Phosphorylation of Mammalian Myosin Light Chain Kinases

The phosphorylation of the calmodulin-dependent enzyme myosin light chain kinase, purified from bovine tracheal smooth muscle and human blood platelets, by the catalytic subunit of CAMP-dependent protein kinase and by cGMP-dependent protein kinase was investigated. When myosin light chain kinase which has calmodulin bound is phosphorylated by the catalytic subunit of CAMP-dependent protein kinase, 1 mol of phosphate is incorporated per mol of tracheal myosin light chain kinase or platelet myosin light chain kinase, with no effect on the catalytic activity. Phosphorylation when calmodulin is not bound results in the incorporation of 2 mol of phosphate and significantly decreases the activity. The decrease in myosin light chain kinase activity is due to a 5 to 7-fold increase in the amount of calmodulin required for halfmaximal activation of both tracheal and platelet myosin light chain kinase. In contrast to the results with the catalytic subunit of CAMP-dependent protein kinase, cGMP-dependent protein kinase cannot phosphorylate tracheal myosin light chain kinase in the presence of bound calmodulin. When calmodulin is not bound to tracheal myosin light chain kinase, cGMP-dependent protein kinase phosphorylates only one site, and this phosphorylation has no effect on myosin light chain kinase activity. On the other hand, cGMP-dependent protein kinase incorporates phosphate into two sites in platelet myosin light chain kinase when calmodulin is not bound. The sites phosphorylated by the two cyclic nucleotide-dependent protein kinases were compared by twodimensional peptide mapping following extensive tryptic digestion of the phosphorylated myosin light chain kinases. With respect to the tracheal myosin light chain kinase, the single site phosphorylated by cGMPdepedent protein kinase when calmodulin is not bound appears to be the same site phosphorylated in the tracheal enzyme by the catalytic subunit of CAMP-dependent protein kinase when calmodulin is bound. With respect to the platelet myosin light chain kinase, the additional site that was phosphorylated by cGMPdependent protein kinase when calmodulin was not bound was different from that phosphorylated by the catalytic subunit of CAMP-dependent protein kinase.