RhoA-GDP regulates RhoB protein stability -: Potential involvement of RhoGDIα

RhoA plays a significant role in actin stress fibers formation. However, silencing RhoA alone or RhoA and RhoC did not completely suppress the stress fibers suggesting a residual Rho-like activity. RhoB, the third member of the Rho subclass, is a short-lived protein barely detectable in basal conditions. In various cell types, the silencing of RhoA induced a strong up-regulation of both total and active RhoB protein levels that were rescued by re-expressing RhoA and related to an enhanced half-life of the protein. The RhoA-dependent regulation of RhoB does not depend on the activity of RhoA but is mediated by its GDP-bound form. The stabilization of RhoB was not dependent on isoprenoid biosynthesis, Rho kinase, extracellular signal-regulated kinase, p38 mitogen-activated kinase, or phosphatidylinositol 3'-OH kinase pathways but required RhoGDI alpha. The forced expression of RhoGDI alpha increased RhoB half-life, whereas its knock-down antagonized the induction of RhoB following RhoA silencing. Moreover, a RhoA mutant (RhoAR68E) unable to bind RhoGDI alpha was significantly less efficient as compared with wild-type RhoA in reversing RhoB up-regulation upon RhoA silencing. These results suggest that, in basal conditions, RhoGDI alpha is rate-limiting and the suppression of RhoA makes it available to stabilize RhoB. Our results highlight RhoGDI alpha dependent cross-talks that regulate the stability of RhoGTPases.