Rho (A, B, & C) proteins are members of the Ras superfamily of GTPases. These proteins regulate a variety of cellular functions, including cell cycle progression, cytoskeletal rearrangement, and gene expression. Rho cycles between the active GTP-bound form and an inactive GDP-bound form. Interconversion between these forms is controlled by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). The Rho proteins RhoA, RhoB, and RhoC are highly homologous and contain the consensus amino acid sequences necessary for GDP/GTP-binding and GTPase activity. Post-translational regulation of Rho activity has been shown specifically for RhoA. This Rho protein is phosphorylated in vitro on serine 188 by cAMP- and cGMP-dependent kinases (PKA and PKG). Ser-188 phosphorylation enhances RhoGDI binding and inhibits RhoA-mediated stress fiber formation. Thus, Ser-188 is an important site for negative regulation of RhoA activity.
