Single-headed Myosin Research Articles

Focal segmental glomerulosclerosis and proteinuria associated with Myo1E mutations: novel variants and histological phenotype analysis.

Pathogenic mutations in the non-muscle single-headed myosin, myosin 1E (Myo1e), are a rare cause of pediatric focal segmental glomerulosclerosis (FSGS). These mutations are biallelic, to date only reported as homozygous variants in consanguineous families. Myo1e regulates the actin cytoskeleton dynamics and cell adhesion, which are especially important for podocyte functions. DNA and RNA sequencing were used to identify novel MYO1E variants associated with FSGS. We studied the effects of these variants on the localization of Myo1e in kidney sections. We then analyzed the clinical and histological observations of all known pathogenic MYO1E variants. We identified a patient compound heterozygote for two novel variants in MYO1E and a patient homozygous for a deletion of exon 19. Computer modeling predicted these variants to be disruptive. In both patients, Myo1e was mislocalized. As a rule, pathogenic MYO1E variants map to the Myo1e motor and neck domain and are most often associated with steroid-resistant nephrotic syndrome in children 1-11years of age, leading to kidney failure in 4-10years in a subset of patients. The ultrastructural features are the podocyte damage and striking diffuse and global Alport-like glomerular basement membrane (GBM) abnormalities. We hypothesize that MYO1E mutations lead to disruption of the function of podocyte contractile actin cables resulting in abnormalities of the podocytes and the GBM and dysfunction of the glomerular filtration barrier. The characteristic clinicopathological data can help to tentatively differentiate this condition from other genetic podocytopathies and Alport syndrome until genetic testing is done. A higher resolution version of the Graphical abstract is available as Supplementary information.

A mutation in the switch 1 region of myosin VA alters the load-dependence of phosphate rebinding

The load-dependence of reversible product release in myosins is key to understanding how it converts chemical energy into mechanical force. Following the powerstroke and phosphate (Pi) release, the rebinding of Pi to the actomyosin⋅ADP complex is highly load-dependent, but the underlying mechanisms/structures involved remain unclear. Therefore, we introduced a mutation (S217A) into the switch I region of the active site to examine its impact on the load dependence of Pi rebinding to single-headed myosin Va (MV) in a mini-ensemble (∼10 heads) three-bead laser trap assay at 100 uM ATP.

Myosin's powerstroke occurs prior to the release of phosphate from the active site.

Myosins are a family of motor proteins responsible for various forms of cellular motility, including muscle contraction and vesicular transport. The most fundamental aspect of myosin is its ability to transduce the chemical energy from the hydrolysis of ATP into mechanical work, in the form of force and/or motion. A key unanswered question of the transduction process is the timing of the force-generating powerstroke relative to the release of phosphate (Pi ) from the active site. We examined the ability of single-headed myosin Va to generate a powerstroke in a single molecule laser trap assay while maintaining Pi in its active site, by either elevating Pi in solution or by introducing a mutation in myosin's active site (S217A) to slow Pi -release from the active site. Upon binding to the actin filament, WT myosin generated a powerstoke rapidly (≥500 s-1 ) and without a detectable delay, both in the absence and presence of 30 mM Pi . The elevated levels of Pi did, however, affect event lifetime, eliminating the longest 25% of binding events, confirming that Pi rebound to myosin's active site and accelerated detachment. The S217A construct also generated a powerstroke similar in size and rate upon binding to actin despite the slower Pi release rate. These findings provide direct evidence that myosin Va generates a powerstroke with Pi still in its active site. Therefore, the findings are most consistent with a model in which the powerstroke occurs prior to the release of Pi from the active site.

A Proposed Mechanism for the Initial Myosin Binding Event on the Cardiac Thin Filament: A Metadynamics Study.

The movement of tropomyosin over filamentous actin regulates the cross-bridge cycle of the thick with thin filament of cardiac muscle by blocking and revealing myosin binding sites. Tropomyosin exists in three, distinct equilibrium states with one state blocking myosin-actin interactions (blocked position) and the remaining two allowing for weak (closed position) and strong myosin binding (open position). However, experimental information illuminating how myosin binds to the thin filament and influences tropomyosin's transition across the actin surface is lacking. Using metadynamics, we mimic the effect of a single myosin head binding by determining the work required to pull small segments of tropomyosin toward the open position in several distinct regions of the thin filament. We find differences in required work due to the influence of cardiac troponin T lead to preferential binding sites and determine the mechanism of further myosin head recruitment.

Myosin'S Powerstroke Occurs with Phosphate Still in the Active Site

A fundamental property of myosin is its ability to harness the power of ATP hydrolysis to generate force. A key unanswered question is the timing of the force-generating powerstroke relative to phosphate (Pi) -release from its active site. We examined the ability of single-headed myosin Va (MV) construct to bind to actin and generate a powerstroke in a single molecule laser trap assay in the presence of elevated Pi. We examined both WT MV and the impact of a mutation (S217A) in the conserved switch I region of the active site which slows actin-activated Pi-release. Upon binding to actin WT myosin rapidly (>500/s) generated a 7±0.5 nm powerstroke. In the presence of 30mM Pi, no decrease in the size (7±0.4 nm) or delay in the rate of the development of the powerstroke (>500/s) was observed, however the average duration of strong actomyosin binding (ton) was reduced in the presence of Pi (240±20 vs 113± ms, p<0.05). The reduced ton is consistent with Pi rebinding to the active site and accelerating detachment from actin. Therefore, this suggests myosin generates the powerstroke with Pi still in the active site. S217A generated a powerstroke with a similar rate (>500/s) and size (9±0.9 nm) to the WT, but ton was shorter, but not significantly different, than WT (143±20 ms, p=0.22 and 62±5 ms, p<0.05 for S217A in the absence and presence of Pi, respectively). Since the S217A mutant slows actin-activated Pi-release from the active site our results fit best to a model in which the powerstroke occurs with Pi still in the active site. Together these findings suggest that actin-binding, and not the release of Pi from the active site, trigger the conformational changes that lead to the powerstroke, and thus force-generation by myosin.

Unusual dynamics of the divergent malaria parasite PfAct1 actin filament

Gliding motility and host cell invasion by the apicomplexan parasite Plasmodium falciparum (Pf), the causative agent of malaria, is powered by a macromolecular complex called the glideosome that lies between the parasite plasma membrane and the inner membrane complex. The glideosome core consists of a single-headed class XIV myosin PfMyoA and a divergent actin PfAct1. Here we use total internal reflection fluorescence microscopy to visualize growth of individual unstabilized PfAct1 filaments as a function of time, an approach not previously used with this actin isoform. Although PfAct1 was thought to be incapable of forming long filaments, filaments grew as long as 30 µm. Polymerization occurs via a nucleation-elongation mechanism, but with an ∼4 µM critical concentration, an order-of-magnitude higher than for skeletal actin. Protomers disassembled from both the barbed and pointed ends of the actin filament with similar fast kinetics of 10 to 15 subunits/s. Rapid treadmilling, where the barbed end of the filament grows and the pointed end shrinks while maintaining an approximately constant filament length, was visualized near the critical concentration. Once ATP has been hydrolyzed to ADP, the filament becomes very unstable, resulting in total dissolution in <40 min. Dynamics at the filament ends are suppressed in the presence of inorganic phosphate or more efficiently by BeFX A chimeric PfAct1 with a mammalian actin D-loop forms a more stable filament. These unusual dynamic properties distinguish PfAct1 from more canonical actins, and likely contribute to the difficultly in visualizing PfAct1 filaments in the parasite.

Development of an Atomistic Structure of Myosin-Bound Cardiac Thin Filament and Free Energy Determination of the Close-to-Open Transition

Myosin represents a superfamily of motor proteins that is diverse both in homology as well as function and whose association with actin filaments in a ATP-dependent manner is often correlated with muscle contraction. With recent advances in Cryo-Electron Microscopy, high resolution images of various morphologies of myosin interacting with actin has allowed for the visualization of the specific interactions between myosin, actin, and tropomyosin, but little is still known how these interactions change as myosin progresses through its ATP hydrolysis cycle. Another issue is the lack of a high resolution structure of human cardiac myosin bound to actin where mutagenic changes in the myosin head have been associated with the development of cardiomyopathies within individuals. In the current study, we employ molecular dynamics (MD) simulations as well as other computational methods to study how the beta isoform of cardiac myosin (MYH7) interacts with the cardiac thin filament (CTF). Utilizing free energy calculations such as the adaptive biasing force (ABF) method, we will probe the mean force that a single myosin head must exert on tropomyosin to transition from the closed to open transition. With these simulations we hope to be able to evaluate the overall energetics of the transition, the possibility of a preferential, initial binding site of myosin on the thin filament, and the overall cooperative effect of myosin binding. Development of this novel atomistic myosin-CTF structure provides a unique insight into the specific structure and dynamics of cardiac myosin and allows for further studies of the development of cardiomyopathies as well as the dynamics of the cross-bridge cycle.

Myosin 1b promotes axon formation by regulating actin wave propagation and growth cone dynamics.

Single-headed myosin 1 has been identified in neurons, but its function in these cells is still unclear. We demonstrate that depletion of myosin 1b (Myo1b), inhibition of its motor activity, or its binding to phosphoinositides impairs the formation of the axon, whereas overexpression of Myo1b increases the number of axon-like structures. Myo1b is associated with growth cones and actin waves, two major contributors to neuronal symmetry breaking. We show that Myo1b controls the dynamics of the growth cones and the anterograde propagation of the actin waves. By coupling the membrane to the actin cytoskeleton, Myo1b regulates the size of the actin network as well as the stability and size of filopodia in the growth cones. Our data provide the first evidence that a myosin 1 plays a major role in neuronal symmetry breaking and argue for a mechanical control of the actin cytoskeleton both in actin waves and in the growth cones by this myosin.

Observing the Behavior of a Single Myosin Head Within a Myosin Filament Moving on Actin

We developed a novel unloaded in vitro motility assay in which myosin filaments move over surface-attached actin (Brizendine et al, PNAS 2015). For three types of muscle myosin II (smooth, skeletal, and cardiac), the relationship between myosin filament velocity (V) and the number of myosin heads available to interact with actin (N) can be accounted for by a novel mixed-kinetic model where V is influenced by both attachment and detachment kinetics (see Brizendine et al, Abstract this meeting). A key model variable is the flexibility and effective length (L) of the subfragment 2 (S2) region of the myosin tail, which can presumably bend or buckle to allow actin-attached myosin heads to place very little drag load on the moving filament. This model predicts that a given myosin head within a moving filament will appear stationary for the duration of time it remains attached to actin (ton), followed by a jump forward a distance L relative to the filament backbone. To test these predictions, we prepared three different smooth muscle myosin (SMM)-derived QD-labeled constructs (regulatory light chain (RLC), light meromyosin (LMM), and S2) and assembled them individually and simultaneously into co-filaments with unlabeled native SMM. We will track the QD motion with nm accuracy during filament motility to compare the behavior of myosin heads (RLC-QD) and the headless S2-QD relative to the global filament motion (LMM-QD). As [ATP] is varied, ton is predicted to change according to ton = (k-AD)−1 + (kt[ATP])−1, where k-AD is the ADP release rate and kT is the rate of attachment of ATP to acto-myosin, whereas L will be invariant at ∼60 nm, a value based upon our previous work (Brizendine et al, PNAS 2015).

Investigation of the Molecular Interactions Regulating the Function of Human Cardiac Myosin

Cardiac myosin interacts cyclically with actin in the presence of ATP in the sarcomere to drive heart contraction. Previous fiber studies suggested that the structure and function of myosin in the thick filaments are regulated by its own regulatory light chain (RLC) phosphorylation and through its interaction with myosin binding protein-C (MyBP-C). Interestingly, ∼80% of mutations that cause hypertrophic cardiomyopathy (HCM), a relatively common genetic cardiac disorder characterized by hypercontractility, have been found in the genes encoding human β-cardiac myosin and human cardiac MyBP-C. It has been hypothesized that the heads of the two-headed motor can adopt a sequestered structure where the heads interact asymmetrically with their own tail as Ser-15 residue of RLC remains dephosphorylated, and the N-terminal domains of MyBP-C binds to myosin to stabilize the structure. We designed two different two-headed human β-cardiac myosin motor constructs differing in the length of their S2 tail domains and co-expressed them with both of the human light chains to test the hypothesis that the number of myosin heads available for interaction with actin is regulated by phosphorylation of the RLC and MyBP-C. The shorter construct (2-hep HMM) is devoid of the tail region needed for the interaction of S2 with its S1 heads whereas the longer one (25-hep HMM) has the required length of S2 tail region to allow such interactions. We found that the maximal actin-activated ATPase rate for 2-hep HMM and the single-headed myosin construct S1 are essentially the same, and are only slightly changed by RLC phosphorylation. On the other hand, de-phosphorylated 25-hep HMM has a significantly lower maximal actin-activated ATPase than the 2-hep HMM, consistent with the former having fewer actin-accessible S1 heads than the latter. Additionally, binding experiments of 2-hep and 25-hep HMM with the N-terminal C0-C2 domain of human cardiac MyBP-C showed that the affinity of complex formation is weakened by phosphorylation of either RLC or MyBP-C.

Direct Measurements of Local Coupling between Myosin Molecules are Consistent with a Model of Muscle Activation

Muscle contracts due to ATP-dependent interactions of myosin motors with thin filaments composed of the proteins actin, troponin, and tropomyosin. Contraction is initiated when calcium binds to troponin, which changes conformation and displaces tropomyosin, a filamentous protein that wraps around the actin filament, thereby exposing myosin binding sites on actin. Myosin motors interact with each other indirectly via tropomyosin, since myosin binding to actin locally displaces tropomyosin and thereby facilitates binding of nearby myosin. Defining and modeling this local coupling between myosin motors is an open problem in muscle modeling and, more broadly, a requirement to understanding the connection between muscle contraction at the molecular and macro scale. It is challenging to directly observe this coupling, and such measurements have only recently been made. Analysis of these data suggests that two myosin heads are required to activate the thin filament. This result contrasts with a theoretical model, which reproduces several indirect measurements of coupling between myosin, that assumes a single myosin head can activate the thin filament. To understand this apparent discrepancy, we incorporated the model into stochastic simulations of the experiments, which generated simulated data that were then analyzed identically to the experimental measurements. By varying a single parameter, good agreement between simulation and experiment was established. The conclusion that two myosin molecules are required to activate the thin filament arises from an assumption, made during data analysis, that the intensity of the fluorescent tags attached to myosin varies depending on experimental condition. We provide an alternative explanation that reconciles theory and experiment without assuming that the intensity of the fluorescent tags varies.

Direct Measurements of Local Coupling between Myosin Molecules Are Consistent with a Model of Muscle Activation.

Muscle contracts due to ATP-dependent interactions of myosin motors with thin filaments composed of the proteins actin, troponin, and tropomyosin. Contraction is initiated when calcium binds to troponin, which changes conformation and displaces tropomyosin, a filamentous protein that wraps around the actin filament, thereby exposing myosin binding sites on actin. Myosin motors interact with each other indirectly via tropomyosin, since myosin binding to actin locally displaces tropomyosin and thereby facilitates binding of nearby myosin. Defining and modeling this local coupling between myosin motors is an open problem in muscle modeling and, more broadly, a requirement to understanding the connection between muscle contraction at the molecular and macro scale. It is challenging to directly observe this coupling, and such measurements have only recently been made. Analysis of these data suggests that two myosin heads are required to activate the thin filament. This result contrasts with a theoretical model, which reproduces several indirect measurements of coupling between myosin, that assumes a single myosin head can activate the thin filament. To understand this apparent discrepancy, we incorporated the model into stochastic simulations of the experiments, which generated simulated data that were then analyzed identically to the experimental measurements. By varying a single parameter, good agreement between simulation and experiment was established. The conclusion that two myosin molecules are required to activate the thin filament arises from an assumption, made during data analysis, that the intensity of the fluorescent tags attached to myosin varies depending on experimental condition. We provide an alternative explanation that reconciles theory and experiment without assuming that the intensity of the fluorescent tags varies.

Coupling of two non-processive myosin 5c dimers enables processive stepping along actin filaments.

Myosin 5c (Myo5c) is a low duty ratio, non-processive motor unable to move continuously along actin filaments though it is believed to participate in secretory vesicle trafficking in vertebrate cells. Here, we measured the ATPase kinetics of Myo5c dimers and tested the possibility that the coupling of two Myo5c molecules enables processive movement. Steady-state ATPase activity and ADP dissociation kinetics demonstrated that a dimer of Myo5c-HMM (double-headed heavy meromyosin 5c) has a 6-fold lower Km for actin filaments than Myo5c-S1 (single-headed myosin 5c subfragment-1), indicating that the two heads of Myo5c-HMM increase F-actin-binding affinity. Nanometer-precision tracking analyses showed that two Myo5c-HMM dimers linked with each other via a DNA scaffold and moved processively along actin filaments. Moreover, the distance between the Myo5c molecules on the DNA scaffold is an important factor for the processive movement. Individual Myo5c molecules in two-dimer complexes move stochastically in 30–36 nm steps. These results demonstrate that two dimers of Myo5c molecules on a DNA scaffold increased the probability of rebinding to F-actin and enabled processive steps along actin filaments, which could be used for collective cargo transport in cells.

The Loop2 Insertion of Type IX Myosin Acts as an Electrostatic Actin Tether that Permits Processive Movement

Although class IX myosins are single-headed, they demonstrate characteristics of processive movement along actin filaments. Double-headed myosins that move processively along actin filaments achieve this by successive binding of the two heads in a hand-over-hand mechanism. This mechanism, obviously, cannot operate in single-headed myosins. However, it has been proposed that a long class IX specific insertion in the myosin head domain at loop2 acts as an F-actin tether, allowing for single-headed processive movement. Here, we tested this proposal directly by analysing the movement of deletion constructs of the class IX myosin from Caenorhabditis elegans (Myo IX). Deletion of the large basic loop2 insertion led to a loss of processive behaviour, while deletion of the N-terminal head extension, a second unique domain of class IX myosins, did not influence the motility of Myo IX. The processive behaviour of Myo IX is also abolished with increasing salt concentrations. These observations directly demonstrate that the insertion located in loop2 acts as an electrostatic actin tether during movement of Myo IX along the actin track.

Understanding the Effects of Cardiomyopathy Causing Mutations on Human Beta Cardiac Myosin Biomechanical Function

Cardiomyopathy is one of the leading causes of sudden cardiac death across the developed world. Over the last twenty years numerous single point mutations in the cardiac contractile apparatus have been implicated in hypertrophic and dilated cardiomyopathy (HCM or DCM), out of which nearly 40% of them are housed in the human β-cardiac myosin. Nonetheless, the underlying molecular effects of most of these mutations remain elusive. The cardiac muscle cyclically contracts under varying loads and it is likely that the mutations in the β-cardiac myosin affect the power output of the heart. We hypothesize that these mutations cause a fundamental mechanistic change in the power output of the cardiac myosin contractile system by either altering force or velocity of contraction, which ultimately compromises heart function and results in the clinical phenotypes. Using a combination of biochemistry and single molecule spectroscopy we show that two HCM-causing mutations (R403Q and R453C) in a truncated version of human β-cardiac myosin ending after the essential light chain (1-808) cause an increase in the power output by two different mechanisms. For R403Q we see a ∼20% increase in the maximum unloaded velocity, whereas for R453C the major change is a ∼50% increase in the intrinsic force generated by a single myosin head. On the other hand, with the DCM-causing S532P β-cardiac myosin mutant both unloaded velocity and intrinsic force is decreased as compared to the wild type, predicting a lower power output. These results indicate that distinct molecular mechanisms for altering power output are operative for different cardiomyopathy-causing mutants. Work is currently being performed to understand these and other cardiomyopathy-causing mutations using an extended version of the human β-cardiac myosin molecule (1-843) with both human light chains.

Myosin-3B and its Light Chains

Unconventional myosin-3B is a single-headed myosin containing an amino-terminal kinase domain. In vitro, we find full length and truncated versions of the molecule to bind the regulatory light chain (RLC) and calmodulin (CaM). The RLC binds to the first IQ-like motif and CaM to the second IQ motif within the neck region of the molecule. Uniquely, we find the RLC to be reversibly exchangeable with CaM in the presence of calcium. Calcium-free conditions favor RLC binding. Exchange of RLC versus CaM to the first IQ motif significantly increases the steady-state ATPase activity in the presence of calcium and modulates the interaction of the molecule with F-actin. This observation shows that the calcium-dependent light chain exchange triggers the activity of human myosin-3B as a molecular motor and suggests that myosin motor function is directly influenced by the set of light chains bound to the neck region.

Purification, crystallization and preliminary X-ray crystallographic analysis of squid heavy meromyosin

All muscle-based movement is dependent upon carefully choreographed interactions between the two major muscle components, myosin and actin. Regulation of vertebrate smooth and molluscan muscle contraction is myosin based (both are in the myosin II class), and requires the double-headed form of myosin. Removal of Ca2+ from these muscles promotes a relatively compact conformation of the myosin dimer, which inhibits its interaction with actin. Although atomic structures of single myosin heads are available, the structure of any double-headed portion of myosin, including the ∼375 kDa heavy meromyosin (HMM), has only been visualized at low (∼20 Å) resolution by electron microscopy. Here, the growth of three-dimensional crystals of HMM with near-atomic resolution (up to ∼5 Å) and their X-ray diffraction are reported for the first time. These crystals were grown in off-state conditions, that is in the absence of Ca2+ and the presence of nucleotide analogs, using HMM from the funnel retractor muscle of squid. In addition to the crystallization conditions, the techniques used to isolate and purify this HMM are also described. Efforts at phasing and improving the resolution of the data in order to determine the structure are ongoing.

Myo1c is Built for Sustained Power Under Load, not Tension Sensing

Myo1c, a widely expressed single-headed myosin, has been proposed to act in a variety of cellular processes, including insulin-stimulated transport of GLUT4 and mechanosensing in sensory hair cells; however, the mechanical and kinetic properties of a transporter and a tension sensor are very different. To better understand the adaptations of myo1c for its molecular roles in the cell, we examined its biochemical and mechanical properties using ensemble kinetic and single molecule techniques. Consistent with previous reports, we found that the myo1c working stroke is comprised of two substeps, which are likely associated with phosphate and ADP release.

Two single-headed myosin V motors bound to a tetrameric adapter protein form a processive complex

Myo4p, one of two class V myosins in budding yeast, continuously transports messenger RNA (mRNA) cargo in the cell but is nonprocessive when characterized in vitro. The adapter protein She3p tightly binds to the Myo4p rod, forming a single-headed motor complex. In this paper, we show that two Myo4p-She3p motors are recruited by the tetrameric mRNA-binding protein She2p to form a processive double-headed complex. The binding site for She3p was mapped to a single α helix that protrudes at right angles from She2p. Processive runs of several micrometers on yeast actin-tropomyosin filaments were observed only in the presence of She2p, and, thus, motor activity is regulated by cargo binding. While moving processively, each head steps ~72 nm in a hand-over-hand motion. Coupling two high-duty cycle monomeric motors via a common cargo-binding adapter protein creates a complex with transport properties comparable with a single dimeric processive motor such as vertebrate myosin Va.

At the Source: Treating Heart Failure by Altering Muscle Motor Function

### Cardiac Myosin Activation: A Potential Therapeutic Approach for Systolic Heart Failure Malik et al Science . 2011;331:1439–1443. A new study in Science provides proof of the principle for directly modulating the cardiac muscle's motor, myosin, as a therapeutic target in heart failure. The human and economic tolls from heart failure (HF) are significant in both developed and undeveloped countries.1–3 Although HF can take many forms,4,5 it is most commonly associated with decreased cardiac contractility and is, therefore, referred to as systolic HF (sHF).6 Current therapeutic strategies rely on blocking neurohormonal activation by inhibiting associated pathways or by receptor blockade using either β-adrenergic or aldosterone blockers.7,8 Alternatively, some therapies are directed at increasing cardiac contractility in an effort to restore sufficient blood flow, usually by activating second messenger signaling pathways that increase cardiomyocyte calcium levels and augment sarcomere contraction.9 Both strategies suffer from a lack of specificity in that they target proteins upstream of the actual contractile events that underlie force production. Affecting important signaling pathways or receptor-ligand interactions can impact cellular function and perturb homeostasis in ways unrelated to the contractile apparatus, leading to undesirable side effects, such as hypotension, tachycardia, and even arrhythmias that can lead to sudden death.10 Recognizing the significant limitations of current therapeutic modalities, Malik and co-workers have directed their efforts at directly modulating cardiac contractility by tuning motor function at the source, the myosin heavy chain. The cardiac myosin heavy chains are encoded by two genes, MYH6 , which encodes α myosin heavy chain, and MYH7 , which encodes β myosin heavy chain, the latter being the predominant isoform in human ventricle. Although having greater than 90% amino acid homology, the two proteins differ significantly at the biochemical level. In comparison with α myosin, β myosin is relatively more efficient in force production as a function of energy consumption, with …