The Fer tyrosine kinase regulates interactions of Rho GDP-Dissociation Inhibitor α with the small GTPase Rac

Background: RhoGDI proteins are important regulators of the small GTPase Rac, because they shuttle Rac from the cytoplasm to membranes and also protect Rac from activation, deactivation and degradation. How the binding and release of Rac from RhoGDI is regulated is not precisely understood. Results: We report that the non-receptor tyrosine kinase Fer is able to phosphorylate RhoGDI alpha and form a direct protein complex with it. This interaction is mediated by the C-terminal end of RhoGDI alpha. Activation of Fer by reactive oxygen species caused increased phosphorylation of RhoGDI alpha and pervanadate treatment further augmented this. Tyrosine phosphorylation of RhoGDI alpha by Fer prevented subsequent binding of Rac to RhoGDI alpha, but once a RhoGDI alpha-Rac complex was formed, the Fer kinase was not able to cause Rac release through tyrosine phosphorylation of preformed RhoGDI alpha-Rac complexes. Conclusions: These results identify tyrosine phosphorylation of RhoGDI alpha by Fer as a mechanism to regulate binding of RhoGDI alpha to Rac.