Orientation dependence of displacements by a single one-headed myosin relative to the actin filament

Matching Mechanics and Energetics of Muscle Contraction Suggests Unconventional Chemomechanical Coupling during the Actin-Myosin Interaction

The mechanical performances of the vertebrate skeletal muscle during isometric and isotonic contractions are interfaced with the corresponding energy consumptions to define the coupling between mechanical and biochemical steps in the myosin-actin energy transduction cycle. The analysis is extended to a simplified synthetic nanomachine in which eight HMM molecules purified from fast mammalian skeletal muscle are brought to interact with an actin filament in the presence of 2 mM ATP, to assess the emergent properties of a minimum number of motors working in ensemble without the effects of both the higher hierarchical levels of striated muscle organization and other sarcomeric, regulatory and cytoskeleton proteins. A three-state model of myosin-actin interaction is able to predict the known relationships between energetics and transient and steady-state mechanical properties of fast skeletal muscle either in vivo or in vitro only under the assumption that during shortening a myosin motor can interact with two actin sites during one ATP hydrolysis cycle. Implementation of the molecular details of the model should be achieved by exploiting kinetic and structural constraints present in the transients elicited by stepwise perturbations in length or force superimposed on the isometric contraction.

A reverse stroke characterizes the force generation of cardiac myofilaments, leading to an understanding of heart function

Changes in the molecular properties of cardiac myosin strongly affect the interactions of myosin with actin that result in cardiac contraction and relaxation. However, it remains unclear how myosin molecules work together in cardiac myofilaments and which properties of the individual myosin molecules impact force production to drive cardiac contractility. Here, we measured the force production of cardiac myofilaments using optical tweezers. The measurements revealed that stepwise force generation was associated with a higher frequency of backward steps at lower loads and higher stall forces than those of fast skeletal myofilaments. To understand these unique collective behaviors of cardiac myosin, the dynamic responses of single cardiac and fast skeletal myosin molecules, interacting with actin filaments, were evaluated under load. The cardiac myosin molecules switched among three distinct conformational positions, ranging from pre- to post-power stroke positions, in 1 mM ADP and 0 to 10 mM phosphate solution. In contrast to cardiac myosin, fast skeletal myosin stayed primarily in the post-power stroke position, suggesting that cardiac myosin executes the reverse stroke more frequently than fast skeletal myosin. To elucidate how the reverse stroke affects the force production of myofilaments and possibly heart function, a simulation model was developed that combines the results from the single-molecule and myofilament experiments. The results of this model suggest that the reversal of the cardiac myosin power stroke may be key to characterizing the force output of cardiac myosin ensembles and possibly to facilitating heart contractions.

Muscle myosin performance measured with a synthetic nanomachine reveals a class-specific Ca2+ -sensitivity of the frog myosin II isoform

KEY POINTS

A nanomachine made of an ensemble of seven heavy-meromyosin (HMM) fragments of muscle myosin interacting with an actin filament is able to mimic the half-sarcomere generating steady force and constant-velocity shortening. To preserve Ca²⁺ as a free parameter, the Ca²⁺ -insensitive gelsolin fragment TL40 is used to attach the correctly oriented actin filament to the laser-trapped bead acting as a force transducer. The new method reveals that the performance of the nanomachine powered by myosin from frog hind-limb muscles depends on [Ca²⁺ ], an effect mediated by a Ca²⁺ -binding site in the regulatory light chain of HMM. The Ca²⁺ -sensitivity is class-specific because the performance of the nanomachine powered by mammalian skeletal muscle myosin is Ca²⁺ independent. A model simulation is able to interface the nanomachine performance with that of the muscle of origin and provides a molecular explanation of the functional diversity of muscles with different orthologue isoforms of myosin.

ABSTRACT

An ensemble of seven heavy-meromyosin (HMM) fragments of myosin-II purified from the hindlimb muscles of the frog (Rana esculenta) is used to drive a synthetic nanomachine that pulls an actin filament in the absence of confounding effects of other sarcomeric proteins. In the present version of the nanomachine the +end of the actin filament is attached to the laser trapped bead via the Ca²⁺ -insensitive gelsolin fragment TL40, making [Ca²⁺ ] a free parameter. Frog myosin performance in 2 mm ATP is affected by Ca²⁺ : in 0.1 mm Ca²⁺ , the isometric steady force (F₀ , 15.25 pN) is increased by 50% (P = 0.004) with respect to that in Ca²⁺ -free solution, the maximum shortening velocity (V₀ , 4.6 μm s^-1 ) is reduced by 27% (P = 0.46) and the maximum power (P_max , 7.6 aW) is increased by 21% (P = 0.17). V₀ reduction is not significant for the paucity of data at low force, although it is solidified by a similar decrease (33%, P < 0.0001) in the velocity of actin sliding as indicated by an in vitro motility assay (V_f ). The rate of ATP-hydrolysis in solution (φ) exhibits a similar calcium dependence. Ca²⁺ titration curves for V_f and φ give K_d values of ∼30 μm. All the above mechanical and kinetic parameters are independent of Ca²⁺ when HMM from rabbit psoas myosin is used, indicating that the Ca²⁺ -sensitivity is a class-specific property of muscle myosin. A unique multiscale model allows interfacing of the nanomachine performance to that of the muscle of origin and identifies the kinetic steps responsible for the Ca²⁺ -sensitivity of frog myosin.

A Myosin II-Based Nanomachine Devised for the Study of Ca2+-Dependent Mechanisms of Muscle Regulation

The emergent properties of the array arrangement of the molecular motor myosin II in the sarcomere of the striated muscle, the generation of steady force and shortening, can be studied in vitro with a synthetic nanomachine made of an ensemble of eight heavy-meromyosin (HMM) fragments of myosin from rabbit psoas muscle, carried on a piezoelectric nanopositioner and brought to interact with a properly oriented actin filament attached via gelsolin (a Ca²⁺-regulated actin binding protein) to a bead trapped by dual laser optical tweezers. However, the application of the original version of the nanomachine to investigate the Ca²⁺-dependent regulation mechanisms of the other sarcomeric (regulatory or cytoskeleton) proteins, adding them one at a time, was prevented by the impossibility to preserve [Ca²⁺] as a free parameter. Here, the nanomachine is implemented by assembling the bead-attached actin filament with the Ca²⁺-insensitive gelsolin fragment TL40. The performance of the nanomachine is determined both in the absence and in the presence of Ca²⁺ (0.1 mM, the concentration required for actin attachment to the bead with gelsolin). The nanomachine exhibits a maximum power output of 5.4 aW, independently of [Ca²⁺], opening the possibility for future studies of the Ca²⁺-dependent function/dysfunction of regulatory and cytoskeletal proteins.

What factors determine the number of nonmuscle myosin II in the sarcomeric unit of stress fibers?

Actin stress fibers (SFs), a contractile apparatus in nonmuscle cells, possess a contractile unit that is apparently similar to the sarcomere of myofibrils in muscles. The function of SFs has thus often been addressed based on well-characterized properties of muscles. However, unlike the fixed number of myosin molecules in myofibrils, the number of nonmuscle myosin II (NMII) within the contractile sarcomeric unit in SFs is quite low and variable for some reason. Here we address what factors may determine the specific number of NMII in SFs. We suggest with a theoretical model that the number lies just in between the function of SFs for bearing cellular tension under static conditions and for promptly disintegrating upon forced cell shortening. We monitored shortening-induced disintegration of SFs in human osteosarcoma U2OS cells expressing mutants of myosin regulatory light chain that virtually regulates the interaction of NMII with actin filaments, and the behaviors observed were indeed consistent with the theoretical consequences. This situation-specific nature of SFs may allow nonmuscle cells to respond adaptively to mechanical stress to circumvent activation of pro-inflammatory signals as previously indicated, i.e., a behavior distinct from that of muscles that are basically specialized for exhibiting contractile activity.

Single-Molecule Biophysical Techniques to Study Actomyosin Force Transduction

Inside the cellular environment, molecular motors can work in concert to conduct a variety of important physiological functions and processes that are vital for the survival of a cell. However, in order to decipher the mechanism of how these molecular motors work, single-molecule microscopy techniques have been popular methods to understand the molecular basis of the emerging ensemble behavior of these motor proteins.In this chapter, we discuss various single-molecule biophysical imaging techniques that have been used to expose the mechanics and kinetics of myosins. The chapter should be taken as a general overview and introductory guide to the many existing techniques; however, since other chapters will discuss some of these techniques more thoroughly, the readership should refer to those chapters for further details and discussions. In particular, we will focus on scattering-based single-molecule microscopy methods, some of which have become more popular in the recent years and around which the work in our laboratories has been centered.

The load dependence and the force-velocity relation in intact myosin filaments from skeletal and smooth muscles

In the present study we evaluated the load dependence of force produced by isolated muscle myosin filaments interacting with fluorescently labeled actin filaments, using for the first time whole native myosin filaments. We used a newly developed approach that allowed the use of physiological levels of ATP. Single filaments composed of either skeletal or smooth muscle myosin and single filaments of actin were attached between pairs of nano-fabricated cantilevers of known stiffness. The filaments were brought into contact to produce force, which caused sliding of the actin filaments over the myosin filaments. We applied load to the system by either pushing or pulling the filaments during interactions and observed that increasing the load increased the force produced by myosin and decreasing the load decreased the force. We also performed additional experiments in which we clamped the filaments at predetermined levels of force, which caused the filaments to slide to adjust the different loads, allowing us to measure the velocity of length changes to construct a force-velocity relation. Force values were in the range observed previously with myosin filaments and molecules. The force-velocity curves for skeletal and smooth muscle myosins resembled the relations observed for muscle fibers. The technique can be used to investigate many issues of interest and debate in the field of muscle biophysics.

The mechanobiology of actin cytoskeletal proteins during cell-cell fusion

Myosin II and spectrin β display mechanosensitive accumulations in invasive protrusions during cell-cell fusion of Drosophila myoblasts. The biochemical inhibition and deactivation of these proteins results in significant fusion defects. Yet, a quantitative understanding of how the protrusion geometry and fusion process are linked to these proteins is still lacking. Here we present a quantitative model to interpret the dependence of the protrusion size and the protrusive force on the mechanical properties and microstructures of the actin cytoskeleton and plasma membrane based on a mean-field theory. We build a quantitative linkage between mechanosensitive accumulation of myosin II and fusion pore formation at the tip of the invasive protrusion through local area dilation. The mechanical feedback loop between myosin II and local deformation suggests that myosin II accumulation possibly reduces the energy barrier and the critical radius of fusion pores. We also analyse the effect of spectrin β on maintaining the proper geometry of the protrusions required for the success of cell-cell fusion.

Functional significance of HCM mutants of tropomyosin, V95A and D175N, studied with in vitro motility assays

The majority of hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere proteins. We examined tropomyosin (Tpm)’s HCM mutants in humans, V95A and D175N, with in vitro motility assay using optical tweezers to evaluate the effects of the Tpm mutations on the actomyosin interaction at the single molecular level. Thin filaments were reconstituted using these Tpm mutants, and their sliding velocity and force were measured at varying Ca²⁺ concentrations. Our results indicate that the sliding velocity at pCa ≥8.0 was significantly increased in mutants, which is expected to cause a diastolic problem. The velocity that can be activated by Ca²⁺ decreased significantly in mutants causing a systolic problem. With sliding force, Ca²⁺ activatable force decreased in V95A and increased in D175N, which may cause a systolic problem. Our results further demonstrate that the duty ratio determined at the steady state of force generation in saturating [Ca²⁺] decreased in V95A and increased in D175N. The Ca²⁺ sensitivity and cooperativity were not significantly affected by the mutations. These results suggest that the two mutants modulate molecular processes of the actomyosin interaction differently, but to result in the same pathology known as HCM.

Step Sizes and Rate Constants of Single-headed Cytoplasmic Dynein Measured with Optical Tweezers

A power stroke of dynein is thought to be responsible for the stepping of dimeric dynein. However, the actual size of the displacement driven by a power stroke has not been directly measured. Here, the displacements of single-headed cytoplasmic dynein were measured by optical tweezers. The mean displacement of dynein interacting with microtubule was ~8 nm at 100 µM ATP, and decreased sigmoidally with a decrease in the ATP concentration. The ATP dependence of the mean displacement was explained by a model that some dynein molecules bind to microtubule in pre-stroke conformation and generate 8-nm displacement, while others bind in the post-stroke one and detach without producing a power stroke. Biochemical assays showed that the binding affinity of the post-stroke dynein to a microtubule was ~5 times higher than that of pre-stroke dynein, and the dissociation rate was ~4 times lower. Taking account of these rates, we conclude that the displacement driven by a power stroke is 8.3 nm. A working model of dimeric dynein driven by the 8-nm power stroke was proposed.

Do Actomyosin Single-Molecule Mechanics Data Predict Mechanics of Contracting Muscle?

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.

Myosin Clusters of Finite Size Develop Contractile Stress in 1D Random Actin Arrays

Myosin-powered force generation and contraction in nonmuscle cells underlies many cell biological processes and is based on contractility of random actin arrays. This contractility must rely on a microscopic asymmetry, the precise mechanism of which is not completely clear. A number of models of mechanical and structural asymmetries in actomyosin contraction have been posited. Here, we examine a contraction mechanism based on a finite size of myosin clusters and anisotropy of force generation by myosin heads at the ends of the myosin clusters. We use agent-based numerical simulations to demonstrate that if average lengths of actin filaments and myosin clusters are similar, then the proposed microscopic asymmetry leads to effective contraction of random 1D actomyosin arrays. We discuss the model's implication for mechanics of contractile rings and stress fibers.

Recent Advances in Biological Single-Molecule Applications of Optical Tweezers and Fluorescence Microscopy

Over the past two decades, single-molecule techniques have evolved into robust tools to study many fundamental biological processes. The combination of optical tweezers with fluorescence microscopy and microfluidics provides a powerful single-molecule manipulation and visualization technique that has found widespread application in biology. In this combined approach, the spatial (~nm) and temporal (~ms) resolution, as well as the force scale (~pN) accessible to optical tweezers is complemented with the power of fluorescence microscopy. Thereby, it provides information on the local presence, identity, spatial dynamics, and conformational dynamics of single biomolecules. Together, these techniques allow comprehensive studies of, among others, molecular motors, protein-protein and protein-DNA interactions, biomolecular conformational changes, and mechanotransduction pathways. In this chapter, recent applications of fluorescence microscopy in combination with optical trapping are discussed. After an introductory section, we provide a description of instrumentation together with the current capabilities and limitations of the approaches. Next we summarize recent studies that applied this combination of techniques in biological systems and highlight some representative biological assays to mark the exquisite opportunities that optical tweezers combined with fluorescence microscopy provide.

Velocity Fluctuations in Kinesin-1 Gliding Motility Assays Originate in Motor Attachment Geometry Variations

Motor proteins such as myosin and kinesin play a major role in cellular cargo transport, muscle contraction, cell division, and engineered nanodevices. Quantifying the collective behavior of coupled motors is critical to our understanding of these systems. An excellent model system is the gliding motility assay, where hundreds of surface-adhered motors propel one cytoskeletal filament such as an actin filament or a microtubule. The filament motion can be observed using fluorescence microscopy, revealing fluctuations in gliding velocity. These velocity fluctuations have been previously quantified by a motional diffusion coefficient, which Sekimoto and Tawada explained as arising from the addition and removal of motors from the linear array of motors propelling the filament as it advances, assuming that different motors are not equally efficient in their force generation. A computational model of kinesin head diffusion and binding to the microtubule allowed us to quantify the heterogeneity of motor efficiency arising from the combination of anharmonic tail stiffness and varying attachment geometries assuming random motor locations on the surface and an absence of coordination between motors. Knowledge of the heterogeneity allows the calculation of the proportionality constant between the motional diffusion coefficient and the motor density. The calculated value (0.3) is within a standard error of our measurements of the motional diffusion coefficient on surfaces with varying motor densities calibrated by landing rate experiments. This allowed us to quantify the loss in efficiency of coupled molecular motors arising from heterogeneity in the attachment geometry.

Poorly understood aspects of striated muscle contraction

Muscle contraction results from cyclic interactions between the contractile proteins myosin and actin, driven by the turnover of adenosine triphosphate (ATP). Despite intense studies, several molecular events in the contraction process are poorly understood, including the relationship between force-generation and phosphate-release in the ATP-turnover. Different aspects of the force-generating transition are reflected in the changes in tension development by muscle cells, myofibrils and single molecules upon changes in temperature, altered phosphate concentration, or length perturbations. It has been notoriously difficult to explain all these events within a given theoretical framework and to unequivocally correlate observed events with the atomic structures of the myosin motor. Other incompletely understood issues include the role of the two heads of myosin II and structural changes in the actin filaments as well as the importance of the three-dimensional order. We here review these issues in relation to controversies regarding basic physiological properties of striated muscle. We also briefly consider actomyosin mutation effects in cardiac and skeletal muscle function and the possibility to treat these defects by drugs.

Kinetic mechanism of Nicotiana tabacum myosin-11 defines a new type of a processive motor

The 175-kDa myosin-11 from Nicotiana tabacum (Nt(175kDa)myosin-11) is exceptional in its mechanical activity as it is the fastest known processive actin-based motor, moving 10 times faster than the structurally related class 5 myosins. Although this ability might be essential for long-range organelle transport within larger plant cells, the kinetic features underlying the fast processive movement of Nt(175kDa)myosin-11 still remain unexplored. To address this, we generated a single-headed motor domain construct and carried out a detailed kinetic analysis. The data demonstrate that Nt(175kDa)myosin-11 is a high duty ratio motor, which remains associated with actin most of its enzymatic cycle. However, different from other processive myosins that establish a high duty ratio on the basis of a rate-limiting ADP-release step, Nt(175kDa)myosin-11 achieves a high duty ratio by a prolonged duration of the ATP-induced isomerization of the actin-bound states and ADP release kinetics, both of which in terms of the corresponding time constants approach the total ATPase cycle time. Molecular modeling predicts that variations in the charge distribution of the actin binding interface might contribute to the thermodynamic fine-tuning of the kinetics of this myosin. Our study unravels a new type of a high duty ratio motor and provides important insights into the molecular mechanism of processive movement of higher plant myosins.

Using fluorescent myosin to directly visualize cooperative activation of thin filaments

Contraction of striated muscle is tightly regulated by the release and sequestration of calcium within myocytes. At the molecular level, calcium modulates myosin's access to the thin filament. Once bound, myosin is hypothesized to potentiate the binding of further myosins. Here, we directly image single molecules of myosin binding to and activating thin filaments. Using this approach, the cooperative binding of myosin along thin filaments has been quantified. We have found that two myosin heads are required to laterally activate a regulatory unit of thin filament. The regulatory unit is found to be capable of accommodating 11 additional myosins. Three thin filament activation states possessing differential myosin binding capacities are also visible. To describe this system, we have formulated a simple chemical kinetic model of cooperative activation that holds across a wide range of solution conditions. The stochastic nature of activation is strongly highlighted by data obtained in sub-optimal activation conditions where the generation of activation waves and their catastrophic collapse can be observed. This suggests that the thin filament has the potential to be turned fully on or off in a binary fashion.

The path to visualization of walking myosin V by high-speed atomic force microscopy

The quest for understanding the mechanism of myosin-based motility started with studies on muscle contraction. From numerous studies, the basic frameworks for this mechanism were constructed and brilliant hypotheses were put forward. However, the argument about the most crucial issue of how the actin-myosin interaction generates contractile force and shortening has not been definitive. To increase the "directness of measurement", in vitro motility assays and single-molecule optical techniques were created and used. Consequently, detailed knowledge of the motility of muscle myosin evolved, which resulted in provoking more arguments to a higher level. In parallel with technical progress, advances in cell biology led to the discovery of many classes of myosins. Myosin V was discovered to be a processive motor, unlike myosin II. The processivity reduced experimental difficulties because it allowed continuous tracing of the motor action of single myosin V molecules. Extensive studies of myosin V were expected to resolve arguments and build a consensus but did not necessarily do so. The directness of measurement was further enhanced by the recent advent of high-speed atomic force microscopy capable of directly visualizing biological molecules in action at high spatiotemporal resolution. This microscopy clearly visualized myosin V molecules walking on actin filaments and at last provided irrefutable evidence for the swinging lever-arm motion propelling the molecules. However, a peculiar foot stomp behavior also appeared in the AFM movie, raising new questions of the chemo-mechanical coupling in this motor and myosin motors in general. This article reviews these changes in the research of myosin motility and proposes new ideas to resolve the newly raised questions.

Stiffness, working stroke, and force of single-myosin molecules in skeletal muscle: elucidation of these mechanical properties via nonlinear elasticity evaluation

In muscles, the arrays of skeletal myosin molecules interact with actin filaments and continuously generate force at various contraction speeds. Therefore, it is crucial for myosin molecules to generate force collectively and minimize the interference between individual myosin molecules. Knowledge of the elasticity of myosin molecules is crucial for understanding the molecular mechanisms of muscle contractions because elasticity directly affects the working and drag (resistance) force generation when myosin molecules are positively or negatively strained. The working stroke distance is also an important mechanical property necessary for elucidation of the thermodynamic efficiency of muscle contractions at the molecular level. In this review, we focus on these mechanical properties obtained from single-fiber and single-molecule studies and discuss recent findings associated with these mechanical properties. We also discuss the potential molecular mechanisms associated with reduction of the drag effect caused by negatively strained myosin molecules.

Single turnovers of fluorescent ATP bound to bipolar myosin filament during actin filaments sliding

The nucleotide turnover rates of bipolar myosin thick filament along which actin filament slides were measured by the displacement of prebound fluorescent ATP analog 2'(3')-O-[N-[2-[(Cy3)]amindo]ethyl] carbamoyl]-adenosine 5' triphosphate (Cy3-EDA-ATP) upon flash photolysis of caged ATP. The fluorescence of the thick filament where actin filament slides decayed with two exponential processes. The slower rate constant was the same as that without actin filament. Along bipolar myosin thick filament, actin filaments slide at a fast speed towards the central bare zone (forward), but more slowly away from the bare zone (backward). The displacement rate constant of fluorescent ATP from the myosin filament where actin filament moved forward was 5.0 s(-1), whereas the rate constant where the actin filament slid backward was 1.7 s(-1). These findings suggest that the slow ADP release rate is responsible for the slow backward sliding movement.

Forces measured with micro-fabricated cantilevers during actomyosin interactions produced by filaments containing different myosin isoforms and loop 1 structures

BACKGROUND

There is evidence that the actin-activated ATP kinetics and the mechanical work produced by muscle myosin molecules are regulated by two surface loops, located near the ATP binding pocket (loop 1), and in a region that interfaces with actin (loop 2). These loops regulate force and velocity of contraction, and have been investigated mostly in single molecules. There is a lack of information of the work produced by myosin molecules ordered in filaments and working cooperatively, which is the actual muscle environment.

METHODS

We use micro-fabricated cantilevers to measure forces produced by myosin filaments isolated from mollusk muscles, skeletal muscles, and smooth muscles containing variations in the structure of loop 1 (tonic and phasic myosins). We complemented the experiments with in-vitro assays to measure the velocity of actin motility.

RESULTS

Smooth muscle myosin filaments produced more force than skeletal and mollusk myosin filaments when normalized per filament overlap. Skeletal muscle myosin propelled actin filaments in a higher sliding velocity than smooth muscle myosin. The values for force and velocity were consistent with previous studies using myosin molecules, and suggest a close correlation with the myosin isoform and structure of surface loop 1.

GENERAL SIGNIFICANCE

The technique using micro-fabricated cantilevers to measure force of filaments allows for the investigation of the relation between myosin structure and contractility, allowing experiments to be conducted with an array of different myosin isoforms. Using the technique we observed that the work produced by myosin molecules is regulated by amino-acid sequences aligned in specific loops.

Recent insights into muscle fatigue at the cross-bridge level

The depression in force and/or velocity associated with muscular fatigue can be the result of a failure at any level, from the initial events in the motor cortex of the brain to the formation of an actomyosin cross-bridge in the muscle cell. Since all the force and motion generated by muscle ultimately derives from the cyclical interaction of actin and myosin, researchers have focused heavily on the impact of the accumulation of intracellular metabolites [e.g., P(i), H(+) and adenosine diphoshphate (ADP)] on the function these contractile proteins. At saturating Ca(++) levels, elevated P(i) appears to be the primary cause for the loss in maximal isometric force, while increased [H(+)] and possibly ADP act to slow unloaded shortening velocity in single muscle fibers, suggesting a causative role in muscular fatigue. However the precise mechanisms through which these metabolites might affect the individual function of the contractile proteins remain unclear because intact muscle is a highly complex structure. To simplify problem isolated actin and myosin have been studied in the in vitro motility assay and more recently the single molecule laser trap assay with the findings showing that both P(i) and H(+) alter single actomyosin function in unique ways. In addition to these new insights, we are also gaining important information about the roles played by the muscle regulatory proteins troponin (Tn) and tropomyosin (Tm) in the fatigue process. In vitro studies, suggest that both the acidosis and elevated levels of P(i) can inhibit velocity and force at sub-saturating levels of Ca(++) in the presence of Tn and Tm and that this inhibition can be greater than that observed in the absence of regulation. To understand the molecular basis of the role of regulatory proteins in the fatigue process researchers are taking advantage of modern molecular biological techniques to manipulate the structure and function of Tn/Tm. These efforts are beginning to reveal the relevant structures and how their functions might be altered during fatigue. Thus, it is a very exciting time to study muscle fatigue because the technological advances occurring in the fields of biophysics and molecular biology are providing researchers with the ability to directly test long held hypotheses and consequently reshaping our understanding of this age-old question.

Quasiperiodic distribution of rigor cross-bridges along a reconstituted thin filament in a skeletal myofibril

Electron microscopy has shown that cross-bridges (CBs) are formed at the target zone that is periodically distributed on the thin filament in striated muscle. Here, by manipulating a single bead-tailed actin filament with optical tweezers, we measured the unbinding events of rigor CBs one by one on the surface of the A-band in rabbit skeletal myofibrils. We found that the spacings between adjacent CBs were not always the same, and instead were 36, 72, or 108 nm. Tropomyosin and troponin did not affect the CB spacing except for a relative increase in the appearance of longer spacing in the presence of Ca(2+). In addition, in an in vitro assay where myosin molecules were randomly distributed, were obtained the same spacing, i.e., a multiple of 36 nm. These results indicate that the one-dimensional distribution of CBs matches with the 36-nm half pitch of a long helical structure of actin filaments. A stereospecific model composed of three actin protomers per target zone was shown to explain the experimental results. Additionally, the unbinding force (i.e., the binding affinity) of CBs for the reconstituted thin filaments was found to be larger and smaller relative to that for actin filaments with and without Ca(2+), respectively.

Single-molecule measurements using microneedles

Myosin is both an enzyme and a molecular motor that hydrolyzes ATP and interacts with actin filaments for force generation. Manipulation techniques with microneedles and laser traps have recently been developed to capture and manipulate the actomyosin interaction for the purpose of revealing the mechanics of this system. Combined with single-molecule imaging techniques, the coupling between chemical processes (ATP hydrolysis) and mechanical processes (myosin force generation) has been directly determined. In this chapter, we describe these two manipulation techniques, especially microneedle method, in detail.

Gene transfer, expression, and sarcomeric incorporation of a headless myosin molecule in cardiac myocytes: evidence for a reserve in myofilament motor function

The purpose of this study was to implement a living myocyte in vitro model system to test whether a motor domain-deleted headless myosin construct could be incorporated into the sarcomere and affect contractility. To this end we used gene transfer to express a "headless" myosin heavy chain (headless-MHC) in complement with the native full-length myosin motors in the cardiac sarcomere. An NH2-terminal Flag epitope was used for unique detection of the motor domain-deleted headless-MHC. Total MHC content (i.e., headless-MHC+endogenous MHC) remained constant, while expression of the headless-MHC in transduced myocytes increased from 24 to 72 h after gene transfer until values leveled off at 96 h after gene transfer, at which time the headless-MHC comprised ∼20% of total MHC. Moreover, immunofluorescence labeling and confocal imaging confirmed expression and demonstrated incorporation of the headless-MHC in the A band of the cardiac sarcomere. Functional measurements in intact myocytes showed that headless-MHC modestly reduced amplitude of dynamic twitch contractions compared with controls (P<0.05). In chemically permeabilized myocytes, maximum steady-state isometric force and the tension-pCa relationship were unaltered by the headless-MHC. These data suggest that headless-MHC can express to 20% of total myosin and incorporate into the sarcomere yet have modest to no effects on dynamic and steady-state contractile function. This would indicate a degree of functional tolerance in the sarcomere for nonfunctional myosin molecules.

A technique for simultaneous measurement of force and overlap between single muscle filaments of myosin and actin

In this study, we show a method for direct measurements of force and simultaneous visualization of isolated muscle filaments. Single actin filaments isolated from chicken skeletal muscle and single thick filaments isolated from Mussels were imaged using fluorescence and dark field microscopy, respectively. Force generated by the filaments was measured using micro-fabricated cantilevers. Force values were in the range observed previously with myosin filaments and molecules. The results suggest that the technique can be used to investigate many issues of interest and debate in the field of muscle biophysics.

Verification of Single-Molecule Imaging and Single-Molecule Measurements

Single-molecule imaging and single-molecule measurements constitute an integral part of life science researches. The research method, which allows direct measurements of individual motions and events of biomolecules, has contributed much to the development of life science research by providing us with numerous findings. It is left up to the judgment of researchers, however, whether the measurements really represent single-molecule events, which make it harder for more researchers to enter into this field. This paper deals with how single-molecule measurements were presented in past research outcomes and illustrates statistical-mechanical methods as well.

Mechanical characterization of one-headed myosin-V using optical tweezers

Class V myosin (myosin-V) is a cargo transporter that moves along an actin filament with large (approximately 36-nm) successive steps. It consists of two heads that each includes a motor domain and a long (23 nm) neck domain. One of the more popular models describing these steps, the hand-over-hand model, assumes the two-headed structure is imperative. However, we previously succeeded in observing successive large steps by one-headed myosin-V upon optimizing the angle of the acto-myosin interaction. In addition, it was reported that wild type myosin-VI and myosin-IX, both one-headed myosins, can also generate successive large steps. Here, we describe the mechanical properties (stepsize and stepping kinetics) of successive large steps by one-headed and two-headed myosin-Vs. This study shows that the stepsize and stepping kinetics of one-headed myosin-V are very similar to those of the two-headed one. However, there was a difference with regards to stability against load and the number of multisteps. One-headed myosin-V also showed unidirectional movement that like two-headed myosin-V required 3.5 k(B)T from ATP hydrolysis. This value is also similar to that of smooth muscle myosin-II, a non-processive motor, suggesting the myosin family uses a common mechanism for stepping regardless of the steps being processive or non-processive. In this present paper, we conclude that one-headed myosin-V can produce successive large steps without following the hand-over-hand mechanism.

Thermal activation energy for bidirectional movement of actin along bipolar tracks of myosin filaments

Previous in vitro motility assays using bipolar myosin thick filaments demonstrated that actin filaments were capable of moving in both directions along the myosin filament tracks. The movements; however, were slower in the direction leading away from the central bare zone than towards it. To understand the mechanism underlying these different direction-dependent motilities, we have examined the effects of temperature on the velocities of the bidirectional movements along reconstituted myosin filaments. Activation energies of the movements were determined by Arrhenius plots at high and low concentrations of ATP. As a result, the thermal activation energy of the movement away from the central bare zone was significantly higher than that of the movement toward the zone. Given that the backward movement away from the central bare zone would cause the myosin heads to be constrained and the stiffness of the cross-bridges to increase, these results suggest that elastic energy required for the cross-bridge transition is supplied by thermal fluctuations.

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.

Direction and speed of microtubule movements driven by kinesin motors arranged on catchin thick filaments

Conventional kinesin (Kinesin-1) is a microtubule-based molecular motor that supports intracellular vesicle/organelle transport in various eukaryotic cells. To arrange kinesin motors similarly to myosin motors on thick filaments in muscles, the motor domain of rat conventional kinesin (amino acid residues 1-430) fused to the C-terminal 829 amino acid residues of catchin (KHC430Cat) was bacterially expressed and attached to catchin filaments that can attach to and arrange myosin molecules in a bipolar manner on their surface. Unlike the case of myosin where actin filaments move toward the center much faster than in the opposite direction along the catchin filaments, microtubules moved at the same speed in both directions. In addition, many microtubules moved across the filaments at the same speed with various angles between the axes of the microtubule and catchin filament. Kinesin/catchin chimera proteins with a shorter kinesin neck domain were also prepared. Those without the whole hinge 1 domain and the C-terminal part of the neck helix moved microtubules toward the center of the catchin filaments significantly, but only slightly, faster than in the opposite direction, although the movements in both directions were slower than those of the KHC430Cat construct. The results suggest that kinesin has substantial mechanical flexibility within the motor domain, possibly within the neck linker, enabling its interaction with microtubules having any orientation.

Single molecule measurements and molecular motors

Single molecule imaging and manipulation are powerful tools in describing the operations of molecular machines like molecular motors. The single molecule measurements allow a dynamic behaviour of individual biomolecules to be measured. In this paper, we describe how we have developed single molecule measurements to understand the mechanism of molecular motors. The step movement of molecular motors associated with a single cycle of ATP hydrolysis has been identified. The single molecule measurements that have sensitivity to monitor thermal fluctuation have revealed that thermal Brownian motion is involved in the step movement of molecular motors. Several mechanisms have been suggested in different motors to bias random thermal motion to directional movement.

Laminar flow cells for single-molecule studies of DNA-protein interactions

Microfluidic flow cells are used in single-molecule experiments, enabling measurements to be made with high spatial and temporal resolution. We discuss the fundamental processes affecting flow cell operation and describe the flow cells in use at present for studying the interaction of optically trapped or mechanically isolated, single DNA molecules with proteins. To assist the experimentalist in flow cell selection, we review the construction techniques and materials used to fabricate both single- and multiple-channel flow cells and the advantages of each design for different experiments.

Measurement system for simultaneous observation of myosin V chemical and mechanical events

Myosin V is an actin-based processive molecular motor driven by the chemical energy of ATP hydrolysis. Although the chemo-mechanical coupling in processive movement has been postulated by separate structural, mechanical and biochemical studies, no experiment has been able to directly test these conclusions. Therefore the relationship between ATP-turnover and force generation remains unclear. Currently, the most direct method to measure the chemo-mechanical coupling in processive motors is to simultaneously observe ATP-turnover cycles and displacement at the single molecule level. In this study, we developed a simultaneous measurement system suitable for mechanical and chemical assays of myosin V in order to directly elucidate its chemo-mechanical coupling.

Smooth muscle myosin phosphorylated at single head shows sustained mechanical activity

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast ( approximately 24 s(-1)) and slow prolonged ( approximately 1 s(-1)) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast ( approximately 29 s(-1)) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

Optical tweezers as a probe for oligodeoxyribonucleotide structuration

The aim of this work is to investigate if the optical tweezers (OT) are suitable as a diagnostic tool for monitoring the oligodeoxyribonucleotide (ODN) structural behavior in solution. Preliminary experiments, performed on the quadruplex formed by the ODN sequence TGGGGT, showed that the OT can be used as a probe for ODN structuration by monitoring the medium viscosity changes associated with ODN folding-unfolding processes.

Characterization of actomyosin bond properties in intact skeletal muscle by force spectroscopy

Force generation and motion in skeletal muscle result from interaction between actin and myosin myofilaments through the cyclical formation and rupture of the actomyosin bonds, the cross-bridges, in the overlap region of the sarcomeres. Actomyosin bond properties were investigated here in single intact muscle fibers by using dynamic force spectroscopy. The force needed to forcibly detach the cross-bridge ensemble in the half-sarcomere (hs) was measured in a range of stretching velocity between 3.4 x 10(3) nm.hs(-1).s(-1) or 3.3 fiber length per second (l(0)s(-1)) and 6.1 x 10(4) nm.hs(-1).s(-1) or 50 l(0).s(-1) during tetanic force development. The rupture force of the actomyosin bond increased linearly with the logarithm of the loading rate, in agreement with previous experiments on noncovalent single bond and with Bell theory [Bell GI (1978) Science 200:618-627]. The analysis permitted calculation of the actomyosin interaction length, x(beta) and the dissociation rate constant for zero external load, k(0). Mean x(beta) was 1.25 nm, a value similar to that reported for single actomyosin bond under rigor condition. Mean k(0) was 20 s(-1), a value about twice as great as that reported in the literature for isometric force relaxation in the same type of muscle fibers. These experiments show, for the first time, that force spectroscopy can be used to reveal the properties of the individual cross-bridge in intact skeletal muscle fibers.

How single molecule detection measures the dynamic actions of life

Biomolecules dynamically work in cells in which a variety of molecules assemble and interact in unique manner. The molecular mechanisms underlying several biological processes have been elucidated from the results obtained from the descriptions of cell function, from the snapshots of the structures of biomolecules involved in these processes, and from the biochemical properties of these reactions in vitro. Recently developed single molecule measurements have revealed the dynamic properties of the biomolecules that have been hidden in the data that have been averaged over large numbers of molecules in both ensemble measurement and in cells. Single molecule imaging and manipulation of single molecules have allowed the visualization of the dynamic operations of molecular motors, enzymatic reactions, structural dynamics of biomolecules, and cell signaling processes. The results have shown that the single molecule techniques are powerful tools to monitor the dynamic actions of biomolecules and their assemblies. This approach has been applied to a variety of fields within the life sciences. As new information emerges about the dynamic actions of biomolecules using methods of single molecule detection new views on how biological processes work will be revealed.

How single molecule detection measures the dynamic actions of life

Biomolecules dynamically work in cells in which a variety of molecules assemble and interact in unique manner. The molecular mechanisms underlying several biological processes have been elucidated from the results obtained from the descriptions of cell function, from the snapshots of the structures of biomolecules involved in these processes, and from the biochemical properties of these reactions in vitro. Recently developed single molecule measurements have revealed the dynamic properties of the biomolecules that have been hidden in the data that have been averaged over large numbers of molecules in both ensemble measurement and in cells. Single molecule imaging and manipulation of single molecules have allowed the visualization of the dynamic operations of molecular motors, enzymatic reactions, structural dynamics of biomolecules, and cell signaling processes. The results have shown that the single molecule techniques are powerful tools to monitor the dynamic actions of biomolecules and their assemblies. This approach has been applied to a variety of fields within the life sciences. As new information emerges about the dynamic actions of biomolecules using methods of single molecule detection new views on how biological processes work will be revealed.

Effect of fluctuation in step size on actin-myosin sliding motion

It is possible that the step size, or power stroke, of a skeletal muscle myosin is not constant; rather, it fluctuates for each force generation. The estimated widths of the fluctuation are as large as the estimated values of the step size. Although such non-negligible fluctuation is presumed to affect the sliding motion, these effects remain unclear. We examined a system driven by a single myosin molecule sliding along an actin filament to reveal its basic effects. First, we calculated the sliding velocity of the system for each fluctuation width and found that the mean velocity increased with the fluctuation width. We also found that the estimated fluctuation widths satisfied the conditions for maximizing the sliding velocity. Next, we examined the sliding motion along a heterogeneous filament, on which binding sites for myosins were distributed randomly. We found that the loss in sliding velocity that was attributable to heterogeneity of the filament became small when fluctuation in the step size existed. This finding implied that the fluctuation stabilized velocity sliding along possible heterogeneous filaments. These benefits of fluctuation in step size might be used in biological systems, such as a muscle system, and are applicable to fabricated micromachines.

Motors and their tethers: the role of secondary binding sites in processive motility

Cytoskeletal motors convert the energy from binding and hydrolyzing ATP into conformational changes that direct movement along a cytoskeletal polymer substrate. These enzymes utilize different mechanisms to generate long-range motion on the order of a micron or more that is required for functions ranging from muscle contraction to transport of growth factors along a nerve axon. Several of the individual cytoskeletal motors are processive, meaning that they have the ability to take sequential steps along their polymer substrate without dissociating from the polymer. This ability to maintain contact with the polymer allows individual motors to move cargos quickly from one cellular location to another. Many of the processive motors have now been found to utilize secondary binding sites that aid in motor processivity.

Selective spatial localization of actomyosin motor function by chemical surface patterning

We have previously described the efficient guidance and unidirectional sliding of actin filaments along nanosized tracks with adsorbed heavy meromyosin (HMM; myosin II motor fragment). In those experiments, the tracks were functionalized with trimethylchlorosilane (TMCS) by chemical vapor deposition (CVD) and surrounded by hydrophilic areas. Here we first show, using in vitro motility assays on nonpatterned and micropatterned surfaces, that the quality of HMM function on CVD-TMCS is equivalent to that on standard nitrocellulose substrates. We further examine the influences of physical properties of different surfaces (glass, SiO(2), and TMCS) and chemical properties of the buffer solution on motility. With the presence of methylcellulose in the assay solution, there was HMM-induced actin filament sliding on both glass/SiO(2) and on TMCS, but the velocity was higher on TMCS. This difference in velocity increased with decreasing contact angles of the glass and SiO(2) surfaces in the range of 20-67 degrees (advancing contact angles for water droplets). The corresponding contact angle of CVD-TMCS was 81 degrees. In the absence of methylcellulose, there was high-quality motility on TMCS but no motility on glass/SiO(2). This observation was independent of the contact angle of the glass/SiO(2) surfaces and of HMM incubation concentrations (30-150 microg mL(-)(1)) and ionic strengths of the assay solution (20-50 mM). Complete motility selectivity between TMCS and SiO(2) was observed for both nonpatterned and for micro- and nanopatterned surfaces. Spectrophotometric analysis of HMM depletion during incubation, K/EDTA ATPase measurements, and total internal reflection fluorescence spectroscopy of HMM binding showed only minor differences in HMM surface densities between TMCS and SiO(2)/glass. Thus, the motility contrast between the two surface chemistries seems to be attributable to different modes of HMM binding with the hindrance of actin binding on SiO(2)/glass.

Brownian molecular motors driven by rotation-translation coupling

We investigated three models of Brownian motors which convert rotational diffusion into directed translational motion by switching on and off a potential. In the first model a spatially asymmetric potential generates directed translational motion by rectifying rotational diffusion. It behaves much like a conventional flashing ratchet. The second model utilizes both rotational diffusion and drift to generate translational motion without spatial asymmetry in the potential. This second model can be driven by a combination of a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven) depending on the chosen parameters. In the third model, elements of both the Brownian motor and powerstroke mechanisms are combined by switching between three distinct states. Relevance of the model to biological motor proteins is discussed.

The stroke size of myosins: a reevaluation

In this article results are reviewed from different experimental approaches to determine the size of the power stroke generated by myosin molecules during their ATPase cycle. While data from fiber studies and protein crystallography predict a stroke size of about 10 nm for skeletal muscle myosins, single molecule studies imply a stroke size for these myosins of only about 5 nm. Single molecule studies also showed the stroke size to be proportional to the length of the light chain binding domain, acting like a lever arm. At the same lever arm length, however, the stroke size of smooth muscle myosin II is found about twice as large and a stroke size of about 14 nm was reported for class-I myosins. It was proposed that such different stroke sizes for molecules with same lever arm length result from different extend of converter domain rotation. Only for class-I myosins, however, an about 30 degrees larger rotation of the converter was found so far by protein crystallography. This, however, is far too small to account for the almost 3-fold larger stroke size reported from single molecule studies. In this contribution we discuss some factors that might account for the apparent discrepancies between single molecule studies on the one hand and protein crystallography as well as some fiber studies on the other hand. In addition, we present some modeling to illustrate that the power stroke very likely is underestimated to a large extent in current single molecule approaches. We further show that differences in the stroke size for various classes of myosins reported from single molecule studies might be related to small differences in the probability to execute the power stroke kinetics. We demonstrate that such small changes in power stroke kinetics can seriously affect the extent to which the 'true' power stroke is underestimated by present single molecule approaches.

Reconciling the working strokes of a single head of skeletal muscle myosin estimated from laser-trap experiments and crystal structures

Myosin generates force by a rotation of its lever arm. Crystal structures of myosin II indicate an unloaded working stroke of 10-12 nm, a range confirmed by recent x-ray interference experiments. However, when an actin filament, held between two weakly, optically trapped beads is made to interact with a single head of skeletal myosin, the bead displacements have often been reported as having a mean value of 5-6 nm, a value that is commonly interpreted as the working stroke. In general, the observed displacement is not expected to be equal to the working stroke because the kinetics of the stroke is necessarily strain-dependent: this effect biases the frequency of binding events to different actin sites so that displacements smaller than the working stroke are preferentially selected. Our analysis is tailored to current trap experiments, in which the time resolution is insufficient to detect pre-rigor states. If the preceding transitions are in equilibrium, the mean displacement is zero, contrary to observations in the presence of ATP. However, under ATP-cycling conditions, we find that the mean displacement is deflated to 0.3-0.7 of the true working stroke, depending on the equilibrium constant of the stroke and the rate at which the first myosin product state can detach from actin. The primary working stroke of processive myosin motors as measured by optical trapping is similarly uncertain.

Two independent mechanical events in the interaction cycle of skeletal muscle myosin with actin

During skeletal muscle contraction, regular arrays of actin and myosin filaments slide past each other driven by the cyclic ATP-dependent interaction of the motor protein myosin II (the cross-bridge) with actin. The rate of the cross-bridge cycle and its load-dependence, defining shortening velocity and energy consumption at the molecular level, vary widely among different isoforms of myosin II. However, the underlying mechanisms remain poorly understood. We have addressed this question by applying a single-molecule approach to rapidly ( approximately 300 mus) and precisely ( approximately 0.1 nm) detect acto-myosin interactions of two myosin isoforms having large differences in shortening velocity. We show that skeletal myosin propels actin filaments, performing its conformational change (working stroke) in two steps. The first step ( approximately 3.4-5.2 nm) occurs immediately after myosin binding and is followed by a smaller step ( approximately 1.0-1.3 nm), which occurs much faster in the fast myosin isoform than in the slow one, independently of ATP concentration. On the other hand, the rate of the second phase of the working stroke, from development of the latter step to dissociation of the acto-myosin complex, is very similar in the two isoforms and depends linearly on ATP concentration. The finding of a second mechanical event in the working stroke of skeletal muscle myosin provides the molecular basis for a simple model of actomyosin interaction. This model can account for the variation, in different fiber types, of the rate of the cross-bridge cycle and provides a common scheme for the chemo-mechanical transduction within the myosin family.

Force generation in single conventional actomyosin complexes under high dynamic load

The mechanical load borne by a molecular motor affects its force, sliding distance, and its rate of energy transduction. The control of ATPase activity by the mechanical load on a muscle tunes its efficiency to the immediate task, increasing ATP hydrolysis as the power output increases at forces less than isometric (the Fenn effect) and suppressing ATP hydrolysis when the force is greater than isometric. In this work, we used a novel 'isometric' optical clamp to study the mechanics of myosin II molecules to detect the reaction steps that depend on the dynamic properties of the load. An actin filament suspended between two beads and held in separate optical traps is brought close to a surface that is sparsely coated with motor proteins on pedestals of silica beads. A feedback system increases the effective stiffness of the actin by clamping the force on one of the beads and moving the other bead electrooptically. Forces measured during actomyosin interactions are increased at higher effective stiffness. The results indicate that single myosin molecules transduce energy nearly as efficiently as whole muscle and that the mechanical control of the ATP hydrolysis rate is in part exerted by reversal of the force-generating actomyosin transition under high load without net utilization of ATP.