Fig. 2. Signalling pathways controlling stress fibre formation. RhoA-mediated activation of mDia1 at the plasma membrane provides the actin-nucleating activity required for stress fibre formation. RhoA also activates ROCK, which can phosphorylate MLC directly but also inhibits myosin phosphatase through a complex set of pathways. Myosin phosphatase is a trimeric complex comprising the M20 subunit, the PPc1 catalytic subunit and the regulatory MBS (MYPT) subunit. Inhibition of myosin phosphatase activity downstream of RhoA increases MLC phosphorylation and hence increases actomyosin contractility. ROCK activity is inhibited by the Rho GTPase RhoE and by the small GTPases Rad and Gem.