Myosin light-chain phosphatase

Membrane Tension Orchestrates Rear Retraction in Matrix-Directed Cell Migration

In development, wound healing, and cancer metastasis, vertebrate cells move through 3D interstitial matrix, responding to chemical and physical guidance cues. Protrusion at the cell front has been extensively studied, but the retraction phase of the migration cycle is not well understood. Here, we show that fast-moving cells guided by matrix cues establish positive feedback control of rear retraction by sensing membrane tension. We reveal a mechanism of rear retraction in 3D matrix and durotaxis controlled by caveolae, which form in response to low membrane tension at the cell rear. Caveolae activate RhoA-ROCK1/PKN2 signaling via the RhoA guanidine nucleotide exchange factor (GEF) Ect2 to control local F-actin organization and contractility in this subcellular region and promote translocation of the cell rear. A positive feedback loop between cytoskeletal signaling and membrane tension leads to rapid retraction to complete the migration cycle in fast-moving cells, providing directional memory to drive persistent cell migration in complex matrices.

Myosin phosphorylation improves contractile economy of mouse fast skeletal muscle during staircase potentiation

Phosphorylation of the myosin regulatory light chain (RLC) by skeletal myosin light chain kinase (skMLCK) potentiates rodent fast twitch muscle but is an ATP-requiring process. Our objective was to investigate the effect of skMLCK-catalyzed RLC phosphorylation on the energetic cost of contraction and the contractile economy (ratio of mechanical output to metabolic input) of mouse fast twitch muscle in vitro (25°C). To this end, extensor digitorum longus (EDL) muscles from wild-type (WT) and from skMLCK-devoid (skMLCK^-/-) mice were subjected to repetitive low-frequency stimulation (10 Hz for 15 s) to produce staircase potentiation of isometric twitch force, after which muscles were quick frozen for determination of high-energy phosphate consumption (HEPC). During stimulation, WT muscles displayed significant potentiation of isometric twitch force while skMLCK^-/- muscles did not (i.e. 23% versus 5% change, respectively). Consistent with this, RLC phosphorylation was increased ∼3.5-fold from the unstimulated control value in WT but not in skMLCK^-/- muscles. Despite these differences, the HEPC of WT muscles was not greater than that of skMLCK^-/- muscles. As a result of the increased contractile output relative to HEPC, the calculated contractile economy of WT muscles was greater than that of skMLCK^-/- muscles. Thus, our results suggest that skMLCK-catalyzed phosphorylation of the myosin RLC increases the contractile economy of WT mouse EDL muscle compared with skMLCK^-/- muscles without RLC phosphorylation.

Drosophila non-muscle myosin II motor activity determines the rate of tissue folding

Non-muscle cell contractility is critical for tissues to adopt shape changes. Although, the non-muscle myosin II holoenzyme (myosin) is a molecular motor that powers contraction of actin cytoskeleton networks, recent studies have questioned the importance of myosin motor activity cell and tissue shape changes. Here, combining the biochemical analysis of enzymatic and motile properties for purified myosin mutants with in vivo measurements of apical constriction for the same mutants, we show that in vivo constriction rate scales with myosin motor activity. We show that so-called phosphomimetic mutants of the Drosophila regulatory light chain (RLC) do not mimic the phosphorylated RLC state in vitro. The defect in the myosin motor activity in these mutants is evident in developing Drosophila embryos where tissue recoil following laser ablation is decreased compared to wild-type tissue. Overall, our data highlights that myosin activity is required for rapid cell contraction and tissue folding in developing Drosophila embryos.

DOI:http://dx.doi.org/10.7554/eLife.20828.001

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55's action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55's attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55. DOI: http://dx.doi.org/10.7554/eLife.01695.001.

The effect of skeletal myosin light chain kinase gene ablation on the fatigability of mouse fast muscle

Contraction-induced activation of a skeletal muscle specific Ca(2+) and calmodulin dependent myosin light chain kinase (skMLCK) catalyzes phosphorylation of the myosin regulatory light chain (RLC), a reaction that potentiates twitch force. The purpose of this study was to test the effect of skMLCK gene ablation on the fatigability of mouse extensor digitorum longus (EDL) muscle (in vitro at 25°C). Muscles were isolated from wildtype (WT, n = 10-12) and skeletal MLCK knockout (skMLCK KO, n = 10-12) mice and fatigued using a protocol consisting of 5 min of repeated tetanic stimulation (150 Hz for 1000 ms every 5 s). Both twitch (P(t)) and tetanic (P(o)) force as well as unloaded shortening velocity (V(o)) were assessed before, during and after fatiguing stimulation. Fatiguing stimulation increased RLC phosphorylation in WT but not skMLCK KO muscles (16 ± 0.01-0.63 ± 0.02 and 0.07 ± 0.02-0.08 ± 0.02 mol phos mol RLC, respectively). Although P(t) was potentiated above baseline in both WT and KO muscles, this increase was greater in WT than in KO muscles (to 1.37 ± 0.05 vs. 1.14 ± 0.02 of unpotentiated values, respectively). The difference in P(t) persisted until P(o) had been diminished to ~60% of baseline and thereafter P(t) declined to similar levels in both WT and KO muscles (to ~35% of initial). Overall, the time-course and decline in P(o) for WT and KO was similar (reduced to 0.20 ± 0.01 and 0.20 ± 0.01 of baseline, respectively) (P < 0.05). Initial values for V(o) were similar between WT and KO muscles and, moreover, the fatigue related decline in Vo was similar for both muscle genotypes (P < 0.05). Thus, our results demonstrate that skMLCK--catalyzed RLC phosphorylation augments isometric twitch force during moderate, but not severe, levels of fatigue.

Myosin regulatory light chain phosphorylation inhibits shortening velocities of skeletal muscle fibers in the presence of the myosin inhibitor blebbistatin

Phosphorylation of skeletal myosin regulatory light chain (RLC) occurs in fatigue and may play a role in the inhibition of shortening velocities observed in vivo. Forces and shortening velocities were measured in permeabilized rabbit psoas fibers with either phosphorylated or dephosphorylated RLCs and in the presence or absence of the myosin inhibitor blebbistatin. Addition of 20 microM blebbistatin decreased tensions by approximately 80% in fibers, independent of phosphorylation. In blebbistatin maximal shortening velocities (V(max)) at 30 degrees C, were decreased by 45% (3.2 +/- 0.34 vs. 5.8 +/- 0.18 lengths/s) in phosphorylated fibers but were not inhibited in dephosphorylated fibers (6.0 +/- 0.30 vs. 5.4 +/- 0.30). In the presence of 20 microM blebbistatin, K(m) for V(max) as a function of [ATP] was lower for phosphorylated fibers than for dephosphorylated fibers (50 +/- 20 vs. 330 +/- 84 microM) indicating that the apparent binding of ATP is stronger in these fibers. Phosphorylation of RLC in situ during fiber preparation or by addition of myosin light chain kinase yielded similar data. RLC phosphorylation inhibited velocity in blebbistatin at both 30 and 10 degrees C, unlike previous reports where RLC phosphorylation only affected shortening velocities at higher temperatures.

Asymmetric cell division in C. elegans: cortical polarity and spindle positioning

The one-cell Caenorhabditis elegans embryo divides asymmetrically into a larger and smaller blastomere, each with a different fate. How does such asymmetry arise? The sperm-supplied centrosome establishes an axis of polarity in the embryo that is transduced into the establishment of anterior and posterior cortical domains. These cortical domains define the polarity of the embryo, acting upstream of the PAR proteins. The PAR proteins, in turn, determine the subsequent segregation of fate determinants and the plane of cell division. We address how cortical asymmetry could be established, relying on data from C. elegans and other polarized cells, as well as from applicable models. We discuss how cortical polarity influences spindle position to accomplish an asymmetric division, presenting the current models of spindle orientation and anaphase spindle displacement. We focus on asymmetric cell division as a function of the actin and microtubule cytoskeletons, emphasizing the cell biology of polarity.

Phosphorylation of the regulatory light chains of myosin affects Ca2+ sensitivity of skeletal muscle contraction

The role of phosphorylation of the myosin regulatory light chains (RLC) is well established in smooth muscle contraction, but in striated (skeletal and cardiac) muscle its role is still controversial. We have studied the effects of RLC phosphorylation in reconstituted myosin and in skinned skeletal muscle fibers where Ca2+ sensitivity and the kinetics of steady-state force development were measured. Skeletal muscle myosin reconstituted with phosphorylated RLC produced a much higher Ca2+ sensitivity of thin filament-regulated ATPase activity than nonphosphorylated RLC (change in -log of the Ca2+ concentration producing half-maximal activation = approximately 0.25). The same was true for the Ca2+ sensitivity of force in skinned skeletal muscle fibers, which increased on reconstitution of the fibers with the phosphorylated RLC. In addition, we have shown that the level of endogenous RLC phosphorylation is a crucial determinant of the Ca2+ sensitivity of force development. Studies of the effects of RLC phosphorylation on the kinetics of force activation with the caged Ca2+, DM-nitrophen, showed a slight increase in the rates of force development with low statistical significance. However, an increase from 69 to 84% of the initial steady-state force was observed when nonphosphorylated RLC-reconstituted fibers were subsequently phosphorylated with exogenous myosin light chain kinase. In conclusion, our results suggest that, although Ca2+ binding to the troponin-tropomyosin complex is the primary regulator of skeletal muscle contraction, RLC play an important modulatory role in this process.

Vanadate oligomer interactions with myosin

'Monovanadate' containing a mixture of at least four different vanadate species and 'decavanadate' containing apparently only two vanadate species, mainly decameric species, inhibit myosin and actomyosin ATPase activities. The addition of myosin to 'monovanadate' and 'decavanadate' solutions promotes differential increases on the 51V NMR spectral linewidths of vanadate oligomers. The relative order of line broadening upon myosin addition, reflecting the interaction of the vanadate oligomers with the protein, was V10 > V4 > V1 = 1, whereas no changes were observed for monomeric vanadate species. It is concluded that decameric and tetrameric vanadate species interact quite potently with the protein and affect myosin as well actomyosin ATPase activities.

Myosin light chain kinase from skeletal muscle regulates an ATP-dependent interaction between actin and myosin by binding to actin

Myosin light chain kinase (MLCK) has been purified from various muscles as an enzyme to phosphorylate myosin light chains. While the regulatory role of smooth muscle MLCK is well understood, the role of skeletal muscle MLCK in the regulation of contraction has not been fully characterized. Such characterization of skeletal muscle MLCK is difficult because skeletal muscle myosin interacts with actin whether or not the myosin is phosphorylated. Taking the hint from our recent finding that smooth muscle MLCK inhibits the actin-myosin interaction by binding to actin (Kohama et al., Biochem Biophys Res Commun 184: 1204-1211, 1992), we investigated the regulatory role of the actin-binding activity of MLCK from chicken breast muscle in the actin-myosin interaction. The amount of MLCK that bound to actin increased with increases in the concentration of MLCK. However, MLCK hardly bound to myosin. The actin-binding activity of MLCK was affected when Ca2+ and calmodulin (Ca2+ -CaM) were present. The effect of MLCK on the actin-myosin interaction was examined by an in vitro motility assay; the movement of actin-filaments on a myosin-coated glass surface was inhibited by increasing the concentration of MLCK. When CaM was present, the inhibition was overcome in a Ca2+ -dependent manner at microM levels. The inhibition of the movement by MLCK and the recovery from the inhibition by Ca2+ -CaM were not altered whether we use phosphorylated or unphosphorylated myosin for the assay, ruling out the involvement of the kinase activity of MLCK.

Myosin light-chain phosphorylation in diabetic cardiomyopathy in rats

The regulatory myosin light chain (MLC) is phosphorylated in cardiac muscle by Ca2+/calmodulin-dependent MLC kinase (MLCK) and is considered to play a modulatory role in the activation of myofibrillar adenosine triphosphatase (ATPase) and the process of force generation. Since the depression in cardiac contractile function in chronic diabetes is associated with a decrease in myofibrillar ATPase activity, we investigated changes in MLC phosphorylation in diabetic heart. Rats were made diabetic by injecting streptozotocin (65 mg/kg intravenously), and the hearts were removed 8 weeks later; some 6-week diabetic animals were injected with insulin (3 U/d) for 2 weeks. Changes in the relative MLC and MLCK protein contents were measured by electrophoresis and immunoblot assay, whereas phosphorylated and unphosphorylated MLCs were separated on 10% acrylamide/urea gel and identified by Western blot. MLC and MLCK contents were decreased markedly (40% to 45%) and MLC phosphorylation was decreased significantly (30% to 45%) in the diabetic rat heart homogenate in comparison to control values. The changes in MLC and MLCK content in diabetic heart were partially reversible, whereas changes in MLC phosphorylation were normalized upon treatment with insulin. These results suggest that decreased protein contents of MLC and MLCK and phosphorylation of MLC may contribute to the depression of cardiac myofibriliar ATPase activity and heart dysfunction in diabetic cardiomyopathy.

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

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

A Mg(2+)-dependent, Ca(2+)-inhibitable serine/threonine protein phosphatase from bovine brain

The Mg(2+)-dependent serine/threonine protein phosphatases, also known as type 2C phosphatases (PP2C), belong to a gene family distinct from the other serine/threonine phosphatases and tyrosine phosphatases. Here we report the purification to apparent homogeneity of a novel Mg(2+)-dependent, Ca(2+)-inhibitable serine/threonine protein phosphatase from bovine brain. It is a type 2C enzyme in view of its Mg2+ requirement, resistance to okadaic acid and calyculin A, inability to use phosphorylase alpha as substrate, and a segment of amino acid sequence typical of all PP2C type phosphatases known to date. However, it differs from the other PP2C enzymes, particularly the mammalian PP2C alpha and -beta isoforms, in that its molecular weight, 76,000, is considerably larger and that it is inhibited by Ca2+, NaF, and polycations, but not by orthovanadate. The Ca2+ inhibition may not be related to its cellular regulation because of Ki values in the 20-90 microM range, but this property permits distinction of this enzyme from the other phosphatases. Although the precise physiological role of this phosphatase is not yet known, its ability to dephosphorylate a wide variety of phosphoproteins and its broad distribution, as shown by a survey of mouse tissues for its activity, suggest that it may serve an important cellular function.

Skeletal muscle myosin regulatory light chains conformation affects the papain cleavage of A1 light chains

In the present study, the influence of magnesium-for-calcium exchange and phosphorylation of regulatory light chain (RLC) on accessibility of myosin and heavy meromyosin alkali light chains (A1) for papain digestion was investigated. The properties of native and papain treated myosin and heavy meromyosin were compared. Exchange of magnesium ions bound to RLCs for calcium ions accelerates the digestion of A1 in the presence of ATP in dephosphorylated myosin, heavy meromyosin, acto-myosin and the acto-heavy meromyosin complex. In the absence of ATP the exchange of magnesium ions bound to RLCs for calcium ions delays the digestion of A1 in the acto-myosin complex. Myosin and heavy meromyosin having shortened A1 by papain cleavage shows decreased K(+)-ATPase and increased actin binding ability in the presence and absence of ATP. The cooperation of RLC and A1 with heavy chains in the changes of structural organization of myosin head during muscle contraction is discussed.

A hypothalamic activator of calmodulin-dependent enzymes is thymosin beta 4 (1-39)

A new class of stimulators of basal activity of a number of calmodulin-dependent enzymes have been previously isolated from bovine hypothalamus. One of these stimulators, denoted as C3, has been purified to homogeneity by reverse phase HPLC and tentatively identified as thymosin beta 4 (1-39) by mass spectrometry and Edman microsequence analysis. The stimulating effect of C3 on rabbit skeletal muscle MLCK basal activity was compared with that of thymosin alpha 1 and thymosin beta 4 (16-38). Evidence is presented that all the indicated compounds are Ca(2+)-independent high-affinity MLCK stimulators. The potency of the stimulators in activating the enzyme was: C3 greater than beta 4 greater than (CaM+Ca2+) greater than alpha 1.

X-ray diffraction study of the structural changes accompanying phosphorylation of tarantula muscle

Electron microscopy of negatively stained isolated thick filaments of tarantula muscle has revealed that phosphorylation of myosin regulatory light chains is accompanied by a loss of the helical order of myosin heads. From equatorial X-ray diffraction patterns of tarantula muscles in the phosphorylated state we have detected a mass movement in the myosin filaments that supports this finding.

Chimaeric myosin regulatory light chains: sub-domain switching experiments to analyse the function of the N-terminal EF hand

The regulatory light chains (RLCs) located on the myosin head, regulate the interaction of myosin with actin in response to either Ca2+ or phosphorylation signals. The RLCs belong to a family of calcium binding proteins and are composed of four "EF hand" ancestral calcium binding motifs (numbered I to IV). To determine the role of the first EF hand (EF hand I) in the regulatory process, chimaeric light chains were constructed by protein engineering, by switching this region between smooth muscle and skeletal muscle myosin RLCs. For example, chimaera G(I)S consisted of EF hand I of the smooth muscle (gizzard) RLC and EF hands II to IV of the skeletal muscle RLC, whereas chimaera S(I)G consisted of EF hand I of the skeletal muscle RLC and EF hands II to IV of the smooth muscle RLC. The chimaeric RLCs were expressed in Escherichia coli using the pLcII expression system, and after isolation and purification their regulatory properties were compared with those of wild-type smooth and skeletal muscle myosin RLCs. The chimaeric RLCs bound to the myosin heads in scallop striated muscle myofibrils from which the endogenous RLCs had been removed ("desensitized" myofibrils) with similar affinities to those of the wild-type smooth and skeletal muscle RLCs. Both chimaeric RLCs were able to regulate the actin-activated Mg(2+)-ATPase activity of scallop myosin: G(I)S inhibited the ATPase in the presence and absence of Ca2+, like the wild-type skeletal muscle RLC, while S(I)G inhibited the myosin ATPase in the absence of Ca2+, and this inhibition was relieved on Ca2+ addition, in the same way as the wild-type smooth muscle RLC. Thus the type of regulation that the RLCs confer on the myosin is determined by the source of EF hands II to IV rather than that of EF hand I.

Formation and contraction of a microfilamentous shell in saponin-permeabilized platelets

To study the mechanism of granule centralization in platelets, we permeabilized with saponin in either EGTA (5 mM) or calcium (1 or 10 microM). Under all conditions, platelets retained 40-50% of their total actin and greater than 70% of their actin-binding protein (ABP) but lost greater than 80% of talin and myosin to the supernatant. Thin sections of platelets permeabilized in EGTA showed a microfilament network under the residual plasma membrane and throughout the cytoplasm. Platelets permeabilized in calcium contained a microfilament shell partly separated from the residual membrane. The shell stained brightly for F-actin. A less dense microfilament shell was also seen in sections of ADP-stimulated intact platelets subsequently permeabilized in EGTA. In the presence of 1 mM ATP gamma S and calcium, myosin was retained (70%) and was localized by indirect immunofluorescence in bright central spots that also stained intensely for F-actin. Electron micrographs showed centralized granules surrounded by a closely packed mass of microfilaments much like the structures seen in thrombin-stimulated intact platelets subsequently permeabilized in EGTA. Permeabilization in calcium, ATP, and okadaic acid, produced the same configuration of centralized granules and packed microfilaments; myosin was retained and the myosin regulatory light chain became phosphorylated. Microtubule coil disassembly before permeabilization did not inhibit granule centralization. These results suggest a possible mechanism for granule centralization in these models. The cytoskeletal network first separates from some of its connections to the plasma membrane by a calcium-dependent mechanism not involving ABP proteolysis. Phosphorylated myosin interacts with the microfilaments to contract the shell moving the granules to the platelet's center.

The effect of myosin light chain phosphorylation and Mg2+ on the conformation of myosin in thick filaments of glycerinated fibers of rabbit skeletal muscle

It has been shown by polarization microfluorimetry that phosphorylation of myosin light chain 2, in stretched single glycerinated fibers of rabbit skeletal muscle, results in changes in polarized fluorescence anisotropy of both the tryptophan residues of myosin molecules and the fluorescent label, N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine, associated with the fast-reacting thiol group in myosin heads. These changes are also dependent on the presence or absence of Mg2+ in the medium: they are most pronounced in the presence of 5 mM MgCl2. It is assumed that both Mg2+ binding to myosin and phosphorylation of light chain 2 associated with myosin heads induce structural changes in myosin filaments of muscle fibres which are expressed as changes in the orientation of myosin heads and in the conformation of myosin rods.

Variations in cross-bridge attachment rate and tension with phosphorylation of myosin in mammalian skinned skeletal muscle fibers. Implications for twitch potentiation in intact muscle

The Ca2+ sensitivities of the rate constant of tension redevelopment (ktr; Brenner, B., and E. Eisenberg. 1986. Proceedings of the National Academy of Sciences. 83:3542-3546) and isometric force during steady-state activation were examined as functions of myosin light chain 2 (LC2) phosphorylation in skinned single fibers from rabbit and rat fast-twitch skeletal muscles. To measure ktr the fiber was activated with Ca2+ and steady isometric tension was allowed to develop; subsequently, the fiber was rapidly (less than 1 ms) released to a shorter length and then reextended by approximately 200 nm per half sarcomere. This maneuver resulted in the complete dissociation of cross-bridges from actin, so that the subsequent redevelopment of tension was related to the rate of cross-bridge reattachment. The time course of tension redevelopment, which was recorded under sarcomere length control, was best fit by a first-order exponential equation (i.e., tension = C(1 - e-kt) to obtain the value of ktr. In control fibers, ktr increased sigmoidally with increases in [Ca2+]; maximum values of ktr were obtained at pCa 4.5 and were significantly greater in rat superficial vastus lateralis fibers (26.1 +/- 1.2 s-1 at 15 degrees C) than in rabbit psoas fibers (18.7 +/- 1.0 s-1). Phosphorylation of LC2 was accomplished by repeated Ca2+ activations (pCa 4.5) of the fibers in solutions containing 6 microM calmodulin and 0.5 microM myosin light chain kinase, a protocol that resulted in an increase in LC2 phosphorylation from approximately 10% in the control fibers to greater than 80% after treatment. After phosphorylation, ktr was unchanged at maximum or very low levels of Ca2+ activation. However, at intermediate levels of Ca2+ activation, between pCa 5.5 and 6.2, there was a significant increase in ktr such that this portion of the ktr-pCa relationship was shifted to the left. The steady-state isometric tension-pCa relationship, which in control fibers was left shifted with respect to the ktr-pCa relationship, was further left-shifted after LC2 phosphorylation. Phosphorylation of LC2 had no effect upon steady-state tension during maximum Ca2+ activation. In fibers from which troponin C was partially extracted to disrupt molecular cooperativity within the thin filament (Moss et al. 1985. Journal of General Physiology. 86:585-600), the effect of LC2 phosphorylation to increase the Ca2+ sensitivity of steady-state isometric force was no longer evident, although the effect of phosphorylation to increase ktr was unaffected by this maneuver.(ABSTRACT TRUNCATED AT 400 WORDS)

Specific dephosphorylation by phosphatases 1 and 2A of a nuclear protein structurally and immunologically related to nucleolin. Possible influence on the regulation of rRNA synthesis

A new nuclear substrate (N-60) for phosphatase 1 and 2Ac has been described. In contrast to nucleolin (C23), to which it is structurally and immunologically related, N-60 becomes dephosphorylated to 51% and 41% by phosphatases 1 and 2Ac, respectively, within 10 min. Incubation up to 20 min led to a complete dephosphorylation of N-60. The two other phosphatases tested (2B and 2C) did not dephosphorylate protein N-60 to the same extent as phosphatases 1 and 2Ac. In the case of nucleolin only 18% phosphate was released by all four phosphatases tested. The activity of both phosphatases, 1 and 2A, could be blocked by tumour promoter okadaic acid (100 nM) when N-60 was used as a substrate. These results support the notion that the observed okadaic-acid-induced hyperphosphorylation of N-60 in intact human fibroblasts may be caused by specific inhibition of phosphatases involved in the process of rDNA transcription.

Inhibitory effect of a marine-sponge toxin, okadaic acid, on protein phosphatases. Specificity and kinetics

The inhibitory effect of a marine-sponge toxin, okadaic acid, was examined on type 1, type 2A, type 2B and type 2C protein phosphatases as well as on a polycation-modulated (PCM) phosphatase. Of the protein phosphatases examined, the catalytic subunit of type 2A phosphatase from rabbit skeletal muscle was most potently inhibited. For the phosphorylated myosin light-chain (PMLC) phosphatase activity of the enzyme, the concentration of okadaic acid required to obtain 50% inhibition (ID50) was about 1 nM. The PMLC phosphatase activities of type 1 and PCM phosphatase were also strongly inhibited (ID50 0.1-0.5 microM). The PMCL phosphatase activity of type 2B phosphatase (calcineurin) was inhibited to a lesser extent (ID50 4-5 microM). Similar results were obtained for the phosphorylase a phosphatase activity of type 1 and PCM phosphatases and for the p-nitrophenyl phosphate phosphatase activity of calcineurin. The following phosphatases were not affected by up to 10 microM-okadaic acid: type 2C phosphatase, phosphotyrosyl phosphatase, inositol 1,4,5-trisphosphate phosphatase, acid phosphatases and alkaline phosphatases. Thus okadaic acid had a relatively high specificity for type 2A, type 1 and PCM phosphatases. Kinetic studies showed that okadaic acid acts as a non-competitive or mixed inhibitor on the okadaic acid-sensitive enzymes.

Reduction of myosin-light-chain phosphorylation and of parvalbumin content in myotonic mouse muscle and its reversal by tocainide

In muscle of the myotonic mouse mutant, 'arrested development of righting response', ADR, a reduced level of fast-myosin-light-chain-2 (LC2f) phosphorylation was observed in addition to a lowered parvalbumin content. In fast muscles, average phosphorylation levels of LC2f (LC2-P/LC2 total) were 0.76 mol/mol for wild type and 0.59 mol/mol for the myotonic mutant. The difference was not due to short-term activity prior to freezing because it was also found in curare-paralyzed muscles. Long-term treatment of genetically myotonic animals with the membrane-stabilizing drug, tocainide, led to an increase of parvalbumin content and LC2-P level. In wild-type mice, tocainide had a similar effect, leading to supranormal parvalbumin concentrations. It is concluded that both the basal level of LC2-P and parvalbumin concentration are regulated by a common factor, related to long-term muscle activity.

Structural changes accompanying phosphorylation of tarantula muscle myosin filaments

Electron microscopy has been used to study the structural changes that occur in the myosin filaments of tarantula striated muscle when they are phosphorylated. Myosin filaments in muscle homogenates maintained in relaxing conditions (ATP, EGTA) are found to have nonphosphorylated regulatory light chains as shown by urea/glycerol gel electrophoresis and [32P]phosphate autoradiography. Negative staining reveals an ordered, helical arrangement of crossbridges in these filaments, in which the heads from axially neighboring myosin molecules appear to interact with each other. When the free Ca2+ concentration in a homogenate is raised to 10(-4) M, or when a Ca2+-insensitive myosin light chain kinase is added at low Ca2+ (10(-8) M), the regulatory light chains of myosin become rapidly phosphorylated. Phosphorylation is accompanied by potentiation of the actin activation of the myosin Mg-ATPase activity and by loss of order of the helical crossbridge arrangement characteristic of the relaxed filament. We suggest that in the relaxed state, when the regulatory light chains are not phosphorylated, the myosin heads are held down on the filament backbone by head-head interactions or by interactions of the heads with the filament backbone. Phosphorylation of the light chains may alter these interactions so that the crossbridges become more loosely associated with the filament backbone giving rise to the observed changes and facilitating crossbridge interaction with actin.

Proteolysis rates of a myosin heavy chain site with papain. Evidence for a combined LC2-filament-mediated mechanism

In striated muscle myosin, a proteolysis site at the 25-50 kDa junction, susceptible in the filament and efficiently protected by nucleotides, is similarly protected when myosin is monomeric. Kinetic studies at low ionic strength show a close relationship between LC2 cleavage or degradation rate and cleavage of the 25-50 kDa heavy chain site. The myosin-[(T)-LC2'] species forms normal reconstituted filaments but its 25-50 kDa site susceptibility is closer to that of monomeric myosin, thus becoming practically ionic strength-independent. In this species the absence of the LC2 N-terminal segment induces a significantly greater susceptibility of the papain-sensitive site in LC1. In an LC2-depleted myosin the 25-50 kDa site susceptibility also becomes ionic strength-independent, however, the cleavage rates are then closer to that of filaments. Susceptibility in HMM and S1 is also much less dependent on ionic strength with rates intermediary between those of filament and monomer. These observations show that the maximum susceptibility to papain of the 25-50 kDa site requires both the integrity of the LC2 light chain and the filament structure and furthermore provide evidence that: (i) the LC2 N-terminus interacts specifically with some part of the filament; (ii) this interaction induces a specific transconformation in a region close to the ATPase active site; (iii) there is an interrelationship between LC1 and LC2 light chain N-terminal extremities, at least in the filament structure.

Myosin light chain kinase and myosin light chain phosphatase from Dictyostelium: effects of reversible phosphorylation on myosin structure and function

We have partially purified myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) from Dictyostelium discoideum. MLCK was purified 4,700-fold with a yield of approximately 1 mg from 350 g of cells. The enzyme is very acidic as suggested by its tight binding to DEAE. Dictyostelium MLCK has an apparent native molecular mass on HPLC G3000SW of approximately 30,000 D. Mg2+ is required for enzyme activity. Ca2+ inhibits activity and this inhibition is not relieved by calmodulin. cAMP or cGMP have no effect on enzyme activity. Dictyostelium MLCK is very specific for the 18,000-D light chain of Dictyostelium myosin and does not phosphorylate the light chain of several other myosins tested. Myosin purified from log-phase amebas of Dictyostelium has approximately 0.3 mol Pi/mol 18,000-D light chain as assayed by glycerol-urea gel electrophoresis. Dictyostelium MLCK can phosphorylate this myosin to a stoichiometry approaching 1 mol Pi/mol 18,000-D light chain. MLCP, which was partially purified, selectively removes phosphate from the 18,000-D light chain but not from the heavy chain of Dictyostelium myosin. Phosphatase-treated Dictyostelium myosin has less than or equal to 0.01 mol Pi/mol 18,000-D light chain. Phosphatase-treated myosin could be rephosphorylated to greater than or equal to 0.96 mol Pi/mol 18,000-D light chain by incubation with MLCK and ATP. We found myosin thick filament assembly to be independent of the extent of 18,000-D light-chain phosphorylation when measured as a function of ionic strength. However, actin-activated Mg2+-ATPase activity of Dictyostelium myosin was found to be directly related to the extent of phosphorylation of the 18,000-D light chain. MLCK-treated myosin moved in an in vitro motility assay (Sheetz, M. P., and J. A. Spudich, 1983, Nature (Lond.), 305:31-35) at approximately 1.4 micron/s whereas phosphatase-treated myosin moved only slowly or not at all. The effects of phosphatase treatment on the movement were fully reversed by subsequent treatment with MLCK.

Binding of phosphorylated and dephosphorylated heavy meromyosin to F-actin

The effect of myosin light chain phosphorylation in skeletal muscle was investigated with respect to the binding affinity of phosphorylated and dephosphorylated heavy meromyosin (HMM) for F-actin in the absence of ATP. For phosphorylated HMM the affinity was 2.5-times weaker in the presence of Ca2+ as in its absence (HMM divalent binding sites saturated only with Mg). For dephosphorylated HMM the reverse was true, the binding being 2.4-times higher in the presence of Ca2+.

Myosin specific phosphatases isolated from Dictyostelium discoideum

Using native myosin phosphorylated on either the heavy chain or the light chain, we have isolated myosin phosphatases from extracts of vegetative Dictyostelium amoeba. Two phosphatases were resolved by DEAE-cellulose chromatography. One of these phosphatases removed phosphate from heavy chain or light chain at approximately the same rate. The other phosphatase appeared to be much more specific for phosphorylated myosin heavy chain. Although these enzymes removed phosphate from other phosphoprotein substrates such as histone or casein, they did so at a much lower rate. Both enzymes required magnesium for activity, but appeared to be independent of calcium.

A simple and rapid preparation of fully phosphorylated and fully dephosphorylated skeletal muscle myosin. Application to the preparation of a phosphorylated LC2-modified artificial isozyme

Fast skeletal myosin LC2 is phosphorylated on ser-15 by a specific myosin light chain kinase (MLCK) in the presence of Ca2+ and calmodulin, and dephosphorylated by a muscle phosphate in the presence of Mg2+. Fully dephosphorylated myosin is obtained by dialysis of muscle crude extract (0.06 M NaCl, 0.01 M Tris-HCl, pH 7.5, 50 microM EGTA); fully phosphorylated myosin is obtained by addition of Ca2+ (0.2 mM), Mg2+ (10 mM) and ATP (3 mM) and 5 min incubation at 28 degrees C. The following reaction characteristics were noted. The crude extract is a very efficient phosphorylating complex and can be diluted to phosphorylate or dephosphorylate purified myosin. Phosphorylation and dephosphorylation appear monophasic, showing no evidence of negative cooperativity in this particular type of myosin and medium. Phosphorylation is 24 times slower in the presence of 0.45 M KCl, 5 mM pyrophosphate. Thiophosphorylated myosin is slowly dephosphorylated by phosphatase. At the crude myosin stage the dephosphorylation reaction is efficiently inhibited (at 0-4 degrees C) by the presence of 70 mM NaF. Myosin-[(T)-LC2'] (a myosin species in which LC2 has been selectively modified by trypsin) is an interesting species refractory to phosphorylation. The myosin-[(T)-LC2'] isozyme can be obtained fully phosphorylated by phosphorylation of myosin followed by limited tryptic proteolysis as described earlier. Urea-PAGE as used separates LC2, phosphoryl-LC2, LC2' and phosphoryl-LC2' effectively and in this order. Through this procedure the (de)-phosphorylating complex is ipso facto specific to the myosin species considered; the method avoids lengthy preparations of purified proteins and is easy, rapid and efficient.

Phosphorylation in skeletal myosin light chain modulates the actin--myosin interaction in the presence of regulatory proteins in vitro

The effect of phosphorylation in skeletal myosin light chain (LC2) on the actomyosin and acto-heavymeromyosin (HMM) ATPase activities was investigated in the presence or absence of regulatory proteins (tropomyosin-troponin complex). Phosphorylation in LC2 did not modulate the actin-myosin and actin-HMM interactions over a wide range of KCl concentrations from 30 to 150 mM without regulatory proteins. In the presence of regulatory proteins, phosphorylation in myosin LC2 enhanced the ATPase activity of actomyosin with calcium ions, but the removal of calcium ions made little difference in the ATPase activity between phosphorylated and dephosphorylated myosins. Ca2+-sensitivity of the regulated actomyosin was slightly changed by phosphorylation in myosin LC2. However, both the ATPase activity and Ca2+-sensitivity of the regulated acto-HMM were unaffected by phosphorylation in HMM LC2.

Protein thiophosphorylation associated with secretory inhibition in permeabilized chromaffin cells

Permeabilized cells treated with the adenosine triphosphate analog, [35S]adenosine-5'-0-(3-thiotriphosphate) ([gamma-35S]ATP), showed thiophosphorylation of a small number of cellular proteins. A 54 kilodalton (kDa) protein was heavily thiophosphorylated in unstimulated control cells and a 43 kilodalton protein was more heavily thiophosphorylated in calcium stimulated cells. Intact cells incorporated 35S into a series of higher molecular weight proteins. Stimulation of prelabelled, permeabilized cells resulted in a loss of 35S from the cells over a 20 min period. Treatment of permeabilized cells with ATP gamma S inhibited secretion and 35S incorporation into the cells. Pretreatment with ATP gamma S resulted in subsequent inhibition of both secretion and the ability of the cells to incorporate 35S from [gamma-35S]ATP. These results indicate that the sites normally available for phosphorylation were inactivated by thiophosphorylation and were unavailable to participate in the secretory process. The inhibition of secretion associated with thiophosphorylation of these proteins suggests that they may play a role in the control of secretion by chromaffin cells.

Influence of the regulatory light chain of fast skeletal muscle myosin on its interaction with actin in the presence and absence of ATP

The effect of myosin LC2 modifications (phosphorylation or selective proteolytic removal of a seven-residue N-terminal peptide) and partial or complete removal of the whole LC2 was studied under various conditions. (1) Actin binding in the absence of ATP is not influenced by the nature of the myosin species (phosphorylated, dephosphorylated or devoid of LC2). (2) A 50% inhibition of K+/EDTA-ATPase was obtained with actin concentrations hardly different when phosphorylated and dephosphorylated myosins were compared (of the order of 5 microM), whereas both myosin devoid of LC2 and myosin in which the LC2 N-terminal peptide has been removed required significantly higher concentrations of actin (13.0 +/- 2 and 12.0 +/- 2.0 microM, respectively). (3) Dissociation of the actomyosin complex at high ionic strength with nucleotides is not influenced by phosphorylation. (4) Actin activation of Mg2+-ATPase is enhanced when LC2 is phosphorylated; no activation enhancement is observed with myosin devoid of LC2. (5) Translational diffusion coefficient measurements of myosin in high-ionic-strength solutions indicate a tendency for LC2-deprived myosin to form autoassociation oligomers. It thus appears that a structural modification (partial cleavage or removal of LC2) induces important structural changes in myosin, pointing to a role for LC2 in the intrinsic conformation of the molecule and its interaction potentialities. Effects of LC2 removal at high ionic strength are best explained by interactions bearing no relationship to physiological functions. A physiologically significant effect of LC2 phosphorylation requires a minimum degree of organization (actomyosin complex) to be expressed in which LC2 could play the role of a return-spring in the cross-bridge mechanism.

Factors influencing interaction of phosphorylated and dephosphorylated myosin with actin

The influence of various factors on the interaction of phosphorylated and dephosphorylated myosin with actin was examined. It was found that the difference between the values of specific activity of the two myosin forms of actin-stimulated Mg2+-ATPase is affected by changes in KCl, MgATP and actin concentration. The effect of increased pH on the differences in the rate of ATP hydrolysis by actomyosin containing phosphorylated myosin as compared with that of the dephosphorylated one, observed in the presence of EGTA, is abolished by addition of Ca2+. Tropomyosin strongly inhibits the actin-stimulated Mg2+-ATPase of phosphorylated myosin (by about 60%). The tropomyosin-troponin complex and native tropomyosin lowered the rate of ATP hydrolysis by actomyosin containing both phosphorylated and dephosphorylated myosin by about of 60% of the value obtained in the absence of those proteins. These results indicate that the change of negative charge on the myosin head due to phosphorylation and dephosphorylation of myosin light chains modulates the actin-myosin interaction at different steps of the ATP hydrolysis cycle. Phosphorylation of myosin seems to be a factor decreasing the rate of ATP hydrolysis by actomyosin under physiological conditions.

Decoration of actin filaments with skeletal muscle heavy meromyosin containing either phosphorylated or dephosphorylated regulatory light chains

Heavy meromyosin containing almost intact regulatory light chains (LC2) was obtained from monomeric phosphorylated and dephosphorylated rabbit fast skeletal muscle myosin by brief chymotryptic digestion in the presence of CaCl2. Actin filaments, complexed with heavy meromyosin, display two different forms of arrowhead, depending on the form of LC2.

Myosin light chain phosphorylation and isometric twitch potentiation in intact human muscle

The effects of a single voluntary contraction of the quadriceps muscle group on phosphate incorporation into the phosphorylatable light chains (P-light chains) of fast and slow myosin isolated from the vastus lateralis muscle and potentiation of the electrically stimulated twitch tension was studied in intact human muscle. Twitch potentiation was maximal 20 s after the voluntary contraction. Thereafter, twitch potentiation declined, but was still significantly higher than pre-contraction values 2 min after the voluntary contraction. Phosphate incorporation into the P-light chain of fast myosin followed a similar time course to twitch potentiation, but no phosphate was incorporated into slow myosin P-light chains. These observations suggest that myosin light chain kinase activity is mainly associated with fast-twitch muscle fibers and, in agreement with previous studies, suggests that twitch potentiation associated with P-light chain phosphorylation is confined to the fast-twitch fibers of human muscle.

Thiophosphorylation and phosphorylation of chromatin proteins from calf thymus in vitro

Thiophosphorylation and phosphorylation of 5% perchloric acid extractable proteins from calf thymus chromatin were studied using a cyclic GMP-dependent protein kinase from bovine lung and a nuclear protein kinase II from rat liver. The phosphorylation reaction catalyzed by nuclear protein kinase II utilized [gamma -35S]ATP as a phosphate donor almost as efficiently as [gamma -32P]ATP, but the cGMP-dependent protein kinase mediated phosphorylation by [35S]ATP was about 20 times less effective than that by [32P]ATP. In addition, using [35S]ATP instead of [32P]ATP changed markedly the cGMP-dependent phosphorylation pattern of the PCA-extractable proteins as examined by gel electrophoresis. Thus, depending on the type of protein kinase, the results from thiophosphorylation and phosphorylation reactions may vary considerably.