Signal Transduction in Smooth Muscle: Invited Review: Regulation of myosin phosphorylation in smooth muscle

Department of Physiology, University of Cologne, D-50931 Koeln, Germany Phosphorylation of the regulatory light chains of myosin II (rMLC) by the Ca 2+ /calmodulin-dependent myosin light-chain kinase (MLCK) and dephosphorylation by a type 1 phosphatase (MLCP), which is targeted to myosin by a regulatory subunit (MYPT1), are the predominant mechanisms of regulation of smooth muscle tone. The activities of both enzymes are modulated by several protein kinases. MLCK is inhibited by the Ca 2+ /calmodulin-dependent protein kinase II, whereas the activity of MLCP is increased by cGMP and perhaps also cAMP-dependent protein kinases. In either case, this results in a decrease in the Ca 2+ sensitivity of rMLC phosphorylation and force production. The activity of MLCP is inhibited by Rho-associated kinase, one of the effectors of the monomeric GTPase Rho, and protein kinase C, leading to an increase in Ca 2+ sensitivity. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades. calcium sensitivity of smooth muscle contraction; myosin light-chain kinase; myosin light-chain phosphatase