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Acceleration of Myosin Light Chain Dephosphorylation and Relaxation of Smooth Muscle by Telokin

Journal of Biological Chemistry, ISSN: 0021-9258, Vol: 273, Issue: 18, Page: 11362-11369
1998
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Incorporation of 32 P into telokin, a smooth muscle-specific, 17–18-kDa, acidic (pI 4.2–4.4) protein, was increased by forskolin (20 μ m ) in intact rabbit ileum smooth muscle (ileum) and by 8-bromo-cyclic GMP (100 μ m ) in α-toxin-permeabilized ileum. Native telokin (5–20 μ m ), purified from turkey gizzard, and recombinant rabbit telokin, expressed in Escherichia coli and purified to >90% purity, induced dose-dependent relaxation, associated with a significant decrease in regulatory myosin light chain phosphorylation, without affecting the rate of thiophosphorylation of regulatory myosin light chain of ileum permeabilized with 0.1% Triton X-100. Endogenous telokin was lost from ileum during prolonged permeabilization (>20 min) with 0.1% Triton X-100, and the time course of loss was correlated with the loss of 8-bromo-cyclic GMP-induced calcium desensitization. Recombinant and native gizzard telokins were phosphorylated, in vitro, by the catalytic subunit of cAMP-dependent protein kinase, cGMP-dependent protein kinase, and p42/44 mitogen-activated protein kinase; the recombinant protein was also phosphorylated by calmodulin-dependent protein kinase II. Exogenous cGMP-dependent protein kinase (0.5 μ m ) activated by 8-bromo-cyclic GMP (50 μ m ) phosphorylated recombinant telokin (10 μ m ) when added concurrently to ileum depleted of its endogenous telokin, and their relaxant effects were mutually potentiated. Forskolin (20 μ m ) also increased phosphorylation of telokin in intact ileum. We conclude that telokin induces calcium desensitization in smooth muscle by enhancing myosin light chain phosphatase activity, and cGMP- and/or cAMP-dependent phosphorylation of telokin up-regulates its relaxant effect.

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