The Rho family of small GTPases comprises some 21 genes in humans, encoding at least 23 signalling proteins. Although these proteins control an amazingly diverse range of cellular functions, one general role is in the establishment of polarity and of polarised structures through dynamic regulation of the actin cytoskeleton. This theme is carried through all three eukaryote kingdoms - from bud formation in S. cerevisiae, to pollen tube elongation in Arabidopsis, to the formation of complex structures such as cochlear stereocilia in mammals. Rho GTPases control the polymerisation, branching and bundling of actin, allowing them to regulate the remodelling of the actin cytoskeleton into distinct architectural elements. Spatial and temporal control of these elements allows Rho GTPases to direct complex mechanical processes such as cell motility and phagocytosis (Bishop and Hall, 2000; Hall, 1998).
The best-characterised family members are RhoA, Rac1 and Cdc42 (human nomenclature)....
