Myosin structure and function

Myofilament-associated proteins with intrinsic disorder (MAPIDs) and their resolution by computational modeling

The cardiac sarcomere is a cellular structure in the heart that enables muscle cells to contract. Dozens of proteins belong to the cardiac sarcomere, which work in tandem to generate force and adapt to demands on cardiac output. Intriguingly, the majority of these proteins have significant intrinsic disorder that contributes to their functions, yet the biophysics of these intrinsically disordered regions (IDRs) have been characterized in limited detail. In this review, we first enumerate these myofilament-associated proteins with intrinsic disorder (MAPIDs) and recent biophysical studies to characterize their IDRs. We secondly summarize the biophysics governing IDR properties and the state-of-the-art in computational tools toward MAPID identification and characterization of their conformation ensembles. We conclude with an overview of future computational approaches toward broadening the understanding of intrinsic disorder in the cardiac sarcomere.

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.

Statistical Mechanics of Non-Muscle Myosin IIA in Human Bone Marrow-Derived Mesenchymal Stromal Cells Seeded in a Collagen Scaffold: A Thermodynamic Near-Equilibrium Linear System Modified by the Tripeptide Arg-Gly-Asp (RGD)

Mesenchymal stromal cells (MSCs) were obtained from human bone marrow and amplified in cultures supplemented with human platelet lysate. Once semi-confluent, cells were seeded in solid collagen scaffolds that were rapidly colonized by the cells generating a 3D cell scaffold. Here, they acquired a myofibroblast phenotype and when exposed to appropriate chemical stimulus, developed tension and cell shortening, similar to those of striated and smooth muscle cells. Myofibroblasts contained a molecular motor-the non-muscle myosin type IIA (NMMIIA) whose crossbridge (CB) kinetics are dramatically slow compared with striated and smooth muscle myosins. Huxley's equations were used to determine the molecular mechanical properties of NMMIIA. Thank to the great number of NMMIIA molecules, we determined the statistical mechanics (SM) of MSCs, using the grand canonical ensemble which made it possible to calculate various thermodynamic entities such as the chemical affinity, statistical entropy, internal energy, thermodynamic flow, thermodynamic force, and entropy production rate. The linear relationship observed between the thermodynamic force and the thermodynamic flow allowed to establish that MSC-laden in collagen scaffolds were in a near-equilibrium stationary state (affinity ≪ RT), MSCs were also seeded in solid collagen scaffolds functionalized with the tripeptide Arg-Gly-Asp (RGD). This induced major changes in NMMIIA SM particularly by increasing the rate of entropy production. In conclusion, collagen scaffolds laden with MSCs can be viewed as a non-muscle contractile bioengineered tissue operating in a near-equilibrium linear regime, whose SM could be substantially modified by the RGD peptide.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

The alpha-helix, an overlooked molecular motor

At first sight the alpha-helix appears as a rigid scaffold braced by hydrogen bonds nearly parallel to the helix axis. Looked at more closely it turned out to be highly dynamic and able to transform chemical into mechanical energy. The hydrogen bonds are fairly weak and compliant bonds. Their length, usually between 0.267 and 0.291 nm (mean value, 0.28 nm), depends on the interaction of the side chains. The most important strong interaction is the electrostatic repelling force between equally charged side chains (Glu-, Asp-, Lys+, Arg+), well known by experiments with polyamino acids. In proteins with different amino acids, repelling forces between charged side chains work in the axial direction and stretch the hydrogen bonds. Extreme shortening of the hydrogen bonds occurs when ions, e.g., Ca2+, H+, or PO3-, are added and discharge side chains. This means a cooperative pitch decrease of the alpha-helix (pitch range between 0.52 and more than 0.55 nm; mean value, 0.54 nm). This pitch change is absolutely connected by steric reasons with torque generation and torsional rotations, as demonstrated by molecular and tubular alpha-helix models. Thus, charged alpha-helices are molecular motors propelled by the electrostatic energy of added ions. The motor effect is most striking with highly charged alpha-helical coiled coils, e.g., tropomyosin, myosin, and alpha-actinin that can rotate actin filaments by winding and unwinding. For example, the shortening of muscle depends on the sliding (drilling) motion of the Ca2+-activated helical actin filaments into the cross-bridges of the A-band. Here, models are presented for the in vitro sliding of actin filaments and for cytoplasmic streaming by winding and unwinding of myosin chains, and for membrane proteins that contain nonhelical domains between membrane-penetrating alpha-helices. They may transport molecules by the described torsional rotations if they perform supercoiling. Winding and supercoiling can lead to displacement of bound ions and to a feed-back-regulated oscillation between two different coiling stages E1 and E2 that explain "eversion". The models need the torque for 1-2 rotations. They explain active and passive transports, the driving-effects of ion gradients, ATP hydrolysis by unwinding, ATP synthesis by winding up of the supercoils, etc.

A cell number counting factor regulates Akt/protein kinase B to regulate Dictyostelium discoideum group size

Little is known about how individual cells can organize themselves to form structures of a given size. During development, Dictyostelium discoideum aggregates in dendritic streams and forms groups of approximately 20,000 cells. D. discoideum regulates group size by secreting and simultaneously sensing a multiprotein complex called counting factor (CF). If there are too many cells in a stream, the associated high concentration of CF will decrease cell-cell adhesion and increase cell motility, causing aggregation streams to break up. The pulses of cyclic AMP (cAMP) that mediate aggregation cause a transient translocation of Akt/protein kinase B (Akt/PKB) to the leading edge of the plasma membrane and a concomitant activation of the kinase activity, which in turn stimulates motility. We found that countin- cells (which lack bioactive CF) and wild-type cells starved in the presence of anticountin antibodies (which block CF activity) showed a decreased level of cAMP-stimulated Akt/PKB membrane translocation and kinase activity compared to parental wild-type cells. Recombinant countin has the bioactivity of CF, and a 1-min treatment of cells with recombinant countin potentiated Akt/PKB translocation to membranes and Akt/PKB activity. Western blotting of total cell lysates indicated that countin does not affect the total level of Akt/PKB. Fluorescence microscopy of cells expressing an Akt/PKB pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion protein indicated that recombinant countin and anti-countin antibodies do not obviously alter the distribution of Akt/PKB PH-GFP when it translocates to the membrane. Our data indicate that CF increases motility by potentiating the cAMP-stimulated activation and translocation of Akt/PKB.

Molecular Motors: Strategies to Get Along

The majority of active transport in the cell is driven by three classes of molecular motors: the kinesin and dynein families that move toward the plus-end and minus-end of microtubules, respectively, and the unconventional myosin motors that move along actin filaments. Each class of motor has different properties, but in the cell they often function together. In this review we summarize what is known about their single-molecule properties and the possibilities for regulation of such properties. In view of new results on cytoplasmic dynein, we attempt to rationalize how these different classes of motors might work together as part of the intracellular transport machinery. We propose that kinesin and myosin are robust and highly efficient transporters, but with somewhat limited room for regulation of function. Because cytoplasmic dynein is less efficient and robust, to achieve function comparable to the other motors it requires a number of accessory proteins as well as multiple dyneins functioning together. This necessity for additional factors, as well as dynein's inherent complexity, in principle allows for greatly increased control of function by taking the factors away either singly or in combination. Thus, dynein's contribution relative to the other motors can be dynamically tuned, allowing the motors to function together differently in a variety of situations.

WD repeat domains target dictyostelium myosin heavy chain kinases by binding directly to myosin filaments

Myosin heavy chain kinase (MHCK) A phosphorylates mapped sites at the C-terminal tail of Dictyostelium myosin II heavy chain, driving disassembly of myosin filaments both in vitro and in vivo. MHCK A is organized into three functional domains that include an N-terminal coiled-coil region, a central kinase catalytic domain unrelated to conventional protein kinases, and a WD repeat domain at the C terminus. MHCK B is a homologue of MHCK A that possesses structurally related catalytic and WD repeat domains. In the current study, we explored the role of the WD repeat domains in defining the activities of both MHCK A and MHCK B using recombinant bacterially expressed truncations of these kinases either with or without their WD repeat domains. We demonstrate that substrate targeting is a conserved function of the WD repeat domains of both MHCK A and MHCK B and that this targeting is specific for Dictyostelium myosin II filaments. We also show that the mechanism of targeting involves direct binding of the WD repeat domains to the myosin substrate. To our knowledge, this is the first report of WD repeat domains physically targeting attached kinase domains to their substrates. The examples presented here may serve as a paradigm for enzyme targeting in other systems.

Functional characterization of vertebrate nonmuscle myosin IIB isoforms using Dictyostelium chimeric myosin II

The alternatively spliced isoform of nonmuscle myosin II heavy chain B (MHC-IIB) with an insert of 21 amino acids in the actin-binding surface loop (loop 2), MHC-IIB(B2), is expressed specifically in the central nervous system of vertebrates. To examine the role of the B2 insert in the motor activity of the myosin II molecule, we expressed chimeric myosin heavy chain molecules using the Dictyostelium myosin II heavy chain as the backbone. We replaced the Dictyostelium native loop 2 with either the noninserted form of loop 2 from human MHC-IIB or the B2-inserted form of loop 2 from human MHC-IIB(B2). The transformant Dictyostelium cells expressing only the B2-inserted chimeric myosin formed unusual fruiting bodies. We then assessed the function of chimeric proteins, using an in vitro motility assay and by measuring ATPase activities and binding to F-actin. We demonstrate that the insertion of the B2 sequence reduces the motor activity of Dictyostelium myosin II, with reduction of the maximal actin-activated ATPase activity and a decrease in the affinity for actin. In addition, we demonstrate that the native loop 2 sequence of Dictyostelium myosin II is required for the regulation of the actin-activated ATPase activity by phosphorylation of the regulatory light chain.

Visualizing myosin's power stroke in muscle contraction

The long-standing swinging crossbridge or lever arm hypothesis for the motor action of myosin heads finds support in recent results from 3-D tomograms of insect flight muscle (IFM) fast frozen during active contraction and from both fluorescence polarization and X-ray diffraction during rapid stretches or releases of isometrically contracting fibers. The latter provide direct evidence for lever arm movements synchronous with force changes. Rebuilding the atomic model of nucleotide-free subfragment 1 (S1) to fit fast-frozen, active IFM crossbridges suggests a two-stage power stroke in which the catalytic domain rolls on actin from weak to strong binding; this is followed by a 5-nm lever arm swing of the light chain domain, which gives a total interaction distance of approx. 12 nm. Comparison of S1 crystal structures with in situ myosin heads suggests that actin binding may be necessary in order to view the full repertoire of myosin motor action. The differing positions of the catalytic domains of actin-attached myosin heads in contracting IFM suggest that both the actin-myosin binding energy and the hydrolysis of ATP may be used to cock the crossbridge and drive the power stroke.

Structural mechanism of muscle contraction

X-ray crystallography shows the myosin cross-bridge to exist in two conformations, the beginning and end of the "power stroke." A long lever-arm undergoes a 60 degrees to 70 degrees rotation between the two states. This rotation is coupled with changes in the active site (OPEN to CLOSED) and phosphate release. Actin binding mediates the transition from CLOSED to OPEN. Kinetics shows that the binding of myosin to actin is a two-step process which affects ATP and ADP affinity. The structural basis of these effects is not explained by the presently known conformers of myosin. Therefore, other states of the myosin cross-bridge must exist. Moreover, cryoelectronmicroscopy has revealed other angles of the cross-bridge lever arm induced by ADP binding. These structural states are presently being characterized by site-directed mutagenesis coupled with kinetic analysis.

Role of Rac in controlling the actin cytoskeleton and chemotaxis in motile cells

We have used the chemotactic ability of Dictyostelium cells to examine the roles of Rho family members, known regulators of the assembly of F-actin, in cell movement. Wild-type cells polarize with a leading edge enriched in F-actin toward a chemoattractant. Overexpression of constitutively active Dictyostelium Rac1B(61L) or disruption of DdRacGAP1, which encodes a Dictyostelium Rac1 GAP, induces membrane ruffles enriched with actin filaments around the perimeter of the cell and increased levels of F-actin in resting cells. Whereas wild-type cells move linearly toward the cAMP source, Rac1B(61L) and Ddracgap1 null cells make many wrong turns and chemotaxis is inefficient, which presumably results from the unregulated activation of F-actin assembly and pseudopod extension. Cells expressing dominant-negative DdRac1B(17N) do not have a well-defined F-actin-rich leading edge and do not protrude pseudopodia, resulting in very poor cell motility. From these studies and assays examining chemoattractant-mediated F-actin assembly, we suggest DdRac1 regulates the basal levels of F-actin assembly, its dynamic reorganization in response to chemoattractants, and cellular polarity during chemotaxis.

The Dictyostelium LIM domain-containing protein LIM2 is essential for proper chemotaxis and morphogenesis

We have identified limB, a gene encoding a novel LIM domain-containing protein, LIM2, in a screen for genes required for morphogenesis. limB null cells aggregate, although poorly, but they are unable to undergo morphogenesis, and the aggregates arrest at the mound stage. limB null cells exhibit an aberrant actin cytoskeleton and have numerous F-actin-enriched microspikes. The cells exhibit poor adhesion to a substratum and do not form tight cell-cell agglomerates in suspension. Furthermore, limB null cells are unable to properly polarize in chemoattractant gradients and move very poorly. Expression of limB from a prestalk-specific but not a prespore-specific promoter complements the morphogenetic defects of the limB null strain, suggesting that the limB null cell developmental defect results from an inability to properly sort prestalk cells. LIM2 protein is enriched in the cortex of wild-type cells, although it does not colocalize with the actin cytoskeleton. Our analysis indicates that LIM2 is a new regulatory protein that functions to control rearrangements of the actin cytoskeleton and is required for cell motility and chemotaxis. Our findings may be generally applicable to understanding pathways that control cell movement and morphogenesis in all multicellular organisms. Structure function studies on the LIM domains are presented.

PAKa, a putative PAK family member, is required for cytokinesis and the regulation of the cytoskeleton in Dictyostelium discoideum cells during chemotaxis

We have identified a Dictyostelium discoideum gene encoding a serine/threonine kinase, PAKa, a putative member of the Ste20/PAK family of p21-activated kinases, with a kinase domain and a long NH(2)-terminal regulatory domain containing an acidic segment, a polyproline domain, and a CRIB domain. PAKa colocalizes with myosin II to the cleavage furrow of dividing cells and the posterior of polarized, chemotaxing cells via its NH(2)-terminal domain. paka null cells are defective in completing cytokinesis in suspension. PAKa is also required for maintaining the direction of cell movement, suppressing lateral pseudopod extension, and proper retraction of the posterior of chemotaxing cells. paka null cells are defective in myosin II assembly, as the myosin II cap in the posterior of chemotaxing cells and myosin II assembly into cytoskeleton upon cAMP stimulation are absent in these cells, while constitutively active PAKa leads to an upregulation of myosin II assembly. PAKa kinase activity against histone 2B is transiently stimulated and PAKa incorporates into the cytoskeleton with kinetics similar to those of myosin II assembly in response to chemoattractant signaling. However, PAKa does not phosphorylate myosin II. We suggest that PAKa is a major regulator of myosin II assembly, but does so by negatively regulating myosin II heavy chain kinase.

Unconventional myosins in cell movement, membrane traffic, and signal transduction

In the past few years genetic, biochemical, and cytolocalization data have implicated members of the myosin superfamily of actin-based molecular motors in a variety of cellular functions including membrane trafficking, cell movements, and signal transduction. The importance of myosins is illustrated by the identification of myosin genes as targets for disease-causing mutations. The task at hand is to decipher how the multitude of myosins function at both the molecular and cellular level-a task facilitated by our understanding of myosin structure and function in muscle.

Structure-function studies of the myosin motor domain: importance of the 50-kDa cleft

We used random mutagenesis to create 21 point mutations in a highly conserved region of the motor domain of Dictyostelium myosin and classified them into three distinct groups based on the ability to complement myosin null cell phenotypes: wild type, intermediate, and null. Biochemical analysis of the mutated myosins also revealed three classes of mutants that correlated well with the phenotypic classification. The mutated myosins that were not fully functional showed defects ranging from ATP nonhydrolyzers to myosins whose enzymatic and mechanical properties are uncoupled. Placement of the mutations onto the three-dimensional structure of myosin showed that the mutated region lay along the cleft that separates the active site from the actin-binding domain and that has been shown to move in response to changes at the active site. These results demonstrate that this region of myosin plays a key role in transduction of chemical energy to mechanical displacement.