Possible involvement of the novel CPI-17 protein in protein kinase C signal transduction of rabbit arterial smooth muscle

• Published in: The Journal of physiology 1998-05, Vol.508 (3), p.871-881
• Main Authors: Li, L., Eto, M., Lee, M. R., Morita, F., Yazawa, M., Kitazawa, T.
• Format: Article
• Language: English
• Subjects: Animals; Aorta - cytology; Calcium - pharmacology; Dose-Response Relationship, Drug; Enzyme Inhibitors - metabolism; Femoral Artery - cytology; Guanosine Diphosphate - analogs & derivatives; Guanosine Diphosphate - pharmacology; Male; Muscle Contraction - drug effects; Muscle Contraction - physiology; Muscle, Smooth, Vascular - chemistry; Muscle, Smooth, Vascular - enzymology; Myosin-Light-Chain Phosphatase; Original; Peptide Fragments - pharmacology; Phosphoprotein Phosphatases - antagonists & inhibitors; Phosphoprotein Phosphatases - metabolism; Phosphorylation; Protein Kinase C - antagonists & inhibitors; Protein Kinase C - metabolism; Protein Kinase C - pharmacology; Protein Phosphatase 2; Proteins - metabolism; Rabbits; Recombinant Proteins - pharmacology; Signal Transduction - drug effects; Signal Transduction - physiology; Sulfur Compounds - metabolism; Swine; Thionucleotides - pharmacology; Vasoconstriction - drug effects; Vasoconstriction - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1111/j.1469-7793.1998.871bp.x

CPI-17 has recently been identified as a novel protein in vascular smooth muscle. In vitro , its phosphorylation and thiophosphorylation by protein kinase C (PKC) specifically inhibits the type 1 class of protein phosphatases, including myosin light chain (MLC) phosphatase. Both of the phosphorylated CPI-17 states dose-dependently potentiated submaximal contractions at constant [Ca 2+ ] in β-escin-permeabilized and Triton X-100-demembranated arterial smooth muscle, but produced no effect in intact and less intensely permeabilized (α-toxin) tissue. Thiophosphorylated CPI-17 (tp-CPI) induced large contractions even under Ca 2+ -free conditions and decreased Ca 2+ EC 50 by more than an order of magnitude. Unphosphorylated CPI-17 produced minimal but significant effects. tp-CPI substantially increased the steady-state MLC phosphorylation to Ca 2+ ratios in β-escin preparations. tp-CPI affected the kinetics of contraction and relaxation and of MLC phosphorylation and dephosphorylation in such a manner that indicates its major physiological effect is to inhibit MLC phosphatase. Results from use of specific inhibitors in concurrence with tp-CPI repudiate the involvement of general G proteins, rho A or PKC itself in the Ca 2+ sensitization by tp-CPI. Our results indicate that phosphorylation of CPI-17 by PKC stimulates binding of CPI-17 to and subsequent inhibition of MLC phosphatase. This implies that CPI-17 accounts largely for the heretofore unknown signalling pathway between PKC and inhibited MLC phosphatase.