An investigation into the role of SH1 and SH2 groups of myosin in calcium binding and tension generation

Modulation of contractile apparatus Ca2+ sensitivity and disruption of excitation-contraction coupling by S-nitrosoglutathione in rat muscle fibres

S-Nitrosoglutathione (GSNO) is generated in muscle and may S-glutathionylate and/or S-nitrosylate various proteins involved in excitation–contraction (EC) coupling, such as Na+-K+-ATPases, voltage-sensors (VSs) and Ca2+ release channels (ryanodine receptors,RyRs), possibly changing their properties. Using mechanically skinned fibres from rat extensor digitorum longus muscle, we sought to identify which EC coupling processes are most susceptible to GSNO-modulated changes and whether these changes could be important in muscle function and fatigue. For comparison, we examined the effect of other oxidation, nitrosylation, or glutathionylation treatments (S-nitroso-N-acetyl-penicillamine (SNAP), hydrogen peroxide,2,2-dithiodipyridine and reduced glutathione) on twitch and tetanic force, action potential (AP) repriming, sarcoplasmic reticulum (SR) Ca2+ loading and leakage, and contractile apparatus properties. None of the treatments detectably altered AP repriming, indicating that t-system excitability was relatively insensitive to such oxidative modification. Importantly, the overall effect on twitch and tetanic force of a given treatment was determined primarily by its action on Ca2+ sensitivity of the contractile apparatus. For example, S-nitrosylation with the NO• donor,SNAP, caused matching decreases in the contractile Ca2+ sensitivity and twitch response, and GSNO applied ∼10 min after preparation had very similar effects. The only exception was when GSNO was applied immediately after preparation, which resulted in irreversible decreases in twitch and tetanic responses even though it concomitantly increased Ca2+ sensitivity by∼0.1 pCaunits, the latter evidently due to S-glutathionylation of the contractile apparatus. This decrease in AP-mediated force responses was due to impaired VS–RyR coupling and was accompanied by increased Ca2+ leakage through RyRs. Such oxidation-related impairment of coupling could be responsible for prolonged low frequency fatigue in certain circumstances.

Cooperativity of thiol-modified myosin filaments. ATPase and motility assays of myosin function

The effects of chemical modifications of myosin's reactive cysteines on actomyosin adenosine triphosphatase (ATPase) activities and sliding velocities in the in vitro motility assays were examined in this work. The three types of modifications studied were 4-[N-[(iodoacetoxy)ethyl]-N-methylamino]-7-nitrobenz-2-oxa-1,3- diazole labeling of SH2 (based on Ajtai and Burghart. 1989. Biochemistry. 28:2204-2210.), phenylmaleimide labeling of SH1, and phenylmaleimide labeling of myosin in myofibrils under rigor conditions. Each type of modified myosin inhibited the sliding of actin in motility assays. The sliding velocities of actin over copolymers of modified and unmodified myosins in the motility assay were slowest with rigor-modified myosin and most rapid with SH2-labeled myosin. The actin-activated ATPase activities of similarly copolymerized myosins were lowest with SH2-labeled myosin and highest with rigor-modified myosin. The actin-activated ATPase activities of myosin subfragment-1 obtained from these modified myosins decreased in the same linear manner with the fraction of modified heads. These results are interpreted using a model in which the sliding of actin filaments over myosin filaments decreases the probability of myosin activation by actin. The sliding velocity of actin over monomeric rigor-modified myosin exceeded that over the filamentous form, which suggests for this myosin that filament structure is important for the inhibition of actin sliding in motility assays. The fact that all cysteine modifications examined inhibited the actomyosin ATPase activities and sliding velocities of actin over myosin poses questions concerning the information about the activated crossbridge obtained from probes attached to SH1 or SH2 on myosin.

TTX-induced muscle disuse alters Ca2+ activation characteristics of myofibril ATPase

1. Previous reports of the effects of disuse induced by tetrodotoxin (TTX) have demonstrated alterations in muscle function suggesting changes in the quality of contractile proteins. 2. We extended these studies to the effects of TTX-induced disuse on the Ca2+ activation characteristics of myofibrillar ATPase of the rat gastrocnemius. 3. Atrophic responses were as previously reported (St-Pierre, D.M.M. and Gardiner P.F. (1985) Effect of disuse on mammalian fast-twitch muscle: joint fixation compared with neurally applied tetrodotoxin. Exp. Neurol. 90, 635-651; St-Pierre, D.M.M. et al. (1987). Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise; 1. Contractile properties. Exp. Neurol. 98, 472-488.) with a significant decrease in left gastrocnemius weight compared to control (C) (1.25 +/- 0.06 for C vs 0.72 +/- 0.04 for TTX, X +/- SEM, P less than or equal to 0.01). 4. Myofibrillar protein yield (mg/g wet weight) was also depressed (92.8 +/- 4.5 for C vs 70.3 +/- 3.7 for TTX; P less than or equal to 0.01). 5. Maximum ATPase of myofibrils (nmol Pi/mg/min) was decreased (441 +/- 28 for C vs 181 +/- 30 for TTX, P less than or equal to 0.01). 6. Furthermore, the Hill n which reflects the cooperative aspects of Ca2+ activation of the myofibrillar ATPase was depressed (1.58 +/- 0.07 for C vs 1.29 +/- 0.09 for TTX; P less than or equal to 0.01). 7. The results suggest that muscle perturbations resulting from disuse are partially related to changes in the myofibril.

Effect of calcium and F-actin on the rate of SH2 modification in skeletal myosin

The rate constant of modification of a specific thiol group, SH2, with N-ethylmaleimide (NEM) has been used to estimate the conformational change in the local area containing SH2 (SH2 region) of skeletal myosin as a structural probe. The rate of Mg2+-ATP-induced SH2 modification of subfragment-1 (S-1) isozymes was regulated by Ca2+ in the pCa range below 6.4 and was not regulated in the pCa range above 6.4. No substantial difference between S-1 containing alkali light chain, A1, (S-1(A1] and S-1 containing alkali light chain, A2, (S-1(A2] was observed in the Ca2+-dependent rate of SH2 modification. Due to the presence of this Ca2+ regulation in myosin (absence in S-1 isozymes) in the pCa range above 6.4, absence of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) light chain in S-1 isozymes, and high affinity of Ca2+ for DTNB light chain, this Ca2+ regulation in the pCa range above 6.4 is possibly related to the Ca2+ binding to DTNB light chain. F-Actin, which is entirely free from tropomyosin and troponin, enhanced the rate of Mg2+-ATP-induced SH2 modification of S-1 isozymes equally and of myosin, and reduced the Ca2+ sensitivity with an increase in F-actin concentration.

On the mechanism of energy transduction in myosin subfragment 1

It is proposed that the myosin subfragment 1 moiety of the muscle contractile apparatus is a self-contained "engine" whose operational plan is based on the interactive nature of ATP (or degradation intermediate) binding and actin binding, made possible by an intersite communication system. It is suggested that the spatial information required for examining this engine can, at least provisionally, be derived from fluorescence resonance energy transfer measurements interpreted by the Förster equation and that the existence of an intersite communication system can be deduced from piece-wise detection of interacting pairs of points.

Calcium sensitivity of vertebrate skeletal muscle myosin

The calcium sensitivity of vertebrate skeletal muscle myosin has been investigated. Adenosinetriphosphatase (ATPase) activity was assayed in a reconstituted system composed of either purified rabbit myosin plus actin or myosin plus actin, tropomyosin, and troponin. The calcium sensitivity of actomyosin Mg-ATPase activity was found to be directly affected by the ionic strength of the assay medium. Actomyosin assayed at approximately physiological ionic strength (120 mM KCl) demonstrated calcium sensitivity which varied between 6 and 52%, depending on the myosin preparation and the age of the myosin. Mg-ATPase activity was increased when calcium was present in the assay medium at physiological ionic strength. Conversely, actomyosin Mg-ATPase activity assayed at a lower ionic strength (15 mM KCl) was inhibited by addition of calcium. Addition of tropomyosin and troponin to the assay increased the calcium sensitivity of the system at the physiological ionic strength still further (up to 99% calcium sensitivity) and conferred calcium sensitivity on the system at the lower ionic strength (greater than 90% calcium sensitivity). A correlation also existed between myosin's calcium sensitivity and the phosphorylated state of light chain 2.

The effects of chemical cross-linking agents on calcium-induced structural changes in skinned muscle fibers. Origin within thick filaments detected by optical diffraction methods

We have reported earlier (Sabbadini, R.A., Rieser, G.D. and Paolini, P.J. (1979) Biochim. Biophys. Acta 578, 526-533) that physiological levels of calcium (pCa 6.95-5.49) can produce structural changes in thick filaments which are detectable as an intensity loss of the first-order optical diffraction lines from chemically skinned skeletal muscle fibers stretched beyond myofilament overlap. We now show that the calcium-induced intensity decrease results from structural changes within, rather than between, thick filaments. Glycerinated, detergent-treated fibers from frog semitendinosus muscle were incubated in 1-10 mM concentrations of dimethylsuberimidate (DMS), dithiobis(succinimidylpropionate) (DTSP) or dimethyl-3,3'-dithiobispropionimidate (DTBP) for 4 h. These substances are homobifunctional lysine-modifying cross-linking reagents known to restrict movement of S-1 heads and limit changes in the association of myosin rods within the core of the thick filament without affecting interfilament lattice spacing. Diffraction patterns from cross-linked cells in relaxing solution were identical to those in control cells, but Ca2+ (pCa 5.49) totally failed to produce the typical 50-70% attenuation of first-order line intensity. Cleavage of the disulfide bond in DTBP-treated cells with dithiothreitol fully restored the Ca2+ sensitivity. Lysine group modification with methylacetimidate, a monofunctional lysine modification reagent equivalent to DMS, did not block the Ca2+ sensitivity. We observed that intensity reductions can also be produced by numerous other agents and mechanisms, such as nonionic polymeric solutions of polyvinylpyrrolidone, which reduces the lattice spacing, and alkaline pH, which probably displaces the S-1 heads from a resting position close to the thick filament surface. However, the prevention of the Ca2+ effect by cross-linkers indicates that intrafilament rather than interfilament changes in structure are responsible for the light diffraction intensity decrease accompanying activation.

Structural aspects of actomyosin interaction

Actin binding to myosin-S1 modulates the limited tryptic cleavage of the COOH-terminal region of the 95K heavy chain at the joint connecting the 75K and 20K peptide units; concomitantly actin affords total protection against the resulting loss of acto-S1 Mg2+-ATPase activity. The specificity of the actin effect is illustrated by the fact that it exerts itself not only on free S1 but also on the intact myosin molecule. Mg2+-ATP and Mg2+-ADP impair the protective action of actin to an extent closely related to their respective affinity for the acto-S1 complex. Tryptic fragmentation of S1 heavy chain under highly controlled conditions, using trypsin to S1 weight ratios in the range 1:1000 - 1:1500 led us to establish that peptide bond cleavage at the 75K-20K junction is a sequential process giving rise first to a 22K peptide intermediate which is subsequently converted to the stable 20K fragment. Most importantly, it is also demonstrated that the loss of S1 activation by actin is not due to the initial scission of the 75K-22K linkage but is intimately associated with the breakdown of the 22K precursor into its 20K moiety. Three trypsin-modified S1 derivatives, the heavy chain of which is a complex of two or three fragments, were purified. A detailed analysis of the C-termini of these fragments, as compared to the C-terminal structure of the intact heavy chain, indicated that the 20K fragment is formed mainly through the degradation of a NH2-terminal 2K segment in the 22K precursor and that this proteolytic event is the only one accounting for the acto-S1 ATPase loss. Cross-linking experiments exploiting the reaction of a carbodiimide reagent with rigor complexes containing either fluorescent actin or fluorescent fragmented S1 revealed unequivocally the attachment of the actin monomer to recognition sites on the 20K and 50K units of S1 heavy chain. Specific interactions between the C-terminal 20K domain and light chain LC2 are proposed as being part of the molecular mechanism of the myosin-linked regulation of actomyosin interaction.