Phosphorylation of the protein phosphatase type 1 inhibitor protein CPI-17 by protein kinase C.
CPI-17 is a cytosolic protein of 17 kDa that becomes a potent inhibitor of certain type 1 protein serine/threonine phosphatases, including smooth muscle myosin light-chain phosphatase (MLCP), when phosphorylated at Thr38. Several protein kinases are capable of phosphorylating CPI-17 at this site in vitro; however, in intact tissue, compelling evidence only exists for phosphorylation by protein kinase C (PKC). Agonist-induced activation of heterotrimeric G proteins of the Gq/11 family via seven-transmembrane domain-containing, G protein-coupled receptors results in phospholipase Cbeta-mediated hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG). IP3 triggers Ca2+ release from the sarcoplasmic reticulum. DAG and Ca2+ together activate classical isoforms of PKC, and DAG activates novel PKC isoforms without a requirement for Ca2+. Activated PKC phosphorylates CPI-17 at Thr38, enhancing its potency of inhibition of MLCP approx 1000-fold. The myosin light-chain kinase (MLCK):MLCP activity ratio is thereby increased at the prevailing cytosolic free-Ca2+ concentration ([Ca2+]i), resulting in an increase in phosphorylation of the 20-kDa light chains of myosin II (LC20) catalyzed by Ca2+- and calmodulin-dependent MLCK and contraction of the smooth muscle. Physiologically, this mechanism can account for some instances of Ca2+ sensitization of smooth muscle contraction (i.e., an increase in force in response to agonist stimulation without a change in [Ca2+]i).
Walsh
Michael P
MP
Smooth Muscle Research Group, Department of Biochemistry & Molecular Biology, University of Calgary, Alberta, Canada.
Susnjar
Marija
M
Deng
Jingti
J
Sutherland
Cindy
C
Kiss
Eniko
E
Wilson
David P
DP
D000818:Animals; D002118:Calcium / Q000378:metabolism; D009219:Myosin-Light-Chain Kinase / Q000378:metabolism; D043263:Myosin-Light-Chain Phosphatase / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D013552:Swine
D010750:Phosphoproteins; D011493:Protein Kinase C; D009219:Myosin-Light-Chain Kinase; D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; D002118:Calcium