Myosin light chain phosphatase regulation

Ca2+ entry, Ca2+-uptake into the SR by SERCA, Ca2+ extrusion from the cell and dephosphorylation of the myosin light chains. The t ype 1 phosphatase, myosin light chain phosphatase (MLCP) dephosphorylates myosin. As with MLCK its activity is physiologically regulated, e.g. its activity is decreased following phosphorylation via Rho associated kinase (Somlyo Somlyo 2000). In the uterus we have found a small but significant reduction of force, but not Ca2+ when Rho-associated kinase is inhibited (Kupittayanant et al 2001b). 

It has been established for several years that the major mechanism for regulation of contraction in smooth muscle is myosin phosphorylation (Hart-shorne, 1987). Phosphorylation of the two 20,000-dalton light chains of myosin (LC20) activates the actin-dependent ATPase activity of myosin and this initiates the contractile response. Dephosphorylated myosin is associated with relaxed muscle. In this scheme there are two key enzymes the myosin light chain kinase (MLCK) and the myosin light chain phosphatase (MLCP). Obviously a balance of these two activities determines the level of myosin phosphorylation. 

Ikebe M, Brozovich FV (1996) Protein kinase C increases force and slows relaxation in smooth muscle evidence for regulation of the myosin light chain phosphatase. Biochem Biophys Res Commun 225 370-376 Ikebe M, Reardon S (1989) Location of the inhibitory region of smooth muscle myosin light chain kinase. J. Biol Chem 264 6967-6971 Ishizaki T, Maekawa M, Fujisawa K, Okawa K, Iwamatsu A, Fujita A, Watanabe N, Saito Y, Kakizuka A, Morri N, Narumiya S. (1996) The small GTP-binding protein Rho binds to and activates a 160 kDa Ser/Thr protein kinase homologous to myotonic dystrophy kinase. EMBO J 15 1885-1893... 

Matsuo M, Reardon S, Ikebe M, Kitazawa T (1994) A novel mechanism for the Ca -sensitizing effect of protein kinase C on vascular smooth muscle inhibition of myosin light chain phosphatase. J Gen Physiol 104 265-286 McDaniel NL, Rembold CM, Murphy RA (1994) Cyclic nucleotide dependent relaxation in vascular smooth muscle. Can J Physiol Pharmacol 72 1380-1385 Murphy RA (1994) What is special about smooth muscle The significance of covalent crossbridge regulation. FASEB J 8 311-318... 

Myosin light chain phosphatase (MLCP) is an important protein in the regulation of cellular motility and division. We developed and synthesized a fluorescent small molecule (17e) that selectively inhibits MLCP. We found the growth of human prostate cancer cell lines with upregulated amounts of MLCP proves to be more sensitive when treated with our selective MLCP inhibitor. We used the fluorescent properties of 17e to image its entry and compartmentalization in human prostate cancer... 

Figure 22.12 Regulation of actin-myosin interaction in smooth muscle via the light-chain kinase and phosphatase and effect on blood pressure. ions bind to calmodulin and the complex stimulates the conversion of inactive myosin light chain kinase (MLCK) to active MLCK which then phosphorylates the light chain. This results in activation of the cross-bridge cycle. The overall effect is vasoconstriction of the arteriole, which increases blood pressure.

Kimura K, Ito M, Amano M, Chihara K, Fukata Y, Nakafuku M, Yamamori B, Feng J, Nakano T, Okawa K, Iwamatsu A, Kaibuchi K (1996) Regulation of myosin phosphatase by Rho and Rho-associated kinase (Rho-kinase). Science 273 245-248 Kitazawa T, Gaylinn BD, Denney GH, Somlyo AP (1991) G-protein-mediated Ca sensitization of smooth muscle contraction through myosin light chain phosphorylation. J Biol Chem 266 1708-1715... 

Figure 9. Schematic of proposed pathways for chemoattractant regulation of myosin iight chain (MLC]-mediated contraction in neutrophils. Chemoattractant-induced activation of Rho activates Rho kinase, which catalyzes MLC phosphorylation leading to assembly of actin-myosin filaments and contraction. Rho kinase may also inactivate myosin phosphatase, further promoting the accumulation of MLC in the active phosphorylated state. PAK activation by Rao and Cdo42 inactivates myosin light chain kinase (MLCK) inhibiting its regulation of MLC activity.

Other signaling pathways activate Rho kinase, which can stimulate myosin activity in two ways. First, Rho kinase can phosphorylate myosin LC phosphatase (see Figure 19-25b), thereby Inhibiting Its activity. With the phosphatase inactivated, the level of myosin LC phosphorylation and thus myosin activity Increase. In addition, Rho kinase direcdy activates myosin by phosphorylating the regulatory light chain. Note that Ca plays no role in the regulation of myosin activity by Rho kinase. 

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