AbstractVascular smooth muscle cell contractile state is the primary determinant of blood vessel tone. Vascular smooth muscle cell contractility is directly related to the phosphorylation of myosin light chains (MLCs), which in turn is tightly regulated by the opposing activities of myosin light chain kinase (MLCK) and myosin phosphatase. Myosin phosphatase is the principal enzyme that dephosphorylates MLCs leading to relaxation. Myosin phosphatase is regulated by both vasoconstrictors that inhibit its activity to cause MLC phosphorylation and contraction, and vasodilators that activate its activity to cause MLC dephosphorylation and relaxation. The RhoA/ROCK pathway is activated by vasoconstrictors to inhibit myosin phosphatase activity. The mechanism by which RhoA and ROCK are localized to and interact with myosin light chain phosphatase (MLCP) is not well understood. We recently found a new member of the myosin phosphatase complex, myosin phosphatase‐rho interacting protein, that directly binds to both RhoA and the myosin‐binding subunit of myosin phosphatase in vitro, and targets myosin phosphatase to the actinomyosin contractile filament in smooth muscle cells. Because myosin phosphatase‐rho interacting protein binds both RhoA and MLCP, we investigated whether myosin phosphatase‐rho interacting protein was required for RhoA/ROCK‐mediated myosin phosphatase regulation. Myosin phosphatase‐rho interacting protein silencing prevented LPA‐mediated myosin‐binding subunit phosphorylation, and inhibition of myosin phosphatase activity. Myosin phosphatase‐rho interacting protein did not regulate the activation of RhoA or ROCK in vascular smooth muscle cells. Silencing of M‐RIP lead to loss of stress fiber‐associated RhoA, suggesting that myosin phosphatase‐rho interacting protein is a scaffold linking RhoA to regulate myosin phosphatase at the stress fiber. J. Cell. Biochem. 103: 1158–1170, 2008. © 2007 Wiley‐Liss, Inc.