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Special Issue "The Actin-Myosin Interaction in Muscle"

A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Molecular Biophysics".

Deadline for manuscript submissions: closed (19 October 2018).

Special Issue Editor

Prof. Dr. John Squire
E-Mail Website
Guest Editor
Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol BS8 1TD, UK
Interests: myosin filament structure; actin filament structure and regulation; muscle contractile mechanism; sarcomere structure; M-band structure; Z-band structure; titin properties and interactions; muscle sarcomere evolution

Special Issue Information

Dear Colleagues,

Research into the mechanism of muscle contraction has reached an intriguing stage. A great deal is known about the structures of the various components that, together, form the muscle contractile machinery. The structures of myosin heads and myosin and actin filaments are fairly well understood at quite high resolutions, as are the geometries of interaction of myosin heads with actin under a variety of conditions. However, in answer to the question ‘how is muscular force actually produced?’, we have to acknowledge that there are still great uncertainties in relating what we know about muscle structure to the observed physiological behaviour of different muscles. Additionally, how do different mutations in the proteins of the contractile machinery affect how muscles actually work?  

The aim of the present Special Issue is to bring together reviews and original papers on the actual mechanism of contraction as understood from a variety of approaches. We are looking for papers or reviews in three main areas: (1) original papers or reviews on what is known about myosin head structures and the way that myosin heads interact with actin; (2) original papers or reviews on the dynamics of myosin crossbridge behaviour in active muscle; and (3) original papers or reviews on the ways that known disease-associated mutations in the various thick and thin filament proteins, including the myosin head, affect the contractile cycle.

Prof. Dr. John Squire
Guest Editor

Manuscript Submission Information

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Published Papers (14 papers)

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Editorial

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Open AccessEditorial
Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview
Int. J. Mol. Sci. 2019, 20(22), 5715; https://doi.org/10.3390/ijms20225715 - 14 Nov 2019
Cited by 1
Abstract
Muscular contraction is a fundamental phenomenon in all animals; without it life as we know it would be impossible. The basic mechanism in muscle, including heart muscle, involves the interaction of the protein filaments myosin and actin. Motility in all cells is also [...] Read more.
Muscular contraction is a fundamental phenomenon in all animals; without it life as we know it would be impossible. The basic mechanism in muscle, including heart muscle, involves the interaction of the protein filaments myosin and actin. Motility in all cells is also partly based on similar interactions of actin filaments with non-muscle myosins. Early studies of muscle contraction have informed later studies of these cellular actin-myosin systems. In muscles, projections on the myosin filaments, the so-called myosin heads or cross-bridges, interact with the nearby actin filaments and, in a mechanism powered by ATP-hydrolysis, they move the actin filaments past them in a kind of cyclic rowing action to produce the macroscopic muscular movements of which we are all aware. In this special issue the papers and reviews address different aspects of the actin-myosin interaction in muscle as studied by a plethora of complementary techniques. The present overview provides a brief and elementary introduction to muscle structure and function and the techniques used to study it. It goes on to give more detailed descriptions of what is known about muscle components and the cross-bridge cycle using structural biology techniques, particularly protein crystallography, electron microscopy and X-ray diffraction. It then has a quick look at muscle mechanics and it summarises what can be learnt about how muscle works based on the other studies covered in the different papers in the special issue. A picture emerges of the main molecular steps involved in the force-producing process; steps that are also likely to be seen in non-muscle myosin interactions with cellular actin filaments. Finally, the remarkable advances made in studying the effects of mutations in the contractile assembly in causing specific muscle diseases, particularly those in heart muscle, are outlined and discussed. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Research

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Open AccessArticle
Myosin Cross-Bridge Behaviour in Contracting Muscle—The T1 Curve of Huxley and Simmons (1971) Revisited
Int. J. Mol. Sci. 2019, 20(19), 4892; https://doi.org/10.3390/ijms20194892 - 02 Oct 2019
Cited by 1
Abstract
The stiffness of the myosin cross-bridges is a key factor in analysing possible scenarios to explain myosin head changes during force generation in active muscles. The seminal study of Huxley and Simmons (1971: Nature 233: 533) suggested that most of the observed [...] Read more.
The stiffness of the myosin cross-bridges is a key factor in analysing possible scenarios to explain myosin head changes during force generation in active muscles. The seminal study of Huxley and Simmons (1971: Nature 233: 533) suggested that most of the observed half-sarcomere instantaneous compliance (=1/stiffness) resides in the myosin heads. They showed with a so-called T1 plot that, after a very fast release, the half-sarcomere tension reduced to zero after a step size of about 60Å (later with improved experiments reduced to 40Å). However, later X-ray diffraction studies showed that myosin and actin filaments themselves stretch slightly under tension, which means that most (at least two-thirds) of the half sarcomere compliance comes from the filaments and not from cross-bridges. Here we have used a different approach, namely to model the compliances in a virtual half sarcomere structure in silico. We confirm that the T1 curve comes almost entirely from length changes in the myosin and actin filaments, because the calculated cross-bridge stiffness (probably greater than 0.4 pN/Å) is higher than previous studies have suggested. Our model demonstrates that the formulations produced by previous authors give very similar results to our model if the same starting parameters are used. However, we find that it is necessary to model the X-ray diffraction data as well as mechanics data to get a reliable estimate of the cross-bridge stiffness. In the light of the high cross-bridge stiffness found in the present study, we present a plausible modified scenario to describe aspects of the myosin cross-bridge cycle in active muscle. In particular, we suggest that, apart from the filament compliances, most of the cross-bridge contribution to the instantaneous T1 response may come from weakly-bound myosin heads, not myosin heads in strongly attached states. The strongly attached heads would still contribute to the T1 curve, but only in a very minor way, with a stiffness that we postulate could be around 0.1 pN/Å, a value which would generate a working stroke close to 100 Å from the hydrolysis of one ATP molecule. The new model can serve as a tool to calculate sarcomere elastic properties for any vertebrate striated muscle once various parameters have been determined (e.g., tension, T1 intercept, temperature, X-ray diffraction spacing results). Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessArticle
The Primary Causes of Muscle Dysfunction Associated with the Point Mutations in Tpm3.12; Conformational Analysis of Mutant Proteins as a Tool for Classification of Myopathies
Int. J. Mol. Sci. 2018, 19(12), 3975; https://doi.org/10.3390/ijms19123975 - 10 Dec 2018
Cited by 3
Abstract
Point mutations in genes encoding isoforms of skeletal muscle tropomyosin may cause nemaline myopathy, cap myopathy (Cap), congenital fiber-type disproportion (CFTD), and distal arthrogryposis. The molecular mechanisms of muscle dysfunction in these diseases remain unclear. We studied the effect of the E173A, R90P, [...] Read more.
Point mutations in genes encoding isoforms of skeletal muscle tropomyosin may cause nemaline myopathy, cap myopathy (Cap), congenital fiber-type disproportion (CFTD), and distal arthrogryposis. The molecular mechanisms of muscle dysfunction in these diseases remain unclear. We studied the effect of the E173A, R90P, E150A, and A155T myopathy-causing substitutions in γ-tropomyosin (Tpm3.12) on the position of tropomyosin in thin filaments, and the conformational state of actin monomers and myosin heads at different stages of the ATPase cycle using polarized fluorescence microscopy. The E173A, R90P, and E150A mutations produced abnormally large displacement of tropomyosin to the inner domains of actin and an increase in the number of myosin heads in strong-binding state at low and high Ca2+, which is characteristic of CFTD. On the contrary, the A155T mutation caused a decrease in the amount of such heads at high Ca2+ which is typical for mutations associated with Cap. An increase in the number of the myosin heads in strong-binding state at low Ca2+ was observed for all mutations associated with high Ca2+-sensitivity. Comparison between the typical conformational changes in mutant proteins associated with different myopathies observed with α-, β-, and γ-tropomyosins demonstrated the possibility of using such changes as tests for identifying the diseases. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessArticle
Myosin Head Configurations in Resting and Contracting Murine Skeletal Muscle
Int. J. Mol. Sci. 2018, 19(9), 2643; https://doi.org/10.3390/ijms19092643 - 06 Sep 2018
Cited by 6
Abstract
Transgenic mouse models have been important tools for studying the relationship of genotype to phenotype for human diseases, including those of skeletal muscle. We show that mouse skeletal muscle can produce high quality X-ray diffraction patterns establishing the mouse intact skeletal muscle X-ray [...] Read more.
Transgenic mouse models have been important tools for studying the relationship of genotype to phenotype for human diseases, including those of skeletal muscle. We show that mouse skeletal muscle can produce high quality X-ray diffraction patterns establishing the mouse intact skeletal muscle X-ray preparation as a potentially powerful tool to test structural hypotheses in health and disease. A notable feature of the mouse model system is the presence of residual myosin layer line intensities in contracting mouse muscle patterns. This provides an additional tool, along with the I1,1/I1,0 intensity ratio, for estimating the proportions of active versus relaxed myosin heads under a given set of conditions that can be used to characterize a given physiological condition or mutant muscle type. We also show that analysis of the myosin layer line intensity distribution, including derivation of the myosin head radius, Rm, may be used to study the role of the super-relaxed state in myosin regulation. When the myosin inhibitor blebbistatin is used to inhibit force production, there is a shift towards a highly quasi-helically ordered configuration that is distinct from the normal resting state, indicating there are more than one helically ordered configuration for resting crossbridges. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessArticle
Different Myosin Head Conformations in Bony Fish Muscles Put into Rigor at Different Sarcomere Lengths
Int. J. Mol. Sci. 2018, 19(7), 2091; https://doi.org/10.3390/ijms19072091 - 18 Jul 2018
Cited by 2
Abstract
At a resting sarcomere length of approximately 2.2 µm bony fish muscles put into rigor in the presence of BDM (2,3-butanedione monoxime) to reduce rigor tension generation show the normal arrangement of myosin head interactions with actin filaments as monitored by low-angle X-ray [...] Read more.
At a resting sarcomere length of approximately 2.2 µm bony fish muscles put into rigor in the presence of BDM (2,3-butanedione monoxime) to reduce rigor tension generation show the normal arrangement of myosin head interactions with actin filaments as monitored by low-angle X-ray diffraction. However, if the muscles are put into rigor using the same protocol but stretched to 2.5 µm sarcomere length, a markedly different structure is observed. The X-ray diffraction pattern is not just a weaker version of the pattern at full overlap, as might be expected, but it is quite different. It is compatible with the actin-attached myosin heads being in a different conformation on actin, with the average centre of cross-bridge mass at a higher radius than in normal rigor and the myosin lever arms conforming less to the actin filament geometry, probably pointing back to their origins on their parent myosin filaments. The possible nature of this new rigor cross-bridge conformation is discussed in terms of other well-known states such as the weak binding state and the ‘roll and lock’ mechanism; we speculate that we may have trapped most myosin heads in an early attached strong actin-binding state in the cross-bridge cycle on actin. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Review

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Open AccessReview
Insights into Actin-Myosin Interactions within Muscle from 3D Electron Microscopy
Int. J. Mol. Sci. 2019, 20(7), 1703; https://doi.org/10.3390/ijms20071703 - 05 Apr 2019
Cited by 4
Abstract
Much has been learned about the interaction between myosin and actin through biochemistry, in vitro motility assays and cryo-electron microscopy (cryoEM) of F-actin, decorated with myosin heads. Comparatively less is known about actin-myosin interactions within the filament lattice of muscle, where myosin heads [...] Read more.
Much has been learned about the interaction between myosin and actin through biochemistry, in vitro motility assays and cryo-electron microscopy (cryoEM) of F-actin, decorated with myosin heads. Comparatively less is known about actin-myosin interactions within the filament lattice of muscle, where myosin heads function as independent force generators and thus most measurements report an average signal from multiple biochemical and mechanical states. All of the 3D imaging by electron microscopy (EM) that has revealed the interplay of the regular array of actin subunits and myosin heads within the filament lattice has been accomplished using the flight muscle of the large water bug Lethocerus sp. The Lethocerus flight muscle possesses a particularly favorable filament arrangement that enables all the myosin cross-bridges contacting the actin filament to be visualized in a thin section. This review covers the history of this effort and the progress toward visualizing the complex set of conformational changes that myosin heads make when binding to actin in several static states, as well as the fast frozen actively contracting muscle. The efforts have revealed a consistent pattern of changes to the myosin head structures as determined by X-ray crystallography needed to explain the structure of the different actomyosin interactions observed in situ. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Actin-Myosin Interaction: Structure, Function and Drug Discovery
Int. J. Mol. Sci. 2018, 19(9), 2628; https://doi.org/10.3390/ijms19092628 - 05 Sep 2018
Cited by 4
Abstract
Actin-myosin interactions play crucial roles in the generation of cellular force and movement. The molecular mechanism involves structural transitions at the interface between actin and myosin’s catalytic domain, and within myosin’s light chain domain, which contains binding sites for essential (ELC) and regulatory [...] Read more.
Actin-myosin interactions play crucial roles in the generation of cellular force and movement. The molecular mechanism involves structural transitions at the interface between actin and myosin’s catalytic domain, and within myosin’s light chain domain, which contains binding sites for essential (ELC) and regulatory light chains (RLC). High-resolution crystal structures of isolated actin and myosin, along with cryo-electron micrographs of actin-myosin complexes, have been used to construct detailed structural models for actin-myosin interactions. However, these methods are limited by disorder, particularly within the light chain domain, and they do not capture the dynamics within this complex under physiological conditions in solution. Here we highlight the contributions of site-directed fluorescent probes and time-resolved fluorescence resonance energy transfer (TR-FRET) in understanding the structural dynamics of the actin-myosin complex in solution. A donor fluorescent probe on actin and an acceptor fluorescent probe on myosin, together with high performance TR-FRET, directly resolves structural states in the bound actin-myosin complex during its interaction with adenosine triphosphate (ATP). Results from these studies have profound implications for understanding the contractile function of actomyosin and establish the feasibility for the discovery of allosteric modulators of the actin-myosin interaction, with the ultimate goal of developing therapies for muscle disorders. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Cardiomyopathies and Related Changes in Contractility of Human Heart Muscle
Int. J. Mol. Sci. 2018, 19(8), 2234; https://doi.org/10.3390/ijms19082234 - 31 Jul 2018
Cited by 7
Abstract
About half of hypertrophic and dilated cardiomyopathies cases have been recognized as genetic diseases with mutations in sarcomeric proteins. The sarcomeric proteins are involved in cardiomyocyte contractility and its regulation, and play a structural role. Mutations in non-sarcomeric proteins may induce changes in [...] Read more.
About half of hypertrophic and dilated cardiomyopathies cases have been recognized as genetic diseases with mutations in sarcomeric proteins. The sarcomeric proteins are involved in cardiomyocyte contractility and its regulation, and play a structural role. Mutations in non-sarcomeric proteins may induce changes in cell signaling pathways that modify contractile response of heart muscle. These facts strongly suggest that contractile dysfunction plays a central role in initiation and progression of cardiomyopathies. In fact, abnormalities in contractile mechanics of myofibrils have been discovered. However, it has not been revealed how these mutations increase risk for cardiomyopathy and cause the disease. Much research has been done and still much is being done to understand how the mechanism works. Here, we review the facts of cardiac myofilament contractility in patients with cardiomyopathy and heart failure. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
The Molecular Mechanisms of Mutations in Actin and Myosin that Cause Inherited Myopathy
Int. J. Mol. Sci. 2018, 19(7), 2020; https://doi.org/10.3390/ijms19072020 - 11 Jul 2018
Cited by 7
Abstract
The discovery that mutations in myosin and actin genes, together with mutations in the other components of the muscle sarcomere, are responsible for a range of inherited muscle diseases (myopathies) has revolutionized the study of muscle, converting it from a subject of basic [...] Read more.
The discovery that mutations in myosin and actin genes, together with mutations in the other components of the muscle sarcomere, are responsible for a range of inherited muscle diseases (myopathies) has revolutionized the study of muscle, converting it from a subject of basic science to a relevant subject for clinical study and has been responsible for a great increase of interest in muscle studies. Myopathies are linked to mutations in five of the myosin heavy chain genes, three of the myosin light chain genes, and three of the actin genes. This review aims to determine to what extent we can explain disease phenotype from the mutant genotype. To optimise our chances of finding the right mechanism we must study a myopathy where there are a large number of different mutations that cause a common phenotype and so are likely to have a common mechanism: a corollary to this criterion is that if any mutation causes the disease phenotype but does not correspond to the proposed mechanism, then the whole mechanism is suspect. Using these criteria, we consider two cases where plausible genotype-phenotype mechanisms have been proposed: the actin “A-triad” and the myosin “mesa/IHD” models. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Do Actomyosin Single-Molecule Mechanics Data Predict Mechanics of Contracting Muscle?
Int. J. Mol. Sci. 2018, 19(7), 1863; https://doi.org/10.3390/ijms19071863 - 25 Jun 2018
Cited by 10
Abstract
In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this difference we compare results from single-molecule studies and muscle mechanics and analyze to what degree data from the [...] Read more.
In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this difference we compare results from single-molecule studies and muscle mechanics and analyze to what degree data from the two types of studies agree with each other. There is reasonable correspondence in estimates of the cross-bridge power-stroke distance (7–13 nm), cross-bridge stiffness (~2 pN/nm) and average isometric force per cross-bridge (6–9 pN). Furthermore, models defined on the basis of single-molecule mechanics and solution biochemistry give good fits to experimental data from muscle. This suggests that the ordered myofilament lattice, accessory proteins and emergent effects of the sarcomere organization have only minor modulatory roles. However, such factors may be of greater importance under e.g., disease conditions. We also identify areas where single-molecule and muscle data are conflicting: (1) whether force generation is an Eyring or Kramers process with just one major power-stroke or several sub-strokes; (2) whether the myofilaments and the cross-bridges have Hookean or non-linear elasticity; (3) if individual myosin heads slip between actin sites under certain conditions, e.g., in lengthening; or (4) if the two heads of myosin cooperate. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Synchrotron Radiation X-ray Diffraction Techniques Applied to Insect Flight Muscle
Int. J. Mol. Sci. 2018, 19(6), 1748; https://doi.org/10.3390/ijms19061748 - 13 Jun 2018
Cited by 4
Abstract
X-ray fiber diffraction is a powerful tool used for investigating the molecular structure of muscle and its dynamics during contraction. This technique has been successfully applied not only to skeletal and cardiac muscles of vertebrates but also to insect flight muscle. Generally, insect [...] Read more.
X-ray fiber diffraction is a powerful tool used for investigating the molecular structure of muscle and its dynamics during contraction. This technique has been successfully applied not only to skeletal and cardiac muscles of vertebrates but also to insect flight muscle. Generally, insect flight muscle has a highly ordered structure and is often capable of high-frequency oscillations. The X-ray diffraction studies on muscle have been accelerated by the advent of 3rd-generation synchrotron radiation facilities, which can generate brilliant and highly oriented X-ray beams. This review focuses on some of the novel experiments done on insect flight muscle by using synchrotron radiation X-rays. These include diffraction recordings from single myofibrils within a flight muscle fiber by using X-ray microbeams and high-speed diffraction recordings from the flight muscle during the wing-beat of live insects. These experiments have provided information about the molecular structure and dynamic function of flight muscle in unprecedented detail. Future directions of X-ray diffraction studies on muscle are also discussed. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Temperature Effects on Force and Actin–Myosin Interaction in Muscle: A Look Back on Some Experimental Findings
Int. J. Mol. Sci. 2018, 19(5), 1538; https://doi.org/10.3390/ijms19051538 - 22 May 2018
Cited by 6
Abstract
Observations made in temperature studies on mammalian muscle during force development, shortening, and lengthening, are re-examined. The isometric force in active muscle goes up substantially on warming from less than 10 °C to temperatures closer to physiological (>30 °C), and the sigmoidal temperature [...] Read more.
Observations made in temperature studies on mammalian muscle during force development, shortening, and lengthening, are re-examined. The isometric force in active muscle goes up substantially on warming from less than 10 °C to temperatures closer to physiological (>30 °C), and the sigmoidal temperature dependence of this force has a half-maximum at ~10 °C. During steady shortening, when force is decreased to a steady level, the sigmoidal curve is more pronounced and shifted to higher temperatures, whereas, in lengthening muscle, the curve is shifted to lower temperatures, and there is a less marked increase with temperature. Even with a small rapid temperature-jump (T-jump), force in active muscle rises in a definitive way. The rate of tension rise is slower with adenosine diphosphate (ADP) and faster with increased phosphate. Analysis showed that a T-jump enhances an early, pre-phosphate release step in the acto-myosin (crossbridge) ATPase cycle, thus inducing a force-rise. The sigmoidal dependence of steady force on temperature is due to this endothermic nature of crossbridge force generation. During shortening, the force-generating step and the ATPase cycle are accelerated, whereas during lengthening, they are inhibited. The endothermic force generation is seen in different muscle types (fast, slow, and cardiac). The underlying mechanism may involve a structural change in attached myosin heads and/or their attachments on heat absorption. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Unconventional Imaging Methods to Capture Transient Structures during Actomyosin Interaction
Int. J. Mol. Sci. 2018, 19(5), 1402; https://doi.org/10.3390/ijms19051402 - 08 May 2018
Cited by 3
Abstract
Half a century has passed since the cross-bridge structure was recognized as the molecular machine that generates muscle tension. Despite various approaches by a number of scientists, information on the structural changes in the myosin heads, particularly its transient configurations, remains scant even [...] Read more.
Half a century has passed since the cross-bridge structure was recognized as the molecular machine that generates muscle tension. Despite various approaches by a number of scientists, information on the structural changes in the myosin heads, particularly its transient configurations, remains scant even now, in part because of their small size and rapid stochastic movements during the power stroke. Though progress in cryo-electron microscopy is eagerly awaited as the ultimate means to elucidate structural details, the introduction of some unconventional methods that provide high-contrast raw images of the target protein assemblies is quite useful, if available, to break the current impasse. Quick-freeze deep–etch–replica electron microscopy coupled with dedicated image analysis procedures, and high-speed atomic-force microscopy are two such candidates. We have applied the former to visualize actin-associated myosin heads under in vitro motility assay conditions, and found that they take novel configurations similar to the SH1–SH2-crosslinked myosin that we characterized recently. By incorporating biochemical and biophysical results, we have revised the cross-bridge mechanism to involve the new conformer as an important main player. The latter “microscopy” is unique and advantageous enabling continuous observation of various protein assemblies as they function. Direct observation of myosin-V’s movement along actin filaments revealed several unexpected behaviors such as foot-stomping of the leading head and unwinding of the coiled-coil tail. The potential contribution of these methods with intermediate spatial resolution is discussed. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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Open AccessReview
Electron Microscopic Recording of the Power and Recovery Strokes of Individual Myosin Heads Coupled with ATP Hydrolysis: Facts and Implications
Int. J. Mol. Sci. 2018, 19(5), 1368; https://doi.org/10.3390/ijms19051368 - 04 May 2018
Cited by 1
Abstract
The most straightforward way to get information on the performance of individual myosin heads producing muscle contraction may be to record their movement, coupled with ATP hydrolysis, electron-microscopically using the gas environmental chamber (EC). The EC enables us to visualize and record ATP-induced [...] Read more.
The most straightforward way to get information on the performance of individual myosin heads producing muscle contraction may be to record their movement, coupled with ATP hydrolysis, electron-microscopically using the gas environmental chamber (EC). The EC enables us to visualize and record ATP-induced myosin head movement in hydrated skeletal muscle myosin filaments. When actin filaments are absent, myosin heads fluctuate around a definite neutral position, so that their time-averaged mean position remains unchanged. On application of ATP, myosin heads are found to move away from, but not towards, the bare region, indicating that myosin heads perform a recovery stroke (average amplitude, 6 nm). After exhaustion of ATP, myosin heads return to their neutral position. In the actin–myosin filament mixture, myosin heads form rigor actin myosin linkages, and on application of ATP, they perform a power stroke by stretching adjacent elastic structures because of a limited amount of applied ATP ≤ 10 µM. The average amplitude of the power stroke is 3.3 nm and 2.5 nm at the distal and the proximal regions of the myosin head catalytic domain (CAD), respectively. The power stroke amplitude increases appreciably at low ionic strength, which is known to enhance Ca2+-activated force in muscle. In both the power and recovery strokes, myosin heads return to their neutral position after exhaustion of ATP. Full article
(This article belongs to the Special Issue The Actin-Myosin Interaction in Muscle)
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