Myosin light-chain phosphatase

Contraction in intact pig aortic strips is not always associated with phosphorylation of myosin light chains

Intact pig aortic strips were incubated in medium containing [32P]P1 and various Ca2+ concentrations. The 32P content of the myosin P-light chain was determined by radioautography after electrophoresis in the presence of sodium dodecyl sulphate. Although treatment of the strips with noradrenaline always caused a rise in tension, this was not necessarily accompanied by increased phosphorylation of the P-light chain. These results indicate that, in aortic smooth muscle, phosphorylation of the P-light chain is not obligatory for contraction.

Biphasic steady-state kinetics of myosin adenosine triphosphatase. Evidence for a substrate effector site

The steady-state kinetics of the K+, Ca2+, and Mg2+-activated adenosine triphosphatase (ATPase) activities of rabbit skeletal myosin were investigated in the substrate concentration range from 0.05 microM to 5 mM and found not to follow Michaelis-Menten kinetics but rather to display biphasic behavior. The Ca2+-ATPase activity of myosin chymotryptic subfragment-1 (S-1), which has only one active site, also exhibits biphasic kinetics, thus excluding the possibility that the biphasic behavior is caused by negative cooperativity between the two active sites of myosin. Myosin K+ and Mg2+-ATPase are both activated by 5'-adenyl methylenediphosphonate (AdoPP[CH2]P) in a competitive manner at high substrate concentrations; i.e. the maximal velocity observed at high substrate concentrations is independent of the AdoPP[CH2]P concentration. This result provides evidence for substrate activation via binding to a regulatory site. Pyrophosphate inhibits myosin ATPase in a competitive manner at low substrate concentrations and in an uncompetitive manner at high substrate concentrations, with the uncompetitive Ki being smaller than the competitive Ki; i.e. pyrophosphate binds more tightly to the effector site than to the active site.

Regulation of myosin light chain kinase by reversible phosphorylation and calcium-calmodulin

1) Myosin light chain kinases from smooth muscle and platelets can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase. 2) Phosphorylation of both kinases, in the absence of calmodulin, markedly decreases kinase activity. 3) The decrease in smooth muscle myosin kinase activity is due to a decreased affinity of the phosphorylated kinase for calmodulin. 4) Dephosphorylation of the smooth muscle kinase by a phosphatase isolated from smooth muscle restores the affinity of the kinase for calmodulin.

Biochemical basis for contraction of vascular smooth muscle

Some of the current facts and theories concerning the contractile mechanism in smooth muscle are summarized. The review is divided into two major sections. One deals with the components of the contractile apparatus (the thick and thin filaments), and the protein components of each filament type are descirbed briefly. The other is devoted to the Ca2+-dependent mechanism of regulation in smooth muscle, and this is restricted to those components that control the activity of the contractile apparatus. There are basically two theories that have been proposed as the regulatory mechanism in smooth muscle. One theory is that the regulatory mechanism is located on the thin filaments and is functional via some modification of the thin filament proteins. This system is termed leiotonin. the second theory is that regulation is achieved by the phosphorylation and dephosphorylation of the light chains of myosin. Since the latter theory represents the author's bias and in general is more widely accepted, it is considered in more detail. A cyclic scheme is presented to illustrate the role of the myosin light chain kinase in the activation and contraction of smooth muscle and the role of the myosin light chain phosphatase in the relaxation process.

Protein phosphorylation: quantitative analysis in vivo and in intact cell systems

Protein phosphorylation-dephosphorylation appears to be an essential component in the regulation of many cellular processes by hormones and drugs. This concept has developed primarily from in vitro biochemical studies in which various purified proteins have been phosphorylated and dephosphorylated by distinct protein kinases and phosphoprotein phosphatases. However, the more difficult, but essential, task of demonstrating the physiological occurrence of these reactions in intact tissue or cell preparations in many cases has not been undertaken in a quantitative manner. There are 4 basic approaches for assessing the extent of protein phosphorylation in vivo and in intact cell systems, each having particular advantages and disadvantages. These are summarized in Table 2. The applicability of any one procedure will be highly dependent upon the protein under investigation. For instance, chemical measurements of total protein-bound phosphate may provide only limited information for proteins which are phosphorylated at multiple sites but could be highly useful for those proteins such as glycogen phosphorylase which are phosphorylated at single sites. The relative ease and the high sensitivity of measuring 32P incorporation into proteins will tempt many investigators to rely heavily on this approach. It is a very powerful procedure, particularly for the initial identification of phosphoproteins, but ultimately quantitative conclusions regarding 32P incorporation must be corroborated by one or more of the other procedures. There is no simple, single experimental approach that may be used under all circumstances, but by integrating these procedures firm conclusions may be drawn regarding the physiological importance of phorphorylation of specific proteins.

Phosphorylation of myosin light chains in perfused rat heart. Effect of adrenaline and increased cytoplasmic calcium ions

1. A method was developed for the isolation of essentially pure myosin light chains from perfused rat heart. The phosphorylation of the P-light chains was estimated by hydrolysis and measurement of phosphate released, by electrophoresis in 8 M-urea and by 32P incorporation in perfusion with [32P]Pi. 2. In control perfusions there was 0.5-0.6 mol of phosphate/mol of P-light chain. This was not changed by perfusion with 5 microM-adrenaline for 10-40s. Perfusion for 1 min with medium containing 7.5 mM-CaCl2, or for 30s with medium containing 118 mM-KCl, also did not change the phosphorylation of P-light chains. 3. It is concluded that phosphorylation of P-light chains is not important in mediating the action of inotropic agents in the heart.

Nucleotide induced head-head interaction in myosin

In isolated myosin the reaction sequence of essential thiol groups with N-ethylmaleimide was studied using the following five approaches: kinetics of the modification reaction, effects of modification on enzyme properties, affinity chromatography of isolated subfragment-1 stemming from modified myosin, isolation of cyanogen bromide peptides and identification of the tryptic thiol peptides thereof. All techniques involved revealed differences whether the modification was performed in the presence or absence of pyrophosphate on the one hand and in the presence of ADP or ATP on the other. In the former cases the two thiol-1 groups per myosin, one per active site, reacted at an equal rate indicating an equivalent microenvironment of these groups and hence a symmetric site-site relationship. In contrast, the nucleotides induce the sequential modification of thiol-1 on one head followed by the thiol-2 on the other head. This indicates non-equivalence in microenvironment of the essential thiols connected with each active site and hence that a form of asymmetric head-head interaction is operative.

Activation of skinned arthropod muscle fibres by Ca2+ and Sr2+

Mechanically skinned skeletal muscle fibres of three crustaceans (barnacle, crayfish and crab) and two insects (cockroach and cricket) were activated in Ca2+- and Sr2+-buffered solutions of different concentrations and the isometric force response was determined. The maximum force response induced by Sr2+ (P0Sr) was only 0-10% of that induced by Ca2+ (P0Ca) in all crustacean muscles, but approached 90% in insects. Experiments on barnacle muscle fibres activated simultaneously by Ca2+ and Sr2+ suggested that Sr2+ competes with Ca2+ for binding onto the regulatory sites without, however, being able to turn all of them 'on' as efficiently as Ca2+. Interestingly, the ratio P0Sr/P0Ca and the sensitivity for both Sr2+ and Ca2+ increased substantially after 4-6 h following the dissection of the animals in most intact decapod muscle fibres and after 24 h in most barnacle muscle fibres. The steepness of the activation curves for both Ca2+ and Sr2+ was similar for each muscle regardless of the age of the fibre and implied that more than 2 Ca2+ (2 Sr2+) were involved in the activation process of each muscle. A Ca2+-induced Ca2+ release mechanism of physiological importance was found to operate in all arthropod muscle fibres investigated.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.

Calcium-sensitivity of pig-carotid-actomyosin ATPase in relation to phosphorylation of the regulatory light chain

Ca2+-dependent phosphorylation of the 20000-Mr regulatory light chain was found to be a necessary condition for the Ca2+-sensitivity of the Mg2+-dependent ATPase activity and superprecipitation of pig carotid actomyosin. Actin-myosin interaction independent of phosphorylation and Ca2+ (ATPase activity and superprecipitation) were demonstrated in aged actomyosin preparations and in preparations from which the regulatory light chains were removed by papain digestion.

Enzymatic preparation of adenosine 5'-O-(3-[35S]thiotriphosphate) by thiophosphorylation of adenosine diphosphate

The very useful radiolabeled ATP analog, adenosine 5-O-(3-[35S] thiotriphosphate) or [35S] ATPgammaS, has been prepared by a technique based on the thiophosphorylation of ADP that results in much higher yields of [35S] ATPgammaS than does a thiophosphate exchange method [1].

Isolation of cardiac myofibrils and myosin light chains with in vivo levels of light chain phosphorylation

Conditions are described for the preparation of functional myofibrils and myosin light chains from freeze-clamped beating hearts with the state of light chain phosphorylation chemically 'frozen' during the extraction procedure. Myofibrils were shown to be functionally intact by measurement of Ca2+ binding and ATPase activity. Highly purified cardiac myosin light chains could be routinely isolated from myofibrillar preparations using ethanol fractionation together with ion-exchange chromatography. Analysis of light chains for covalent phosphate indicated that basal levels of phosphorylation of the 18--20 000 dalton light chain of myosin in rabbit hearts beating in situ or in a perfusion apparatus were 0.3--0.4 mol/mol. Covalent phosphate content of the light chain fraction did not change during perfusion of hearts with 10 microM epinephrine.

Purification and properties of myosin light-chain kinase from fast skeletal muscle

1. A procedure is described for the isolation of myosin light-chain kinase from rabbit fast skeletal muscle as a homogeneous protein. 2. Myosin light-chain kinase is a monomeric enzyme of mol.wt. 77000. Under some conditions of storage it is converted into components of mol.wts. about 50000 and 30000 that possess enzymic activity. 3. The enzyme is clearly different in structure and properties from any other protein kinase so far isolated from muscle. 4. The enzyme is highly specific for the P-light chain (18000-20000-dalton light chain) of myosin and requires Ca2+ for activity. 5. The P-light chain is phosphorylated at a similar rate whether isolated or associated with the rest of the myosin molecule. 6. The effects of pH, bivalent cation and other nucleotides on the enzymic activity are described. 7. The role of the phosphorylation of the P-light chain of myosin in muscle function is discussed.

Enzymatic synthesis of adenosine 5'-O-(3-[35S]-thiotriphosphate)

A method for the enzymatic synthesis of adenosine 5'-O-(3-[35S]thiotriphosphate) is described. The method involves the thiophosphate exchange of adenosine 5'-O-(3-thiotriphosphate) with [35S]thiophosphate with the aid of glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase in the presence of 3-phosphoglycerate. The method is also applicable for the synthesis of guanosine 5'-O-(3-[35S]thiotriphosphate).

Ca++ activation of ATPase activity, ATP-Pi exchange, and tension in briefly glycerinated heart muscle

In the course of MG-dependent ATP splitting by heart actomyosin, an "energy rich" actomyosin-ADP complex is formed, which promotes the incorporation of phosphate 32P into ATP in myofibrils. The rate of this ATP-phosphate exchange reaction depends on the extent of actin-myosin overlap which can be decreased by stretching glycerinated muscle fibres. In heart muscle, the calcium-ion dependence of this reaction is similar to that of the actomyosin ATPase, the tension, and "immediate fibre stiffness" (which is "hookean" and which is a measure for the number of myosin cross-bridges attached to and interacting with actin). These findings suggest that calcium increases the amount of "contractile" actomyosin-ADP complexes. The proportionality between tension and ATPase activity further suggests that the rate-limiting step of the cross-bridge cycle (which determines the molecular turnover number, the "Wechselzahl" of the ATPase) is only little affected by calcium ions. These ions act by recruiting more bridges rather than by accelerating their reactions. In addition, the depressing effect of inorganic phosphate on the contractile tension and its presumable role in energetic insuffciency will be discussed.

Effect of phosphorylation of smooth muscle myosin on actin activation and Ca2+ regulation

A 35--70% ammonium sulfate fraction of smooth muscle actomyosin was prepared from guinea pig vas deferens. This fraction also contains a smooth muscle myosin kinase and a phosphatase that phosphorylates and dephosphorylates, respectively, the 20,000-dalton light chain of smooth muscle myosin. Phosphorylated and dephosphorylated smooth muscle myosin. Phosphorylated and dephosphorylated smooth muscle myosin were purified from this ammonium sulfate fraction by gel filtration, which also separated the kinase and the phosphatase from the myosin. Purified phosphorylated and dephosphorylated myosin have identical stained patterns after sodium dodecyl sulfate/polyacrylamide gel electrophoresis. They also have similar ATPase activities measured in 0.5 M KCl in the presence of K+-EDTA and Ca2+. However, the actin-activated myosin ATPase activity is markedly increased after phosphorylation. Moreover, the actin-activated ATPase activity of phosphorylated myosin is inhibited by the removal of Ca2+ in the absence of any added regulatory proteins. Dephosphorylation of myosin results in a decrease in the actin-activated ATPase activity. Skeletal muscle tropomyosin markedly increased the actin-activated ATPase activity of phosphorylated but not dephosphorylated myosin in the presence, but not in the absence, of Ca2+.