Structure and Function of Smooth Muscle Myosin Light Chain Kinase

Abstract

Myosin light chain kinase (MLCK) plays a central role in regulating the actin-myosin interaction of smooth muscle. MLCK phosphorylates the light chain of myosin in the presence of Ca²⁺ and calmodulin (CaM) thereby activating myosin so that it can interact with actin. Besides this kinase activity, MLCK shows i) actin-binding activity that can assemble actin filaments into their bundles and ii) myosin-binding activity that can form myosin filaments. To localize the actin- and myosin-binding activities in the MLCK molecule and to examine their possible role in regulating the actin-myosin interaction, we expressed various fragments of cDNA encoding MLCK in Escherichia coli as recombinant proteins. We found that MLCK consists of an N-terminal actin-binding domain, a central kinase domain, and a C-terminal myosin-binding domain. The Met¹-Pro⁴¹ sequence is responsible for Ca²⁺/CaM-sensitive binding to actin. This binding site exerts an inhibitory effect on the actin-myosin interaction only when myosin is phosphorylated. MLCK binds to myosin at the C-terminal domain, the sequence of which is identical to telokin, an abundant myosin-binding protein in smooth muscle cells. This domain itself has no regulatory role in the interaction. However, the interaction was stimulated when this domain was extended to include the sequence known to regulate the activity of the kinase domain. The stimulation was observed only when myosin was unphosphorylated.

Myosin light chain kinase (MLCK) has been characterized as an enzyme that phosphorylates the 20 kDa light chain of smooth muscle myosin in the presence of Ca²⁺ and calmodulin (Ca²⁺/CaM). Upon phosphorylation, myosin turns from the inactive form to the active form and can interact with actin filaments (1 for review). The kinase domain, which begins with the consensus ATP-binding sequence of GXGXXGX₁₆K, is located in the central part of the MLCK molecule (Fig. 1). The site that regulates the kinase activity by binding CaM adjoins the C-terminal of the kinase domain. The regulatory site consists of the CaM-binding sequence and the pseudosubstrate sequence, which inhibits the kinase activity in the absence of Ca²⁺/CaM by binding to the active site of the kinase domain (2 for review).

Besides its kinase activity, MLCK has been found to bind actin filaments, and its role in regulating the actin-myosin interaction has recently been revealed in the studies by Kohama and colleagues (3 for review). However, details about the amino acid sequence responsible for the actin binding are not known yet.

Telokin is an abundant smooth muscle protein that has an amino acid sequence identical to the C-terminal portion of MLCK (4), and assembles smooth muscle myosin by binding myosin (5). However, the importance of the myosin-binding, telokin domain of the MLCK molecule needs to be clarified.

In this review, we present the structure and function of the N-terminal, actin-binding domain and the C-terminal, myosin-binding domain as shown in our recent study, and include our preliminary results.