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

Diffusion of myosin light chain kinase on actin: A mechanism to enhance myosin phosphorylation rates in smooth muscle

Smooth muscle myosin (SMM) light chain kinase (MLCK) phosphorylates SMM, thereby activating the ATPase activity required for muscle contraction. The abundance of active MLCK, which is tightly associated with the contractile apparatus, is low relative to that of SMM. SMM phosphorylation is rapid despite the low ratio of MLCK to SMM, raising the question of how one MLCK rapidly phosphorylates many SMM molecules. We used total internal reflection fluorescence microscopy to monitor single molecules of streptavidin-coated quantum dot-labeled MLCK interacting with purified actin, actin bundles, and stress fibers of smooth muscle cells. Surprisingly, MLCK and the N-terminal 75 residues of MLCK (N75) moved on actin bundles and stress fibers of smooth muscle cell cytoskeletons by a random one-dimensional (1-D) diffusion mechanism. Although diffusion of proteins along microtubules and oligonucleotides has been observed previously, this is the first characterization to our knowledge of a protein diffusing in a sustained manner along actin. By measuring the frequency of motion, we found that MLCK motion is permitted only if acto-myosin and MLCK-myosin interactions are weak. From these data, diffusion coefficients, and other kinetic and geometric considerations relating to the contractile apparatus, we suggest that 1-D diffusion of MLCK along actin (a) ensures that diffusion is not rate limiting for phosphorylation, (b) allows MLCK to locate to areas in which myosin is not yet phosphorylated, and (c) allows MLCK to avoid getting "stuck" on myosins that have already been phosphorylated. Diffusion of MLCK along actin filaments may be an important mechanism for enhancing the rate of SMM phosphorylation in smooth muscle.

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.

Structural limits on force production and shortening of smooth muscle

This study determined the factors that limit force production and shortening in two smooth muscles having very different relationships between active and passive force as a function of muscle length. The rat anococcygeus muscle develops active force over the range of lengths 0.2-2.0× the optimum length for force production (Lo). Passive tension due to extension of the resting muscle occurs only at lengths exceeding Lo. In contrast, the rabbit taenia coli develops force in the range of lengths 0.4-1.1 Lo, and passive force which is detectable at 0.56 Lo, increases to ~0.45 maximum active force at Lo, and increases sharply with further extension. The anococcygeus muscle can shorten to 0.2 Lo and the taenia coli to 0.4 Lo. Dynamic stiffness and energy usage at short muscle lengths suggest that the limit of shortening in the taenia coli, in contrast to the anococcygeus muscle, is not due to a failure of cross bridge interaction. Phosphorylation of the regulatory myosin light chains in intact muscles decreased to a small extent at short lengths compared to the decrease in force production. The differences in force production and the extent of shortening in the two muscles was maintained even when, following permeabilization, the myosin light chains were irreversibly phosphorylated with ATPγS, indicating that differences in activation played little, if any role. Ultrastructural studies on resting and activated muscles show that the taenia coli, which is rich in connective tissue (unlike the anococcygeus muscle) undergoes marked cellular twisting and contractile filament misalignment at short lengths with compression of the extracellular matrix. As a result, force is not transmitted in the longitudinal axis of the muscle, but is dissipated against an internal load provided by the compressed extracellular matrix. These observations on two very different normal smooth muscles reveal how differences in the relative contribution of active and passive structural elements determine their mechanical behavior, and how this is potentially modified by remodeling that occurs in disease and in response to changes in functional demand.

Novel role for non-muscle myosin light chain kinase (MLCK) in hyperoxia-induced recruitment of cytoskeletal proteins, NADPH oxidase activation, and reactive oxygen species generation in lung endothelium

We recently demonstrated that hyperoxia (HO) activates lung endothelial cell NADPH oxidase and generates reactive oxygen species (ROS)/superoxide via Src-dependent tyrosine phosphorylation of p47(phox) and cortactin. Here, we demonstrate that the non-muscle ~214-kDa myosin light chain (MLC) kinase (nmMLCK) modulates the interaction between cortactin and p47(phox) that plays a role in the assembly and activation of endothelial NADPH oxidase. Overexpression of FLAG-tagged wild type MLCK in human pulmonary artery endothelial cells enhanced interaction and co-localization between cortactin and p47(phox) at the cell periphery and ROS production, whereas abrogation of MLCK using specific siRNA significantly inhibited the above. Furthermore, HO stimulated phosphorylation of MLC and recruitment of phosphorylated and non-phosphorylated cortactin, MLC, Src, and p47(phox) to caveolin-enriched microdomains (CEM), whereas silencing nmMLCK with siRNA blocked recruitment of these components to CEM and ROS generation. Exposure of nmMLCK(-/-) null mice to HO (72 h) reduced ROS production, lung inflammation, and pulmonary leak compared with control mice. These results suggest a novel role for nmMLCK in hyperoxia-induced recruitment of cytoskeletal proteins and NADPH oxidase components to CEM, ROS production, and lung injury.

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.

Skeletal myosin light chain kinase regulates skeletal myogenesis by phosphorylation of MEF2C

The MEF2 factors regulate transcription during cardiac and skeletal myogenesis. MEF2 factors establish skeletal muscle commitment by amplifying and synergizing with MyoD. While phosphorylation is known to regulate MEF2 function, lineage-specific regulation is unknown. Here, we show that phosphorylation of MEF2C on T(80) by skeletal myosin light chain kinase (skMLCK) enhances skeletal and not cardiac myogenesis. A phosphorylation-deficient MEF2C mutant (MEFT80A) enhanced cardiac, but not skeletal myogenesis in P19 stem cells. Further, MEFT80A was deficient in recruitment of p300 to skeletal but not cardiac muscle promoters. In gain-of-function studies, skMLCK upregulated myogenic regulatory factor (MRF) expression, leading to enhanced skeletal myogenesis in P19 cells and more efficient myogenic conversion. In loss-of-function studies, MLCK was essential for efficient MRF expression and subsequent myogenesis in embryonic stem (ES) and P19 cells as well as for proper activation of quiescent satellite cells. Thus, skMLCK regulates MRF expression by controlling the MEF2C-dependent recruitment of histone acetyltransferases to skeletal muscle promoters. This work identifies the first kinase that regulates MyoD and Myf5 expression in ES or satellite 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.

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.

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.

Calcium transients regulate patterned actin assembly during myofibrillogenesis

The highly ordered arrangement of sarcomeric myosin during striated muscle development requires spontaneous calcium (Ca(2+)) transients. Here, we show that blocking transients also compromises patterned assembly of actin thin filaments, titin, and capZ. Because a conserved temporal assembly pattern has been described for these proteins, selective inhibitors of either thick or thin filament formation were used to determine their relative temporal interdependencies. For example, inhibition of myosin light chain kinase (MLCK) by application of a specific inhibitory peptide or phorbol myistate acetate (PMA) disrupts myosin assembly without significantly affecting formation of actin bands. The MLCK inhibitor ML-7, however, disrupted actin as well as myosin. Surprisingly, agents that interfere with actin dynamics, such as cytochalasin D, produced only minor organizational disruptions in actin, capZ, and titin staining. However, cytochalasin D and other actin disrupting compounds significantly perturbed myosin organization. The results indicate that (1) Ca(2+) transients regulate one or more of the earliest steps in sarcomere formation, (2) mature actin filaments can assemble independently of myosin band formation, and (3) myosin thick filament assembly is extremely sensitive to disruption of either the actin or titin filament systems.

Translocation of telokin by cGMP signaling in smooth muscle cells

Telokin is an acidic protein with a sequence identical to the COOH-terminal domain of myosin light chain kinase (MLCK) produced by an alternate promoter of the MLCK gene. Although it is abundantly expressed in smooth muscle, its physiological function is not understood. In the present study, we attempted to clarify the function of telokin by analyzing its spatial and temporal localization in living single smooth muscle cells. Primary cultured smooth muscle cells were transfected with green fluorescent protein (GFP)-tagged telokin. The telokin-GFP localized mostly diffusely in cytosol. Stimulation with both sodium nitroprusside (SNP) and 8-bromo-cyclic GMP induced translocation of GFP-tagged telokin to near plasma membrane in living single smooth muscle cells. The translocation was slow, and it took more than 10 min at room temperature. Mutation of the phosphorylation sites of telokin (S13A, S19A, and S13A/S19A) significantly attenuated SNP-induced translocation. Both KT-5823 (cGMP-dependent protein kinase inhibitor) and PD-98059 (mitogen-activated protein kinase inhibitor) diminished the telokin-GFP translocation. These results suggest that telokin changes its intracellular localization because of phosphorylation at Ser13 and/or Ser19 via the cGMP-signaling pathway.

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.

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.

The C terminus of annexin II mediates binding to F-actin

Annexin II heterotetramer (AIIt) is a multifunctional Ca(2+)-binding protein composed of two 11-kDa subunits and two annexin II subunits. The annexin II subunit contains the binding sites for anionic phospholipids, heparin, and F-actin, whereas the p11 subunit provides a regulatory function. The F-actin-binding site is presently unknown. In the present study we have utilized site-directed mutagenesis to create annexin II mutants with truncations in the C terminus of the molecule. Interestingly, a mutant annexin II lacking its C-terminal 16, 13, or 9 amino acids was unable to bind to F-actin but still retained its ability to interact with both anionic phospholipids and heparin. Recombinant AIIt, composed of wild-type p11 subunits and the mutant annexin II subunits, was also unable to bundle F-actin. This loss of F-actin bundling activity was directly attributable to the inability of mutant AIIt to bind F-actin. These results establish for the first time that the annexin II C-terminal amino acid residues, LLYLCGGDD, participate in F-actin binding.

Smooth muscle myosin light chain kinase expression in cardiac and skeletal muscle

The purpose of this study was to characterize myosin light chain kinase (MLCK) expression in cardiac and skeletal muscle. The only classic MLCK detected in cardiac tissue, purified cardiac myocytes, and in a cardiac myocyte cell line (AT1) was identical to the 130-kDa smooth muscle MLCK (smMLCK). A complex pattern of MLCK expression was observed during differentiation of skeletal muscle in which the 220-kDa-long or "nonmuscle" form of MLCK is expressed in undifferentiated myoblasts. Subsequently, during myoblast differentiation, expression of the 220-kDa MLCK declines and expression of this form is replaced by the 130-kDa smMLCK and a skeletal muscle-specific isoform, skMLCK in adult skeletal muscle. These results demonstrate that the skMLCK is the only tissue-specific MLCK, being expressed in adult skeletal muscle but not in cardiac, smooth, or nonmuscle tissues. In contrast, the 130-kDa smMLCK is ubiquitous in all adult tissues, including skeletal and cardiac muscle, demonstrating that, although the 130-kDa smMLCK is expressed at highest levels in smooth muscle tissues, it is not a smooth muscle-specific protein.

Phosphorylation of myosin light chain kinase by p21-activated kinase PAK2

Phosphorylation of myosin II regulatory light chains (RLC) by Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) is a critical step in the initiation of smooth muscle and non-muscle cell contraction. Post-translational modifications to MLCK down-regulate enzyme activity, suppressing RLC phosphorylation, myosin II activation, and tension development. Here we report that PAK2, a member of the Rho family of GTPase-dependent kinases, regulates isometric tension development and myosin II RLC phosphorylation in saponin permeabilized endothelial monolayers. PAK2 blunts tension development by 75% while inhibiting diphosphorylation of myosin II RLC. Cdc42-activated placenta and recombinant, constitutively active PAK2 phosphorylate MLCK in vitro with a stoichiometry of 1.71 +/- 0. 21 mol of PO(4)/mol of MLCK. This phosphorylation inhibits MLCK phosphorylation of myosin II RLC. PAK2 catalyzes MLCK phosphorylation on serine residues 439 and 991. Binding calmodulin to MLCK blocks phosphorylation of Ser-991 by PAK2. These results demonstrate that PAK2 can directly phosphorylate MLCK, inhibiting its activity and limiting the development of isometric tension.

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).

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

Phosphorylation of the 20-kDa regulatory light chain of myosin catalyzed by a Ca(2+)/calmodulin-dependent myosin light chain kinase is important in the initiation of smooth muscle contraction and other contractile processes in non-muscle cells. It has been previously shown that residues 1-142 of smooth muscle myosin light chain kinase are necessary for high-affinity binding to actin-containing filaments in cells (1). To further localize the region of the kinase required for binding, a series of N-terminal deletion mutants as well as several N-terminal glutathione S-transferase fusion proteins were constructed. Cosedimentation assays showed that a peptide containing residues 1-75 binds to purified smooth muscle myofilaments. Furthermore, the N-terminal peptide was sufficient for high-affinity binding to actin stress fibers in smooth muscle cells in vivo. Alanine scanning mutagenesis in the fusion protein identified residues Asp-30, Phe-31, Arg-32, and Leu-35 as important for binding in vitro. There are two additional DFRXXL motifs located at residues 2-7 and 58-63. The DFR residues in these three motifs were individually replaced by alanine residues in the full-length kinase. Each of these mutations significantly decreased myosin light chain kinase binding to myofilaments in vitro, and each abolished high-affinity binding to actin-containing filaments in smooth muscle cells in vivo. These results identify a unique structural motif comprised of three repeat consensus sequences in the N terminus of myosin light chain kinase necessary for high-affinity binding to actin-containing filaments.

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.

Properties of filament-bound myosin light chain kinase

Myosin light chain kinase binds to actin-containing filaments from cells with a greater affinity than to F-actin. However, it is not known if this binding in cells is regulated by Ca2+/calmodulin as it is with F-actin. Therefore, the binding properties of the kinase to stress fibers were examined in smooth muscle-derived A7r5 cells. Full-length myosin light chain kinase or a truncation mutant lacking residues 2-142 was expressed as chimeras containing green fluorescent protein at the C terminus. In intact cells, the full-length kinase bound to stress fibers, whereas the truncated kinase showed diffuse fluorescence in the cytoplasm. After permeabilization with saponin, the fluorescence from the truncated kinase disappeared, whereas the fluorescence of the full-length kinase was retained on stress fibers. Measurements of fluorescence intensities and fluorescence recovery after photobleaching of the full-length myosin light chain kinase in saponin-permeable cells showed that Ca2+/calmodulin did not dissociate the kinase from these filaments. However, the filament-bound kinase was sufficient for Ca2+-dependent phosphorylation of myosin regulatory light chain and contraction of stress fibers. Thus, dissociation of myosin light chain kinase from actin-containing thin filaments is not necessary for phosphorylation of myosin light chain in thick filaments. We note that the distance between the N terminus and the catalytic core of the kinase is sufficient to span the distance between thin and thick filaments.