Regulation of Drosophila myosin ATPase activity by phosphorylation of myosin light chains--I. Wild-type fly

1. Two types of myosins with phosphorylated and dephosphorylated myosin light chains were prepared from Drosophila flies. The former had ATPase (Ca2(+)- and Mg2(+)-activited) activities twice those of the latter. 2. The myosin phosphorylated with crude myosin light chain kinase from flies showed ATPase (Ca2(+)- and Mg2(+)-activated) activaties twice those of the dephosphorylated myosin. 3. It is suggested that phosphorylation of myosin light chains several hours after emergence stimulates myosin ATPase activity so as to facilitate the flight function of the fruitfly.     

Macrophage myosin. Regulation of actin-activated ATPase, activity by phosphorylation of the 20,000-dalton light chain.

Myosin was purified from rabbit alveolar macrophages in a form that could not be activated by actin. This myosin could be phosphorylated by an endogenous myosin light chain kinase, up to 2 mol of phosphate being incorporated/mol of myosin. The site phosphorylated was located on the 20,000-dalton myosin light chain. Phosphorylation of macrophage myosin was found to be necessary for actin activation of myosin ATPase activity. Moreover, the actin-activated ATPase activity was found to vary directly with the extent of myosin phosphorylation, maximal phosphorylation (2 mol of Pi/mol of myosin) resulting in an actin-activated MgATPase activity of approximately 200 nmol of Pi/mg of myosin/min at 37 degrees C. These results establish that phosphyoyration of the 20,000-dalton light chain of myosin is sufficient to regulate the actin-activated ATPase activity of macrophage myosin.     

Correlation between multiple phosphorylation of gizzard myosin light chains and actin-activated myosin ATPase activity

With large amounts of gizzard Mr 135,000 calmodulin-binding protein (myosin light chain kinase), the phosphate incorporation into myosin light chains was determined to be 2 mol/mol of myosin light chain. The actin-activated ATPase activity was dramatically enhanced when myosin light chains were phosphorylated by more than 1 mol of phosphate incorporated/mol of myosin light chain.     

The effect of myosin light chain phosphorylation on the actin-stimulated ATPase activity of myosin minifilaments

It has been shown that in the absence of KCl, the actin-stimulated Mg2+-ATPase activity of rabbit skeletal myosin minifilaments with phosphorylated regulatory lights chains (LC2) exceeds 3-4-fold that of myosin minifilaments with dephosphorylated LC2. Addition of KCl leads to a decrease in the difference between the two ATPase activities. LC2 phosphorylation considerably increases the rate of ATPase reaction and only slightly decreases the affinity of myosin minifilaments for F-actin. It is suggested that the unusual effect of LC2 phosphorylation on the kinetic parameters of the actin-stimulated ATPase reaction of myosin minifilaments can be accounted for by its influence on the interaction between myosin heads which results in the ordered self-assembly of minifilaments.     

Activation by actin of ATPase activity of chemically modified gizzard myosin without phosphorylation.

The ATPase activity of myosin from chicken gizzard measured in the presence of either Mg²⁺ or Ca²⁺ is increased in the absence of dithiothreitol or upon reaction with Cu²⁺, o-iodosobenzoate, or N-ethylmaleimide. Iodosobenzoate or Cu²⁺ produce no change in K⁺(EDTA)-ATPase while N-ethylmaleimide produces a decrease. These treatments also make the actin-activated ATPase insensitive to Ca²⁺ when assayed in the presence of tropomyosin and a partially purified myosin light chain kinase. Phosphorylation of N-ethylmaleimide modified myosin remains dependent on Ca²⁺ and therefore appears not to be required for activation by actin of the ATPase activity of modified myosin.     

Regulation in vitro of brush border myosin by light chain phosphorylation

Myosin was purified from chicken brush border cells to greater than 95% homogeneity and in a predominantly non-phosphorylated state. The effects of light chain phosphorylation by a Ca2+-calmodulin-dependent myosin light chain kinase on the conformational, enzymatic and filament assembly properties of this myosin were investigated. The actin-activated MgATPase activity of the non-phosphorylated myosin was low, and upon light chain phosphorylation an eight- to ninefold increase in this activity was observed, which was further potentiated by tropomyosin. Light chain phosphorylation was shown to control the assembly and disassembly of brush border myosin filaments. For example, turbidity measurements and electron microscopy demonstrated that MgATP disassembled non-phosphorylated myosin filaments; the disassembled myosin could reassemble when the light chains were phosphorylated, and could be disassembled again by dephosphorylating the light chains with phosphatase. In the electron microscope, the disassembled non-phosphorylated myosin molecules appeared in a folded conformation, and they were extended when phosphorylated. Proteolytic digestion was used to probe further the conformation of these folded and extended molecules, and their subunit organizations were characterized by a gel overlay technique. Quantitative analysis further demonstrated that light chain phosphorylation alters dramatically the monomer/polymer equilibrium of brush border myosin, shifting it towards filament formation. Comparison of analogous data for myosin from gizzard and thymus shows that each myosin has distinct solubility properties.     

Regulation of cytoskeletal mechanics and cell growth by myosin light chain phosphorylation

The role ofmyosin light chain phosphorylation in regulating the mechanicalproperties of the cytoskeleton was studied in NIH/3T3 fibroblastsexpressing a truncated, constitutively active form of smooth musclemyosin light chain kinase (tMK). Cytoskeletal stiffness determined byquantifying the force required to indent the apical surface of adherentcells showed that stiffness was increased twofold in tMK cells comparedwith control cells expressing the empty plasmid (Neo cells).Cytoskeletal stiffness quantified using magnetic twisting cytometryshowed an ~1.5-fold increase in stiffness in tMK cells compared withNeo cells. Electronic volume measurements on cells in suspensionrevealed that tMK cells had a smaller volume and are more resistant toosmotic swelling than Neo cells. tMK cells also have smaller nuclei,and activation of mitogen-activated protein kinase (MAP kinase) andtranslocation of MAP kinase to the nucleus are slower in tMK cells thanin control cells. In tMK cells, there is also lessbromodeoxyuridine incorporation, and the doubling time isincreased. These data demonstrate that increased myosin light chainphosphorylation correlates with increased cytoskeletal stiffness andsuggest that changing the mechanical characteristics of thecytoskeleton alters the intracellular signaling pathways that regulatecell growth and division.

     

Phosphorylated and dephosphorylated myosin light chains of Drosophila fly and larva

1. The present study confirmed that light chains of Drosophila adult fibrillar (flight) muscle myosin consist of Lf1, Lf2, Lf2' and Lf3, and tubular muscle myosin light chains contain Lt1, Lt2, Lt2' and Lt3, as revealed by two-dimensional (isoelectric focusing and SDS-gel electrophoresis) gel electrophoresis. 2. Larva myosin light chains were of all the tubular type. However, it was found that Lt1 and Lt2' are produced by phosphorylation of Lt2, and Lf1 is produced by phosphorylation of Lf2'. 3. Injection of radioactive phosphate into Drosophila fly resulted in phosphorylations of Lf1 and Lt1. When larva or late pupa myosin was incubated with myosin light chain kinase from chicken gizzard or adult flies, phosphorylation of Lt1, Lf2' and Lt2' occurred. Drosophila myosin light chain kinase phosphorylated Lf1 in addition to Lt1 and L2' (Lf2' + Lt2') of adult myosin. 4. Dephosphorylation of adult myosin by potato acid and calf intestine alkaline phosphatases led to the shift of Lf1 (34,000), Lt1 (31,000) and L2' (Lf2' + Lt2') (30,000) to L2 (Lf2 + Lt2) positions (30,000). 5. Peptide mapping analyses revealed that larva Lt1, Lt2', Lt2 and adult Lt1 were all the same; therefore, it is thought that a single species of Lt2 specific to the tubular type of myosin and its phosphorylated isoforms (Lt1, Lt2') exist. 6. The peptide map of Lf1 was slightly different from that of Lt1, but very similar to that of L2' in adult myosin. L2 and L2' of adult myosin showed very similar peptide maps, but there were several different peptide fragments.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

肌球蛋白轻链激酶CaM结合位点突变体对肌球蛋白ATP酶活性的影响

目的:探讨肌球蛋白轻链激酶(MLCK)钙调蛋白(CaM)结合位点突变体对肌球蛋白ATP酶活性的影响.方法:构建牛胃重组全长野生型MLCK CaM结合位点突变型蛋白(△CaM/MLCK);孔雀绿方法检测△CaM/MLCK对肌球蛋白的Mg2+-ATP酶活性的影响.结果:在无Ca2+/CaM存在时,随着△△CaM/MLCK浓度的增加,非磷酸化肌球蛋白的Mg2+-ATP酶活性明显增加;而磷酸化肌球蛋白的Mg2+-ATP酶活性明显降低.结论:△CaM/MLCK对肌球蛋白Mg2+-ATP酶活性的影响表明MLCK具有非激酶活性.     

Phosphorylation of myosin light chain and the actin-activated ATPase activity of adrenal medullary myosin

Myosin light chain kinase was partially purified from bovine adrenal medulla. A polypeptide of Mr 165,000 dalton was identified as kinase by using anti-gizzard myosin light chain kinase IgG on immunoreplica. Phosphorylation of medullary myosin was Ca2+- and calmodulin-dependent. The phosphorylated myosin was showed to enhance the actin-activated Mg2+-ATPase activity. In contrast, the myosin ATPase activity was dramatically decreased by dephosphorylation of myosin.     

Effects of relaxin on rat uterine myosin light chain kinase activity and myosin light chain phosphorylation

Isometrically suspended uteri from estrogen-primed rats were stimulated with prostaglandin F2 alpha and then exposed to relaxin. Relaxin-dependent decreases in the ratio of phosphorylated to total myosin light chains (MLC) and in MLC kinase activity, measured in the presence of 0.5 mg/ml of uterine myosin and the absence and presence of Ca2+-calmodulin (CaM), were observed. The time-course and concentration-response of these biochemical effects of relaxin paralleled the hormone-induced inhibition of uterine contractile activity. Relaxin treatment resulted in a change in the requirements of MLC kinase for Ca2+, CaM, and myosin. Titrations of MLC kinase activity showed a shift in K50 values for Ca2+ from 82 to 260 nM and for CaM from 2.2 to 25 nM in extracts from control and relaxin-treated tissues, respectively. The myosin Km values of MLC kinase from control and relaxin-treated tissues were 0.33 and 0.71 mg/ml, respectively. Under optimal assay conditions (100 microM Ca2+, 1 microM CaM, and 1.2 mg/ml of myosin) the activities of MLC kinase in both extracts were identical, regardless of hormone concentration or exposure time. These data suggest that relaxin-treatment results in a change in the affinity of MLC kinase for its substrate and modulator and that relaxin inhibits uterine contractile activity by a mechanism which involves a decrease in MLC kinase activity and, in turn, a decrease in phosphorylation of the 20,000-dalton light chains of myosin.     

Regulation of the actin-activated ATPase activity of Acanthamoeba myosin I by cross-linking actin filaments

The actin-activated Mg2+-ATPase activity of phosphorylated Acanthamoeba myosin I was previously shown to be cooperatively dependent on the myosin concentration (Albanesi, J. P., Fujisaki, H., and Korn, E. D. (1985) J. Biol. Chem. 260, 11174-11179). This observation was rationalized by assuming that myosin I contains a high-affinity and a low-affinity F-actin-binding site and that binding at the low-affinity site is responsible for the actin-activated ATPase activity. Therefore, enzymatic activity would correlate with the cross-linking of actin filaments by myosin I, and the cooperative increase in specific activity at high myosin:actin ratios would result from the fact that cross-linking by one myosin molecule would increase the effective F-actin concentration for neighboring myosin molecules. This model predicts that high specific activity should occur at myosin:actin ratios below that required for cooperative interactions if the actin filaments are cross-linked by catalytically inert cross-linking proteins. This prediction has been confirmed by cross-linking actin filaments with either of three gelation factors isolated from Acanthamoeba, one of which has not been previously described, or by enzymatically inactive unphosphorylated Acanthamoeba myosin I.     

Regulation of myosin filament assembly by light-chain phosphorylation

Myosins isolated from vertebrate smooth muscles and non-muscle cells such as lymphocytes and platelets contain regulatory light chains (Mr = 20000), which are phosphorylated by a Ca2+-calmodulin-dependent kinase and dephosphorylated by a Ca2+-insensitive phosphatase. Phosphorylation of the regulatory light chains of these myosins in vitro regulates not only their interactions with actin but also their assembly into filaments. Under approximately physiological conditions (0.15 M NaCl, pH 7.0) stoichiometric levels of Mg-ATP disassemble these non-phosphorylated myosin filaments into species with sedimentation coefficients (So20,w) of approximately 11S. Hydrodynamic and electron microscope observations have indicated that this 11S species is a monomer with a folded conformation (Trybus et al., Proc. natn. Acad. Sci. U.S.A. 79, 6151 (1982)). Rotary shadowing reveals that the tails of disassembled gizzard and thymus myosins are folded twice at two hinge points to form a folded three-segment structure. Phosphorylation of the regulatory light chains of these myosins causes these folded 11S molecules to unfold into the conventional extended monomeric form (6S), which is able to assemble into filaments. Thus in vitro these myosin filaments can be assembled or disassembled by phosphorylation or dephosphorylation of their light chains. Whether these results have any relevance to the situation within living non-muscle and smooth muscle cells remains to be established.     

Role of myosin II activity and the regulation of myosin light chain phosphorylation in astrocytomas

The generation of contractile force mediated by actin-myosin interactions is essential for cell motility. Myosin activity is promoted by phosphorylation of myosin light chain (MLC). MLC phosphorylation in large part is controlled by kinases that are effectors of Rho family GTPases. Accordingly, in this study we examined the effects of ROCK and Rac1 inhibition on MLC phosphorylation in astrocytoma cells. We found that low concentrations of the ROCK inhibitor Y27632 increased the phosphorylation state of the Triton X-100 soluble fraction of MLC, whereas higher concentrations of Y27632 decreased soluble phospho-MLC. These effects of Y27632 were dependent on Rac1. The soluble form of phospho-MLC comprises about 10% of total phospho-MLC in control cells. Interestingly, ROCK inhibition led to a decrease in the phosphorylation state of total MLC, whereas Rac1 inhibition had little effect. Thus, the soluble form of MLC is differentially regulated by ROCK and Rac1 compared with MLC examined in a total cell extract. We also observed that astrocytoma migration is stimulated by low concentrations of the myosin II inhibitor blebbistatin. However, higher concentrations of blebbistatin inhibit migration leading us to believe that migration has a biphasic dependence on myosin II activity. Taken together, our data show that modulation of myosin II activity is important in determining optimal astrocytoma migration. In addition, these findings suggest that there are at least two populations of MLC that are differentially regulated.     

Dynamic regulation of myosin light chain phosphorylation by Rho-kinase

Myosin light chain (MLC) phosphorylation plays important roles in various cellular functions such as cellular morphogenesis, motility, and smooth muscle contraction. MLC phosphorylation is determined by the balance between activities of Rho-associated kinase (Rho-kinase) and myosin phosphatase. An impaired balance between Rho-kinase and myosin phosphatase activities induces the abnormal sustained phosphorylation of MLC, which contributes to the pathogenesis of certain vascular diseases, such as vasospasm and hypertension. However, the dynamic principle of the system underlying the regulation of MLC phosphorylation remains to be clarified. Here, to elucidate this dynamic principle whereby Rho-kinase regulates MLC phosphorylation, we developed a mathematical model based on the behavior of thrombin-dependent MLC phosphorylation, which is regulated by the Rho-kinase signaling network. Through analyzing our mathematical model, we predict that MLC phosphorylation and myosin phosphatase activity exhibit bistability, and that a novel signaling pathway leading to the auto-activation of myosin phosphatase is required for the regulatory system of MLC phosphorylation. In addition, on the basis of experimental data, we propose that the auto-activation pathway of myosin phosphatase occurs in vivo. These results indicate that bistability of myosin phosphatase activity is responsible for the bistability of MLC phosphorylation, and the sustained phosphorylation of MLC is attributed to this feature of bistability.     

Regulation of myosin II dynamics by phosphorylation and dephosphorylation of its light chain in epithelial cells

Nonmuscle myosin II, an actin-based motor protein, plays an essential role in actin cytoskeleton organization and cellular motility. Although phosphorylation of its regulatory light chain (MRLC) is known to be involved in myosin II filament assembly and motor activity in vitro, it remains unclear exactly how MRLC phosphorylation regulates myosin II dynamics in vivo. We established clones of Madin Darby canine kidney II epithelial cells expressing MRLC-enhanced green fluorescent protein or its mutants. Time-lapse imaging revealed that both phosphorylation and dephosphorylation are required for proper dynamics of myosin II. Inhibitors affecting myosin phosphorylation and MRLC mutants indicated that monophosphorylation of MRLC is required and sufficient for maintenance of stress fibers. Diphosphorylated MRLC stabilized myosin II filaments and was distributed locally in regions of stress fibers where contraction occurs, suggesting that diphosphorylation is involved in the spatial regulation of myosin II assembly and contraction. We further found that myosin phosphatase or Zipper-interacting protein kinase localizes to stress fibers depending on the activity of myosin II ATPase.