The relaxing protein system of striated muscle. Resolution of the troponin complex into inhibitory and calcium ion-sensitizing factors and their relationship to tropomyosin

1. A method involving isoelectric precipitation and chromatography on SE-Sephadex (sulphoethyl-Sephadex) is described for the preparation of the troponin complex free of tropomyosin from low-ionic-strength extracts of natural actomyosin and myofibrils. 2. Purified troponin complex required tropomyosin to inhibit the Mg²⁺-stimulated adenosine triphosphatase activity and superprecipitation of desensitized actomyosin in the presence of ethanedioxybis(ethylamine)tetra-acetate. An upper limit of 35000 for the `molecular weight' of the troponin complex was derived from the amounts required to bring about 50% of the maximum inhibition of the Mg²⁺-stimulated adenosine triphosphatase activity of desensitized actomyosin of known concentration. 3. In the presence of dissociating reagents the troponin complex could be dissociated into inhibitory and Ca²⁺-sensitizing factors, which could be isolated separately on SE-Sephadex. The inhibitory factor inhibited the Mg²⁺-stimulated adenosine triphosphatase activity and superprecipitation of desensitized actomyosin independently of the concentration of free Ca²⁺ in the medium. 4. The Ca²⁺-sensitizing factor changed its electrophoretic mobility on polyacrylamide gel in the presence of ethanedioxybis(ethylamine)tetra-acetate. It formed a complex with the inhibitory factor at low ionic strength and the original biological activity of the troponin complex could be restored on mixing the inhibitory factor with the Ca²⁺-sensitizing factor in the ratio of about 3:2. 5. Evidence is presented indicating that the ability of tropomyosin preparations to restore relaxing-protein-system activity to the troponin complex and their inhibitory effect on the Ca²⁺-stimulated adenosine triphosphatase activity of desensitized actomyosin are two properties of different stability to preparative procedures and tryptic digestion. This suggests that the relaxing protein system of muscle may contain another as yet uncharacterized component.     

Effect of bivalent cations on the adenosine triphosphatase of actomyosin and its modification by tropomyosin and troponin

1. After removal of tropomyosin and troponin from the `natural'' actomyosin complex, the adenosine triphosphatase activity of the resulting `desensitized'' actomyosin is stimulated to the same extent by various bivalent cations with an ionic radius in the range 0·65–0·99å when tested at optimum concentration of the metal ion in the presence of 2·5mm-ATP at low ionic strength and pH7·6. Under identical conditions the adenosine triphosphatase activity of myosin alone is stimulated to an appreciable extent only by Ca²⁺ (ionic radius 0·99å). 2. Tropomyosin narrows the range of size of the stimulatory cations by inhibiting specifically the adenosine triphosphatase activity of `desensitized'' actomyosin when stimulated by Ca<sup>2+</sup> or the slightly smaller Cd<sup>2+</sup> (ionic radius 0·97å). Tropomyosin has no effect on the adenosine triphosphatase activity of `desensitized'' actomyosin when stimulated by the smaller cations, nor on the Ca²⁺-activated adenosine triphosphatase activity of myosin alone. 3. The adenosine triphosphatase activity of the `natural'' actomyosin system (containing tropomyosin and troponin) stimulated by the smallest cation, Mg²⁺ (ionic radius 0·65å), is low when the system is deprived of Ca²⁺ but high in the presence of small amounts of Ca²⁺. This sensitivity to Ca²⁺ seems to be a unique feature of the Mg²⁺-stimulated system. 4. The changes in specificity of the myosin adenosine triphosphatase activity in its requirement for bivalent cations caused by interaction with actin, tropomyosin and troponin primarily concern the size of the metal ions. The effects on enzymic properties of myofibrils due to tropomyosin and troponin can be demonstrated at low and at physiological ionic strength.     

Interactions of desensitized actomyosin with tropomyosin, troponin A, troponin B, and polyanions

Troponin B is an inhibitor of the Mg⁺⁺-activated ATPase activity of actomyosin. The inhibitory effect, which is observed, however, depends upon whether tropomyosin is also present. In the absence of tropomyosin the inhibition by troponin B is markedly reduced by increasing the ionic strength from 0.03 to 0.07, but is not affected by calcium up to a concentration of 10^-4 M. Troponin A relieves the inhibition in both the absence and presence of calcium, an effect which is also shown by many polyanions and is illustrated by using RNA. Tropomyosin enhances the inhibitory effect of troponin B and renders it more resistant to increasing ionic strength but it does not make the inhibition calcium-sensitive. However, when troponin A or low concentrations of polyanions are added to troponin B and tropomyosin, the actomyosin ATPase activity becomes calcium-sensitive; i.e., in the presence of tropomyosin, troponin A or polyanions do not relieve the inhibitory action of troponin B in the absence of calcium but only in its presence. In marked contrast to this is the effect of troponin A in the absence of tropomyosin where it neutralizes the effect of troponin B under all conditions. Thus troponin A and the polyanions both confer calcium regulation on the troponin B-tropomyosin system. The similar effects exhibited by troponin A and the polyanions suggest that the addition of net negative charge to troponin B is an important factor in the conferral of calcium sensitivity. It is also clear that tropomyosin is an essential component of the regulatory mechanism.     

The functional characteristics conserved in tropomyosins.

1. Tropomyosin, one of the regulatory proteins in muscle contraction, was prepared from chickens, rabbits, frogs, shrimps, and shellfish, and conserved characteristics were studied using an enzymological technique. 2. All tropomyosins tested, irrespective of their sources, were found to have the ability to mediate the inhibitory activity of rabbit troponin toward rabbit Mg2+-activated actomyosin ATPase (Mg2+-ATPase) activity in the absence of Ca2+ ions. 3. The effect of tropomyosin on the Mg2+-ATPase activity in the presence of Ca2+ ions varied, depending on the source, and this variation appeared to reflect the evolutionary course of this protein. 4. Tropomyosin from shellfish adductor muscle had the ability to bind to rabbit skeletal muscle troponin and actin. This ability is also considered to be a basic characteristic of tropomyosin which has been conserved during evolution.     

Activation of the Adenosine Triphosphatase of Limulus polyphemus Actomyosin by Tropomyosin

Purified actin does not stimulate the adenosine triphosphatase (ATPase) activity of Limulus myosin greatly. The ATPase activity of such reconstituted preparations is only about one-fourth the ATPase of myofibrils or of natural actomyosin. Actin preparations containing tropomyosin, however, activate Limulus myosin fully. Both the tropomyosin and the actin preparations appear to be pure when tested by different techniques. Tropomyosin combines with actin but not with myosin and full activation is reached at a tropomyosin-to-actin ratio likely to be present in muscle. Tropomyosin and actin of several different animals stimulate the ATPase of Limulus myosin. Tropomyosin, however, is not required for the ATPases of scallop and rabbit myosin which are fully activated by pure actin alone. Evidence is presented that Limulus myosin, in the presence of ATP at low ionic strength, has a higher affinity for actin modified by tropomyosin than for pure actin.     

Effect of cardiac and skeletal tropomyosin on Mg2+-actomyosin ATPase.

Tropomyosin, one of the proteins regulating the sarcomere, was prepared from pig heart and rabbit skeletal muscles. The effect of these two different tropomyosins was studied between 0.5 and 10 mM of Mg2+ at a constant ATP concentration (1 mM) on reconstituted actomyosin prepared from pig heart myosin and rabbit skeletal actin. Cardiac and skeletal tropomyosin both activated the ATPase at low Mg2+ concentrations and inhibited it above 3 mM. The pig heart and rabbit skeletal tropomyosins which contain two isomers, alpha alpha and alpha beta, respectively has very similar effects on actomyosin ATPase.     

The contractile and regulatory proteins of insect flight muscle

1. Myosin, actin and the regulatory proteins were prepared from insect flight muscle. 2. The light subunit composition of the myosin differed from that of vertebrate muscle myosin. The ionic strength and pH dependence of the myosin adenosine triphosphatase (ATPase) were measured. 3. Actin was associated with a protein of subunit molecular weight 55000 and was purified by gel filtration. Impure actin had protein bound at a periodicity of about 40nm. 4. Regulatory protein extracts had tropomyosin and troponin components of subunit molecular weight 18000, 27000 and 30000. Crude extracts of regulatory proteins inhibited the ATPase activity of desensitized or synthetic actomyosin; this inhibition was relatively insensitive to high Ca(2+) concentrations. Purified insect regulatory protein produced as much sensitivity to Ca(2+) as did the rabbit troponin-tropomyosin complex. 5. Synthetic actomyosins were made from rabbit and insect proteins. Actomyosins containing insect myosin had a low ATPase activity that was activated by tropomyosin. The Ca(2+) sensitivity of actomyosins containing insect myosin or actin, with added troponin-tropomyosin complex from rabbit, was comparable with that of rabbit actomyosin.     

Myosin and actomyosin from human skeletal muscle

Human skeletal natural actomyosin contained actin, tropomyosin, troponin and myosin components as judged by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Purified human myosin contained at least three light chains having molecular weights (±2000) of 25 000, 18 000 and 15 000. Inhibitory and calcium binding components of troponin were identified in an actin-tropomyosin-troponin complex extracted from acetone-dried muscle powder at 37°C. Activation of the Mg-ATPase activity of Ca²⁺-sensitive human natural or reconstituted actomyosin was half maximal at approximately 3.4 μM Ca²⁺ concentration (CaEGTA binding constant = 4.4 · 10⁵ at pH 6.8). Subfragment 1, isolated from the human heavy meromyosin by digestion with papain, appeared as a single peak after DEAE-cellulose chromatography. In the pH 6–9 range, the Ca²⁺-ATPase activity of the subfragment 1 was 1.8-and 4-fold higher that the original heavy meromyosin and myosin, respectively. The ATPase activities of human myosin and its fragments were 6–10 fold lower than those of corresponding proteins from rabbit fast skeletal muscle. Human myosin lost approximately 60% of the Ca²⁺-ATPase activity at pH 9 without a concomitant change in the number of distribution of its light chains. These findings indicate that human skeletal muscle myosin resembles other slow and fast mammalian muscles. Regulation of human skeletal actomyosin by Ca²⁺ is similar to that of rabbit fast or slow muscle     

The contractile proteins of smooth muscle. Properties and components of a Ca2+-sensitive actomyosin from chicken gizzard.

The preparation and characterization of a Ca²⁺-sensitive actomyosin from chicken gizzard is described. The pH curve of the Mg²⁺ ATPase activity of the actomyosin was dominated by the activity of the myosin component, and this gave rise to the acid and alkaline optima. Skeletal muscle myosin showed a similar curve. Both the activation of myosin ATPase by actin, and the Ca²⁺ sensitivity were confined to the neutral pH region. The subunit composition of the Ca²⁺-sensitive actomyosin was interesting in that no components corresponding to skeletal muscle troponin were obvious. It is suggested that the activity of gizzard actomyosin is regulated by a protein on the thin filaments with a subunit weight of ~130,000.     

Comparison of the reaction of N-ethylmaleimide with myosin and heavy meromyosin subfragment 1

Myosin reacted at low ionic strength with NEM forms an actomyosin which is Ca⁺⁺ insensitive. With HMM S-1 the reaction with NEM causes a marked loss of the actin activated ATPase activity and the Ca⁺⁺ sensitivity is reduced but not eliminated. The presence of actin during the sulfhydryl reaction does not significantly alter this result. HMM S-1 prepared from myosin previously desensitized by NEM regains Ca⁺⁺ sensitivity. These results indicate that the conformations of myosin and HMM S-1 are different and could reflect a difference between insoluble (filamentous) myosin and myosin, or its fragments, in solution.     

Superprecipitation of an actomyosin-like complex isolated from Naegleria gruberi amoebae

A protein complex similar to muscle actomyosin and plasmodial myosin B has been isolated from Naegleria gruberi amoebae. This extract, which comprises approximately 0.7% of the total cell protein, has the solubility properties of actomyosin, displays Ca²⁺-activated, Mg²⁺-inhibited ATPase activity, forms microfilaments, and undergoes a strong superprecipitation reaction. Superprecipitation is initiated by ATP and is preceded by a very brief clearing phase. Although added Mg²⁺ is not essential for superprecipitation of the extract, the reaction proceeds maximally when 7 mM Mg²⁺ is provided. This extract does not appear to have a Ca²⁺ requirement, and superprecipitation is in fact inhibited by added Ca²⁺ ion at all concentrations greater than 0.1 mM. Both ATPase activity and superprecipitation of the actomyosin-like complex are inhibited by the sulfhydryl inhibitor salyrgan.     

Functional effects of substitutions I92T and V95A in actin-binding period 3 of tropomyosin

Tropomyosin polymerizes along actin filaments and together with troponin regulates muscle contraction in a Ca-dependent manner. Actin-binding periods are homologous residues, which repeat along tropomyosin sequence, form tropomyosin-actin interface and determine regulatory functions. To learn how period 3 is involved in tropomyosin functions we examined effects of two mutations in Tpm1.1, I92T and V95A, which have been linked to dilated and hypertrophic cardiomyopathies characterized respectively by hyper- and hypocontractile phenotypes. In this work the functional consequences of both mutations were studied in vitro by using actin thin filaments reconstituted in the presence of mutant Tpm1.1 homodimers carrying the substitutions in both tropomyosin chains, Tpm1.1 heterodimers with substitution only in one Tpm1.1 chain, and Tpm1.1/Tpm2.2 heterodimers with substitution in Tpm1.1 chain and wild type Tpm2.2 in the second chain. The presence of the substitution I92T decreased the tropomyosin affinity for actin, abolished Ca²⁺-dependent activation of the actomyosin ATPase, decreased the sensitivity of the tropomyosin-troponin complex to subsaturating Ca²⁺ concentrations and reduced the cooperativity of the myosin-induced transition of the thin filament to a fully active state. The substitution V95A had opposite effects: increased actin affinity, increased the actomyosin ATPase activity above the level observed for wild type Tpm and increased cooperativity of myosin-induced activation of the thin filaments reconstructed with homo- and heterodimers of tropomyosin. Substitutions I92T and V95A were dominant, but the formation of heterodimers modified the effects observed for homodimers.     

Structural Basis for the Activation of Muscle Contraction by Troponin and Tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca²⁺ on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca²⁺ and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca²⁺ allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca²⁺-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca²⁺, troponin operates as an inhibitor, while in activated muscles at high Ca²⁺, it acts as a promoter to initiate contraction.     

Contractile Proteins of a Benthic Fish. II. Composition and ATPase Properties of Actomyosin

SYNOPSIS. Actomyosin was extracted from skeletal muscle of Coryphaenoides,a benthic fish living at 2,200 meters depth, at a temperatureof 2°C, or less, and at pressure of 3,000 psi. On SDS-ureaelectrophoresis on acrylamide gel, the actomyosin extracts yieldcomponents of apparent molecular weight 210,000 (myosin heavychains), 47,000 (actin), 35,000 (tropomyosin and/or troponinsubunits), and 13,000 (myosin light chains). The Mg²⁺-ATPaseof Coryphaenoides actomyosin shows a complex Arrhenius plot,with marked denaturation at temperatures above 30°C, anddiminished temperature sensitivity at temperatures below 15°C.Mg²⁺-ATPase is inhibited by pressure, with activation volumesof + 160 cc/mole at 25°C, and + 230 cc/mole at 2°C.Ca²⁺-ATPase of actomyosin exhibits the same pH, temperature,and pressure dependence as Ca²⁺-ATPase of myosin. The overalldata would be consistent with a positive activation volume thatis independent of temperature (to first approximation) and isrelated to the interaction of actin and myosin, and a negativeactivation volume that is temperature dependent and is relateddirectly to activation of myosin ATPase. The net effect appearsto be an adaptive mechanism whereby Mg²⁺-ATPase of Coryphaenoidesactomyosin is relatively insensitive to pressure and temperatureunder conditions encountered by the living fish.     

Evidence for a role for cardiac myosin in regulating the contractile response.

Dinitrophenylated bovine cardiac myosin incorporates 1.3 mol of 1-fluoro-2,4-dinitro-benzene per 5 × 10⁵ g of protein. Concomitantly there was an activation of the Ca²⁺-ATPase activity and an inhibition of the K⁺(EDTA)-ATPase activity. The dinitrophenyl group is located in the smallest active proteolytic fragment, subfragment 1. Virtually all of the labeling occurs in the region containing the heavy chains of cardiac myosin as judged by dissociation experiments in sodium dodecyl sulfate. Dinitrophenylated myosin failed to form calcium-sensitive actomyosin when tested in an ATPase assay system containing actin, tropomyosin, troponin and ethylene glycol-bis(β-aminoethyl ether)N,N′-tetraacetic acid. Thiolysis of the dinitrophenyl group from myosin with 2-mercaptoethanol restored its ability to form a calcium-sensitive actomyosin. The Ca²⁺ and K⁺(EDTA)-ATPase activities were also restored to control values. These results indicate that cardiac myosin participates in the regulation of the interaction between the contractile proteins.     

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.     

The functional properties of full length and mutant chicken gizzard smooth muscle caldesmon expressed in Escherichia coli

Wild type chicken gizzard caldesmon (756 amino acids) was expressed in a T7 RNA polymerase-based bacterial expression system at a yield of 1 mg pure caldesmon per litre bacterial culture. A mutant composed of amino acids 1-578 was also constructed and expressed. The wild type and mutant caldesmon were purified and compared with native chicken gizzard caldesmon. Native and wild type expressed caldesmon were indistinguishable in assays for inhibition of actin-tropomyosin activation of myosin ATPase, reversal of inhibition by Ca²⁺-calmodulin and binding to actin, actin-tropomyosin, Ca²⁺-calmodulin, tropomyosin and myosin. The mutant missing the C-terminal 178 amino acids had no inhibitory effect and did not bind to actin or Ca²⁺-calmodulin. It bound to tropomyosin with a 5-fold reduced affinity and to myosin with a greater than 10-fold reduced affinity.     

The regulatory proteins of the myofibril. Characterization and properties of the inhibitory factor (troponin B)

1. Gel-filtration results indicate that the major component of inhibitory-factor preparations isolated by dissociation of the troponin complex consisted of a protein of subunit weight 23000 daltons. By the same procedure a molecular weight of 18000 was obtained for the calcium-sensitizing factor. 2. The inhibitory factor is specific for the actomyosin type of ATPase and ITPase. It is effective on desensitized actomyosin, natural actomyosin and intact myofibrils. 3. For inhibition, the actomyosin ATPase must be stimulated by Mg(2+), Ca(2+) or Mn(2+). The Co(2+)-, Cd(2+)- or Zn(2+)-stimulated ATPases are not affected. 4. Biological activity is stable to treatment with dissociating agents, heat, pH11, pH1 and carboxymethylation. 5. Increasing amounts of actin, but not myosin or tropomyosin, progressively neutralize the inhibitory activity when added to desensitized actomyosin or myofibrils.     

Concentration-dependent inhibition of myosin ATPase by an indole metabolite of epinephrine

An indole metabolite of epinephrine (an isomer of adrenochrome) was shown to be a potent inhibitor (EC₅₀ of 1.50 μM to 1.85 μM) of myosin, actomyosin, and myofibrillar ATPase when assayed at or near physiologic ionic strength and pH. The inhibition of actomyosin ATPase by this epinephrine derivative was demonstrated to be competetive in nature. Complete inhibition of ATPase activity was never achieved under physiological conditions; maximum inhibition was 50% to 60%. It is concluded that the inhibitor reduced ATPase activity by reversibly attaching to sulfhydryl groups associated with ATPase activity. The reduction of ATPase activity by 50% may be explained by the known heterogeneity of the ATPase sites on myosin; only one-half of these sites may be sensitive or accessible to the inhibitor in the state of aggregation of myosin at physiologic ionic strength. The inhibitor was found to have no effect on hog cerebral cortex Na⁺,K⁺-activated ATPase, suggesting that it may be selective for contractile protein ATPase. These results further support the hypothesis proposed earlier from this laboratory that this inhibitory indole metabolite of epinephrine (which is formed only in smooth muscles relaxed by epinephrine) may be part of the mechanism by which epinephrine produces relaxation in certain smooth muscles.     

Role of actin C-terminus in regulation of striated muscle thin filament

In striated muscle, regulation of actin-myosin interactions depends on a series of conformational changes within the thin filament that result in a shifting of the tropomyosin-troponin complex between distinct locations on actin. The major factors activating the filament are Ca²⁺ and strongly bound myosin heads. Many lines of evidence also point to an active role of actin in the regulation. Involvement of the actin C-terminus in binding of tropomyosin-troponin in different activation states and the regulation of actin-myosin interactions were examined using actin modified by proteolytic removal of three C-terminal amino acids. Actin C-terminal modification has no effect on the binding of tropomyosin or tropomyosin-troponin + Ca²⁺, but it reduces tropomyosin-troponin affinity in the absence of Ca²⁺. In contrast, myosin S1 induces binding of tropomyosin to truncated actin more readily than to native actin. The rate of actin-activated myosin S1 ATPase activity is reduced by actin truncation both in the absence and presence of tropomyosin. The Ca²⁺-dependent regulation of the ATPase activity is preserved. Without Ca²⁺ the ATPase activity is fully inhibited, but in the presence of Ca²⁺ the activation does not reach the level observed for native actin. The results suggest that through long-range allosteric interactions the actin C-terminus participates in the thin filament regulation.