Fig. 1. Regulation of actin-based motility by the Rho family GTPases. Activated Cdc42, Rac and Rho bind to and specifically activate their downstream effectors, which are either kinases (such as ROCK, PAK and PI5K) or scaffolding proteins (such as mDia, WASP and IRSp53). These effector proteins activate diverse signalling pathways with distinct effects on the actin cytoskeleton and cellular morphology. An important aspect for the cell motility is the equilibrium between the myosin light chain (MLC) and the phosphorylated MLC, which is tightly regulated.