[Formation and regulation of myosin light chain kinase and phosphatase complex in smooth muscle: the outlook].

Myosin kinase and phosphatase, phosphorylating and dephosphorylating regulatory light chain of myosin, are the key regulatory enzymes of smooth muscle. They are tightly associated not only with myosin filament but also with each other and, therefore, appear to form a functional complex which is responsible for regulation of contraction and relaxation of smooth muscle. In our recent studies we have shown that the complex includes the kinase with its activator (calmodulin; CM) and the phosphatase in a form of catalytic and targeting subunits. The targeting subunit, being a CM binding protein, links the catalytic subunit to the kinase and to CM in a Ca-independent manner. In solution, the kinase is not exclusively monomeric but also dimerise and forms small amounts of oligomers, and all these forms are in equilibrium with each other. The dimers are responsible for a cooperative activation of the kinase by CM as well as for its intramolecular autophosphorylation, while the oligomeric form is involved in kinase localization on the myosin filament and also participates in formation of the complex with phosphatase. A kinase related protein (telokin) acts as a very effective modulator of the oligomeric state of the kinase by transferring the oligomers into the dimers and/or monomers. Telokin was effectively releasing the kinase from myosin filaments and the phosphatase from the complex with resulting inhibition of myosin phosphorylation and acceleration of myosin dephosphorylation. These modulating effects were reversed by a very slow phosphorylation of telokin by the kinase.