissue Distribution
  • Two-Hybrid System Techniques
  • cdc42 GTP-Binding Protein / metabolism
  • rho GTP-Binding Proteins / metabolism*


  • Blood Proteins
  • CDC42EP2 protein, human
  • CDC42EP4 protein, human
  • Cdc42ep5 protein, mouse
  • Cytoskeletal Proteins
  • GTP Phosphohydrolase Activators
  • GTP-Binding Protein Regulators
  • RNA-Binding Proteins
  • Recombinant Proteins
  • JNK Mitogen-Activated Protein Kinases
  • Mitogen-Activated Protein Kinases
  • GTP Phosphohydrolases
  • RHOQ protein, human
  • Rhoq protein, mouse
  • cdc42 GTP-Binding Protein
  • rho GTP-Binding Proteins

Associated data

  • GENBANK/AF163840
  • GENBANK/AF164118
  • GENBANK/AF164119
  • GENBANK/AF165114