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

Early signs of architectural and biomechanical failure in isolated myofibers and immortalized myoblasts from desmin-mutant knock-in mice

In striated muscle, desmin intermediate filaments interlink the contractile myofibrillar apparatus with mitochondria, nuclei, and the sarcolemma. The desmin network’s pivotal role in myocytes is evident since mutations in the human desmin gene cause severe myopathies and cardiomyopathies. Here, we investigated skeletal muscle pathology in myofibers and myofibrils isolated from young hetero- and homozygous R349P desmin knock-in mice, which carry the orthologue of the most frequent human desmin missense mutation R350P. We demonstrate that mutant desmin alters myofibrillar cytoarchitecture, markedly disrupts the lateral sarcomere lattice and distorts myofibrillar angular axial orientation. Biomechanical assessment revealed a high predisposition to stretch-induced damage in fiber bundles of R349P mice. Notably, Ca²⁺-sensitivity and passive myofibrillar tension were decreased in heterozygous fiber bundles, but increased in homozygous fiber bundles compared to wildtype mice. In a parallel approach, we generated and subsequently subjected immortalized heterozygous R349P desmin knock-in myoblasts to magnetic tweezer experiments that revealed a significantly increased sarcolemmal lateral stiffness. Our data suggest that mutated desmin already markedly impedes myocyte structure and function at pre-symptomatic stages of myofibrillar myopathies.

Dual effect of actin on the accessibility of myosin essential light chain A1 to papain cleavage

The influence of various amounts of actin on the proteolytic susceptibility of myosin essential light chain (ELC) A1, the binding of isolated A1 light chain and the N-peptide spanning N-terminal sequence of A1 to actin is studied to obtain more information on the role of the N-terminus of A1 light chain in the myosin-actin interaction. Low ratios of actin to myosin (1:1) lead to stimulate cleavage, whereas higher ratios (4:1) lead to protection of A1. Exposure of A1 by actin is especially seen in heavy meromyosin (HMM) and monomeric myosin and this is probably related to the full saturation of actin protomers with myosin heads. The protecting action of actin on A1 cleavage is more pronounced in myosin filaments. Conditions favoring the saturation of myosin regulatory light chain (RLC) with calcium ions instead of magnesium ions promotes the protection of A1. Cross-linking of HMM and actin results in higher yields of A1-actin product at high actin to myosin heads ratios. Isolated A1 light chain is pelleted by actin. A synthetic peptide spanning the N-terminal sequence of A1 can be cross-linked to actin. It is postulated that the protective action of actin on A1 papain cleavage is caused by the binding of the A1 N-terminus to actin. Changes in the RLC phosphorylation level and magnesium-for-calcium exchange in RLC may affect the probability of this interaction.

The possible role of myosin A1 light chain in the weakening of actin-myosin interaction

The effects resulting from the removal of the N-terminus of myosin A1 by limited papain cleavage are investigated. The myosin and heavy meromyosin K+-ATPase and Ca2+-ATPase activities, and actin-activated ATPase activity of heavy meromyosin (HMM) and subfragment-1, are studied. Myosin and HMM preparations devoid of the A1 N-terminus exhibits lower Ca2+-ATPase activities at low ionic strength whereas no differences in K+- or Ca2+-ATPase activities are observed at high ionic strength. Direct binding of actin to monomeric myosin under K+-activated ATPase conditions is much more effective for myosin containing a shortened A1 light chain. The kinetic constants K(app) for actin and V(max) are calculated from actin-activation curves for HMM and subfragment-1. The kinetic constants for HMM are determined under conditions assuring saturation of regulatory light chains (RLC) either with Mg2+ or Ca2+. The removal of the A1 N-terminus influences the actin-myosin interaction in a Ca2+- and phosphorylation-dependent manner; in most cases, this leads to an increase in affinity. In the case of subfragment-1, the removal of the N-terminus of A1 led to a decrease in affinity. It is reasonable to assume that the intact A1 light chain may cause weakening of the actin-myosin interaction under certain conditions. This weakening may be regulated by RLC phosphorylation and RLC-bound calcium-for-magnesium exchange. Such an effect requires a structural minimum that is present in HMM but not in subfragment-1. Implications of such a role for the A1 N-terminus in the myosin-actin interaction are discussed.

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.

Regulatory light chain influences alterations of myosin head induced by actin

The effect of magnesium-for-calcium exchange and phosphorylation of regulatory light chain (LC2) on structural organization of rabbit skeletal myosin head was studied by limited tryptic digestion. In the presence of actin, exchange of magnesium bound to LC2 by calcium in dephosphorylated myosin accelerates the digestion of myosin and heavy meromyosin heavy chain and increases the accumulation of a 50 kDa fragment. This effect is significantly diminished in the case of phosphorylated myosin. Thus, both phosphorylation and cation exchange influences the effect of actin binding on the structural organization of myosin head.

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.

Conformational changes of F-actin in myosin-free ghost single fibre induced by either phosphorylated or dephosphorylated heavy meromyosin

The changes in F-actin conformation of myosin-free single ghost fibre induced by binding of phosphorylated or dephosphorylated heavy meromyosin have been studied by measuring polarized fluorescence of F-actin intrinsic tryptophan and of phalloidin-rhodamine bound to F-actin. The changes of polarization of both fluorescences were found to be dependent on low or high Ca2+ concentration and on the phosphorylated or dephosphorylated form of heavy meromyosin. Computer analysis of polarized fluorescence has shown that binding of phosphorylated heavy meromyosin with divalent ion binding sites saturated with Mg2 (in the presence of 1 mM MgCl2 and 1 mM EGTA) and dephosphorylated heavy meromyosin with divalent ion binding sites saturated with Ca2+ (in the presence of 1 mM MgCl2 and 0.1 mM Ca2+) decreases the angles of emission and absorption dipoles and the angle between the F-actin axis and the fibre axis, thus suggesting that F-actin in ghost fibre becomes more flexible. On the other hand, the above-mentioned angles increase when phosphorylated heavy meromyosin at high and dephosphorylated heavy meromyosin at low Ca2+ concentration were bound to thin filaments, thus showing the decrease of F-actin flexibility under these conditions.

Effect of phosphorylation of myosin light chains on interaction of heavy meromyosin with regulated F-actin in ghost fibers

The binding of phosphorylated heavy meromyosin to regulated F-actin in ghost fibers at high Ca2+ concentration increases, and at low Ca2+ concentration decreases, the anisotropy of intrinsic tryptophan fluorescence of F-actin. The effect is opposite to the effect of the binding of dephosphorylated heavy meromyosin.

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+.

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.