The Rho family of small guanine nucleotide binding proteins is one of five generally recognized branches of the Ras superfamily. Like most Ras superfamily members, typical Rho proteins function as binary switches controlling a variety of biological processes. They perform this function by cycling between active GTP-bound and inactive GDP-bound conformations. Mammalian Rho GTPases include RhoA, RhoB and RhoC (Rho proteins), Rac1 3 (Rac proteins), Cdc42, TC10, TCL, Wrch1, Chp/Wrch2, RhoD and RhoG, to name some. The family also includes RhoH and Rnd1-3, which lack GTPase activity and are predicted to exist in a constitutively active state.
Members of the Rho family have been identified in all eukaryotes. Including the atypical RHOBTB1-3 and RHOT1-2 proteins, 24 Rho family members have been identified in mammals (Jaffe and Hall, 2005; Bernards, 2005; Ridley, 2006). Among Rho GTPases, RhoA, Rac1 and Cdc42 have been most extensively studied. These proteins are best known for their ability to induce dynamic rearrangements of the plasma membrane-associated actin cytoskeleton (Aspenstrom et al, 2004; Murphy et al, 1999; Govek et al, 2005). Beyond this function, Rho GTPases also regulate actomyosin contractility and microtubule dynamics. Rho mediated effects on transcription and membrane trafficking are believed to be secondary to these functions. At the more macroscopic level, Rho GTPases have been implicated in many important cell biological processes, including cell growth control, cytokinesis, cell motility, cell cell and cell extracellular matrix adhesion, cell transformation and invasion, and development (Govek et al., 2005). The illustration below lists Rho GTPase effectors implicated in actin and microtubule dynamics (courtesy: Govek et al., 2005, Genes and Development, CSHL Press). Detailed annotations of various biological processes regulated by Rho GTPases will be available in future releases.