Physiological role of myosin light chain kinase in regulating smooth muscle contraction : An approach by gene targeting followed by rescue.

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Smooth muscle contraction is induced by an ATE-dependent interaction between actin and myosin, on which Ca^<2+> exerts regulatory activity, The site of action of Ca^<2+> is calmodulin (CaM). The role of CaM is to activate myosin light chain kinase (MLCK), which is able to phosphorylate the 20 kDa regulatory light chain of myosin. The myosin thus phosphorylated is in an active form that is able to interact with actin ATP-dependently. But the effect of Ca^<2+> on the actual contraction of smooth muscle is much more complex. There are regulatory ways by Ca^<2+> which are not subject to phosphorylation.To approach such a problem, we are interested in the actin binding activity of MLCK, a property that has been known for many years. We examined the effect of the actin-binding activity, and found that the activity regulates the interaction in association with CaM.Thus, MLCK regulates the interaction by its kinase activity as well as its actin-binding activity.We transfected plasmid containing antisence DNA of MLCK into smooth muscle cells and observed a reduction in the amount of MLCK.We also produced MLCK fragments by transfecting expression plasmieds containing various length of sence DNA of MLCIK into E.Coli. We obtained a few fragments that regulate the actin-myosin interaction. These results enable the study to examine physiological role of MLCK by introducing the recombinant fragments into the cells where endogenouse levels of MLCK is lowered.