Identification of a novel actin binding motif in smooth muscle myosin light chain kinase

The myosin light-chain kinase MLCK-1 relocalizes during Caenorhabditis elegans ovulation to promote actomyosin bundle assembly and drive contraction

We identify the Caenorhabditis elegans myosin light-chain kinase, MLCK-1, required for contraction of spermathecae. During contraction, MLCK-1 moves from the apical cell boundaries to the basal actomyosin bundles, where it stabilizes myosin downstream of calcium signaling. MLCK and ROCK act in distinct subsets of cells to coordinate the timing of contraction.

Myosin Light Chain Kinase MYLK1: Anatomy, Interactions, Functions, and Regulation

This review discusses and summarizes the results of molecular and cellular investigations of myosin light chain kinase (MLCK, MYLK1), the key regulator of cell motility. The structure and regulation of a complex mylk1 gene and the domain organization of its products is presented. The interactions of the mylk1 gene protein products with other proteins and posttranslational modifications of the mylk1 gene protein products are reviewed, which altogether might determine the role and place of MLCK in physiological and pathological reactions of cells and entire organisms. Translational potential of MLCK as a drug target is evaluated.

Phosphorylation Regulates Interaction of 210-kDa Myosin Light Chain Kinase N-terminal Domain with Actin Cytoskeleton

High molecular weight myosin light chain kinase (MLCK210) is a multifunctional protein involved in myosin II activation and integration of cytoskeletal components in cells. MLCK210 possesses actin-binding regions both in the central part of the molecule and in its N-terminal tail domain. In HeLa cells, mitotic protein kinase Aurora B was suggested to phosphorylate MLCK210 N-terminal tail at serine residues (Dulyaninova, N. G., and Bresnick, A. R. (2004) Exp. Cell Res., 299, 303-314), but the functional significance of the phosphorylation was not established. We report here that in vitro, the N-terminal actin-binding domain of MLCK210 is located within residues 27-157 (N27-157, avian MLCK210 sequence) and is phosphorylated by cAMP-dependent protein kinase (PKA) and Aurora B at serine residues 140/149 leading to a decrease in N27-157 binding to actin. The same residues are phosphorylated in a PKA-dependent manner in transfected HeLa cells. Further, in transfected cells, phosphomimetic mutants of N27-157 showed reduced association with the detergent-stable cytoskeleton, whereas in vitro, the single S149D mutation reduced N27-157 association with F-actin to a similar extent as that achieved by N27-157 phosphorylation. Altogether, our results indicate that phosphorylation of MLCK210 at distinct serine residues, mainly at S149, attenuates the interaction of MLCK210 N-terminus with the actin cytoskeleton and might serve to regulate MLCK210 microfilament cross-linking activity in cells.

The ZEB1/miR-200c feedback loop regulates invasion via actin interacting proteins MYLK and TKS5

Epithelial to mesenchymal transition (EMT) is a developmental process which is aberrantly activated during cancer invasion and metastasis. Elevated expression of EMT-inducers like ZEB1 enables tumor cells to detach from the primary tumor and invade into the surrounding tissue. The main antagonist of ZEB1 in controlling EMT is the microRNA-200 family that is reciprocally linked to ZEB1 in a double negative feedback loop. Here, we further elucidate how the ZEB1/miR-200 feedback loop controls invasion of tumor cells. The process of EMT is attended by major changes in the actin cytoskeleton. Via in silico screening of genes encoding for actin interacting proteins, we identified two novel targets of miR-200c - TKS5 and MYLK (MLCK). Co-expression of both genes with ZEB1 was observed in several cancer cell lines as well as in breast cancer patients and correlated with low miR-200c levels. Depletion of TKS5 or MYLK in breast cancer cells reduced their invasive potential and their ability to form invadopodia. Whereas TKS5 is known to be a major component, we could identify MYLK as a novel player in invadopodia formation. In summary, TKS5 and MYLK represent two mediators of invasive behavior of cancer cells that are regulated by the ZEB1/miR-200 feedback loop.

Role of myosin light chain kinase in inflammatory bowel disease

Myosin light chain kinase (MLCK) dependent myosin light chain (MLC) phosphorylation plays an important role in the pathogenesis of inflammatory bowel disease (IBD). Cytoskeletal contraction induced by the phosphorylation of MLC is essential for the destruction of the intestinal epithelial barrier, and increased expression of MLCK can mediate epithelial barrier dysfunction. Currently studies focused mainly on the mechanism of MLCK in the intestinal epithelial barrier. This review discusses the role of MLCK in IBD.

Myosin light chain kinase (MLCK) regulates cell migration in a myosin regulatory light chain phosphorylation-independent mechanism

Myosin light chain kinase (MLCK) has long been implicated in the myosin phosphorylation and force generation required for cell migration. Here, we surprisingly found that the deletion of MLCK resulted in fast cell migration, enhanced protrusion formation, and no alteration of myosin light chain phosphorylation. The mutant cells showed reduced membrane tether force and fewer membrane F-actin filaments. This phenotype was rescued by either kinase-dead MLCK or five-DFRXXL motif, a MLCK fragment with potent F-actin-binding activity. Pull-down and co-immunoprecipitation assays showed that the absence of MLCK led to attenuated formation of transmembrane complexes, including myosin II, integrins and fibronectin. We suggest that MLCK is not required for myosin phosphorylation in a migrating cell. A critical role of MLCK in cell migration involves regulating the cell membrane tension and protrusion necessary for migration, thereby stabilizing the membrane skeleton through F-actin-binding activity. This finding sheds light on a novel regulatory mechanism of protrusion during cell migration.

Myosin light-chain kinase is necessary for membrane homeostasis in cochlear inner hair cells

The structural homeostasis of the cochlear hair cell membrane is critical for all aspects of sensory transduction, but the regulation of its maintenance is not well understood. In this report, we analyzed the cochlear hair cells of mice with specific deletion of myosin light chain kinase (MLCK) in inner hair cells. MLCK-deficient mice showed impaired hearing, with a 5- to 14-dB rise in the auditory brainstem response (ABR) thresholds to clicks and tones of different frequencies and a significant decrease in the amplitude of the ABR waves. The mutant inner hair cells produced several ball-like structures around the hair bundles in vivo, indicating impaired membrane stability. Inner hair cells isolated from the knockout mice consistently displayed less resistance to hypoosmotic solution and less membrane F-actin. Myosin light-chain phosphorylation was also reduced in the mutated inner hair cells. Our results suggest that MLCK is necessary for maintaining the membrane stability of inner hair cells.

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.

Association of aorta intima permeability with myosin light chain kinase expression in hypercholesterolemic rabbits

The development of hypercholesterolemia is a multifactorial process in which elevated plasma cholesterol levels play a central role. This study analyzed the variability of the expression and activity of myosin light chain kinase (MLCK) and endothelial permeability in the artery wall of rabbits after feeding the animals with a normal or a high-cholesterol diet. Hypercholesterolemia was induced by a high-cholesterol diet for 4 weeks. Aortas were removed and analyzed for endothelial permeability and MLCK expression. Samples of the arterial media were analyzed for MLCK activity and expression. A selective MLCK inhibitor 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML7) were used in hypercholesterolemia rabbit (1 mg/kg body weight). The aortas of high-cholesterol diet rabbits showed an increase in MLCK expression and activity (nearly threefold compare with control) as well as endothelial permeability. ML7 inhibit MLC phosphorylation and MLCK activity (nearly twofold compare with control) and endothelial permeability stimulated by cholesterol. These results indicate for the first time that hypercholesterolemia may be associated with MLCK expression and activity through which endothelial permeability is increased.

The N-terminal region of twitchin binds thick and thin contractile filaments: redundant mechanisms of catch force maintenance

Catch force maintenance in invertebrate smooth muscles is probably mediated by a force-bearing tether other than myosin cross-bridges between thick and thin filaments. The phosphorylation state of the mini-titin twitchin controls catch. The C-terminal phosphorylation site (D2) of twitchin with its flanking Ig domains forms a phosphorylation-sensitive complex with actin and myosin, suggesting that twitchin is the tether (Funabara, D., Osawa, R., Ueda, M., Kanoh, S., Hartshorne, D. J., and Watabe, S. (2009) J. Biol. Chem. 284, 18015-18020). Here we show that a region near the N terminus of twitchin also interacts with thick and thin filaments from Mytilus anterior byssus retractor muscles. Both a recombinant protein, including the D1 and DX phosphorylation sites with flanking 7th and 8th Ig domains, and a protein containing just the linker region bind to thin filaments with about a 1:1 mol ratio to actin and K(d) values of 1 and 15 μM, respectively. Both proteins show a decrease in binding when phosphorylated. The unphosphorylated proteins increase force in partially activated permeabilized muscles, suggesting that they are sufficient to tether thick and thin filaments. There are two sites of thin filament interaction in this region because both a 52-residue peptide surrounding the DX site and a 47-residue peptide surrounding the D1 site show phosphorylation-dependent binding to thin filaments. The peptides relax catch force, confirming the region's central role in the mechanism of catch. The multiple sites of thin filament interaction in the N terminus of twitchin in addition to those in the C terminus provide an especially secure and redundant mechanical link between thick and thin filaments in catch.

MLCK-independent phosphorylation of MLC20 and its regulation by MAP kinase pathway in human bladder smooth muscle cells

Myosins are a superfamily of actin-based molecular motor proteins, which hydrolyze ATP and generate various forms of eukaryotic motility and muscle contraction. Myosin light chain 20 (MLC20) is small ring around the neck region of heavy chain of myosins. Phosphorylation of MLC20 is thought to play a key role in regulation of smooth muscle contraction. Calcium- and calmodulin-dependent myosin light chain kinase (MLCK) is considered the primary regulator of MLC20 phosphorylation. However, several observations in smooth muscle contraction cannot be explained by the mode of phosphorylation. By performing a series of experiments in vitro and in vivo, we report here MLCK-independent MLC20 phosphorylation. Gene expression study reveals that expression of MLCK in smooth muscles is inconsistent with MLC20 phosphorylation at Ser19. None of inactivating calmodulin/MLCK, depriving of calcium and silencing MLCK expression by siRNA blocks effectively the phosphorylation of MLC20 at Ser19. In addition, by overexpressing active human MAP (mitogen-activated protein)-ERK kinase kinase-1 (MEKK1) and blocking its downstream messengers, we have demonstrated a new regulatory system of MLC phosphorylation via MEKK1, which downregulates Ser19 phosphorylation of MLC20 through its downstream molecules, p38, JNK, and ERK in human bladder smooth muscle cells.

Mechanism of catch force: tethering of thick and thin filaments by twitchin

Catch is a mechanical state occurring in some invertebrate smooth muscles characterized by high force maintenance and resistance to stretch during extremely slow relaxation. During catch, intracellular calcium is near basal concentration and myosin crossbridge cyctng rate is extremely slow. Catch force is relaxed by a protein kinase A-mediated phosphorylation of sites near the N- and C- temini of the minititin twitchin (~526 kDa). Some catch force maintenance car also occur together with cycling myosin crossbridges at submaximal calcium concentrations, but not when the muscle is maximally activated. Additionally, the link responsible for catch can adjust during shortening of submaximally activated muscles and maintain catch force at the new shorter length. Twitchin binds to both thick and thin filaments, and the thin filament binding shown by both the N- and Cterminal portions of twitchin is decreased by phosphorylation of the sites that regulate catch. The data suggest that the twitchin molecule itself is the catch force beanng tether between thick and thin filaments. We present a model for the regulation of catch in which the twitchin tether can be displaced from thin filaments by both (a) the phosphorylation of twitchin and (b) the attachment of high force myosin crossbridges.

Effect of phosphorylation in the motor domain of human myosin IIIA on its ATP hydrolysis cycle

Previous findings suggested that the motor activity of human myosin IIIA (HM3A) is influenced by phosphorylation [Kambara, T., et al. (2006) J. Biol. Chem. 281, 37291-37301]; however, how phosphorylation controls the motor activity of HM3A is obscure. In this study, we clarify the kinetic basis of the effect of phosphorylation on the ATP hydrolysis cycle of the motor domain of HM3A (huM3AMD). The affinity of human myosin IIIA for filamentous actin in the presence of ATP is more than 100-fold decreased by phosphorylation, while the maximum rate of ATP turnover is virtually unchanged. The rate of release of ADP from acto-phosphorylated huM3AMD is 6-fold greater than the overall cycle rate, and thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form is markedly increased by phosphorylation by 30-fold. The dissociation constant for dissociation of the ATP-bound form of huM3AMD from actin is greatly increased by phosphorylation, and this result agrees well with the significant increase in the K(actin) value of the steady-state ATPase reaction. The rate constant of the P(i) off step is greater than 60 s(-1), suggesting that this step does not limit the overall ATP hydrolysis cycle rate. Our kinetic model indicates that phosphorylation induces the dissociation of huM3AMD from actin during the ATP hydrolysis cycle, and this is due to the phosphorylation-dependent marked decrease in the affinity of huM3AMD.ATP for actin and the increase in the ATP hydrolysis rate of huM3AMD in the actin-dissociated state. These results suggest that the phosphorylation of myosin IIIA significantly lowers the duty ratio, which may influence the cargo transporting ability of the native form of myosin IIIA that contains the ATP-independent actin binding site in the tail.

Modular structure of smooth muscle Myosin light chain kinase: hydrodynamic modeling and functional implications

Smooth muscle myosin light chain kinase (smMLCK) is a calcium-calmodulin complex-dependent enzyme that activates contraction of smooth muscle. The polypeptide chain of rabbit uterine smMLCK (Swiss-Prot entry P29294) contains the catalytic/regulatory domain, three immunoglobulin-related motifs (Ig), one fibronectin-related motif (Fn3), a repetitive, proline-rich segment (PEVK), and, at the N-terminus, a unique F-actin-binding domain. We have evaluated the spatial arrangement of these domains in a recombinant 125 kDa full-length smMLCK and its two catalytically active C-terminal fragments (77 kDa, residues 461-1147, and 61 kDa, residues 461-1002). Electron microscopic images of smMLCK cross-linked to F-actin show particles at variable distances (11-55 nm) from the filament, suggesting that a well-structured C-terminal segment of smMLCK is connected to the actin-binding domain by a long, flexible tether. We have used structural homology and molecular dynamics methods to construct various all-atom representation models of smMLCK and its two fragments. The theoretical sedimentation coefficients computed with HYDROPRO were compared with those determined by sedimentation velocity. We found agreement between the predicted and observed sedimentation coefficients for models in which the independently folded catalytic domain, Fn3, and Ig domains are aligned consecutively on the long axis of the molecule. The PEVK segment is modeled as an extensible linker that enables smMLCK to remain bound to F-actin and simultaneously activate the myosin heads of adjacent myosin filaments at a distance of >or=40 nm. The structural properties of smMLCK may contribute to the elasticity of smooth muscle cells.

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.

Arrest defective-1 controls tumor cell behavior by acetylating myosin light chain kinase

Background

The enhancement of cell motility is a critical event during tumor cell spreading. Since myosin light chain kinase (MLCK) regulates cell behavior, it is regarded as a promising target in terms of preventing tumor invasion and metastasis. Since MLCK was identified to be associated with human arrest defective-1 (hARD1) through yeast two-hybrid screening, we here tested the possibility that hARD1 acts as a regulator of MLCK and by so doing controls tumor cell motility.

Methodology/Principal Findings

The physical interaction between MLCK and hARD1 was confirmed both in vivo and in vitro by immunoprecipitation assay and affinity chromatography. hARD1, which is known to have the activity of protein lysine ε-acetylation, bound to and acetylated MLCK activated by Ca²⁺ signaling, and by so doing deactivated MLCK, which led to a reduction in the phosphorylation of MLC. Furthermore, hARD1 inhibited tumor cell migration and invasion MLCK-dependently. Our mutation study revealed that hARD1 associated with an IgG motif of MLCK and acetylated the Lys608 residue in this motif. The K608A-mutated MLCK was neither acetylated nor inactivated by hARD1, and its stimulatory effect on cell motility was not inhibited by hARD1.

Conclusion/Significance

These results indicate that hARD1 is a bona fide regulator of MLCK, and that hARD1 plays a crucial role in the balance between tumor cell migration and stasis. Thus, hARD1 could be a therapeutic target in the context of preventing tumor invasion and metastasis.

Characterization of tightly associated smooth muscle myosin-myosin light-chain kinase-calmodulin complexes

A current popular model to explain phosphorylation of smooth muscle myosin (SMM) by myosin light-chain kinase (MLCK) proposes that MLCK is bound tightly to actin but weakly to SMM. We found that MLCK and calmodulin (CaM) co-purify with unphosphorylated SMM from chicken gizzard, suggesting that they are tightly bound. Although the MLCK:SMM molar ratio in SMM preparations was well below stoichiometric (1:73+/-9), the ratio was approximately 23-37% of that in gizzard tissue. Fifteen to 30% of MLCK was associated with CaM at approximately 1 nM free [Ca(2+)]. There were two MLCK pools that bound unphosphorylated SMM with K(d) approximately 10 and 0.2 microM and phosphorylated SMM with K(d) approximately 20 and 0.2 microM. Using an in vitro motility assay to measure actin sliding velocities, we showed that the co-purifying MLCK-CaM was activated by Ca(2+) and phosphorylation of SMM occurred at a pCa(50) of 6.1 and at a Hill coefficient of 0.9. Similar properties were observed from reconstituted MLCK-CaM-SMM. Using motility assays, co-sedimentation assays, and on-coverslip enzyme-linked immunosorbent assays to quantify proteins on the motility assay coverslip, we provide strong evidence that most of the MLCK is bound directly to SMM through the telokin domain and some may also be bound to both SMM and to co-purifying actin through the N-terminal actin-binding domain. These results suggest that this MLCK may play a role in the initiation of contraction.

Myosin light chain kinase is central to smooth muscle contraction and required for gastrointestinal motility in mice

BACKGROUND & AIMS

Smooth muscle is essential for maintaining homeostasis for many body functions and provides adaptive responses to stresses imposed by pathologic disorders. Identified cell signaling networks have defined many potential mechanisms for initiating smooth muscle contraction with or without myosin regulatory light chain (RLC) phosphorylation by myosin light chain kinase (MLCK). We generated tamoxifen-inducible and smooth muscle-specific MLCK knockout (KO) mice and provide direct loss-of-function evidence that shows the primary importance of MLCK in phasic smooth muscle contractions.

METHODS

We used the Cre-loxP system to establish Mlck floxed mice in which exons 23, 24, and 25 were flanked by 2 loxP sites. Smooth muscle-specific MLCK KO mice were generated by crossing Mlck floxed mice with SM-CreER(T2) (ki) mice followed by tamoxifen treatment. The phenotype was assessed by histologic, biochemical, molecular, cell biological, and physiologic analyses.

RESULTS

Targeted deletion of MLCK in adult mouse smooth muscle resulted in severe gut dysmotility characterized by weak peristalsis, dilation of the digestive tract, and reduction of feces excretion and food intake. There was also abnormal urinary bladder function and lower blood pressure. Isolated muscles showed a loss of RLC phosphorylation and force development induced by K(+)-depolarization. The kinase knockout also markedly reduced RLC phosphorylation and force development with acetylcholine which activates Ca(2+)-sensitizing signaling pathways.

CONCLUSIONS

MLCK and its phosphorylation of RLC are required physiologically for smooth muscle contraction and are essential for normal gastrointestinal motility.

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.

Human myosin III is a motor having an extremely high affinity for actin

Myosin IIIA is expressed in photoreceptor cells and thought to play a critical role in phototransduction processes, yet its function on a molecular basis is largely unknown. Here we clarified the kinetic mechanism of the ATPase cycle of human myosin IIIA. The steady-state ATPase activity was markedly activated approximately 10-fold with very low actin concentration. The rate of ADP off from actomyosin IIIA was 10 times greater than the overall cycling rate, thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form was very slow, but the rate was markedly accelerated by actin binding. The dissociation constant of the ATP-bound form of myosin IIIA from actin is submicromolar, which agrees well with the low K(actin). These results indicate that ATP hydrolysis predominantly takes place in the actin-bound form for actomyosin IIIA ATPase reaction. The obtained K(actin) was much lower than the previously reported one, and we found that the autophosphorylation of myosin IIIA dramatically increased the K(actin), whereas the V(max) was unchanged. Our kinetic model indicates that both the actin-attached hydrolysis and the P(i) release steps determine the overall cycle rate of the dephosphorylated form. Although the stable steady-state intermediates of actomyosin IIIA ATPase reaction are not typical strong actin-binding intermediates, the affinity of the stable intermediates for actin is much higher than conventional weak actin binding forms. The present results suggest that myosin IIIA can spend a majority of its ATP hydrolysis cycling time on actin.

Microfilament-binding properties of N-terminal extension of the isoform of smooth muscle long myosin light chain kinase

Myosin light chain kinases (MLCK) phosphorylate the regulatory light chain of myosin II in thick filaments and bind to F-actin-containing thin filaments with high affinity. The ability of short myosin light chain kinase (S-MLCK) to bind F-actin is structurally attributed to the DFRXXL regions in its N-terminus. The long myosin light chain kinase (L-MLCK) has two additional DFRXXL motifs and six Ig-like modules in its N-terminal extension. The six Ig-like modules are capable of binding to stress fibers independently. Our results from the imaging analysis demonstrated that the first two intact Ig-like modules (2Ig) in N-terminal extension of L-MLCK is the minimal binding module required for microfilament binding. Binding assay confirmed that F-actin was able to bind 2Ig. Stoichiometries of 2Ig peptide were similar for myofilament or pure F-actin. The binding affinities were slightly lower than 5DFRXXL peptide as reported previously. Similar to DFRXXL peptides, the 2Ig peptide also caused efficient F-actin bundle formation in vitro. In the living cell, over-expression of 2Ig fragment increased "spike"-like protrusion formation with over-bundled F-actin. Our results suggest that L-MLCK may act as a potent F-actin bundling protein via its DFRXXL region and the 2Ig region, implying that L-MLCK plays a role in cytoskeleton organization.

Integrin-linked kinase is responsible for Ca2+-independent myosin diphosphorylation and contraction of vascular smooth muscle

Smooth muscle contraction is activated by phosphorylation at Ser-19 of LC20 (the 20 kDa light chains of myosin II) by Ca2+/calmodulin-dependent MLCK (myosin light-chain kinase). Diphosphorylation of LC20 at Ser-19 and Thr-18 is observed in smooth muscle tissues and cultured cells in response to various contractile stimuli, and in pathological circumstances associated with hypercontractility. MLCP (myosin light-chain phosphatase) inhibition can lead to LC20 diphosphorylation and Ca2+-independent contraction, which is not attributable to MLCK. Two kinases have emerged as candidates for Ca2+-independent LC20 diphosphorylation: ILK (integrin-linked kinase) and ZIPK (zipper-interacting protein kinase). Triton X-100-skinned rat caudal arterial smooth muscle was used to investigate the relative importance of ILK and ZIPK in Ca2+-independent, microcystin (phosphatase inhibitor)-induced LC20 diphosphorylation and contraction. Western blotting and in-gel kinase assays revealed that both kinases were retained in this preparation. Ca2+-independent contraction of calmodulin-depleted tissue in response to microcystin was resistant to MLCK inhibitors [AV25 (a 25-amino-acid peptide derived from the autoinhibitory domain of MLCK), ML-7, ML-9 and wortmannin], protein kinase C inhibitor (GF109203X) and Rho-associated kinase inhibitors (Y-27632 and H-1152), but blocked by the non-selective kinase inhibitor staurosporine. ZIPK was inhibited by AV25 (IC50 0.63+/-0.05 microM), whereas ILK was insensitive to AV25 (at concentrations as high as 100 microM). AV25 had no effect on Ca2+-independent, microcystin-induced LC20 mono- or di-phosphorylation, with a modest effect on force. We conclude that direct inhibition of MLCP in the absence of Ca2+ unmasks ILK activity, which phosphorylates LC20 at Ser-19 and Thr-18 to induce contraction. ILK is probably the kinase responsible for myosin diphosphorylation in vascular smooth muscle cells and tissues.

Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin

Troponin and tropomyosin on actin filaments constitute a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle through a series of conformational changes within the actin-based thin filament. Troponin consists of three subunits: an inhibitory subunit (TnI), a Ca2+-binding subunit (TnC), and a tropomyosin-binding subunit (TnT). Ca2+-binding to TnC is believed to weaken interactions between troponin and actin, and triggers a large conformational change of the troponin complex. However, the atomic details of the actin-binding sites of troponin have not been determined. Ternary troponin complexes have been reconstituted from recombinant chicken skeletal TnI, TnC, and TnT2 (the C-terminal region of TnT), among which only TnI was uniformly labelled with 15N and/or 13C. By applying NMR spectroscopy, the solution structures of a "mobile" actin-binding domain (approximately 6.1 kDa) in the troponin ternary complex (approximately 52 kDa) were determined. The mobile domain appears to tumble independently of the core domain of troponin. Ca2+-induced changes in the chemical shift and line shape suggested that its tumbling was more restricted at high Ca2+ concentrations. The atomic details of interactions between actin and the mobile domain of troponin were defined by docking the mobile domain into the cryo-electron microscopy (cryo-EM) density map of thin filament at low [Ca2+]. This allowed the determination of the 3D position of residue 133 of TnI, which has been an important landmark to incorporate the available information. This enabled unique docking of the entire globular head region of troponin into the thin filament cryo-EM map at a low Ca2+ concentration. The resultant atomic model suggests that troponin interacted electrostatically with actin and caused the shift of tropomyosin to achieve muscle relaxation. An important feature is that the coiled-coil region of troponin pushed tropomyosin at a low Ca2+ concentration. Moreover, the relationship between myosin and the mobile domain on actin filaments suggests that the latter works as a fail-safe latch.

The N-terminus of the long MLCK induces a disruption in normal spindle morphology and metaphase arrest

We have shown previously that only the long myosin light chain kinase (MLCK), which is the predominant MLCK isoform expressed in nonmuscle cells, localizes to the cleavage furrow. To further examine the in vivo localization of the long MLCK in HeLa cells and the mechanisms responsible for kinase targeting during the cell cycle, we examined the distribution of the endogenous kinase and constructed green fluorescent protein (GFP) fusions of long HeLa MLCK truncations. A GFP fusion containing the N-terminal IgG domain and the five DXR motifs localized to stress fibers during interphase and the cleavage furrow during mitosis. Although individual fusions of the five DXRs and IgG domain both independently localized to stress fibers, only the five DXRs demonstrated a cortical localization in mitotic cells. Thus, robust targeting of the long MLCK to the cleavage furrow required the five DXRs and additional sequences from the IgG domain. Expression of the IgG domain alone or with five DXRs increased the number of multinucleate cells tenfold, whereas expression of the five DXRs or GFP had no effect. Furthermore, expression of the IgG domain alone or with five DXRs disrupted normal spindle morphology during mitosis. Extended astral microtubules and increased bundling of kinetochore microtubules, and spindle pole fragmentation were detected in mitotic cells. These microtubule defects were associated with abnormalities in metaphase chromosome alignment and a subsequent metaphase arrest caused by activation of the spindle assembly checkpoint at the kinetochores of mono-oriented chromosomes. Together, these results suggest that MLCK has an unexpected regulatory function during mitosis.

Myosin 3A transgene expression produces abnormal actin filament bundles in transgenic Xenopus laevis rod photoreceptors

Myo3A, a class III myosin, localizes to the distal (plus) ends of inner segment actin filament bundles that form the core of microvillus-like calycal processes encircling the base of the photoreceptor outer segment. To investigate Myo3A localization and function, we expressed green fluorescent protein-tagged bass Myo3A and related constructs in transgenic Xenopus rods using a modified opsin promoter. Tagged intact Myo3A localized to rod calycal processes, as previously reported for native bass Myo3A. Transgenic rods developed abnormally large calycal processes and subsequently degenerated. Modified Myo3A expression constructs demonstrated that calycal process localization required an active motor domain and the tail domain. Expressed tail domain alone localized to actin bundles along the entire inner segment length, rather than to the distal end. This tail domain localization required the conserved C-terminal domain (3THDII) previously shown to possess an actin-binding motif. Our findings suggest that Myo3A plays a role in the morphogenesis and maintenance of calycal processes of vertebrate photoreceptors.

Myosin light chain kinase (210 kDa) is a potential cytoskeleton integrator through its unique N-terminal domain

Recently discovered 210-kDa myosin light chain kinase (MLCK-210) is identical to 108-130 kDa MLCK, the principal regulator of the myosin II molecular motor, except for the presence of a unique amino terminal extension. Our in vitro experiments and transfected cell studies demonstrate that the N-terminal half of MLCK-210 unique tail domain has novel microfilament and microtubule binding activity. Consistent with this activity, the MLCK-210 domain codistributes with microfilaments and microtubules in cultured cells and with soluble tubulin in nocodazole-treated cells. This domain is capable of aggregating tubulin dimers in vitro, causing bundling and branching of microtubules induced by taxol. The N-terminal actin-binding region of MLCK-210 has lower affinity to actin (K(d) = 7.4 microM) than its central D(F/V)RXXL repeat-based actin-binding site and does not protect stress fibers from disassembly triggered by MLCK inhibition in transfected cells. Obtained results suggest that while being resident on microfilaments, MLCK-210 may interact with other cytoskeletal components through its N-terminal domain. Based on available evidence, we propose a model in which MLCK-210 could organize cell motility by simultaneous control of cytoskeleton architecture and actomyosin activation through the novel protein scaffold function of the unique tail domain and the classical MLCK catalytic function of the kinase domain.

Real-time evaluation of myosin light chain kinase activation in smooth muscle tissues from a transgenic calmodulin-biosensor mouse

Ca(2+)/calmodulin (CaM)-dependent phosphorylation of myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) initiates smooth muscle contraction and regulates actomyosin-based cytoskeletal functions in nonmuscle cells. The net extent of RLC phosphorylation is controlled by MLCK activity relative to myosin light chain phosphatase activity. We have constructed a CaM-sensor MLCK where Ca(2+)-dependent CaM binding increases the catalytic activity of the kinase domain, whereas coincident binding to the biosensor domain decreases fluorescence resonance energy transfer between two fluorescent proteins. We have created transgenic mice expressing this construct specifically in smooth muscle cells to perform real-time evaluations of the relationship between smooth muscle contractility and MLCK activation in intact tissues and organs. Measurements in intact bladder smooth muscle demonstrate that MLCK activation increases rapidly during KCl-induced contractions but is not maximal, consistent with a limiting amount of cellular CaM. Carbachol treatment produces the same amount of force development and RLC phosphorylation, with much smaller increases in [Ca(2+)](i) and MLCK activation. A Rho kinase inhibitor suppresses RLC phosphorylation and force but not MLCK activation in carbachol-treated tissues. These observations are consistent with a model in which the magnitude of an agonist-mediated smooth muscle contraction depends on a rapid but limited Ca(2+)/CaM-dependent activation of MLCK and Rho kinase-mediated inhibition of myosin light chain phosphatase activity. These studies demonstrate the feasibility of producing transgenic biosensor mice for investigations of signaling processes in intact systems.

LFA-1-induced T cell migration on ICAM-1 involves regulation of MLCK-mediated attachment and ROCK-dependent detachment

This study analyzes signaling events initiated through binding of the leukocyte integrin LFA-1 to ICAM-1, which leads to T cell attachment, polarization and random migration. These events are critically dependent on dynamic changes in the acto-myosin cytoskeleton under the regulation of myosin light chain kinase and ROCK (Rho kinase). A key finding is that the activity of these two kinases is spatially segregated. Myosin light chain kinase (MLCK) must operate at the leading edge of the T cell because blocking its activity causes the polarized T cell to retract from the front of the cell. These activities are mirrored by inhibiting calmodulin, the activator of MLCK. In contrast inhibition of ROCK (and RhoA) has the effect of preventing detachment of the T cell trailing edge, showing that this kinase operates at the rear of the cell. This compartmentalized activity of the two kinases is reflected in their localization within the T cell. Myosin light chain kinase is concentrated at the leading edge, overlapping F-actin, whereas ROCK is more widely distributed in the trailing edge of the T cell. Thus these two kinases perform two different functions in the migrating T cell, with myosin light chain kinase activity important for attachment and movement at the leading edge and ROCK activity required for the detachment of the trailing edge. These two actomyosin-dependent processes operate coordinately to cause forward migration of a T cell.

Twitchin from molluscan catch muscle: primary structure and relationship between site-specific phosphorylation and mechanical function

The phosphorylation state of the myosin thick filament-associated mini-titin, twitchin, regulates catch force maintenance in molluscan smooth muscle. The full-length cDNA for twitchin from the anterior byssus retractor muscle of the mussel Mytilus was obtained using PCR and 5'rapid amplification of cDNA ends, and its derived amino acid sequence showed a large molecule ( approximately 530 kDa) with a motif arrangement as follows: (Ig)11(IgFn2)2Ig(Fn)3Ig(Fn)2Ig(Fn)3(Ig)2(Fn)2(Ig)2 FnKinase(Ig)4. Other regions of note include a 79-residue sequence between Ig domains 6 and 7 (from the N terminus) in which more than 60% of the residues are Pro, Glu, Val, or Lys and between the 7th and 8th Ig domains, a DFRXXL motif similar to that thought to be necessary for high affinity binding of myosin light chain kinase to F-actin. Two major phosphorylation sites, i.e. D1 and D2, were located in linker regions between Ig domains 7 and 8 and Ig domains 21 and 22, respectively. Correlation of the phosphorylation state of twitchin, using antibodies specific to D1 and D2, with mechanical properties suggested that phosphorylation of both D1 and D2 is required for relaxation from the catch state.

Localization of a class III myosin to filopodia tips in transfected HeLa cells requires an actin-binding site in its tail domain

Bass Myo3A, a class III myosin, was expressed in HeLa cells as a GFP fusion in order to study its cellular localization. GFP-Myo3A localized to the cytoplasm and to the tips of F-actin bundles in filopodia, a localization that is consistent with the observed concentration toward the distal ends of F-actin bundles in photoreceptor cells. A mutation in the motor active site resulted in a loss of filopodia localization, suggesting that Myo3A motor activity is required for filopodial tip localization. Deletion analyses showed that the NH2-terminal kinase domain is not required but the CO2H-terminal 22 amino acids of the Myo3A tail are required for filopodial localization. Expression of this tail fragment alone produced fluorescence associated with F-actin throughout the cytoplasm and filopodia and a recombinant tail fragment bound to F-actin in vitro. An actin-binding motif was identified within this tail fragment, and a mutation within this motif abolished both filopodia localization by Myo3A and F-actin binding by the tail fragment alone. Calmodulin localized to filopodial tips when coexpressed with Myo3A but not in the absence of Myo3A, an observation consistent with the previous proposal that class III myosins bind calmodulin and thereby localize it in certain cell types.

Novel interaction of cortactin with endothelial cell myosin light chain kinase

Inflammatory mediators such as thrombin evoke increases in vascular permeability through activation of endothelial contractile mechanisms which involve increased levels of MLC phosphorylation catalyzed by Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK). We previously noted that the high molecular weight endothelial MLCK isoform (EC MLCK) is stably associated with a complex containing p60(src) and 80kDa cortactin, an actin-binding protein and known p60(src) target. In this study we have utilized in vitro binding assays to confirm specific interaction between EC MLCK and cortactin. Tyrosine phosphorylation of either EC MLCK (Y(464), Y(471)) or cortactin (Y(421), Y(466), and Y(482)) by p60(src) significantly increased this direct association. Site-specific antibody and peptide studies subsequently confirmed EC MLCK AA #972-979 and 1019-1025 as sites of cortactin interaction. EC MLCK-cortactin interaction in vitro failed to modulate MLCK enzymatic activity but appeared to inhibit EC MLCK binding to F-actin, while EC MLCK abolished cortactin-mediated augmentation of Arp2/3-stimulated actin polymerization. These data suggest that cortactin-EC MLCK interaction may be a novel determinant of endothelial cortical actin-based cytoskeletal rearrangement.

Properties of long myosin light chain kinase binding to F-actin in vitro and in vivo

Short and long myosin light chain kinases (MLCKs) are Ca(2+)/calmodulin-dependent enzymes that phosphorylate the regulatory light chain of myosin II in thick filaments but bind with high affinity to actin thin filaments. Three repeats of a motif made up of the sequence DFRXXL at the N terminus of short MLCK are necessary for actin binding (Smith, L., Su, X., Lin, P., Zhi, G., and Stull, J. T. (1999) J. Biol. Chem. 274, 29433-29438). The long MLCK has two additional DFRXXL motifs and six Ig-like modules in an N-terminal extension, which may confer unique binding properties for cellular localization. Two peptides containing either five or three DFRXXL motifs bound to F-actin and smooth muscle myofilaments with maximal binding stoichiometries consistent with each motif binding to an actin monomer in the filaments. Both peptides cross-linked F-actin and bound to stress fibers in cells. Long MLCK with an internal deletion of the five DFRXXL motifs and the unique NH(2)-terminal fragment containing six Ig-like motifs showed weak binding. Cell fractionation and extractions with MgCl(2) indicate that the long MLCK has a greater affinity for actin-containing filaments than short MLCK in vitro and in vivo. Whereas DFRXXL motifs are necessary and sufficient for short MLCK binding to actin-containing filaments, the DFRXXL motifs and the N-terminal extension of long MLCK confer high affinity binding to stress fibers in cells.

Smoothelin contains a novel actin cytoskeleton localization sequence with similarity to troponin T

Smoothelin, a cytoskeletal protein, exists in a large isoform specifically expressed in vascular smooth muscle cells, and a small visceral isoform generated by a downstream transcriptional start site. Using fusions to the green fluorescent protein, we could show that both smoothelin isoforms are localized at actin containing filaments and mapped two domains that are each sufficient for localization at the actin cytoskeleton. The first domain is located in the vascular-specific, N-terminal half of smoothelin and the second in the common, C-terminal half. The second domain shares clear sequence similarity with a domain of troponin T involved in actin filament association. These results suggest that the tissue-specific expression of smoothelin isoforms might contribute to the different contractile properties of smooth muscle cells.

Role of the short isoform of myosin light chain kinase in the contraction of cultured smooth muscle cells as examined by its down-regulation

GbaSM-4 cells, smooth muscle cells derived from brain basilar artery, which express both 210-kDa long and 130-kDa short isoforms of myosin light chain kinase (MLCK), were infected with an adenovirus vector carrying a 1.4-kb catalytic portion of MLCK-cDNA in an antisense orientation. Western blot analysis showed that the expression of short MLCK was depressed without affecting long MLCK expression. The contraction of the down-regulated cells was measured by the cell-populated collagen-fiber method. The tension development after stimulation with norepinephrine or was depressed. The additional infection of the down-regulated cells with the adenovirus construct containing the same insert in a sense direction rescued not only the short MLCK expression but also contraction, confirming the physiological role of short MLCK in the contraction. To examine the role of long MLCK in the residual contraction persisting in the short MLCK-deficient cells, long MLCK was further down-regulated by increasing the multiplicity of infection of the antisense construct. The additional down-regulation of long MLCK expression, however, did not alter the residual contraction, ruling out the involvement of long MLCK in the contractile activity. Further, in the cells where short MLCK was down-regulated specifically, the extent of phosphorylation of 20-kDa myosin light chain (MLC20) after the agonist stimulation was not affected. This finding suggests that there are additional factors to MLC20 phosphorylation that contribute to regulate smooth muscle contraction.

220- and 130-kDa MLCKs have distinct tissue distributions and intracellular localization patterns

To better understand the distinct functional roles of the 220- and 130-kDa forms of myosin light chain kinase (MLCK), expression and intracellular localization were determined during development and in adult mouse tissues. Northern blot, Western blot, and histochemical studies show that the 220-kDa MLCK is widely expressed during development as well as in several adult smooth muscle and nonmuscle tissues. The 130-kDa MLCK is highly expressed in all adult tissues examined and is also detectable during embryonic development. Colocalization studies examining the distribution of 130- and 220-kDa mouse MLCKs revealed that the 130-kDa MLCK colocalizes with nonmuscle myosin IIA but not with myosin IIB or F-actin. In contrast, the 220-kDa MLCK did not colocalize with either nonmuscle myosin II isoform but instead colocalizes with thick interconnected bundles of F-actin. These results suggest that in vivo, the physiological functions of the 220- and 130-kDa MLCKs are likely to be regulated by their intracellular trafficking and distribution.

Activation of smooth muscle myosin light chain kinase by calmodulin. Role of LYS(30) and GLY(40)

Calmodulin (CaM)-dependent myosin light chain kinase (MLCK) plays a key role in activation of smooth muscle contraction. A soybean isoform of CaM, SCaM-4 (77% identical to human CaM) fails to activate MLCK, whereas SCaM-1 (90.5% identical to human CaM) is as effective as CaM. We exploited this difference to gain insights into the structural requirements in CaM for activation of MLCK. A chimera (domain I of SCaM-4 and domains II-IV of SCaM-1) behaved like SCaM4, and analysis of site-specific mutants of SCaM-1 indicated that K30E and G40D mutations were responsible for the reduction in activation of MLCK. Competition experiments showed that SCaM-4 binds to the CaM-binding site of MLCK with high affinity. Replacement of CaM in skinned smooth muscle by exogenous CaM or SCaM-1, but not SCaM-4, restored Ca(2+)-dependent contraction. K30E/M36I/G40D SCaM-1 was a poor activator of contraction, but site-specific mutants, K30E, M36I and G40D, each restored Ca(2+)-induced contraction to CaM-depleted skinned smooth muscle, consistent with their capacity to activate MLCK. Interpretation of these results in light of the high-resolution structures of (Ca(2+))(4)-CaM, free and complexed with the CaM-binding domain of MLCK, indicates that a surface domain containing Lys(30) and Gly(40) and residues from the C-terminal domain is created upon binding to MLCK, formation of which is required for activation of MLCK. Interactions between this activation domain and a region of MLCK distinct from the known CaM-binding domain are required for removal of the autoinhibitory domain from the active site, i.e., activation of MLCK, or this domain may be required to stabilize the conformation of (Ca(2+))(4)-CaM necessary for MLCK activation.

Functional plasticity of CH domains

With the refinement of algorithms for the identification of distinct motifs from sequence databases, especially those using secondary structure predictions, new protein modules have been determined in recent years. Calponin homology (CH) domains were identified in a variety of proteins ranging from actin cross-linking to signaling and have been proposed to function either as autonomous actin binding motifs or serve a regulatory function. Despite the overall structural conservation of the unique CH domain fold, the individual modules display a quite striking functional variability. Analysis of the actopaxin/parvin protein family suggests the existence of novel (type 4 and type 5) CH domain families which require special attention, as they appear to be a good example for how CH domains may function as scaffolds for other functional motifs of different properties.

A fluorescent resonant energy transfer-based biosensor reveals transient and regional myosin light chain kinase activation in lamella and cleavage furrows

Approaches with high spatial and temporal resolution are required to understand the regulation of nonmuscle myosin II in vivo. Using fluorescence resonance energy transfer we have produced a novel biosensor allowing simultaneous determination of myosin light chain kinase (MLCK) localization and its [Ca2+]4/calmodulin-binding state in living cells. We observe transient recruitment of diffuse MLCK to stress fibers and its in situ activation before contraction. MLCK is highly active in the lamella of migrating cells, but not at the retracting tail. This unexpected result highlights a potential role for MLCK-mediated myosin contractility in the lamella as a driving force for migration. During cytokinesis, MLCK was enriched at the spindle equator during late metaphase, and was maximally activated just before cleavage furrow constriction. As furrow contraction was completed, active MLCK was redistributed to the poles of the daughter cells. These results show MLCK is a myosin regulator in the lamella and contractile ring, and pinpoints sites where myosin function may be mediated by other kinases.

Ca2+ activation of smooth muscle contraction: evidence for the involvement of calmodulin that is bound to the triton insoluble fraction even in the absence of Ca2+

Smooth muscle contraction is activated by phosphorylation of the 20-kDa light chains of myosin catalyzed by Ca(2+)/calmodulin (CaM)-dependent myosin light chain kinase (MLCK). According to popular current theory, the CaM involved in MLCK regulation is Ca(2+)-free and dissociated from the kinase at resting cytosolic free Ca(2+) concentration ([Ca(2+)](i)). An increase in [Ca(2+)](i) saturates the four Ca(2+)-binding sites of CaM, which then binds to and activates actin-bound MLCK. The results of this study indicate that this theory requires revision. Sufficient CaM was retained after skinning (demembranation) of rat tail arterial smooth muscle in the presence of EGTA to support Ca(2+)-evoked contraction, as observed previously with other smooth muscle tissues. This tightly bound CaM was released by the CaM antagonist trifluoperazine (TFP) in the presence of Ca(2+). Following removal of the (Ca(2+))(4)-CaM-TFP(2) complex, Ca(2+) no longer induced contraction. The addition of exogenous CaM to TFP-treated tissue at a [Ca(2+)] subthreshold for contraction or even in the absence of Ca(2+) (presence of 5 mm EGTA), followed by washout of unbound CaM, restored Ca(2+)-induced contraction; this required MLCK activation, since it was blocked by the MLCK inhibitor ML-9. The data suggest, therefore, that a specific pool of cellular CaM, tightly bound to myofilaments at resting [Ca(2+)](i), or even in the absence of Ca(2+), is responsible for activation of contraction following a local increase in [Ca(2+)]. This mechanism would allow for localized changes in [Ca(2+)] in regions of the cell distant from the myofilaments to regulate distinct Ca(2+)-dependent processes without triggering a contractile response. Immobilized CaM, therefore, resembles troponin C, the Ca(2+)-binding regulatory protein of striated muscle, which is also bound to the thin filament in a Ca(2+)-independent manner.

Myosin light chain kinase binding to a unique site on F-actin revealed by three-dimensional image reconstruction

Ca2+-calmodulin-dependent phosphorylation of myosin regulatory light chains by the catalytic COOH-terminal half of myosin light chain kinase (MLCK) activates myosin II in smooth and nonmuscle cells. In addition, MLCK binds to thin filaments in situ and F-actin in vitro via a specific repeat motif in its NH2 terminus at a stoichiometry of one MLCK per three actin monomers. We have investigated the structural basis of MLCK-actin interactions by negative staining and helical reconstruction. F-actin was decorated with a peptide containing the NH2-terminal 147 residues of MLCK (MLCK-147) that binds to F-actin with high affinity. MLCK-147 caused formation of F-actin rafts, and single filaments within rafts were used for structural analysis. Three-dimensional reconstructions showed MLCK density on the extreme periphery of subdomain-1 of each actin monomer forming a bridge to the periphery of subdomain-4 of the azimuthally adjacent actin. Fitting the reconstruction to the atomic model of F-actin revealed interaction of MLCK-147 close to the COOH terminus of the first actin and near residues 228-232 of the second. This unique location enables MLCK to bind to actin without interfering with the binding of any other key actin-binding proteins, including myosin, tropomyosin, caldesmon, and calponin.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.

Myosin light chain kinase binding to actin filaments

Smooth muscle myosin light chain kinase (MLCK) plays important roles in contractile-motile processes of a variety of cells. Three DFRxxL motifs at the kinase N-terminus (residues 2-63) are critical for high-affinity binding to actin-containing filaments [Smith et al. (1999) J. Biol. Chem. 274, 29433-29438]. A GST fusion protein containing residues 1-75 of MLCK (GST75-MLCK) bound maximally to both smooth muscle myofilaments and F-actin at 0.28 and 0.31 mol GST75-MLCK/mol actin with respective K(D) values of 0.1 microM and 0.8 microM. High-affinity binding of MLCK to actin-containing filaments may be due to each DFRxxL motif binding to one actin monomer in filaments.

Unique sequence of a high molecular weight myosin light chain kinase is involved in interaction with actin cytoskeleton

Myosin light chain kinase (MLCK) is the key regulator of cell motility and smooth muscle contraction in higher vertebrates. We searched for the features of the high molecular weight MLCK (MLCK-210) associated with its unique N-terminal sequence not found in a more ubiquitous lower molecular weight MLCK (MLCK-108). MLCK-210 demonstrates stronger association with the Triton-insoluble cytoskeletons than MLCK-108, suggesting the role for this sequence in subcellular targeting. Indeed, the expressed unique domain of MLCK-210 binds and bundles F-actin in vitro and colocalises with the microfilaments in transfected cells reproducing endogenous MLCK-210 distribution. Thus, MLCK-210 features an extensive actin binding interface and, perhaps, acts as an actin cytoskeleton stabiliser.