POSSIBLE INVOLVEMENT OF THE NOVEL CPI-17 PROTEIN IN PROTEIN-KINASE-C SIGNAL-TRANSDUCTION OF RABBIT ARTERIAL SMOOTH-MUSCLE

1. CPI-17 has recently been identified as a novel protein in vascular smooth muscle. In vitro, its phosphorylation and thiophosphorylation b y protein kinase C (PKC) specifically inhibits the type I class of pro tein phosphatases, including myosin light chain (MLC) phosphatase. 2. Both of the phosphorylated CPI-17 states dose-dependently potentiated submaximal contractions at constant [Ca2+] in beta-escin-permeabilized and Triton X-100-demembranated arterial smooth muscle, but produced n o effect in intact and less intensely permeabilized (or-toxin) tissue. Thiophosphorylated CPI-17 (tp-CPI) induced large contractions even un der Ca2+-free conditions and decreased Ca2+ EC50 by more than an order of magnitude. Unphosphorylated CPI-17 produced minimal but significan t effects. 3. tp-CPI substantially increased the steady-state MLC phos phorylation to Ca2+ ratios in beta-escin preparations. 4. tp-CPI affec ted the kinetics of contraction and relaxation and of MLC phosphorylat ion and dephosphorylation in such a manner that indicates its major ph ysiological effect is to inhibit MLC phosphatase. 5. Results from use of specific inhibitors in concurrence with tp-CPI repudiate the involv ement of general. G proteins, rho A or PKC itself in the Ca2+ sensitiz ation by tp-CPI. 6. Our results indicate that phosphorylation of CPI-1 7 by PKC stimulates binding of CPI-17 to and subsequent inhibition of MLC phosphatase. This implies that CPI-17 accounts largely for the her etofore unknown signalling pathway between PKC and inhibited MLC phosp hatase.