The swinging lever-arm hypothesis of muscle contraction

Myosin folding boosts solubility in cardiac muscle sarcomeres

The polymerization of myosin molecules into thick filaments in muscle sarcomeres is essential for cardiac contractility, with the attenuation of interactions between the heads of myosin molecules within the filaments being proposed to result in hypercontractility, as observed in hypertrophic cardiomyopathy (HCM). However, experimental evidence demonstrates that the structure of these giant macromolecular complexes is highly dynamic, with molecules exchanging between the filaments and a pool of soluble molecules on the minute timescale. Therefore, we sought to test the hypothesis that the enhancement of interactions between the heads of myosin molecules within thick filaments limits the mobility of myosin by taking advantage of mavacamten, a small molecule approved for the treatment of HCM. Myosin molecules were labeled in vivo with a green fluorescent protein (GFP) and imaged in intact hearts using multiphoton microscopy. Treatment of the intact hearts with mavacamten resulted in an unexpected > 5-fold enhancement in GFP-myosin mobility within the sarcomere. In vitro biochemical assays suggested that mavacamten enhanced the mobility of GFP-myosin by increasing the solubility of myosin molecules, through the stabilization of a compact/folded conformation of the molecules, once disassociated from the thick filaments. These findings provide alternative insight into the mechanisms by which molecules exchange into and out of thick filaments and have implications for how mavacamten may affect cardiac contractility.

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

The mechanics of the heart: zooming in on hypertrophic cardiomyopathy and cMyBP-C

Hypertrophic cardiomyopathy (HCM), a disease characterized by cardiac muscle hypertrophy and hypercontractility, is the most frequently inherited disorder of the heart. HCM is mainly caused by variants in genes encoding proteins of the sarcomere, the basic contractile unit of cardiomyocytes. The most frequently mutated among them is MYBPC3, which encodes cardiac myosin-binding protein C (cMyBP-C), a key regulator of sarcomere contraction. In this review, we summarize clinical and genetic aspects of HCM and provide updated information on the function of the healthy and HCM sarcomere, as well as on emerging therapeutic options targeting sarcomere mechanical activity. Building on what is known about cMyBP-C activity, we examine different pathogenicity drivers by which MYBPC3 variants can cause disease, focussing on protein haploinsufficiency as a common pathomechanism also in nontruncating variants. Finally, we discuss recent evidence correlating altered cMyBP-C mechanical properties with HCM development.

Discovery of ultrafast myosin, its amino acid sequence, and structural features

Cytoplasmic streaming with extremely high velocity (∼70 μm s^-1) occurs in cells of the characean algae (Chara). Because cytoplasmic streaming is caused by myosin XI, it has been suggested that a myosin XI with a velocity of 70 μm s^-1, the fastest myosin measured so far, exists in Chara cells. However, the velocity of the previously cloned Chara corallina myosin XI (CcXI) was about 20 μm s^-1, one-third of the cytoplasmic streaming velocity in Chara Recently, the genome sequence of Chara braunii has been published, revealing that this alga has four myosin XI genes. We cloned these four myosin XI (CbXI-1, 2, 3, and 4) and measured their velocities. While the velocities of CbXI-3 and CbXI-4 motor domains (MDs) were similar to that of CcXI MD, the velocities of CbXI-1 and CbXI-2 MDs were 3.2 times and 2.8 times faster than that of CcXI MD, respectively. The velocity of chimeric CbXI-1, a functional, full-length CbXI-1 construct, was 60 μm s^-1 These results suggest that CbXI-1 and CbXI-2 would be the main contributors to cytoplasmic streaming in Chara cells and show that these myosins are ultrafast myosins with a velocity 10 times faster than fast skeletal muscle myosins in animals. We also report an atomic structure (2.8-Å resolution) of myosin XI using X-ray crystallography. Based on this crystal structure and the recently published cryo-electron microscopy structure of acto-myosin XI at low resolution (4.3-Å), it appears that the actin-binding region contributes to the fast movement of Chara myosin XI. Mutation experiments of actin-binding surface loops support this hypothesis.

High-resolution structures of the actomyosin-V complex in three nucleotide states provide insights into the force generation mechanism

The molecular motor myosin undergoes a series of major structural transitions during its force-producing motor cycle. The underlying mechanism and its coupling to ATP hydrolysis and actin binding are only partially understood, mostly due to sparse structural data on actin-bound states of myosin. Here, we report 26 high-resolution cryo-EM structures of the actomyosin-V complex in the strong-ADP, rigor, and a previously unseen post-rigor transition state that binds the ATP analog AppNHp. The structures reveal a high flexibility of myosin in each state and provide valuable insights into the structural transitions of myosin-V upon ADP release and binding of AppNHp, as well as the actomyosin interface. In addition, they show how myosin is able to specifically alter the structure of F-actin.

The molecular basis for sarcomere organization in vertebrate skeletal muscle

Sarcomeres are force-generating and load-bearing devices of muscles. A precise molecular picture of how sarcomeres are built underpins understanding their role in health and disease. Here, we determine the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography. Our reconstruction reveals molecular details of the three-dimensional organization and interaction of actin and myosin in the A-band, I-band, and Z-disc and demonstrates that α-actinin cross-links antiparallel actin filaments by forming doublets with 6-nm spacing. Structures of myosin, tropomyosin, and actin at ~10 Å further reveal two conformations of the "double-head" myosin, where the flexible orientation of the lever arm and light chains enable myosin not only to interact with the same actin filament, but also to split between two actin filaments. Our results provide unexpected insights into the fundamental organization of vertebrate skeletal muscle and serve as a strong foundation for future investigations of muscle diseases.

FRET and optical trapping reveal mechanisms of actin activation of the power stroke and phosphate release in myosin V

Myosins generate force and motion by precisely coordinating their mechanical and chemical cycles, but the nature and timing of this coordination remains controversial. We utilized a FRET approach to examine the kinetics of structural changes in the force-generating lever arm in myosin V. We directly compared the FRET results with single-molecule mechanical events examined by optical trapping. We introduced a mutation (S217A) in the conserved switch I region of the active site to examine how myosin couples structural changes in the actin- and nucleotide-binding regions with force generation. Specifically, S217A enhanced the maximum rate of lever arm priming (recovery stroke) while slowing ATP hydrolysis, demonstrating that it uncouples these two steps. We determined that the mutation dramatically slows both actin-induced rotation of the lever arm (power stroke) and phosphate release (≥10-fold), whereas our simulations suggest that the maximum rate of both steps is unchanged by the mutation. Time-resolved FRET revealed that the structure of the pre- and post-power stroke conformations and mole fractions of these conformations were not altered by the mutation. Optical trapping results demonstrated that S217A does not dramatically alter unitary displacements or slow the working stroke rate constant, consistent with the mutation disrupting an actin-induced conformational change prior to the power stroke. We propose that communication between the actin- and nucleotide-binding regions of myosin assures a proper actin-binding interface and active site have formed before producing a power stroke. Variability in this coupling is likely crucial for mediating motor-based functions such as muscle contraction and intracellular transport.

Millisecond Conformational Dynamics of Skeletal Myosin II Power Stroke Studied by High-Speed Atomic Force Microscopy

Myosin-based molecular motors are responsible for a variety of functions in the cells. Myosin II is ultimately responsible for muscle contraction and can be affected by multiple mutations, that may lead to myopathies. Therefore, it is essential to understand the nanomechanical properties of myosin II. Due to the lack of technical capabilities to visualize rapid changes in nonprocessive molecular motors, there are several mechanistic details in the force-generating steps produced by myosin II that are poorly understood. In this study, high-speed atomic force microscopy was used to visualize the actin-myosin complex at high temporal and spatial resolutions, providing further details about the myosin mechanism of force generation. A two-step motion of the double-headed heavy meromyosin (HMM) lever arm, coupled to an 8.4 nm working stroke was observed in the presence of ATP. HMM heads attached to an actin filament worked independently, exhibiting different lever arm configurations in given time during experiments. A lever arm rotation was associated with several non-stereospecific long-lived and stereospecific short-lived (∼1 ms) HMM conformations. The presence of free P_i increased the short-lived stereospecific binding events in which the power stroke occurred, followed by release of P_i after the power stroke.

Characterization of Contractile Proteins from Skeletal Muscle Using Gel-Based Top-Down Proteomics

The mass spectrometric analysis of skeletal muscle proteins has used both peptide-centric and protein-focused approaches. The term 'top-down proteomics' is often used in relation to studying purified proteoforms and their post-translational modifications. Two-dimensional gel electrophoresis, in combination with peptide generation for the identification and characterization of intact proteoforms being present in two-dimensional spots, plays a critical role in specific applications of top-down proteomics. A decisive bioanalytical advantage of gel-based and top-down approaches is the initial bioanalytical focus on intact proteins, which usually enables the swift identification and detailed characterisation of specific proteoforms. In this review, we describe the usage of two-dimensional gel electrophoretic top-down proteomics and related approaches for the systematic analysis of key components of the contractile apparatus, with a special focus on myosin heavy and light chains and their associated regulatory proteins. The detailed biochemical analysis of proteins belonging to the thick and thin skeletal muscle filaments has decisively improved our biochemical understanding of structure-function relationships within the contractile apparatus. Gel-based and top-down proteomics has clearly established a variety of slow and fast isoforms of myosin, troponin and tropomyosin as excellent markers of fibre type specification and dynamic muscle transition processes.

The load dependence of muscle's force-velocity curve is modulated by alternative myosin converter domains

The hyperbolic shape of the muscle force-velocity relationship (FVR) is characteristic of all muscle fiber types. The degree of curvature of the hyperbola varies between muscle fiber types and is thought to be set by force-dependent properties of different myosin isoforms. However, the structural elements in myosin and the mechanism that determines force dependence are unresolved. We tested our hypothesis that the myosin converter domain plays a critical role in the force-velocity relationship (FVR) mechanism. Drosophila contains a single myosin heavy chain gene with five converters encoded by alternative exons. We measured FVR properties of Drosophila jump muscle fibers from five transgenic lines each expressing a single converter. Consistent with our hypothesis, we observed up to 2.4-fold alterations in FVR curvature. Maximum shortening velocity (v₀) and optimal velocity for maximum power generation were also altered, but isometric tension and maximum power generation were unaltered. Converter 11a, normally found in the indirect flight muscle (IFM), imparted the highest FVR curvature and v₀, whereas converter 11d, found in larval body wall muscle, imparted the most linear FVR and slowest v₀. Jump distance strongly correlated with increasing FVR curvature and v₀, meaning flies expressing the converter from the IFM jumped farther than flies expressing the native jump muscle converter. Fitting our data with Huxley's two-state model and a biophysically based four-state model suggest a testable hypothesis that the converter sets muscle type FVR curvature by influencing the detachment rate of negatively strained myosin via changes in the force dependence of product release.

Cross-bridge mechanism of residual force enhancement after stretching in a skeletal muscle

A muscle model that uses a modified Langevin equation with actomyosin potentials was used to describe the residual force enhancement after active stretching. Considering that the new model uses cross-bridge theory to describe the residual force enhancement, it is different from other models that use passive stretching elements. Residual force enhancement was simulated using a half sarcomere comprising 100 myosin molecules. In this paper, impulse is defined as the integral of an excess force from the steady isometric force over the time interval for which a stretch is applied. The impulse was calculated from the force response due to fast and slow muscle stretches to demonstrate the viscoelastic property of the cross-bridges. A cross-bridge mechanism was proposed as a way to describe the residual force enhancement on the basis of the impulse results with reference to the compliance of the actin filament. It was assumed that the period of the actin potential increased by 0.5% and the amplitude of the potential decreased by 0.5% when the half sarcomere was stretched by 10%. The residual force enhancement after 21.0% sarcomere stretching was 6.9% of the maximum isometric force of the muscle; this value was due to the increase in the number of cross-bridges.

Radial stiffness characteristics of the overlap regions of sarcomeres in isolated skeletal myofibrils in pre-force generating state

We have studied the stiffness of myofilament lattice in sarcomeres in the pre-force generating state, which was realized by a relaxing reagent, BDM (butane dione monoxime). First, the radial stiffness for the overlap regions of sarcomeres of isolated single myofibrils was estimated from the resulting decreases in diameter by osmotic pressure applied with the addition of Dextran. Then, the radial stiffness was also estimated from force-distance curve measurements with AFM technology. The radial stiffness for the overlap regions thus obtained was composed of a soft and a rigid component. The soft component visco-elastically changed in a characteristic fashion depending on the physiological conditions of myofibrils, suggesting that it comes from cross-bridge structures. BDM treatments significantly affected the soft radial component of contracting myofibrils depending on the approach velocity of cantilever: It was nearly equal to that in the contracting state at high approach velocity, whereas as low as that in the relaxing state at low approach velocity. However, comparable BDM treatments greatly suppressed the force production and the axial stiffness in contracting glycerinated muscle fibers and also the sliding velocity of actin filaments in the in vitro motility assay. Considering that BDM shifts the cross-bridge population from force generating to pre-force generating states in contracting muscle, the obtained results strongly suggest that cross-bridges in the pre-force generating state are visco-elastically attached to the thin filaments in such a binding manner that the axial stiffness is low but the radial stiffness significantly high similar to that in force generating state.

Cryo-EM structures reveal specialization at the myosin VI-actin interface and a mechanism of force sensitivity

Despite extensive scrutiny of the myosin superfamily, the lack of high-resolution structures of actin-bound states has prevented a complete description of its mechanochemical cycle and limited insight into how sequence and structural diversification of the motor domain gives rise to specialized functional properties. Here we present cryo-EM structures of the unique minus-end directed myosin VI motor domain in rigor (4.6 Å) and Mg-ADP (5.5 Å) states bound to F-actin. Comparison to the myosin IIC-F-actin rigor complex reveals an almost complete lack of conservation of residues at the actin-myosin interface despite preservation of the primary sequence regions composing it, suggesting an evolutionary path for motor specialization. Additionally, analysis of the transition from ADP to rigor provides a structural rationale for force sensitivity in this step of the mechanochemical cycle. Finally, we observe reciprocal rearrangements in actin and myosin accompanying the transition between these states, supporting a role for actin structural plasticity during force generation by myosin VI.

Like miniature motors, proteins called myosins generate the forces needed for cells to move and for muscles to contract. Myosins use the energy stored in a chemical called ATP to move along filaments made from another protein called actin and produce force. The same part of the myosin protein that binds to and uses ATP also contacts actin. As a myosin protein consumes ATP, it cycles through a series of shape changes to drive the motor protein forward, altering how it interacts with the actin filament in the process.

Although all myosins use ATP in fundamentally the same way, individual members of this protein family have specialized properties that enable them to carry out different roles. It is not clear whether each type of myosin makes unique contacts with the actin filament, which could help determine these properties. Furthermore, mechanical forces can control the activity of myosin motors in ways that are poorly understood.

Gurel et al. have now looked at a family member called myosin VI, which moves in the opposite direction along actin filaments relative to other myosins, to better understand the properties of these proteins. An imaging technique called cryo-electron microscopy (cryo-EM) was used to determine the three-dimensional structure of myosin VI bound to actin at two steps in its cycle. Gurel et al. found that myosin VI formed specific interactions with actin that were very different from another myosin family member called myosin IIc, whose structure bound to actin was already known. In addition, the structural changes observed between the two stages of myosin VI’s cycle provided insight into how force could be used to control the motor.

Together these findings give a more detailed picture of how myosins work. They suggest that the surface of myosin that contacts actin can evolve to change the properties of a specific myosin. Studies of other myosins bound to actin will provide further insight into how distinct interactions relate to motor-specific properties. Future studies could also help scientists to understand how mutations in genes for myosins – which have been linked to a number of diseases in humans – alter the way in which myosins interact with actin filaments. This in turn could give insight into how these mutations disrupt the proteins’ activities.

Physical driving force of actomyosin motility based on the hydration effect

We propose a driving force hypothesis based on previous thermodynamics, kinetics and structural data as well as additional experiments and calculations presented here on water-related phenomena in the actomyosin systems. Although Szent-Györgyi pointed out the importance of water in muscle contraction in 1951, few studies have focused on the water science of muscle because of the difficulty of analyzing hydration properties of the muscle proteins, actin, and myosin. The thermodynamics and energetics of muscle contraction are linked to the water-mediated regulation of protein-ligand and protein-protein interactions along with structural changes in protein molecules. In this study, we assume the following two points: (1) the periodic electric field distribution along an actin filament (F-actin) is unidirectionally modified upon binding of myosin subfragment 1 (M or myosin S1) with ADP and inorganic phosphate Pi (M.ADP.Pi complex) and (2) the solvation free energy of myosin S1 depends on the external electric field strength and the solvation free energy of myosin S1 in close proximity to F-actin can become the potential force to drive myosin S1 along F-actin. The first assumption is supported by integration of experimental reports. The second assumption is supported by model calculations utilizing molecular dynamics (MD) simulation to determine solvation free energies of a small organic molecule and two small proteins. MD simulations utilize the energy representation method (ER) and the roughly proportional relationship between the solvation free energy and the solvent-accessible surface area (SASA) of the protein. The estimated driving force acting on myosin S1 is as high as several piconewtons (pN), which is consistent with the experimentally observed force.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

Myosin XI-I is Mechanically and Enzymatically Unique Among Class-XI Myosins in Arabidopsis

Arabidopsis possesses 13 genes encoding class-XI myosins. Among these, myosin XI-I is phylogenetically distant. To examine the molecular properties of Arabidopsis thaliana myosin XI-I (At myosin XI-I), we performed in vitro mechanical and enzymatic analyses using recombinant constructs of At myosin XI-I. Unlike other biochemically studied class-XI myosins, At myosin XI-I showed extremely low actin-activated ATPase activity (Vmax = 3.7 Pi s(-1) head(-1)). The actin-sliding velocity of At myosin XI-I was 0.25 µm s(-1), >10 times lower than those of other class-XI myosins. The ADP dissociation rate from acto-At myosin XI-I was 17 s(-1), accounting for the low actin-sliding velocity. In contrast, the apparent affinity for actin in the presence of ATP, estimated from Kapp (0.61 µM) of actin-activated ATPase, was extremely high. The equilibrium dissociation constant for actin was very low in both the presence and absence of ATP, indicating a high affinity for actin. To examine At myosin XI-I motility in vivo, green fluorescent protein-fused full-length At myosin XI-I was expressed in cultured Arabidopsis cells. At myosin XI-I localized not only on the nuclear envelope but also on small dots moving slowly (0.23 µm s(-1)) along actin filaments. Our results show that the properties of At myosin XI-I differ from those of other Arabidopsis class-XI myosins. The data suggest that At myosin XI-I does not function as a driving force for cytoplasmic streaming but regulates the organelle velocity, supports processive organelle movement or acts as a tension generator.

A model of muscle contraction based on the Langevin equation with actomyosin potentials

We propose a muscle contraction model that is essentially a model of the motion of myosin motors as described by a Langevin equation. This model involves one-dimensional numerical calculations wherein the total force is the sum of a viscous force proportional to the myosin head velocity, a white Gaussian noise produced by random forces and other potential forces originating from the actomyosin structure and intra-molecular charges. We calculate the velocity of a single myosin on an actin filament to be 4.9-49 μm/s, depending on the viscosity between the actomyosin molecules. A myosin filament with a hundred myosin heads is used to simulate the contractions of a half-sarcomere within the skeletal muscle. The force response due to a quick release in the isometric contraction is simulated using a process wherein crossbridges are changed forcibly from one state to another. In contrast, the force response to a quick stretch is simulated using purely mechanical characteristics. We simulate the force-velocity relation and energy efficiency in the isotonic contraction and adenosine triphosphate consumption. The simulation results are in good agreement with the experimental results. We show that the Langevin equation for the actomyosin potentials can be modified statistically to become an existing muscle model that uses Maxwell elements.

Dynamics of cross-bridge cycling, ATP hydrolysis, force generation, and deformation in cardiac muscle

Despite extensive study over the past six decades the coupling of chemical reaction and mechanical processes in muscle dynamics is not well understood. We lack a theoretical description of how chemical processes (metabolite binding, ATP hydrolysis) influence and are influenced by mechanical processes (deformation and force generation). To address this need, a mathematical model of the muscle cross-bridge (XB) cycle based on Huxley's sliding filament theory is developed that explicitly accounts for the chemical transformation events and the influence of strain on state transitions. The model is identified based on elastic and viscous moduli data from mouse and rat myocardial strips over a range of perturbation frequencies, and MgATP and inorganic phosphate (Pi) concentrations. Simulations of the identified model reproduce the observed effects of MgATP and MgADP on the rate of force development. Furthermore, simulations reveal that the rate of force re-development measured in slack-restretch experiments is not directly proportional to the rate of XB cycling. For these experiments, the model predicts that the observed increase in the rate of force generation with increased Pi concentration is due to inhibition of cycle turnover by Pi. Finally, the model captures the observed phenomena of force yielding suggesting that it is a result of rapid detachment of stretched attached myosin heads.

Temperature effect on the chemomechanical regulation of substeps within the power stroke of a single Myosin II

Myosin IIs in the skeletal muscle are highly efficient nanoscale machines evolved in nature. Understanding how they function can not only bring insights into various biological processes but also provide guidelines to engineer synthetic nanoscale motors working in the vicinity of thermal noise. Though it was clearly demonstrated that the behavior of a skeletal muscle fiber, or that of a single myosin was strongly affected by the temperature, how exactly the temperature affects the kinetics of a single myosin is not fully understood. By adapting the newly developed transitional state model, which successfully explained the intriguing motor force regulation during skeletal muscle contraction, here we systematically explain how exactly the power stroke of a single myosin proceeds, with the consideration of the chemomechanical regulation of sub-steps within the stroke. The adapted theory is then utilized to investigate the temperature effect on various aspects of the power stroke. Our analysis suggests that, though swing rates, the isometric force, and the maximal stroke size all strongly vary with the temperature, the temperature can have a very small effect on the releasable elastic energy within the power stroke.

Beam finite-element model of a molecular motor for the simulation of active fibre networks

Molecular motors are proteins that excessively increase the efficiency of subcellular transport processes. They allow for cell division, nutrient transport and even macroscopic muscle movement. In order to understand the effect of motors in large biopolymer networks, e.g. the cytoskeleton, we require a suitable model of a molecular motor. In this contribution, we present such a model based on a geometrically exact beam finite-element formulation. We discuss the numerical model of a non-processive motor such as myosin II, which interacts with actin filaments. Based on experimental data and inspired by the theoretical understanding offered by the power-stroke model and the swinging-cross-bridge model, we parametrize our numerical model in order to achieve the effect that a physiological motor has on its cargo. To this end, we introduce the mechanical and mathematical foundations of the model, then discuss its calibration, prove its usefulness by conducting finite-element simulations of actin-myosin motility assays and assess the influence of motors on the rheology of semi-flexible biopolymer networks.

Direct measurements of the coordination of lever arm swing and the catalytic cycle in myosin V

Myosins use a conserved structural mechanism to convert the energy from ATP hydrolysis into a large swing of the force-generating lever arm. The precise timing of the lever arm movement with respect to the steps in the actomyosin ATPase cycle has not been determined. We have developed a FRET system in myosin V that uses three donor-acceptor pairs to examine the kinetics of lever arm swing during the recovery and power stroke phases of the ATPase cycle. During the recovery stroke the lever arm swing is tightly coupled to priming the active site for ATP hydrolysis. The lever arm swing during the power stroke occurs in two steps, a fast step that occurs before phosphate release and a slow step that occurs before ADP release. Time-resolved FRET demonstrates a 20-Å change in distance between the pre- and postpower stroke states and shows that the lever arm is more dynamic in the postpower stroke state. Our results suggest myosin binding to actin in the ADP.Pi complex triggers a rapid power stroke that gates the release of phosphate, whereas a second slower power stroke may be important for mediating strain sensitivity.

Orientation of the N- and C-terminal lobes of the myosin regulatory light chain in cardiac muscle

The orientations of the N- and C-terminal lobes of the cardiac isoform of the myosin regulatory light chain (cRLC) in the fully dephosphorylated state in ventricular trabeculae from rat heart were determined using polarized fluorescence from bifunctional sulforhodamine probes. cRLC mutants with one of eight pairs of surface-accessible cysteines were expressed, labeled with bifunctional sulforhodamine, and exchanged into demembranated trabeculae to replace some of the native cRLC. Polarized fluorescence data from the probes in each lobe were combined with RLC crystal structures to calculate the lobe orientation distribution with respect to the filament axis. The orientation distribution of the N-lobe had three distinct peaks (N1–N3) at similar angles in relaxation, isometric contraction, and rigor. The orientation distribution of the C-lobe had four peaks (C1–C4) in relaxation and isometric contraction, but only two of these (C2 and C4) remained in rigor. The N3 and C4 orientations are close to those of the corresponding RLC lobes in myosin head fragments bound to isolated actin filaments in the absence of ATP (in rigor), but also close to those of the pair of heads folded back against the filament surface in isolated thick filaments in the so-called J-motif conformation. The N1 and C1 orientations are close to those expected for actin-bound myosin heads with their light chain domains in a pre-powerstroke conformation. The N2 and C3 orientations have not been observed previously. The results show that the average change in orientation of the RLC region of the myosin heads on activation of cardiac muscle is small; the RLC regions of most heads remain in the same conformation as in relaxation. This suggests that the orientation of the dephosphorylated RLC region of myosin heads in cardiac muscle is primarily determined by an interaction with the thick filament surface.

Phosphorylation of myosin regulatory light chain controls myosin head conformation in cardiac muscle

The effect of phosphorylation on the conformation of the regulatory light chain (cRLC) region of myosin in ventricular trabeculae from rat heart was determined by polarized fluorescence from thiophosphorylated cRLCs labelled with bifunctional sulforhodamine (BSR). Less than 5% of cRLCs were endogenously phosphorylated in this preparation, and similarly low values of basal cRLC phosphorylation were measured in fresh intact ventricle from both rat and mouse hearts. BSR-labelled cRLCs were thiophosphorylated by a recombinant fragment of human cardiac myosin light chain kinase, which was shown to phosphorylate cRLCs specifically at serine 15 in a calcium- and calmodulin-dependent manner, both in vitro and in situ. The BSR-cRLCs were exchanged into demembranated trabeculae, and polarized fluorescence intensities measured for each BSR-cRLC in relaxation, active isometric contraction and rigor were combined with RLC crystal structures to calculate the orientation distribution of the C-lobe of the cRLC in each state. Only two of the four C-lobe orientation populations seen during relaxation and active isometric contraction in the unphosphorylated state were present after cRLC phosphorylation. Thus cRLC phosphorylation alters the equilibrium between defined conformations of the cRLC regions of the myosin heads, rather than simply disordering the heads as assumed previously. cRLC phosphorylation also changes the orientation of the cRLC C-lobe in rigor conditions, showing that the orientation of this part of the myosin head is determined by its interaction with the thick filament even when the head is strongly bound to actin. These results suggest that cRLC phosphorylation controls the contractility of the heart by modulating the interaction of the cRLC region of the myosin heads with the thick filament backbone.

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The orientation of the phosphorylated cRLC was measured by polarized fluorescence.

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Phosphorylated myosin heads are not disordered on the level of the cRLC region.

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cRLC phosphorylation induces a new conformational state of myosin.

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cRLC phosphorylation controls contractility at the myosin head–backbone interface.

A mutation in the converter subdomain of Aspergillus nidulans MyoB blocks constriction of the actomyosin ring in cytokinesis

We have identified a mutant allele of the Aspergillus nidulans homologue of myosin II (myoB; AN4706), which prevents normal septum formation. This is the first reported myosin II mutation in a filamentous fungus. Strains expressing the myoB(G843D) allele produce mainly aberrant septa at 30 °C and are completely aseptate at temperatures above 37 °C. Conidium formation is greatly reduced at 30 °C and progressively impaired with increasing temperature. Sequencing of the myoB(G843D) allele identified a point mutation predicted to result in a glycine-to-aspartate amino acid substitution at residue 843 in the myosin II converter domain. This residue is conserved in all fungal, plant, and animal myosin sequences that we have examined. The mutation does not prevent localization of the myoB(G843D) gene product to contractile rings, but it does block ring constriction. MyoB(G843D) rings at sites of abortive septation disassemble after an extended period and dissipate into the cytoplasm. During contractile ring formation, both wild type and mutant MyoB::GFP colocalize with actin--an association that begins at the pre-ring "string" stage. Down-regulation of wild-type myoB expression under control of the alcA promoter blocks septation but does not prevent actin from aggregating at putative septation sites--the actin rings, however, do not fully coalesce. Both septation and targeting of MyoB are blocked by disruption of filamentous actin using latrunculin B. We propose a model in which myosin assembly at septation sites depends upon the presence of F-actin, but assembly of the actin component of contractile rings depends upon normal levels of myosin only for the final stages of ring compaction.

3-D structural analysis of the crucial intermediate of skeletal muscle myosin and its role in revised actomyosin cross-bridge cycle

Skeletal myosin S1 consists of two functional segments, a catalytic-domain and a lever-arm. Since the crystal structure of ADP/Vi-bound S1 exhibits a strong intramolecular flexure between two segments, inter-conversion between bent and extended forms; i.e. "tilting of the lever-arm" has been accepted as the established molecular mechanism of skeletal muscle contraction. We utilized quick-freeze deep-etch replica electron microscopy to directly visualize the structure of in vitro actin-sliding myosin, and found the existence of a novel oppositely-bent configuration, instead of the expected ADP/Vi-bound form. We also noticed that SH1-SH2 cross-linked myosin gives an aberrant appearance similar to the above structure. Since SH1-SH2-cross-linked myosin is a well-studied analogue of the transient intermediate of the actomyosin cross-bridge cycle, we devised a new image-processing procedure to define the relative view-angles between the catalytic-domain and the lever-arm from those averaged images, and built a 3-D model of the new conformer. The lever-arm in that model was bent oppositely to the ADP/Vi-bound form, in accordance with observed actin-sliding cross-bridge structure. Introducing this conformer as the crucial intermediate that transiently appears during sliding, we propose a revised scheme of the cross-bridge cycle. In the scenario, the novel conformer keeps actin-binding in two different modes until it forms a primed configuration. The final extension of the lever-arm back to the original rigor-state constitutes the "power-stroke". Various images observed during sliding could be easily interpreted by the new conformer. Even the enigmatic behavior of the cross-bridges reported as "loose chemo-mechanical coupling" might be adequately explained under some assumptions.

RISAP is a TGN-associated RAC5 effector regulating membrane traffic during polar cell growth in tobacco

RAC/ROP GTPases coordinate actin dynamics and membrane traffic during polar plant cell expansion. In tobacco (Nicotiana tabacum), pollen tube tip growth is controlled by the RAC/ROP GTPase RAC5, which specifically accumulates at the apical plasma membrane. Here, we describe the functional characterization of RISAP, a RAC5 effector identified by yeast (Saccharomyces cerevisiae) two-hybrid screening. RISAP belongs to a family of putative myosin receptors containing a domain of unknown function 593 (DUF593) and binds via its DUF593 to the globular tail domain of a tobacco pollen tube myosin XI. It also interacts with F-actin and is associated with a subapical trans-Golgi network (TGN) compartment, whose cytoplasmic position at the pollen tube tip is maintained by the actin cytoskeleton. In this TGN compartment, apical secretion and endocytic membrane recycling pathways required for tip growth appear to converge. RISAP overexpression interferes with apical membrane traffic and blocks tip growth. RAC5 constitutively binds to the N terminus of RISAP and interacts in an activation-dependent manner with the C-terminal half of this protein. In pollen tubes, interaction between RAC5 and RISAP is detectable at the subapical TGN compartment. We present a model of RISAP regulation and function that integrates all these findings.

A new mechanokinetic model for muscle contraction, where force and movement are triggered by phosphate release

The atomic structure of myosin-S1 suggests that its working stroke, which generates tension and shortening in muscle, is triggered by the release of inorganic phosphate from the active site. This mechanism is the basis of a new mechanokinetic model for contractility, using the biochemical actomyosin ATPase cycle, strain-dependent kinetics and dimeric myosins on buckling rods. In this model, phosphate-dependent aspects of contractility arise from a rapid reversible release of phosphate from the initial bound state (A.M.ADP.Pi), which triggers the stroke. Added phosphate drives bound myosin towards this initial state, and the transient tension response to a phosphate jump reflects the rate at which it detaches from actin. Predictions for the tensile and energetic properties of striated muscle as a function of phosphate level, including the tension responses to length steps and Pi-jumps, are compared with experimental data from rabbit psoas fibres at 10 °C. The phosphate sensitivity of isometric tension is maximal when the actin affinity of M.ADP.Pi is near unity. Hence variations in actin affinity modulate the phosphate dependence of isometric tension, and may explain why phosphate sensitivity is temperature-dependent or absent in different muscles.

Why muscle is an efficient shock absorber

Skeletal muscles power body movement by converting free energy of ATP hydrolysis into mechanical work. During the landing phase of running or jumping some activated skeletal muscles are subjected to stretch. Upon stretch they absorb body energy quickly and effectively thus protecting joints and bones from impact damage. This is achieved because during lengthening, skeletal muscle bears higher force and has higher instantaneous stiffness than during isometric contraction, and yet consumes very little ATP. We wish to understand how the actomyosin molecules change their structure and interaction to implement these physiologically useful mechanical and thermodynamical properties. We monitored changes in the low angle x-ray diffraction pattern of rabbit skeletal muscle fibers during ramp stretch compared to those during isometric contraction at physiological temperature using synchrotron radiation. The intensities of the off-meridional layer lines and fine interference structure of the meridional M3 myosin x-ray reflection were resolved. Mechanical and structural data show that upon stretch the fraction of actin-bound myosin heads is higher than during isometric contraction. On the other hand, the intensities of the actin layer lines are lower than during isometric contraction. Taken together, these results suggest that during stretch, a significant fraction of actin-bound heads is bound non-stereo-specifically, i.e. they are disordered azimuthally although stiff axially. As the strong or stereo-specific myosin binding to actin is necessary for actin activation of the myosin ATPase, this finding explains the low metabolic cost of energy absorption by muscle during the landing phase of locomotion.

Three-dimensional structure of the human myosin thick filament: clinical implications

High resolution information about the three-dimensional (3D) structure of myosin filaments has always been hard to obtain. Solving the 3D structure of myosin filaments is very important because mutations in human cardiac muscle myosin and its associated proteins (e.g. titin and myosin binding protein C) are known to be associated with a number of familial human cardiomyopathies (e.g. hypertrophic cardiomyopathy and dilated cardiomyopathy). In order to understand how normal heart muscle works and how it fails, as well as the effects of the known mutations on muscle contractility, it is essential to properly understand myosin filament 3D structure and properties in both healthy and diseased hearts.

The aim of this review is firstly to provide a general overview of the 3D structure of myosin thick filaments, as studied so far in both vertebrates and invertebrate striated muscles. Knowledge of this 3D structure is the starting point from which myosin filaments isolated from human cardiomyopathic samples, with known mutations in either myosin or its associated proteins (titin or C-protein), can be studied in detail. This should, in turn, enable us to relate the structure of myosin thick filament to its function and to understanding the disease process. A long term objective of this research would be to assist the design of possible therapeutic solutions to genetic myosin-related human cardiomyopathies.

Novel configuration of a myosin II transient intermediate analogue revealed by quick-freeze deep-etch replica electron microscopy

In the present paper, we described our attempt to characterize the rough three-dimensional features of the structural analogue of the key intermediate of myosin's cross-bridge cycle. Using quick-freeze deep-etch replica electron microscopy, we observed that actin-attached myosin during in vitro sliding was bent superficially as postulated by the conventional hypothesis, but in the opposite direction of the putative pre-power-stroke configuration, as for ADP·Vi (inorganic vanadate)-bound myosin. We searched for the conformational species with a similar appearance and found that SH1-SH2 (thiols 1 and 2)-cross-linked myosin is a good candidate. To characterize such small asymmetric structures, we employed a new pattern-recognition procedure that accommodates the metal-replicated samples. In this method, the best-matched views of the target microscopic images were selected from a comprehensive set of images simulated from known atomic co-ordinates of relevant proteins. Together with effective morphological filtering, we could define the conformational species and the view angles of the catalytic domain and the lever arm cropped from averaged images of disulfide-cross-linked myosin. Whereas the catalytic domain of the new conformer closely resembled the pPDM (N,N'-p-phenylenedimaleimide)-treated, but SH2 Lys705-cross-linked, structure (PDB code 1L2O), a minor product of the same cross-linking reaction, the lever arm projected differently. Using separately determined view angles of the catalytic domain and the lever arm, we built a model of disulfide-cross-linked myosin. Further combination with the 'displacement-mapping' procedure enabled us to reconstruct the global three-dimensional envelope of the unusual structure whose lever arm orientation is compatible with our reports on the actin-sliding cross-bridge structure. Assuming this conformer as the structural analogue of the transient intermediate during actin sliding, the power stroke of the lever arm might accompany the reversal of the disorganized SH1 helix.

Structure of androcam supports specialized interactions with myosin VI

Androcam replaces calmodulin as a tissue-specific myosin VI light chain on the actin cones that mediate D. melanogaster spermatid individualization. We show that the androcam structure and its binding to the myosin VI structural (Insert 2) and regulatory (IQ) light chain sites are distinct from those of calmodulin and provide a basis for specialized myosin VI function. The androcam N lobe noncanonically binds a single Ca(2+) and is locked in a "closed" conformation, causing androcam to contact the Insert 2 site with its C lobe only. Androcam replacing calmodulin at Insert 2 will increase myosin VI lever arm flexibility, which may favor the compact monomeric form of myosin VI that functions on the actin cones by facilitating the collapse of the C-terminal region onto the motor domain. The tethered androcam N lobe could stabilize the monomer through contacts with C-terminal portions of the motor or recruit other components to the actin cones. Androcam binds the IQ site at all calcium levels, constitutively mimicking a conformation adopted by calmodulin only at intermediate calcium levels. Thus, androcam replacing calmodulin at IQ will abolish a Ca(2+)-regulated, calmodulin-mediated myosin VI structural change. We propose that the N lobe prevents androcam from interfering with other calmodulin-mediated Ca(2+) signaling events. We discuss how gene duplication and mutations that selectively stabilize one of the many conformations available to calmodulin support the molecular evolution of structurally and functionally distinct calmodulin-like proteins.

Molecular mechanism of actin-myosin motor in muscle

The interaction of actin and myosin powers striated and smooth muscles and some other types of cell motility. Due to its highly ordered structure, skeletal muscle is a very convenient object for studying the general mechanism of the actin-myosin molecular motor. The history of investigation of the actin-myosin motor is briefly described. Modern concepts and data obtained with different techniques including protein crystallography, electron microscopy, biochemistry, and protein engineering are reviewed. Particular attention is given to X-ray diffraction studies of intact muscles and single muscle fibers with permeabilized membrane as they give insight into structural changes that underlie force generation and work production by the motor. Time-resolved low-angle X-ray diffraction on contracting muscle fibers using modern synchrotron radiation sources is used to follow movement of myosin heads with unique time and spatial resolution under near physiological conditions.

Structural and functional insights on the Myosin superfamily

The myosin superfamily is a versatile group of molecular motors involved in the transport of specific biomolecules, vesicles and organelles in eukaryotic cells. The processivity of myosins along an actin filament and transport of intracellular ‘cargo’ are achieved by generating physical force from chemical energy of ATP followed by appropriate conformational changes. The typical myosin has a head domain, which harbors an ATP binding site, an actin binding site, and a light-chain bound ‘lever arm’, followed often by a coiled coil domain and a cargo binding domain. Evolution of myosins started at the point of evolution of eukaryotes, S. cerevisiae being the simplest one known to contain these molecular motors. The coiled coil domain of the myosin classes II, V and VI in whole genomes of several model organisms display differences in the length and the strength of interactions at the coiled coil interface. Myosin II sequences have long-length coiled coil regions that are predicted to have a highly stable dimeric interface. These are interrupted, however, by regions that are predicted to be unstable, indicating possibilities of alternate conformations, associations to make thick filaments, and interactions with other molecules. Myosin V sequences retain intermittent regions of strong and weak interactions, whereas myosin VI sequences are relatively devoid of strong coiled coil motifs. Structural deviations at coiled coil regions could be important for carrying out normal biological function of these proteins.

The effect of the Asp175Asn and Glu180Gly TPM1 mutations on actin-myosin interaction during the ATPase cycle

Hypertrophic cardiomyopathy (HCM), characterized by cardiac hypertrophy and contractile dysfunction, is a major cause of heart failure. HCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). To understand how the HCM-causing Asp175Asn and Glu180Gly mutations in α-tropomyosin affect on actin-myosin interaction during the ATPase cycle, we labeled the SH1 helix of myosin subfragment-1 and the actin subdomain-1 with the fluorescent probe N-iodoacetyl-N'-(5-sulfo-1-naphtylo)ethylenediamine. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. Wild-type α-tropomyosin increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of the work of cross-bridges. Both mutant TMs increase the proportion of the strong-binding sub-states, with the effect of the Glu180Gly mutation being greater than that of Asp175Asn. It is suggested that the alteration in the concerted conformational changes of actomyosin is likely to provide the structural basis for the altered cardiac muscle contraction.

Evolutionary traces decode molecular mechanism behind fast pace of myosin XI

Background

Cytoplasmic class XI myosins are the fastest processive motors known. This class functions in high-velocity cytoplasmic streaming in various plant cells from algae to angiosperms. The velocities at which they process are ten times faster than its closest class V homologues.

Results

To provide sequence determinants and structural rationale for the molecular mechanism of this fast pace myosin, we have compared the sequences from myosin class V and XI through Evolutionary Trace (ET) analysis. The current study identifies class-specific residues of myosin XI spread over the actin binding site, ATP binding site and light chain binding neck region. Sequences for ET analysis were accumulated from six plant genomes, using literature based text search and sequence searches, followed by triple validation viz. CDD search, string-based searches and phylogenetic clustering. We have identified nine myosin XI genes in sorghum and seven in grape by sequence searches. Both the plants possess one gene product each belonging to myosin type VIII as well. During this process, we have re-defined the gene boundaries for three sorghum myosin XI genes using fgenesh program.

Conclusion

Molecular modelling and subsequent analysis of putative interactions involving these class-specific residues suggest a structural basis for the molecular mechanism behind high velocity of plant myosin XI. We propose a model of a more flexible switch I region that contributes to faster ADP release leading to high velocity movement of the algal myosin XI.

The effect of the dilated cardiomyopathy-causing Glu40Lys TPM1 mutation on actin-myosin interactions during the ATPase cycle

Dilated cardiomyopathy (DCM), characterized by cardiac dilatation and contractile dysfunction, is a major cause of heart failure. DCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). In order to understand how the dilated cardiomyopathy-causing Glu40Lys mutation in TM affects actomyosin interactions, thin filaments have been reconstituted in muscle ghost fibers by incorporation of labeled Cys707 of myosin subfragment-1 and Cys374 of actin with fluorescent probe 1.5-IAEDANS and α-tropomyosin (wild-type or Glu40Lys mutant). For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. The Glu40Lys mutant TM inhibited these movements at the transition from AM(∗∗)·ADP·Pi to AM state, indicating a decrease of the proportion of the strong-binding sub-states in the actomyosin population. These structural changes are likely to underlie the contractile deficit observed in human dilated cardiomyopathy.

Geometrical conditions indispensable for muscle contraction

Computer simulation has uncovered the geometrical conditions under which the vertebrate striated muscle sarcomere can contract. First, all thick filaments should have identical structure, namely: three myosin cross-bridges, building a crown, should be aligned at angles of 0°, 120°, 180°, and the successive crowns and the two filament halves should be turned around 120°. Second, all thick filaments should act simultaneously. Third, coordination in action of the myosin cross-bridges should exist, namely: the three cross-bridges of a crown should act simultaneously and the cross-bridge crowns axially 43 and 14.333 nm apart should act, respectively, simultaneously and with a phase shift. Fifth, six thin filaments surrounding the thick filament should be turned around 180° to each other in each sarcomere half. Sixth, thin filaments should be oppositely oriented in relation to the sarcomere middle. Finally, the structure of each of the thin filaments should change in consequence of strong interaction with myosin heads, namely: the axial distance and the angular alignment between neighboring actin monomers should be, respectively, 2.867 nm and 168° instead of 2.75 nm and 166.15°. These conditions ensure the stereo-specific interaction between actin and myosin and good agreement with the data gathered by electron microscopy and X-ray diffraction methods. The results suggest that the force is generated not only by the myosin cross-bridges but also by the thin filaments; the former acts by cyclical unwrapping and wrapping the thick filament backbone, and the latter byelongation.

Second harmonic generation microscopy probes different states of motor protein interaction in myofibrils

The second harmonic generation (SHG) signal intensity sourced from skeletal muscle myosin II strongly depends on the polarization of the incident laser beam relative to the muscle fiber axis. This dependence is related to the second-order susceptibility χ((2)), which can be described by a single component ratio γ under generally assumed symmetries. We precisely extracted γ from SHG polarization dependence curves with an extended focal field model. In murine myofibrillar preparations, we have found two distinct polarization dependencies: With the actomyosin system in the rigor state, γ(rig) has a mean value of γ(rig) = 0.52 (SD = 0.04, n = 55); in a relaxed state where myosin is not bound to actin, γ(rel) has a mean value of γ(rel) = 0.24 (SD = 0.07, n = 70). We observed a similar value in an activated state where the myosin power stroke was pharmacologically inhibited using N-benzyl-p-toluene sulfonamide. In summary, different actomyosin states can be visualized noninvasively with SHG microscopy. Specifically, SHG even allows us to distinguish different actin-bound states of myosin II using γ as a parameter.

Visualizing key hinges and a potential major source of compliance in the lever arm of myosin

We have determined the 2.3-Å-resolution crystal structure of a myosin light chain domain, corresponding to one type found in sea scallop catch ("smooth") muscle. This structure reveals hinges that may function in the "on" and "off" states of myosin. The molecule adopts two different conformations about the heavy chain "hook" and regulatory light chain (RLC) helix D. This conformational change results in extended and compressed forms of the lever arm whose lengths differ by 10 Å. The heavy chain hook and RLC helix D hinges could thus serve as a potential major and localized source of cross-bridge compliance during the contractile cycle. In addition, in one of the molecules of the crystal, part of the RLC N-terminal extension is seen in atomic detail and forms a one-turn alpha-helix that interacts with RLC helix D. This extension, whose sequence is highly variable in different myosins, may thus modulate the flexibility of the lever arm. Moreover, the relative proximity of the phosphorylation site to the helix D hinge suggests a potential role for conformational changes about this hinge in the transition between the on and off states of regulated myosins.

Crossbridge and filament compliance in muscle: implications for tension generation and lever arm swing

The stiffness of myosin heads attached to actin is a crucial parameter in determining the kinetics and mechanics of the crossbridge cycle. It has been claimed that the stiffness of myosin heads in the anterior tibialis muscle of the common frog (Rana temporaria) is as high as 3.3 pN/nm, substantially higher than its value in rabbit muscle (~1.7 pN/nm). However, the crossbridge stiffness measurement has a large error since the contribution of crossbridges to half-sarcomere compliance is obtained by subtracting from the half-sarcomere compliance the contributions of the thick and thin filaments, each with a substantial error. Calculation of its value for isometric contraction also depends on the fraction of heads that are attached, for which there is no consensus. Surprisingly, the stiffness of the myosin head from the edible frog, Rana esculenta, determined in the same manner, is only 60% of that in Rana temporaria. In our view it is unlikely that the value of such a crucial parameter could differ so substantially between two frog species. Since the means of the myosin head stiffness in these two species are not significantly different, we suggest that the best estimate of the stiffness of the myosin heads for frog muscle is the average of these data, a value similar to that for rabbit muscle. This would allow both frog and rabbit muscles to operate the same low-cooperativity mechanism for the crossbridge cycle with only one or two tension-generating steps. We review evidence that much of the compliance of the myosin head is located in the pliant region where the lever arm emerges from the converter and propose that tension generation ("tensing") caused by the rotation and movement of the converter is a separate event from the passive swinging of the lever arm in its working stroke in which the strain energy stored in the pliant region is used to do work.

The influence of myosin converter and relay domains on cross-bridge kinetics of Drosophila indirect flight muscle

We are investigating the influence of the converter and relay domains on elementary rate constants of the actomyosin cross-bridge cycle. The converter and relay domains vary between Drosophila myosin heavy chain isoforms due to alternative mRNA splicing. Previously, we found that separate insertions of embryonic myosin isoform (EMB) versions of these domains into the indirect flight muscle (IFM) myosin isoform (IFI) both decreased Drosophila IFM power and slowed muscle kinetics. To determine cross-bridge mechanisms behind the changes, we employed sinusoidal analysis while varying phosphate and MgATP concentrations in skinned Drosophila IFM fibers. Based on a six-state cross-bridge model, the EMB converter decreased myosin rate constants associated with actin attachment and work production, k(4), but increased rates related to cross-bridge detachment and work absorption, k(2). In contrast, the EMB relay domain had little influence on kinetics, because only k(4) decreased. The main alteration was mechanical, in that work production amplitude decreased. That both domains decreased k(4) supports the hypothesis that these domains are critical to lever-arm-mediated force generation. Neither domain significantly influenced MgATP affinity. Our modeling suggests the converter domain is responsible for the difference in rate-limiting cross-bridge steps between EMB and IFI myosin--i.e., a myosin isomerization associated with MgADP release for EMB and Pi release for IFI.

Probing myosin structural conformation in vivo by second-harmonic generation microscopy

Understanding of complex biological processes requires knowledge of molecular structures and measurement of their dynamics in vivo. The collective chemomechanical action of myosin molecules (the molecular motors) in the muscle sarcomere represents a paradigmatic example in this respect. Here, we describe a label-free imaging method sensitive to protein conformation in vivo. We employed the order-based contrast enhancement by second-harmonic generation (SHG) for the functional imaging of muscle cells. We found that SHG polarization anisotropy (SPA) measurements report on the structural state of the actomyosin motors, with significant sensitivity to the conformation of myosin. In fact, each physiological/biochemical state we probed (relaxed, rigor, isometric contraction) produced a distinct value of polarization anisotropy. Employing a full reconstruction of the contributing elementary SHG emitters in the actomyosin motor array at atomic scale, we provide a molecular interpretation of the SPA measurements in terms of myosin conformations. We applied this method to the discrimination between attached and detached myosin heads in an isometrically contracting intact fiber. Our observations indicate that isometrically contracting muscle sustains its tetanic force by steady-state commitment of 30% of myosin heads. Applying SPA and molecular structure modeling to the imaging of unstained living tissues provides the basis for a generation of imaging and diagnostic tools capable of probing molecular structures and dynamics in vivo.

Unidirectional Brownian motion observed in an in silico single molecule experiment of an actomyosin motor

The actomyosin molecular motor, the motor composed of myosin II and actin filament, is responsible for muscle contraction, converting chemical energy into mechanical work. Although recent single molecule and structural studies have shed new light on the energy-converting mechanism, the physical basis of the molecular-level mechanism remains unclear because of the experimental limitations. To provide a clue to resolve the controversy between the lever-arm mechanism and the Brownian ratchet-like mechanism, we here report an in silico single molecule experiment of an actomyosin motor. When we placed myosin on an actin filament and allowed myosin to move along the filament, we found that myosin exhibits a unidirectional Brownian motion along the filament. This unidirectionality was found to arise from the combination of a nonequilibrium condition realized by coupling to the ATP hydrolysis and a ratchet-like energy landscape inherent in the actin-myosin interaction along the filament, indicating that a Brownian ratchet-like mechanism contributes substantially to the energy conversion of this molecular motor.

Structures of kinesin motor proteins

Almost 25 years of kinesin research have led to the accumulation of a large body of knowledge about this widespread superfamily of motor and nonmotor proteins present in all eukaryotic cells. This review covers developments in kinesin research with an emphasis on structural aspects obtained by X-ray crystallography and cryoelectron microscopy 3-D analysis on kinesin motor domains complexed to microtubules.