Velocity of myosin-based actin sliding depends on attachment and detachment kinetics and reaches a maximum when myosin-binding sites on actin saturate

Deciphering Mechanochemical Influences of Emergent Actomyosin Crosstalk Using QCM-D

Purpose

Cytoskeletal protein ensembles exhibit emergent mechanics where behavior in teams is not necessarily the sum of the components' single molecule properties. In addition, filaments may act as force sensors that distribute feedback and influence motor protein behavior. To understand the design principles of such emergent mechanics, we developed an approach utilizing QCM-D to measure how actomyosin bundles respond mechanically to environmental variables that alter constituent myosin II motor behavior.

Methods

QCM-D is used for the first time to probe alterations in actin-myosin bundle viscoelasticity due to changes in skeletal myosin II concentration and motor nucleotide state. Actomyosin bundles were constructed on a gold QCM-D sensor using a microfluidic setup, and frequency and dissipation change measurements were recorded for each component addition to decipher which assay constituents lead to changes in bundle structural compliancy.

Results

Lowering myosin concentration is detected as lower shifts in frequency and dissipation, while the relative changes in frequency and dissipation shifts for both the first and second actin additions are relatively similar. Strikingly, buffer washes with different nucleotides (ATP vs. ADP) yielded unique signatures in frequency and dissipation shifts. As myosin II's ADP-bound state tightly binds actin filaments, we observe an increase in frequency and decrease in dissipation change, indicating a decrease in viscoelasticity, likely due to myosin's increased affinity for actin, conversion from an active motor to a static crosslinker, and ability to recruit additional actin filaments from the surface, making an overall more rigid sensor coating. However, lowering the ADP concentration results in increased system compliancy, indicating that transient crosslinking and retaining a balance of motor activity perhaps results in a more cooperative and productive force generating system.

Conclusions

QCM-D can detect changes in actomyosin viscoelasticity due to molecular-level alterations, such as motor concentration and nucleotide state. These results provide support for actin's role as a mechanical force-feedback sensor and demonstrate a new approach for deciphering the feedback mechanisms that drive emergent cytoskeletal ensemble crosstalk and intracellular mechanosensing. This approach can be adapted to investigate environmental influences on more complex cytoskeletal ensemble mechanics, including addition of other motors, crosslinkers, and filament types.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00835-w.

Four phases of a force transient emerge from a binary mechanical system

Accurate models of muscle contraction are important for understanding both muscle performance and the therapeutics that enhance physiological function. However, models are only accurate and meaningful if they are consistent with physical laws. A single muscle fiber contains billions of randomly fluctuating atoms that on the spatial scale of a muscle fiber generate unidirectional force and power output. This thermal system is formally constrained by the laws of thermodynamics, and a recently developed thermodynamic model of muscle force generation provides qualitative descriptions of the muscle force-velocity relationship, muscle force generation, muscle force transients, and the thermodynamic work loop of muscle with a thermodynamic (not molecular) power stroke mechanism. To demonstrate the accuracy of this model requires that its outputs be quantitatively compared with experimentally observed muscle function. Here I show that a two-state thermodynamic model accurately describes the experimentally observed four-phase force transient response to both mechanical and chemical perturbations. This is the simplest possible model of one of the most complex characteristic signatures of muscle mechanics.

Deciphering Mechanochemical Influences of Emergent Actomyosin Crosstalk using QCM-D

Purpose

Cytoskeletal protein ensembles exhibit emergent mechanics where behavior exhibited in teams is not necessarily the sum of the components' single molecule properties. In addition, filaments may act as force sensors that distribute feedback and influence motor protein behavior. To understand the design principles of such emergent mechanics, we developed an approach utilizing QCM-D to measure how actomyosin bundles respond mechanically to environmental variables that alter constituent myosin II motor behavior.

Methods

QCM-D is used for the first time to probe alterations in actin-myosin bundle viscoelasticity due to changes in skeletal myosin II concentration and motor nucleotide state. Actomyosin bundles were constructed on a gold QCM-D sensor using a microfluidic setup, and frequency and dissipation change measurements were recorded for each component addition to decipher which assay constituents lead to changes in bundle structural compliancy.

Results

Lowering myosin concentration is detected as lower shifts in frequency and dissipation, while the relative changes in frequency and dissipation shifts for both the first and second actin additions are relatively similar. Strikingly, buffer washes with different nucleotides (ATP vs. ADP) yielded unique signatures in frequency and dissipation shifts. As myosin II's ADP-bound state tightly binds actin filaments, we observe an increase in frequency and decrease in dissipation change, indicating a decrease in viscoelasticity, likely due to myosin's increased affinity for actin, conversion from an active motor to a static crosslinker, and ability to recruit additional actin filaments from the surface, making an overall more rigid sensor coating. However, lowering the ADP concentration results in increased system compliancy, indicating that transient crosslinking and retaining a balance of motor activity perhaps results in a more cooperative and productive force generating system.

Conclusions

QCM-D can detect changes in actomyosin viscoelasticity due to molecular-level alterations, such as motor concentration and nucleotide state. These results provide support for actin's role as a mechanical force-feedback sensor and demonstrate a new approach for deciphering the feedback mechanisms that drive emergent cytoskeletal ensemble crosstalk and intracellular mechanosensing. This approach can be adapted to investigate environmental influences on more complex cytoskeletal ensemble mechanics, including addition of other motors, crosslinkers, and filament types.

Duality in disease: How two amino acid substitutions at actin residue 312 result in opposing forms of cardiomyopathy

Two common types of cardiovascular disease are hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) which occur from changes to sarcomere contractile mechanisms and activity. Actin amino acid substitutions R312C and R312H have been found in HCM and DCM patients, respectively. Previously, we observed that R312C/H variants display both hyperactivity and hypoactivity in vitro, contradicting traditional characterizations of HCM- and DCM-causing variants. Here, we further characterized R312C/H actin variants in vitro and conducted in silico modeling to better understand the mechanisms differentiating HCM and DCM. Our results suggest that R312C/H substitutions cause structural changes that differentially impact actomyosin activity. A gradient of altered interactions with regulatory proteins troponin, tropomyosin, and the C0C2 domains of myosin-binding protein C was also observed, influencing the accessibility of active and inhibitory conformations of these proteins. The results presented here support our previous suggestion of a gradient of factors that differentiate between HCM and DCM. Further characterization of HCM- and DCM-causing actin variants using in vitro and in silico methods is required for better understanding cardiomyopathy and improving clinical outcomes.

Dilated cardiomyopathy mutation in beta-cardiac myosin enhances actin activation of the power stroke and phosphate release

Inherited mutations in human beta-cardiac myosin (M2β) can lead to severe forms of heart failure. The E525K mutation in M2β is associated with dilated cardiomyopathy (DCM) and was found to stabilize the interacting heads motif (IHM) and autoinhibited super-relaxed (SRX) state in dimeric heavy meromyosin. However, in monomeric M2β subfragment 1 (S1) we found that E525K enhances (threefold) the maximum steady-state actin-activated ATPase activity (k cat) and decreases (eightfold) the actin concentration at which ATPase is one-half maximal (K ATPase). We also found a twofold to fourfold increase in the actin-activated power stroke and phosphate release rate constants at 30 μM actin, which overall enhanced the duty ratio threefold. Loaded motility assays revealed that the enhanced intrinsic motor activity translates to increased ensemble force in M2β S1. Glutamate 525, located near the actin binding region in the so-called activation loop, is highly conserved and predicted to form a salt bridge with another conserved residue (lysine 484) in the relay helix. Enhanced sampling molecular dynamics simulations predict that the charge reversal mutation disrupts the E525-K484 salt bridge, inducing conformations with a more flexible relay helix and a wide phosphate release tunnel. Our results highlight a highly conserved allosteric pathway associated with actin activation of the power stroke and phosphate release and suggest an important feature of the autoinhibited IHM is to prevent this region of myosin from interacting with actin. The ability of the E525K mutation to stabilize the IHM likely overrides the enhanced intrinsic motor properties, which may be key to triggering DCM pathogenesis.

Glioblastoma Cells Use an Integrin- and CD44-Mediated Motor-Clutch Mode of Migration in Brain Tissue

Purpose

Glioblastoma (GBM) is an aggressive malignant brain tumor with 2 year survival rates of 6.7% (Stupp et al. in J Clin Oncol Off J Am Soc Clin Oncol 25:4127-4136, 2007; Mohammed et al. in Rep Pract Oncol Radiother 27:1026-1036, 2002). One key characteristic of the disease is the ability of glioblastoma cells to migrate rapidly and spread throughout healthy brain tissue (Lefranc et al. in J Clin Oncol Off J Am Soc Clin Oncol 23:2411-2422, 2005; Hoelzinger et al. in J Natl Cancer Inst 21:1583-1593, 2007). To develop treatments that effectively target cell migration, it is important to understand the fundamental mechanism driving cell migration in brain tissue. Several models of cell migration have been proposed, including the motor-clutch, bleb-based motility, and osmotic engine models.

Methods

Here we utilized confocal imaging to measure traction dynamics and migration speeds of glioblastoma cells in mouse organotypic brain slices to identify the mode of cell migration.

Results

We found that nearly all cell-vasculature interactions reflected pulling, rather than pushing, on vasculature at the cell leading edge, a finding consistent with a motor-clutch mode of migration, and inconsistent with an osmotic engine model or confined bleb-based migration. Reducing myosin motor activity, a key component in the motor-clutch model, was found to decrease migration speed at high doses for all cell types including U251 and 6 low-passage patient-derived xenograft lines (3 proneural and 3 mesenchymal subtypes). Variable responses were found at low doses, consistent with a motor-clutch mode of migration which predicts a biphasic relationship between migration speed and motor-to-clutch ratio. Targeting of molecular clutches including integrins and CD44 slowed migration of U251 cells.

Conclusions

Overall we find that glioblastoma cell migration is most consistent with a motor-clutch mechanism to migrate through brain tissue ex vivo, and that both integrins and CD44, as well as myosin motors, play an important role in constituting the adhesive clutch.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00799-x.

Geldanaycin-encapsulated magnetic nanoparticle for isolation of myosin in proteomics

The grafting of a drug molecule, i.e., geldanamycin (GA) onto polyethyleneimine (PEI)-coated magnetic nanoparticle produces a novel composite, GA@Fe3O4-NH2. The composite is confirmed by characterizations with FT-IR, Raman, SEM, EDS, VSM and TEM. Due to the high binding-affinity of GA with myosin heavy chain (MYH), GA@Fe3O4-NH2 exhibits excellent adsorption performance towards myosin. Consequently, a solid-phase extraction procedure is established for highly efficient and selective separation of myosin from pig heart extract. At pH 6.0, an adsorption efficiency of 97.1 % is achieved for treating 100 μg mL-1 myosin (0.1 mL) with 0.1 mg GA@Fe3O4-NH2 as adsorbent. The adsorption behavior of myosin onto GA@Fe3O4-NH2 fits Langmuir model, corresponding to a theoretical adsorption capacity of 518.1 mg g-1. The adsorbed myosin can be readily recycled by the SDS solution (1 %, m/m) with an elution efficiency of 91.8 %. According to circular dichroism spectroscopy, the conformational changes of myosin during adsorption and elution are reversible. For practical application, myosin is successfully isolated from the pig left ventricular protein extract with GA@Fe3O4-NH2, and SDS-PAGE and LC-MS/MS showed that myosin had high purity and a total of 716 proteins could be identified. Significantly, Geldamycin-encapsulated magnetic nanoparticle for the separation of myosin well exploits the potential of the nanomaterials modified by drug molecules in the separation and purification of target proteins.

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

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

Dilated cardiomyopathy mutation in beta-cardiac myosin enhances actin activation of the power stroke and phosphate release

Inherited mutations in human beta-cardiac myosin (M2β) can lead to severe forms of heart failure. The E525K mutation in M2β is associated with dilated cardiomyopathy (DCM) and was found to stabilize the interacting heads motif (IHM) and autoinhibited super-relaxed (SRX) state in dimeric heavy meromyosin. However, in monomeric M2β subfragment 1 (S1) we found that E525K enhances (3-fold) the maximum steady-state actin-activated ATPase activity (kcat) and decreases (6-fold) the actin concentration at which ATPase is one-half maximal (KATPase). We also found a 3 to 4-fold increase in the actin-activated power stroke and phosphate release rate constants at 30 μM actin, which overall enhanced the duty ratio 3-fold. Loaded motility assays revealed that the enhanced intrinsic motor activity translates to increased ensemble force in M2β S1. Glutamate 525, located near the actin binding region in the so-called activation loop, is highly conserved and predicted to form a salt-bridge with another conserved residue (lysine 484) in the relay helix. Enhanced sampling molecular dynamics simulations predict that the charge reversal mutation disrupts the E525-K484 salt-bridge, inducing conformations with a more flexible relay helix and a wide phosphate release tunnel. Our results highlight a highly conserved allosteric pathway associated with actin activation of the power stroke and phosphate release and suggest an important feature of the autoinhibited IHM is to prevent this region of myosin from interacting with actin. The ability of the E525K mutation to stabilize the IHM likely overrides the enhanced intrinsic motor properties, which may be key to triggering DCM pathogenesis.

Glioblastoma cells use an integrin- and CD44-mediated motor-clutch mode of migration in brain tissue

Glioblastoma (GBM) is an aggressive malignant brain tumor with 2-year survival rates of 6.7% [1], [2]. One key characteristic of the disease is the ability of glioblastoma cells to migrate rapidly and spread throughout healthy brain tissue[3], [4]. To develop treatments that effectively target cell migration, it is important to understand the fundamental mechanism driving cell migration in brain tissue. Here we utilized confocal imaging to measure traction dynamics and migration speeds of glioblastoma cells in mouse organotypic brain slices to identify the mode of cell migration. Through imaging cell-vasculature interactions and utilizing drugs, antibodies, and genetic modifications to target motors and clutches, we find that glioblastoma cell migration is most consistent with a motor-clutch mechanism to migrate through brain tissue ex vivo, and that both integrins and CD44, as well as myosin motors, play an important role in constituting the adhesive clutch.

The Problem with Inventing Molecular Mechanisms to Fit Thermodynamic Equations of Muscle

Almost every model of muscle contraction in the literature to date is a molecular power stroke model, even though this corpuscular mechanism is opposed by centuries of science, by 85 years of unrefuted evidence that muscle is a thermodynamic system, and by a quarter century of direct observations that the molecular mechanism of muscle contraction is a molecular switch, not a molecular power stroke. An ensemble of molecular switches is a binary mechanical thermodynamic system from which A.V. Hill's muscle force-velocity relationship is directly derived, where Hill's parameter a is the internal force against which unloaded muscle shortens, and Hill's parameter b is the product of the switch displacement, d, and the actin-myosin ATPase rate. Ignoring this model and the centuries of thermodynamics that preceded it, corpuscularians continue to develop molecular power stroke models, adding to their 65-year jumble of "new", "innovative", and "unconventional" molecular mechanisms for Hill's a and b parameters, none of which resemble the underlying physical chemistry. Remarkably, the corpuscularian community holds the thermodynamicist to account for these discrepancies, which, as outlined here, I have done for 25 years. It is long past time for corpuscularians to be held accountable for their mechanisms, which by all accounts have no foundation in science. The stakes are high. Molecular power stroke models are widely used in research and in clinical decision-making and have, for over half a century, muddied our understanding of the inner workings of one of the most efficient and clean-burning machines on the planet. It is problematic that corpuscularians present these models to stakeholders as science when in fact corpuscularians have been actively defending these models against science for decades. The path forward for scientists is to stop baseless rejections of muscle thermodynamics and to begin testing corpuscular and thermodynamic mechanisms with the goal of disproving one or the other of these hypotheses.

Cells solved the Gibbs paradox by learning to contain entropic forces

As Nature's version of machine learning, evolution has solved many extraordinarily complex problems, none perhaps more remarkable than learning to harness an increase in chemical entropy (disorder) to generate directed chemical forces (order). Using muscle as a model system, here I describe the basic mechanism by which life creates order from disorder. In short, evolution tuned the physical properties of certain proteins to contain changes in chemical entropy. As it happens these are the "sensible" properties Gibbs postulated were needed to solve a paradox that has intrigued and challenged scientists and philosophers for over 100 years.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than that of slow anchoring myosin-1s found on endosomal membranes. We, therefore, propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

Four Phases of a Force Transient Emerge from a Binary Mechanical System

Models of muscle contraction are important for guiding drug discovery, drug validation, and clinical decision-making with the goal of improving human health. Models of muscle contraction are also key to discovering clean energy technologies from one of the most efficient and clean-burning machines on the planet. However, these important goals can only be met through muscle models that are based on science. Most every model and mechanism (e.g., a molecular power stroke) of muscle contraction described in the literature to date is based on a corpuscular mechanic philosophy that has been challenged by science for over two decades. A thermodynamic model and mechanisms (e.g., a molecular switch) of muscle contraction is supported by science but has not yet been tested against experimental data. Here, I show that following a rapid perturbation to the free energy of a thermodynamic muscle system, a transient force response emerges with four phases, each corresponding to a different clearly-defined thermodynamic (not molecular) process. I compare these four phases to those observed in two classic muscle transient experiments. The observed consistency between model and data implies that the simplest possible model of muscle contraction (a binary mechanical system) accurately describes muscle contraction.

Drosophila class-I myosins that can impact left-right asymmetry have distinct ATPase kinetics

Myosin-1D (myo1D) is important for Drosophila left-right asymmetry, and its effects are modulated by myosin-1C (myo1C). De novo expression of these myosins in nonchiral Drosophila tissues promotes cell and tissue chirality, with handedness depending on the paralog expressed. Remarkably, the identity of the motor domain determines the direction of organ chirality, rather than the regulatory or tail domains. Myo1D, but not myo1C, propels actin filaments in leftward circles in in vitro experiments, but it is not known if this property contributes to establishing cell and organ chirality. To further explore if there are differences in the mechanochemistry of these motors, we determined the ATPase mechanisms of myo1C and myo1D. We found that myo1D has a 12.5-fold higher actin-activated steady-state ATPase rate, and transient kinetic experiments revealed myo1D has an 8-fold higher MgADP release rate compared to myo1C. Actin-activated phosphate release is rate limiting for myo1C, whereas MgADP release is the rate-limiting step for myo1D. Notably, both myosins have among the tightest MgADP affinities measured for any myosin. Consistent with ATPase kinetics, myo1D propels actin filaments at higher speeds compared to myo1C in in vitro gliding assays. Finally, we tested the ability of both paralogs to transport 50 nm unilamellar vesicles along immobilized actin filaments and found robust transport by myo1D and actin binding but no transport by myo1C. Our findings support a model where myo1C is a slow transporter with long-lived actin attachments, whereas myo1D has kinetic properties associated with a transport motor.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation, and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than like that of slow anchoring myosin-1s found on endosomal membranes. We therefore propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

Summary

Pedersen, Snoberger et al. measure the force-sensitivity of the yeast endocytic the myosin-1 called Myo5 and find that it is more likely to generate power than to serve as a force-sensitive anchor in cells. Implications for Myo5's role in clathrin-mediated endocytosis are discussed.

Cells Solved the Gibbs Paradox by Learning to Contain Entropic Forces

As Nature's version of machine learning, evolution has solved many extraordinarily complex problems, none perhaps more remarkable than learning to harness an increase in chemical entropy (disorder) to generate directed chemical forces (order). Using muscle as a model system, here I unpack the basic mechanism by which life creates order from disorder. In short, evolution tuned the physical properties of certain proteins to contain changes in chemical entropy. As it happens, these are the "sensible" properties Gibbs postulated were needed to solve his paradox.

Thermodynamics and Kinetics of a Binary Mechanical System: Mechanisms of Muscle Contraction

Biological motors function at the interface of biology, physics, and chemistry, and it remains unsettled what rules from which disciplines account for how these motors work. Myosin motors are enzymes that catalyze the hydrolysis of ATP through a mechanism involving a switch-like myosin structural change (a lever arm rotation) induced by actin binding that generates a small displacement of an actin filament. In muscle, individual myosin motors are widely assumed to function as molecular machines having mechanical properties that resemble those of muscle. In a fundamental departure from this perspective, here, I show that muscle more closely resembles a heat engine with mechanical properties that emerge from the thermodynamics of a myosin motor ensemble. The transformative impact of thermodynamics on our understanding of how a heat engine works guides a parallel transformation in our understanding of how muscle works. I consider the simplest possible model of force generation: a binary mechanical system. I develop the mechanics, energetics, and kinetics of this system and show that a single binding reaction generates force when muscle is held at a fixed length and performs work when muscle is allowed to shorten. This creates a network of thermodynamic binding pathways that resembles many of the characteristic mechanical and energetic behaviors of muscle including the muscle force-velocity relationship, heat output by shortening muscle, four phases of a muscle tension transient, spontaneous oscillatory contractions, and force redevelopment. Analogous to the thermodynamic (Carnot) cycle for a heat engine, isothermal and adiabatic binding and detachment reactions create a thermodynamic cycle for muscle that resembles cardiac pressure-volume loops (i.e., how the heart works). This paper provides an outline for how to re-interpret muscle mechanic data using thermodynamics - an ongoing effort that will continue providing novel insights into how muscle and molecular motors work.

Myosin II Adjusts Motility Properties and Regulates Force Production Based on Motor Environment

Introduction

Myosin II has been investigated with optical trapping, but single motor-filament assay arrangements are not reflective of the complex cellular environment. To understand how myosin interactions propagate up in scale to accomplish system force generation, we devised a novel actomyosin ensemble optical trapping assay that reflects the hierarchy and compliancy of a physiological environment and is modular for interrogating force effectors.

Methods

Hierarchical actomyosin bundles were formed in vitro. Fluorescent template and cargo actin filaments (AF) were assembled in a flow cell and bundled by myosin. Beads were added in the presence of ATP to bind the cargo AF and activate myosin force generation to be measured by optical tweezers.

Results

Three force profiles resulted across a range of myosin concentrations: high force with a ramp-plateau, moderate force with sawtooth movement, and baseline. The three force profiles, as well as high force output, were recovered even at low solution concentration, suggesting that myosins self-optimize within AFs. Individual myosin steps were detected in the ensemble traces, indicating motors are taking one step at a time while others remain engaged in order to sustain productive force generation.

Conclusions

Motor communication and system compliancy are significant contributors to force output. Environmental conditions, motors taking individual steps to sustain force, the ability to backslip, and non-linear concentration dependence of force indicate that the actomyosin system contains a force-feedback mechanism that senses the local cytoskeletal environment and communicates to the individual motors whether to be in a high or low duty ratio mode.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-022-00731-1.

Myosin essential light chain 1sa decelerates actin and thin filament gliding on β-myosin molecules

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the "ventricular" essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

A chemical thermodynamic model of motor enzymes unifies chemical-Fx and powerstroke models

Molecular motors play a central role in many biological processes, ranging from pumping blood and breathing to growth and wound healing. Through motor-catalyzed chemical reactions, these nanomachines convert the chemical free energy from ATP hydrolysis into two different forms of mechanical work. Motor enzymes perform reversible work, wrev, through an intermediate step in their catalyzed reaction cycle referred to as a working step, and they perform Fx work when they move a distance, x, against a force, F. In a powerstroke model, wrev is performed when the working step stretches a spring within a given motor enzyme. In a chemical-Fx model, wrev is performed in generating a conserved Fx potential defined external to the motor enzyme. It is difficult to find any common ground between these models even though both have been shown to account for mechanochemical measurements of motor enzymes with reasonable accuracy. Here, I show that, by changing one simple assumption in each model, the powerstroke and chemical-Fx model can be reconciled through a chemical thermodynamic model. The formal and experimental justifications for changing these assumptions are presented. The result is a unifying model for mechanochemical coupling in motor enzymes first presented by A.V. Hill in 1938 that is consistent with single-molecule structural and mechanical data.

Polymer-Carrying Ability of Actin Filaments Interacting with Myosin Motors in a Biological Motility System In Vitro

The reconstituted motility system of actin-myosin is expected to be used in bioinspired transport devices, in which carried materials are attached to either moving actin filaments or walking myosin molecules. However, the dependence of the ability to transport on the size of the attached materials is still inadequately understood. Here, as carried materials, polyethylene glycols (PEGs) of various sizes are covalently bound to actin filaments, and the motility of PEG-attached filaments on a heavy meromyosin immobilized on a glass surface is observed via fluorescence microscopy. Full attachment of 2 kDa PEG, with an approximately 2 nm gyration radius, decreases the velocity and fraction of moving actin filaments by approximately 10% relative to unattached filaments. For the 5 kDa PEG, the fraction of moving filaments is decreased by approximately 70% even when the filaments contain only 20% PEG-attached actin. The attachment of both sizes of PEGs suppresses the actin-activated ATPase activity at the same level. These results suggest that actin filaments can carry PEGs up to 2 kDa having the same size as actin monomers, while the rate of ATP hydrolysis is limited. The size-dependence may provide a criterion for material delivery via actin filaments in nanotransport applications. This article is protected by copyright. All rights reserved.