Abstract
CPI-17 is a phosphorylation-dependent inhibitory protein for smooth muscle myosin phosphate. Phosphorylation at Thr38, in vitro, by protein kinase C or Rho-kinase enhances the inhibitory potency toward myosin phosphatase. Phosphorylation of CPI-17 by protein kinase N (PKN), a fatty acid- and Rho-activated serine/threonine kinase, and its effect on smooth muscle myosin phosphatase activity were investigated. CPI-17 was phosphorylated by GST-PKN-CAT, a constitutively active GST-fusion fragment of PKN, to 1.46 mol of P/mol of CPI-17, in vitro. The Km value of CPI-17 for PKN was 0.96 μM. Phosphorylation of PKN dramatically increased the inhibitory effect of CPI-17 on myosin phosphatase activity. The major and inhibitory phosphorylation site was identified as Thr38 using a point mutant of CPI-17 and a phosphorylation-state specific antibody. Thus, CPI-17 is a substrate of PKN and might be involved in the Ca2+ sensitization of smooth muscle contraction as a downstream effector of Rho and/or arachidonic acid.
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