Treatment of bovine tracheal smooth muscle with carbachol or KCl increased site A phosphorylation of myosin light chain kinase, correlating with increased Ca2+/calmodulin required for activation.
Phosphorylation of myosin light chain kinase at site A in contracting tracheal smooth muscle may play a role in desensitizing contractile elements to Ca2+ activation.
Purified myosin light chain kinase from smooth muscle is phosphorylated by cyclic AMP-dependent protein kinase, protein kinase C, and the multifunctional calmodulin-dependent protein kinase II. Because phosphorylation in a specific site (site A) by any one of these kinases desensitizes myosin light chain kinase to activation by Ca2+/calmodulin, kinase phosphorylation could play an important role in regulating smooth muscle contractility. This possibility was investigated in 32P-labeled bovine tracheal smooth muscle. Treatment of tissues with carbachol, KCl, isoproterenol, or phorbol 12,13-dibutyrate increased the extent of kinase phosphorylation. Six primary phosphopeptides (A-F) of myosin light chain kinase were identified. Site A was phosphorylated to an appreciable extent only with carbachol or KCl, agents which contract tracheal smooth muscle. The extent of site A phosphorylation correlated to increases in the concentration of Ca2+/calmodulin required for activation. These results show that cyclic AMP-dependent protein kinase and protein kinase C do not affect smooth muscle contractility by phosphorylating site A in myosin light chain kinase. It is proposed that phosphorylation of myosin light chain kinase in site A in contracting tracheal smooth muscle may play a role in the reported desensitization of contractile elements to activation by Ca2+.
Stull et al. (1990) studied this question. Carbachol, KCl, isoproterenol, or phorbol 12,13-dibutyrate was evaluated on Extent of kinase phosphorylation and site A phosphorylation. Treatment of bovine tracheal smooth muscle with carbachol or KCl increased site A phosphorylation of myosin light chain kinase, correlating with increased Ca2+/calmodulin required for activation.