A novel regulatory effect of myosin light chain kinase from smooth muscle on the ATP-dependent interaction between actin and myosin

Biphasic myosin II light chain activation during clot retraction

Clot retraction is an essential step during primary haemostasis, thereby promoting thrombus stability and wound healing. Integrin αIIbβ3 plays a critical role in clot retraction, by inducing acto-myosin interactions that allow platelet cytoskeleton reorganisation. However, the signalling pathways that lead to clot retraction are still misunderstood. In this study, we report the first data on the kinetics of myosin II light chain (MLC) phosphorylation during clot retraction. We found an early phosphorylation peak followed by a second peak. By using specific inhibitors of kinases and small G proteins, we showed that MLC kinase (MLCK), RhoA/ROCK, and Rac-1 were involved in clot retraction and in the early MLC phosphorylation peak. Only Rac-1 and actin polymerisation, controlled by outside-in signalling, were crucial to the second MLC phosphorylation peak.

Typical and atypical domain combinations in human protein kinases: functions, disease causing mutations and conservation in other primates

A twist in the evolution of human kinases resulting in kinases with hybrid and rogue properties.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Calcium regulation of non-kinase and kinase activities of recombinant myosin light-chain kinase and its mutants

Myosin light-chain kinase (MLCK) comprised of N-terminal actin-binding domain, central catalytic domain, and C-terminal myosin-binding domain. It exerted not only kinase activity to phosphorylate 20 kDa regulatory light chain of smooth muscle but also exerted non-kinase activity on myosin motor and myosin ATPase activities (Nakamura et al., Biochem. Biophys. Res. Commun. 2008, 369, 135). The previous studies on the multiple MLCK functions were done using MLCK fragments. The present study reported the expression of whole MLCK molecules in Escherichia coli in a large amount. The construct in which the calmodulin (CaM) binding domain for regulating kinase activity was mutated lost the kinase activity. However, the mutant exerted non-kinase activity and inhibited both myosin motor and ATPase activities. The domain that regulated kinase activity was also shown to be involved in the Ca(2+) regulation of non-kinase activity. The deletion mutants of actin-binding domain which located at N-terminal 1-41 amino acids demonstrated that non-kinase activity was mediated through actin filaments.

Role of non-kinase activity of myosin light-chain kinase in regulating smooth muscle contraction, a review dedicated to Dr. Setsuro Ebashi

Myosin light-chain kinase (MLCK) of smooth muscle consists of an actin-binding domain at the N-terminal, the catalytic domain in the central portion, and the myosin-binding domain at the C-terminal. The kinase activity is mediated by the catalytic domain that phosphorylates the myosin light-chain of 20kDa (MLC20), activating smooth muscle myosin to interact with actin. Although the regulatory role of the kinase activity is well established, the role of non-kinase activity derived from actin-binding and myosin-binding domains remains unknown. This review is dedicated to Dr. Setsuro Ebashi, who devoted himself to elucidating the non-kinase activity of MLCK after establishing calcium regulation through troponin in skeletal and cardiac muscles. He proposed that the actin-myosin interaction of smooth muscle could be activated by the non-kinase activity of MLCK, a mechanism that is quite independent of MLC20 phosphorylation. The authors will extend his proposal for the role of non-kinase activity. In this review, we express MLCK and its fragments as recombinant proteins to examine their effects on the actin-myosin interaction in vitro. We also down-regulate MLCK in the cultured smooth muscle cells, and propose that MLC20 phosphorylation is not obligatory for the smooth muscle to contract.

Troponin I inhibitory peptide suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation

To explore possible mechanisms involving the thin filament-linked regulation of contraction in living smooth muscles, we studied the effects of a synthetic peptide of rabbit cardiac troponin I [residues 136-147] (TnIp), which is a minimal sequence required to inhibit striated muscle acto-tropomyosin-myosin ATPase activity, on the mechanical properties of beta-escin skinned preparations of taenia caeci from guinea pig. TnIp reversibly suppressed the Ca(2+)-activated force without significant effects on the Ca(2+) sensitivity and on the phosphorylation level of myosin regulatory light chain (MLC(20)). TnIp also reversibly suppressed the Ca(2+)/calmodulin-independent contraction induced by 30mM Mg(2+). An analogue of TnIp, which lost inhibiting action on acto-tropomyosin-myosin ATPase activity, affected neither Ca(2+)-activated nor 30mM Mg(2+)-induced contraction. These results indicate that TnIp suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation rather than inhibiting the Ca(2+)/calmodulin-dependent thick and thin filament activating processes.

Myosin light chain kinase stimulates smooth muscle myosin ATPase activity by binding to the myosin heads without phosphorylating the myosin light chain

Myosin light chain kinase (MLCK) is a multifunctional regulatory protein of smooth muscle contraction [IUBMB Life 51 (2001) 337, for review]. The well-established mode for its regulation is to phosphorylate the 20 kDa myosin light chain (MLC 20) to activate myosin ATPase activity. MLCK exhibits myosin-binding activity in addition to this kinase activity. The myosin-binding activity also stimulates myosin ATPase activity without phosphorylating MLC 20 [Proc. Natl. Acad. Sci. USA 96 (1999) 6666]. We engineered an MLCK fragment containing the myosin-binding domain but devoid of a catalytic domain to explore how myosin is stimulated by this non-kinase pathway. The recombinant fragment thus obtained stimulated myosin ATPase activity by V(max)=5.53+/-0.63-fold with K(m)=4.22+/-0.58 microM (n=4). Similar stimulation figures were obtained by measuring the ATPase activity of HMM and S1. Binding of the fragment to both HMM and S1 was also verified, indicating that the fragment exerts stimulation through the myosin heads. Since S1 is in an active form regardless of the phosphorylated state of MLC 20, we conclude that the non-kinase stimulation is independent of the phosphorylating mode for activation of myosin.

A myosin light chain kinase inhibitor, ML-9, lowers the intraocular pressure in rabbit eyes

The role of myosin light chain kinase (MLCK) in regulating the intraocular pressure (IOP) and outflow facility in rabbit eyes were studied. The IOP and pupil diameter were determined before and after intracameral and intravitreal administration of ML-9, a specific MLCK inhibitor. Total outflow facility and uveoscleral outflow facility was determined 3hr after intracameral administration of ML-9. Immunoblotting was performed to identify MLCK and the 20-kDa light chain of myosin (MLC) isoforms in human trabecular meshwork (TM) cells. The phosphorylation status of MLC was examined following ML-9 treatment. The effects of ML-9 on the morphology and actin and vinculin distribution in cultured TM cells were also studied. In rabbit eyes, administration of ML-9 resulted in a dose-dependent decrease in IOP. An increase of the outflow facility was also observed. Immunoblot analysis revealed the presence of MLCK in human TM cells. Exposure to ML-9 dose-dependently inhibited MLC phosphorylation/activation. The inhibitor caused retraction and dissociation of cells, disruption of actin bundles and impairment of focal adhesion formation in TM cells. ML-9 induces a reduction in IOP and an increase in the outflow facility in rabbit eyes. The IOP-lowering effects may be related to alterations in TM cell shapes. Inhibitors of MLCK may potentially be developed into novel medications for glaucoma.

Stable transfectants of smooth muscle cell line lacking the expression of myosin light chain kinase and their characterization with respect to the actomyosin system

We constructed a plasmid vector having a 1.4-kilobase pair insert of myosin light chain kinase (MLCK) cDNA in an antisense direction to express antisense mRNA. The construct was then transfected to SM3, a cell line from vascular smooth muscle cells, producing a few stable transfectants. The down-regulation of MLCK expression in the transfectants was confirmed by both Northern and Western blots. The control SM3 showed chemotaxic motility to platelet-derived growth factor-BB, which was supported by lamellipodia. However, the transfectants showed neither chemotaxic motility nor developed lamellipodia, indicating the essential role of MLCK in the motility. The specificity for the targeting was assessed by a few tests including the rescue experiment. Despite this importance of MLCK, platelet-derived growth factor-BB failed to induce MLC20 phosphorylation in not only the transfectants but also in SM3. The mode in which MLCK was involved in the development of membrane ruffling is discussed with special reference to the novel property of MLCK that stimulates the ATPase activity of smooth muscle myosin without phosphorylating its light chain (Ye, L.-H., Kishi, H., Nakamura, A., Okagaki, T., Tanaka, T., Oiwa, K., and Kohama, K. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6666-6671).

Inhibitory effect of phosphorylated myosin light chain kinase on the ATP-dependent actin-myosin interaction

Myosin light chain kinase (MLCK) phosphorylates the regulatory light chain of myosin in the presence of Ca(2+) and calmodulin (Ca(2+)-CaM) so that myosin can interact with actin filaments. MLCK has another activity that is not attributable to this kinase activity, i.e., it inhibits the ATP-dependent movement of actin filaments on a myosin-coated glass surface. MLCK itself can be phosphorylated at site A and site B with a few kinases. The phosphorylation at site A reduces kinase activity. However, we have no knowledge as to how phosphorylation of MLCK affects the inhibitory activity of MLCK. When MLCK was phosphorylated at site B, it exerted an inhibitory effect on the movement in much lower concentrations. When Ca(2+)-CaM or ML-9 was present, the inhibition was reduced. The reduction was less when the movement was arrested by the MLCK phosphorylated at site B. This observation was explained by the increase in the affinity of MLCK to myosin upon the phosphorylation at site B.

Myosin light-chain kinase of smooth muscle stimulates myosin ATPase activity without phosphorylating myosin light chain

Myosin light-chain kinase (MLCK) of smooth muscle is multifunctional, being composed of N-terminal actin-binding domain, central kinase domain, and C-terminal myosin-binding domain. The kinase domain is the best characterized; this domain activates the interaction of smooth-muscle myosin with actin by phosphorylating the myosin light chain. We have recently shown that the Met-1-Pro-41 sequence of MLCK binds to actin to inhibit this interaction. However, it is not known whether the myosin-binding domain modifies the actin-myosin interaction. We designed MLCK.cDNA to overexpress the Asp-777-Glu-972 sequence in Escherichia coli. The purified Asp-777-Glu-972 fragment, although devoid of the kinase activity, exerted a stimulatory effect on the ATPase activity of dephosphorylated myosin (Vmax = 7.36 +/- 0.44-fold, Km = 1.06 +/- 0. 20 microM, n = 4). When the N-terminal 39 residues of the fragment were deleted from the fragment, the resultant fragment, Met-816-Glu-972, lost the stimulatory activity. We synthesized the Ala-777-Ser-815 peptide that was deleted from the fragment and confirmed its stimulatory effect of the peptide (Vmax = 3.03 +/- 0. 22-fold, Km = 6.93 +/- 1.61 microM, n = 3). When this peptide was further divided into Asp-777-Met-795 and Ala-796-Ser-815 peptides, the stimulatory activity was found in the latter. We confirmed that the myosin phosphorylation did not occur during the experiments with the above fragments and peptides. Therefore, we suggest that phosphorylation is not obligatory for smooth-muscle myosin not to be active.

Characterization of the myosin light chain kinase from smooth muscle as an actin-binding protein that assembles actin filaments in vitro

In addition to its kinase activity, myosin light chain kinase has an actin-binding activity, which results in bundling of actin filaments [Hayakawa et al., Biochem. Biophys. Res. Commun. 199, 786-791, 1994]. There are two actin-binding sites on the kinase: calcium- and calmodulin-sensitive and insensitive sites [Ye et al., J. Biol. Chem. 272, 32182-32189, 1997]. The calcium/calmodulin-sensitive, actin-binding site is located at Asp2-Pro41 and the insensitive site is at Ser138-Met213. The cyanogen bromide fragment, consisting of Asp2-Met213, is furnished with both sites and is the actin-binding core of myosin light chain kinase. Cross-linking between the two sites assembles actin filaments into bundles. Breaking of actin-binding at the calcium/calmodulin-sensitive site by calcium/calmodulin disassembles the bundles.

Structure and function of smooth muscle myosin light chain kinase

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.

Biochemical regulation of the nonmuscle myosin light chain kinase isoform in bovine endothelium

Specific models of vascular permeability are critically dependent on myosin light chain phosphorylation, a reaction catalyzed by a novel high molecular-weight (214 kD) Ca2+/calmodulin (CaM)-dependent myosin light chain kinase (MLCK) isoform recently cloned in human endothelium (Am. J. Respir. Cell Mol. Biol., 1997;16:489-494). To evaluate mechanisms of endothelial cell (EC) barrier dysfunction evoked by the serine protease thrombin, we studied the regulation of the 214-kD EC MLCK isoform expressed in bovine endothelium. The EC MLCK isoform bound biotinylated CaM in a Ca2+-dependent manner and co-immunoprecipitated in a functional complex with myosin, actin, and CaM. Thrombin rapidly increased MLCK activity in concert with time-dependent translocation of the enzyme to the actin cytoskeleton. To evaluate whether EC MLCK activity was regulated by direct phosphorylation, amino acid sequence analysis identified multiple potential EC MLCK sites for Ser/Thr phosphorylation, including highly conserved phosphorylation sites for cyclic adenosine monophosphate-dependent protein kinase A (PKA) adjacent to the CaM-binding region. EC MLCK activity was attenuated by either PKA-mediated MLCK phosphorylation or inhibition of Ser/Thr phosphatase activity (fluoride or calyculin), which significantly increased MLCK phosphorylation while decreasing MLCK activity (3- to 4-fold decrease). In summary, although the EC MLCK isoform exhibits multiple features intrinsic to this family of kinases, thrombin-mediated EC contraction and barrier dysfunction requires increased EC MLCK-actin interaction and MLCK translocation to the cytoskeleton. EC MLCK activity appears to be highly dependent upon the phosphorylation status of this key contractile effector.

The structure and function of the actin-binding domain of myosin light chain kinase of smooth muscle

In addition to its kinase activity, the myosin light chain kinase (MLCK) of smooth muscle has an actin binding activity through which it can regulate the actin-myosin interaction of smooth muscle (Kohama, K., Okagaki, T., Hayakawa, K., Lin, Y., Ishikawa, R., Shimmen, T., and Inoue, A. (1992) Biochem. Biophys. Res. Commun. 184, 1204-1211). In this study, we have analyzed the actin binding activity of MLCK and related it to its amino acid sequence by producing native and recombinant fragments of MLCK. Parent MLCK exhibited both calcium ion (Ca2+) and calmodulin (Ca2+/CaM)-sensitive and Ca2+/CaM-insensitive binding to actin filaments. The native fragment, which consists of the Met1-Lys114 sequence (Kanoh, S., Ito, M., Niwa, E., Kawano, Y., and Hartshorne, D. J. (1993) Biochemistry 32, 8902-8907), and the recombinant NN fragment, which contains this 1-114 sequence, showed only Ca2+/CaM-sensitive binding. An inhibitory effect of the NN fragment on the actin-myosin interaction was observed by assaying in vitro motility and by measuring the actin-activated ATPase activity of myosin. The recombinant NN/41 fragment, which is constructed without the Met1-Pro41 sequence of the NN fragment, lost both the actin binding activity and the inhibitory effect. We confirmed the importance of the 1-41 sequence by using a few synthetic peptides to compete against the NN fragment in binding to actin filaments. The experiments using recombinant fragments and synthetic peptides also revealed that the site for CaM-binding is the Pro26-Pro41 sequence. The site for the Ca2+/CaM-insensitive binding, which is shown to be localized between the Ca2+/CaM-sensitive site and the central kinase domain of MLCK, exerted no regulatory effects on the actin-myosin interaction.

Localization of an actin binding domain in smooth muscle myosin light chain kinase

Phosphorylation of the regulatory light chain of myosin II by myosin light chain kinase is important for regulating many contractile processes. Smooth muscle myosin light chain kinase has been shown to be associated with both actin and myosin filaments in vitro and in vivo. In this report we define an actin binding region by using molecular deletions to generate recombinant mutant proteins that were analyzed by co-sedimentation with F-actin. An actin binding region restricted to residues 2-42 in the amino terminus of the rabbit smooth muscle myosin light chain kinase was identified.

Myosin light chain kinase: an actin-binding protein that regulates an ATP-dependent interaction with myosin

Myosin light chain kinase (MLCK) is a key regulator of smooth muscle contraction. The most conspicuous form of regulation is achieved by phosphorylation of the myosin light chain, allowing myosin to interact with actin. This interaction is regulated by actin-binding proteins that modulate actin filaments. In this review Kazuhiro Kohama and colleagues consider MLCK as an actin-binding protein and attempt to shed light on the cross-talk between the different kinds of regulation of the actin-myosin interaction in smooth muscle. An understanding of these mechanisms will assist the development of compounds with therapeutic importance in muscular disorders.

Actin-binding peptide from smooth muscle myosin light chain kinase

The objective of this study was to localize the actin-binding site in the smooth muscle myosin light chain kinase. Limited proteolysis by thermolysin indicated that hydrolysis of the kinase at the N-terminal end of the molecule resulted in loss of actin-binding ability. Various methods of cleavage were investigated for the generation of a discrete actin-binding peptide. The method chosen was cleavage at the cysteine residues by the 5,5'-dithiobis(2-nitrobenzoic acid)-cyanide complex. This procedure yielded an actin-binding peptide of approximate M(r) 17,000. The peptide was purified and shown to possess the actin-binding properties of the native myosin light chain kinase. The binding constant of the isolated peptide and parent enzyme to actin was estimated as 7.5 x 10(4) M-1. From the amino acid composition of the peptide and comparison with the sequence of gizzard myosin light chain kinase, it was suggested that the actin-binding site is located within the N-terminal sequence 1-114. Comparison with other actin-binding proteins shows some similarities to gizzard alpha-actinin and caldesmon.

A novel regulatory protein that affects the functions of caldesmon and myosin light chain kinase

A caldesmon (CaD)-binding protein of about 65 kDa (by SDS-PAGE) was purified from smooth muscle of chicken gizzard. The 65-kDa protein prevented the inhibitory effect of CaD on the ATP-dependent interaction between actin and myosin. Unlike the case with calmodulin (CaM), Ca2+ was not required for this effect. As reported in the preceding communication, myosin light chain kinase (MLCK), another well characterized protein that binds CaM, has CaD-like activity that modulates the interaction by binding to actin. The 65-kDa protein was also effective in relieving the modulation, while leaving unaffected the kinase activity that phosphorylates the light chain of smooth muscle myosin.