title: Rac1 regulates myosin II phosphorylation through regulation of myosin light chain phosphatase.
abstract: Phosphorylation of regulatory light chain (MLC) activates myosin II, which enables it to promote contractile and motile activities of cells. We report here a novel signaling mechanism that activates MLC phosphorylation and smooth muscle contraction. Contractile agonists activated Rac1, and Rac1 inhibition diminished agonist-induced MLC phosphorylation, thus inhibiting smooth muscle contraction. Rac1 inhibits the activity of MLC phosphatase (MLCP) but not that of MLC kinase, through a phosphatase that targets MYPT1 (a regulatory subunit of MLCP) and CPI-17 (a MLCP specific inhibitor) rather than through the RhoA-Rho dependent kinase (ROCK) pathway. Rac1 inhibition decreased the activity of protein kinase C (PKC), which also contributes to the change in CPI-17 phosphorylation. We propose that activation of Rac1 increases the activity of PKC, which increases the phosphorylation of CPI-17 and MYPT1 by inhibiting the phosphatase that targets these proteins, thereby decreasing the activity of MLCP and increasing phosphorylation of MLC. Our results suggest that Rac1 coordinates with RhoA to increase MLC phosphorylation by inactivation of CPI-17/MYPT1 phosphatase, which decreases MLCP activity thus promoting MLC phosphorylation and cell contraction.
authors.affiliation: Department of Microbiology and Physiological Systems, University of Massachusetts Medical School, Worcester, Massachusetts.
authors.forename: Keita
authors.initials: K
authors.lastname: Shibata
authors.forename: Hiroyasu
authors.initials: H
authors.lastname: Sakai
authors.forename: Qian
authors.initials: Q
authors.lastname: Huang
authors.forename: Hirotoshi
authors.initials: H
authors.lastname: Kamata
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Chiba
authors.forename: Miwa
authors.initials: M
authors.lastname: Misawa
authors.forename: Reiko
authors.initials: R
authors.lastname: Ikebe
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Ikebe
mesh_terms: D000818:Animals; D009119:Muscle Contraction / Q000502:physiology; D009124:Muscle Proteins / Q000378:metabolism*; D009130:Muscle, Smooth / Q000378:metabolism; D024462:Myosin Type II / Q000378:metabolism*; D043263:Myosin-Light-Chain Phosphatase / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism; D054645:Protein Phosphatase 1 / Q000378:metabolism*; D051381:Rats; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*
chemical_list: D009124:Muscle Proteins; D010750:Phosphoproteins; C491519:Ppp1r14a protein, rat; D011493:Protein Kinase C; C499850:Ppp1r12a protein, rat; D054645:Protein Phosphatase 1; D043263:Myosin-Light-Chain Phosphatase; D024462:Myosin Type II; C499134:Rac1 protein, rat; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein