Deletion of the myopathy loop of Dictyostelium myosin II and its impact on motor functions

Myosin-1C uses a novel phosphoinositide-dependent pathway for nuclear localization

Accurate control of macromolecule transport between nucleus and cytoplasm underlines several essential biological processes, including gene expression. According to the canonical model, nuclear import of soluble proteins is based on nuclear localization signals and transport factors. We challenge this view by showing that nuclear localization of the actin-dependent motor protein Myosin-1C (Myo1C) resembles the diffusion-retention mechanism utilized by inner nuclear membrane proteins. We show that Myo1C constantly shuttles in and out of the nucleus and that its nuclear localization does not require soluble factors, but is dependent on phosphoinositide binding. Nuclear import of Myo1C is preceded by its interaction with the endoplasmic reticulum, and phosphoinositide binding is specifically required for nuclear import, but not nuclear retention, of Myo1C. Our results therefore demonstrate, for the first time, that membrane association and binding to nuclear partners is sufficient to drive nuclear localization of also soluble proteins, opening new perspectives to evolution of cellular protein sorting mechanisms.

Cryo-EM structure of a human cytoplasmic actomyosin complex at near-atomic resolution

The interaction of myosin with actin filaments is the central feature of muscle contraction and cargo movement along actin filaments of the cytoskeleton. The energy for these movements is generated during a complex mechanochemical reaction cycle. Crystal structures of myosin in different states have provided important structural insights into the myosin motor cycle when myosin is detached from F-actin. The difficulty of obtaining diffracting crystals, however, has prevented structure determination by crystallography of actomyosin complexes. Thus, although structural models exist of F-actin in complex with various myosins, a high-resolution structure of the F-actin–myosin complex is missing. Here, using electron cryomicroscopy, we present the structure of a human rigor actomyosin complex at an average resolution of 3.9 Å. The structure reveals details of the actomyosin interface, which is mainly stabilized by hydrophobic interactions. The negatively charged amino (N) terminus of actin interacts with a conserved basic motif in loop 2 of myosin, promoting cleft closure in myosin. Surprisingly, the overall structure of myosin is similar to rigor-like myosin structures in the absence of F-actin, indicating that F-actin binding induces only minimal conformational changes in myosin. A comparison with pre-powerstroke and intermediate (Pi-release) states of myosin allows us to discuss the general mechanism of myosin binding to F-actin. Our results serve as a strong foundation for the molecular understanding of cytoskeletal diseases, such as autosomal dominant hearing loss and diseases affecting skeletal and cardiac muscles, in particular nemaline myopathy and hypertrophic cardiomyopathy.

How actin initiates the motor activity of Myosin

Fundamental to cellular processes are directional movements driven by molecular motors. A common theme for these and other molecular machines driven by ATP is that controlled release of hydrolysis products is essential for using the chemical energy efficiently. Mechanochemical transduction by myosin motors on actin is coupled to unknown structural changes that result in the sequential release of inorganic phosphate (Pi) and MgADP. We present here a myosin structure possessing an actin-binding interface and a tunnel (back door) that creates an escape route for Pi with a minimal rotation of the myosin lever arm that drives movements. We propose that this state represents the beginning of the powerstroke on actin and that Pi translocation from the nucleotide pocket triggered by actin binding initiates myosin force generation. This elucidates how actin initiates force generation and movement and may represent a strategy common to many molecular machines.

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.

ER sheet persistence is coupled to myosin 1c-regulated dynamic actin filament arrays

The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse structural and functional domains. Proper ER operation requires an intricate balance within and between dynamics, morphology, and functions, but how these processes are coupled in cells has been unclear. Using live-cell imaging and 3D electron microscopy, we identify a specific subset of actin filaments localizing to polygons defined by ER sheets and tubules and describe a role for these actin arrays in ER sheet persistence and, thereby, in maintenance of the characteristic network architecture by showing that actin depolymerization leads to increased sheet fluctuation and transformations and results in small and less abundant sheet remnants and a defective ER network distribution. Furthermore, we identify myosin 1c localizing to the ER-associated actin filament arrays and reveal a novel role for myosin 1c in regulating these actin structures, as myosin 1c manipulations lead to loss of the actin filaments and to similar ER phenotype as observed after actin depolymerization. We propose that ER-associated actin filaments have a role in ER sheet persistence regulation and thus support the maintenance of sheets as a stationary subdomain of the dynamic ER network.

All-atom molecular dynamics simulations of actin-myosin interactions: a comparative study of cardiac α myosin, β myosin, and fast skeletal muscle myosin

Myosins are a superfamily of actin-binding motor proteins with significant variations in kinetic properties (such as actin binding affinity) between different isoforms. It remains unknown how such kinetic variations arise from the structural and dynamic tuning of the actin-myosin interface at the amino acid residue level. To address this key issue, we have employed molecular modeling and simulations to investigate, with atomistic details, the isoform dependence of actin-myosin interactions in the rigor state. By combining electron microscopy-based docking with homology modeling, we have constructed three all-atom models for human cardiac α and β and rabbit fast skeletal muscle myosin in complex with three actin subunits in the rigor state. Starting from these models, we have performed extensive all-atom molecular dynamics (MD) simulations (total of 100 ns per system) and then used the MD trajectories to calculate actin-myosin binding free energies with contributions from both electrostatic and nonpolar forces. Our binding calculations are in good agreement with the experimental finding of isoform-dependent differences in actin binding affinity between these myosin isoforms. Such differences are traced to changes in actin-myosin electrostatic interactions (i.e., hydrogen bonds and salt bridges) that are highly dynamic and involve several flexible actin-binding loops. By partitioning the actin-myosin binding free energy to individual myosin residues, we have also identified key myosin residues involved in the actin-myosin interactions, some of which were previously validated experimentally or implicated in cardiomyopathy mutations, and the rest make promising targets for future mutational experiments.

Conformationally trapping the actin-binding cleft of myosin with a bifunctional spin label

We have trapped the catalytic domain of Dictyostelium (Dicty) myosin II in a weak actin-binding conformation by chemically crosslinking two engineered cysteines across the actin-binding cleft, using a bifunctional spin label (BSL). By connecting the lower and upper 50 kDa domains of myosin, the crosslink restricts the conformation of the actin-binding cleft. Crosslinking has no effect on the basal ATPase activity of isolated myosin, but it impairs rigor actin binding and actin-activation of myosin ATPase. EPR spectra of BSL provide insight into actomyosin structural dynamics. BSL is highly immobilized within the actin-binding cleft and is thus exquisitely sensitive to the global orientation and rotational motions of the myosin head. Conventional EPR shows that myosin heads bound to oriented actin filaments are highly disordered with respect to the actin filament axis, in contrast to the nearly crystalline order of myosin heads in rigor. This disorder is similar to that of weakly bound heads induced by ATP, but saturation transfer EPR shows that the disorder of crosslinked myosin is at least 100 times slower. Thus this cleft-crosslinked myosin is remarkably similar, in both actin affinity and rotational dynamics, to SH1-SH2 crosslinked BSL-myosin S1. We conclude that, whether myosin is trapped at the actin-myosin interface or in the force-generating region between the active site and lever arm, the structural state of myosin is intermediate between the weak-binding state preceding phosphate release and the strong-binding state that succeeds it. We propose that it represents the threshold of force generation.

Structure of the rigor actin-tropomyosin-myosin complex

Regulation of myosin and filamentous actin interaction by tropomyosin is a central feature of contractile events in muscle and nonmuscle cells. However, little is known about molecular interactions within the complex and the trajectory of tropomyosin movement between its "open" and "closed" positions on the actin filament. Here, we report the 8 Å resolution structure of the rigor (nucleotide-free) actin-tropomyosin-myosin complex determined by cryo-electron microscopy. The pseudoatomic model of the complex, obtained from fitting crystal structures into the map, defines the large interface involving two adjacent actin monomers and one tropomyosin pseudorepeat per myosin contact. Severe forms of hereditary myopathies are linked to mutations that critically perturb this interface. Myosin binding results in a 23 Å shift of tropomyosin along actin. Complex domain motions occur in myosin, but not in actin. Based on our results, we propose a structural model for the tropomyosin-dependent modulation of myosin binding to actin.

Structural and functional insights into the Myosin motor mechanism

The general structural features of the motor region of myosin superfamily members are now well established, as is a subset of the structural and kinetic transitions of the actin-myosin catalytic cycle. Not yet visualized are the structural rearrangements triggered by actin binding that are coupled to force generation and product release. In this review we describe the recent progress in understanding these missing components of the mechanism of chemomechanical transduction by myosin motors. These insights come from a combination of kinetic and single-molecule studies on multiple classes of myosins, with additional insights from contracting muscle fibers. These recent studies have explored the effects of intermediate and high loads on the kinetics of the actin-bound myosin state transitions. We also describe studies that delineate how some classes of myosin motors are adapted for processive movement on actin.

The myosin motor domain of fungal chitin synthase V is dispensable for vesicle motility but required for virulence of the maize pathogen Ustilago maydis

Class V chitin synthases are fungal virulence factors required for plant infection. They consist of a myosin motor domain fused to a membrane-spanning chitin synthase region that participates in fungal cell wall formation. The function of the motor domain is unknown, but it might deliver the myosin chitin synthase-attached vesicles to the growth region. Here, we analyze the importance of both domains in Mcs1, the chitin synthase V of the maize smut fungus Ustilago maydis. By quantitative analysis of disease symptoms, tissue colonization, and single-cell morphogenic parameters, we demonstrate that both domains are required for fungal virulence. Fungi carrying mutations in the chitin synthase domain are rapidly recognized and killed by the plant, whereas fungi carrying a deletion of the motor domain show alterations in cell wall composition but can invade host tissue and cause a moderate plant response. We also show that Mcs1-bound vesicles exhibit long-range movement for up to 20 microm at a velocity of approximately 1.75 microm/s. Apical Mcs1 localization depends on F-actin and the motor domain, whereas Mcs1 motility requires microtubules and persists when the Mcs1 motor domain is deleted. Our results suggest that the myosin motor domain of ChsV supports exocytosis but not long-range delivery of transport vesicles.

Visualizing myosin-actin interaction with a genetically-encoded fluorescent strain sensor

Many proteins have been shown to undergo conformational changes in response to externally applied force in vitro, but whether the force-induced protein conformational changes occur in vivo remains unclear. To reveal the force-induced conformational changes, or strains, within proteins in living cells, we have developed a genetically encoded fluorescent "strain sensor," by combining the proximity imaging (PRIM) technique, which uses spectral changes of 2 GFP molecules that are in direct contact, and myosin-actin as a model system. The developed PRIM-based strain sensor module (PriSSM) consists of the tandem fusion of a normal and circularly permuted GFP. To apply strain to PriSSM, it was inserted between 2 motor domains of Dictyostelium myosin II. In the absence of strain, the 2 GFP moieties in PriSSM are in contact, whereas when the motor domains are bound to F-actin, PriSSM has a strained conformation, leading to the loss of contact and a concomitant spectral change. Using the sensor system, we found that the position of the lever arm in the rigor state was affected by mutations within the motor domain. Moreover, the sensor was used to visualize the interaction between myosin II and F-actin in Dictyostelium cells. In normal cells, myosin was largely detached from F-actin, whereas ATP depletion or hyperosmotic stress increased the fraction of myosin bound to F-actin. The PRIM-based strain sensor may provide a general approach for studying force-induced protein conformational changes in cells.

A closer look at energy transduction in muscle

Muscular force is the sum of unitary force interactions generated as filaments of myosins move forcibly along parallel filaments of actins, understanding that the free energy required comes from myosin-catalyzed ATP hydrolysis. Using results from conventional biochemistry, our own mutational studies, and diffraction images from others, we attempt, in molecular detail, an account of a unitary interaction, i.e., what happens after a traveling myosin head, bearing an ADP-P(i), reaches the next station of an actin filament in its path. We first construct a reasonable model of the myosin head and actin regions that meet to form the "weakly bound state". Separately, we consider Holmes' model of the rigor state [Holmes, K. C., Angert, I., Kull, F. J., Jahn, W. & Schröder, R. R. (2003) Nature 425, 423-427], supplemented with several heretofore missing residues, thus realizing the "strongly bound state." Comparing states suggests how influences initiated at the interface travel elsewhere in myosin to discharge various functions, including striking the actins. Overall, state change seems to occur by attachment of a hydrophobic triplet (Trp-546, Phe-547, and Pro-548) of myosin to an actin conduit with a hydrophobic guiding rail (Ile-341, Ile-345, Leu-349, and Phe-352) and the subsequent linear movement of the triplet along the rail.

From transcription to transport: emerging roles for nuclear myosin IThis paper is one of a selection of papers published in this Special Issue, entitled 27th International West Coast Chromatin and Chromosome Conference, and has undergone the Journal's usual peer review process

Myosins are a superfamily of actin-activated ATPases that, in the cytoplasm, work together with actin as molecular motors. The presence of actin in the nucleus has been known for many years. The demonstration of a nuclear isoform of a myosin, nuclear myosin I (NMI), stimulated a great deal of interest in possible intranuclear motor functions of an acto–NMI complex. NMI has been shown to be involved in transcription by RNA polymerases I and II. In both cases, NMI interacts with the respective polymerase and is critically involved in the basic process of transcription. A recent study on intranuclear long-range chromosome movement has now demonstrated a role for NMI in the translocation of chromosome regions as well. Moreover, this movement is based on an active and directed process that is facilitated by an acto–NMI complex, establishing for the first time a functional role for a motor complex consisting of actin and a myosin in the nucleus.

Long-range directional movement of an interphase chromosome site

Increasing evidence suggests functional compartmentalization of interphase nuclei. This includes preferential interior localization of gene-rich and early replicating chromosome regions versus peripheral localization of gene-poor and late replicating chromosome regions , association of some active genes with nuclear speckles or transcription "factories", and association of transcriptionally repressed genes with heterochromatic regions. Dynamic changes in chromosome compartmentalization imply mechanisms for long-range interphase chromatin movements. However, live cell imaging in mammalian cells has revealed limited chromatin mobility, described as "constrained diffusion". None of these studies, though, have examined a chromosome locus undergoing an inducible repositioning between two different nuclear compartments. Here we demonstrate migration of an interphase chromosome site from the nuclear periphery to the interior 1-2 hr after targeting a transcriptional activator to this site. Spot redistribution is perturbed by specific actin or nuclear myosin I mutants. Extended periods of chromosome immobility are interspersed with several minute periods in which chromosomes move unidirectionally along curvilinear paths oriented roughly perpendicular to the nuclear envelope at velocities of 0.1-0.9 microm/min over distances of 1-5 microm. Our results suggest an active mechanism for fast and directed long-range interphase chromosome movements dependent directly or indirectly on actin/myosin.

Structural dynamics of the actin-myosin interface by site-directed spectroscopy

We have used site-directed spin and fluorescence labeling to test molecular models of the actin-myosin interface. Force is generated when the actin-myosin complex undergoes a transition from a disordered weak-binding state to an ordered strong-binding state. Actomyosin interface models, in which residues are classified as contributing to either weak or strong binding, have been derived by fitting individual crystallographic structures of actin and myosin into actomyosin cryo-EM maps. Our goal is to test these models using site-directed spectroscopic probes on actin and myosin. Starting with Cys-lite constructs of both yeast actin (ActC) and the Dictyostelium myosin II motor domain (S1dC), site-directed labeling (SDL) mutants were generated by mutating residues to Cys in the proposed weak and strong-binding interfaces. This report focuses on the effects of forming the strong-binding complex on four SDL mutants, two located in the proposed weak-binding interface (ActC5 and S1dC619) and two located in the proposed strong-binding interface (ActC345 and S1dC401). Neither the mutations nor labeling prevented strong actomyosin binding or actin-activation of myosin ATPase. Formation of the strong-binding complex resulted in decreased spin and fluorescence probe mobility at all sites, but both myosin-bound probes showed remarkably high mobility even after complex formation. Complex formation decreased solvent accessibility for both actin-bound probes, but increased it for the myosin-bound probes. These results are not consistent with a simple model in which there are discrete weak and strong interfaces, with only the strong interface forming under strong-binding conditions, nor are they consistent with a model in which surface residues become rigid and inaccessible upon complex formation. We conclude that all four of these residues are involved in the strong actin-myosin interface, but this interface is remarkably dynamic, especially on the surface of myosin.

Biological, biochemical, and kinetic effects of mutations of the cardiomyopathy loop of Dictyostelium myosin II: importance of ALA400

The cardiomyopathy (CM)-loop of the heavy chain of class-II myosins begins with a highly conserved Arg residue (whose mutation in human beta-cardiac myosin II results in familial hypertrophic cardiomyopathy). The CM-loop of Dictyostelium myosin II (Arg397-Gln407) is essential for its biological functions and biochemical activities. We found that the CM-loop of smooth muscle myosin II substituted partially, and the CM-loop of beta-cardiac myosin II less well, for growth, capping of surface receptors and development, and the actin-activated MgATPase and in vitro motility activities of purified myosins. There was little correlation between the biochemical and biological activities of the two chimeras and 19 point mutants, but only the five mutants with k cat/K actin values equivalent to wild-type myosin supported essentially full biological function. The three point mutations of Arg397 equivalent to those that result in hypertrophic cardiomyopathy in humans had minimal biological effects and different biochemical effects. The A400V mutation rendered full-length wild-type myosin almost completely inactive, both in vitro and in vivo, and the reverse V400A mutation in the cardiac CM-loop chimera restored almost full activity, even though the sequence still differed from wild-type in 7 of 11 positions. Transient kinetic studies of acto-subfragment-1 (S1) showed that the chimeras and the Ala/Val, Val/Ala mutations do not affect the equilibrium or the association and dissociation rate constants for either ATP or ADP binding to acto-S1 or the rate of ATP-induced dissociation of acto-S1. We conclude that the Ala/Val, Val/Ala mutations affect the release of Pi from acto-S1.ADP.Pi. In addition, Val at position 400 substantially reduces the affinity of actin for S1 in the absence of nucleotide.

The regulatory effects of tropomyosin and troponin-I on the interaction of myosin loop regions with F-actin

The N terminus of skeletal myosin light chain 1 and the cardiomyopathy loop of human cardiac myosin have been shown previously to bind to actin in the presence and absence of tropomyosin (Patchell, V. B., Gallon, C. E., Hodgkin, M. A., Fattoum, A., Perry, S. V., and Levine, B. A. (2002) Eur. J. Biochem. 269, 5088-5100). We have extended this work and have shown that segments corresponding to other regions of human cardiac beta-myosin, presumed to be sites of interaction with F-actin (residues 554-584, 622-646, and 633-660), likewise bind independently to actin under similar conditions. The binding to F-actin of a peptide spanning the minimal inhibitory segment of human cardiac troponin I (residues 134-147) resulted in the dissociation from F-actin of all the myosin peptides bound to it either individually or in combination. Troponin C neutralized the effect of the inhibitory peptide on the binding of the myosin peptides to F-actin. We conclude that the binding of the inhibitory region of troponin I to actin, which occurs during relaxation in muscle when the calcium concentration is low, imposes conformational changes that are propagated to different locations on the surface of actin. We suggest that the role of tropomyosin is to facilitate the transmission of structural changes along the F-actin filament so that the monomers within a structural unit are able to interact with myosin.

CsmA, a class V chitin synthase with a myosin motor-like domain, is localized through direct interaction with the actin cytoskeleton in Aspergillus nidulans

One of the essential features of fungal morphogenesis is the polarized synthesis of cell wall components such as chitin. The actin cytoskeleton provides the structural basis for cell polarity in Aspergillus nidulans, as well as in most other eukaryotes. A class V chitin synthase, CsmA, which contains a myosin motor-like domain (MMD), is conserved among most filamentous fungi. The DeltacsmA null mutant showed remarkable abnormalities with respect to cell wall integrity and the establishment of polarity. In this study, we demonstrated that CsmA tagged with 9x HA epitopes localized near actin structures at the hyphal tips and septation sites and that its MMD was able to bind to actin. Characterization of mutants bearing a point mutation or deletion in the MMD suggests that the interaction between the MMD and actin is not only necessary for the proper localization of CsmA, but also for CsmA function. Thus, the finding of a direct interaction between the chitin synthase and the actin cytoskeleton provides new insight into the mechanisms of polarized cell wall synthesis and fungal morphogenesis.

Dictyostelium myosin II as a model to study the actin-myosin interactions during force generation

During steady-state ATP hydrolysis by actomyosin, myosin cyclically passes through strong actin-binding states and weak actin-binding states, depending on the nature of a nucleotide in the ATPase site. This cyclic change of actin-myosin affinity is coupled with the lever-arm swing and is critical for the sliding motion and force generation of actomyosin. To understand the structure-function relationship of this ATPase-dependent actin-myosin interaction, Dictyostelium myosin II has been extensively used for site-directed mutagenesis. By generating a large number of mutant myosins, two hydrophobic actin-binding sites have been revealed, located at the tip of the upper and lower 50 K subdomains of Dictyostelium myosin, one of which is the 'cardiomyopathy loop'. Furthermore, the slight change in relative orientation of these two hydrophobic sites around the 'strut loop' has been shown to work as a switch to turn on and off the strong binding to actin. Once the switch is turned off, myosin enters in the weak-binding state, where ionic interactions between actin and the 'loop 2' of myosin become the dominant force to maintain the actin-myosin association. The details of actin-myosin interactions revealed by the Dictyostelium system can serve as a framework for further examinations of the myosin superfamily proteins.

Amino acids 519-524 of Dictyostelium myosin II form a surface loop that aids actin binding by facilitating a conformational change

Residues 519-524 of Dictyostelium myosin II form a small surface loop on the actin binding face, and have been suggested to bind directly to actin through high affinity hydrophobic interactions. To test this hypothesis, we have characterized mutant myosins that lack this loop in vivo and in vitro. A mutant myosin in which this loop was replaced by an Ala residue (delta519-524/+A) was non-functional in vivo. Replacement with a single Gly residue instead of Ala yielded partial function, suggesting that structural flexibility, rather than hydrophobicity, is the key feature of the loop. The in vivo phenotype of the mutant enabled us to identify a number of additional amino acid changes that restore function to the delta519-524/+A mutation. Intriguingly, many of these, including L596S, were located at some distances away from the 519-524 loop. We have also isolated suppressors for the L596S mutant myosin, which was not functional in vivo. The suppressors for delta519-524/+A and those for L596S showed complementary charge patterns. In ATPase assays, delta519-524/+A S1 showed very low activity and little enhancement by actin, whereas L596S S1 was hyper active and displayed enhanced affinity for actin. In motility assays, delta519-524/+A myosin released actin filaments upon addition of ATP and was unable to support movements. L596S myosin was also inactive, but in this case actin filaments stayed immobile even after the addition of ATP. Transient kinetic measurements demonstrated that delta519-524/+A S1 is not only slower than wild type to bind actin filaments, but also slower to dissociate from actin filaments. Based on these results, we concluded that the 519-524 loop is not a major actin binding site but aids actin binding by facilitating a critical conformational change.

Electron cryo-microscopy shows how strong binding of myosin to actin releases nucleotide

Muscle contraction involves the cyclic interaction of the myosin cross-bridges with the actin filament, which is coupled to steps in the hydrolysis of ATP. While bound to actin each cross-bridge undergoes a conformational change, often referred to as the "power stroke", which moves the actin filament past the myosin filaments; this is associated with the release of the products of ATP hydrolysis and a stronger binding of myosin to actin. The association of a new ATP molecule weakens the binding again, and the attached cross-bridge rapidly dissociates from actin. The nucleotide is then hydrolysed, the conformational change reverses, and the myosin cross-bridge reattaches to actin. X-ray crystallography has determined the structural basis of the power stroke, but it is still not clear why the binding of actin weakens that of the nucleotide and vice versa. Here we describe, by fitting atomic models of actin and the myosin cross-bridge into high-resolution electron cryo-microscopy three-dimensional reconstructions, the molecular basis of this linkage. The closing of the actin-binding cleft when actin binds is structurally coupled to the opening of the nucleotide-binding pocket.

A structural state of the myosin V motor without bound nucleotide

The myosin superfamily of molecular motors use ATP hydrolysis and actin-activated product release to produce directed movement and force. Although this is generally thought to involve movement of a mechanical lever arm attached to a motor core, the structural details of the rearrangement in myosin that drive the lever arm motion on actin attachment are unknown. Motivated by kinetic evidence that the processive unconventional myosin, myosin V, populates a unique state in the absence of nucleotide and actin, we obtained a 2.0 A structure of a myosin V fragment. Here we reveal a conformation of myosin without bound nucleotide. The nucleotide-binding site has adopted new conformations of the nucleotide-binding elements that reduce the affinity for the nucleotide. The major cleft in the molecule has closed, and the lever arm has assumed a position consistent with that in an actomyosin rigor complex. These changes have been accomplished by relative movements of the subdomains of the molecule, and reveal elements of the structural communication between the actin-binding interface and nucleotide-binding site of myosin that underlie the mechanism of chemo-mechanical transduction.

Characterization of three regulatory states of the striated muscle thin filament

The troponin-tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca(2+) and myosin. The thin filament fluctuates between two extreme states: the inactive "off" state and the active "on" state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin(1)-S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca(2+). In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin(2)-S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin(2)-S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin-myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca(2+) regulation that exists between the biochemical and mechanical cycles of the actin-myosin motor.

The inhibitory region of troponin-I alters the ability of F-actin to interact with different segments of myosin

Peptides corresponding to the N-terminus of skeletal myosin light chain 1 (rsMLC1 1-37) and the short loop of human cardiac beta-myosin (hcM398-414) have been shown to interact with skeletal F-actin by NMR and fluorescence measurements. Skeletal tropomyosin strengthens the binding of the myosin peptides to actin but does not interact with the peptides. The binding of peptides corresponding to the inhibitory region of cardiac troponin I (e.g. hcTnI128-153) to F-actin to form a 1 : 1 molar complex is also strengthened in the presence of tropomyosin. In the presence of inhibitory peptide at relatively lower concentrations the myosin peptides and a troponin I peptide C-terminal to the inhibitory region, rcTnI161-181, all dissociate from F-actin. Structural and fluorescence evidence indicate that the troponin I inhibitory region and the myosin peptides do not bind in an identical manner to F-actin. It is concluded that the binding of the inhibitory region of troponin I to F-actin produces a conformational change in the actin monomer with the result that interaction at different locations of F-actin is impeded. These observations are interpreted to indicate that a major conformational change occurs in actin on binding to troponin I that is fundamental to the regulatory process in muscle. The data are discussed in the context of tropomyosin's ability to stabilize the actin filament and facilitate the transmission of the conformational change to actin monomers not in direct contact with troponin I.

Ca(2+)-induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo-microscopy

Muscle contraction is regulated by the intracellular Ca(2+ )concentration. In vertebrate striated muscle, troponin and tropomyosin on actin filaments comprise a Ca(2+)-sensitive switch that controls contraction. Ca(2+ )binds to troponin and triggers a series of changes in actin-containing filaments that lead to cyclic interactions with myosin that generate contraction. However, the precise location of troponin relative to actin and tropomyosin and how its structure changes with Ca(2+ )have been not determined. To understand the regulatory mechanism, we visualized the location of troponin by determining the three-dimensional structure of thin filaments from electron cryo-micrographs without imposing helical symmetry to approximately 35 A resolution. With Ca(2+), the globular domain of troponin was gourd-shaped and was located over the inner domain of actin. Without Ca(2+), the main body of troponin was shifted by approximately 30 A towards the outer domain and bifurcated, with a horizontal branch (troponin arm) covering the N and C-terminal regions of actin. The C-terminal one-third of tropomyosin shifted towards the outer domain of actin by approximately 35 A supporting the steric blocking model, however it is surprising that the N-terminal half of tropomyosin shifted less than approximately 12 A. Therefore tropomyosin shifted differentially without Ca(2+). With Ca(2+), tropomyosin was located entirely over the inner domain thereby allowing greater access of myosin for force generation. The interpretation of three-dimensional maps was facilitated by determining the three-dimensional positions of fluorophores labelled on specific sites of troponin or tropomyosin by applying probabilistic distance geometry to data from fluorescence resonance energy transfer measurements.

Myosin motors: missing structures and hidden springs

High-resolution structures of the motor domain of myosin II and lower resolution actin-myosin structures have led to the "swinging lever arm" model for myosin force generation. The available kinetic data are not all easily reconciled with this model and understanding the final details of the myosin motor mechanism must await actin-myosin co-crystals. The observation that myosin can populate multiple states in the absence of actin has nonetheless led to significant insights. The currently known myosin structures correspond to defined kinetic states that bind weakly (K(d)>microM) to actin. It is possible that the myosin lever arm could complete its swing before strong binding to actin and force generation--a process that would correspond, in the absence of load, to a Brownian ratchet. We further suggest that, under load, internal springs within the myosin head could decouple force generation and lever arm movement.

Regulation of Dictyostelium myosin I and II

Dictyostelium expresses 12 different myosins, including seven single-headed myosins I and one conventional two-headed myosin II. In this review we focus on the signaling pathways that regulate Dictyostelium myosin I and myosin II. Activation of myosin I is catalyzed by a Cdc42/Rac-stimulated myosin I heavy chain kinase that is a member of the p21-activated kinase (PAK) family. Evidence that myosin I is linked to the Arp2/3 complex suggests that pathways that regulate myosin I may also influence actin filament assembly. Myosin II activity is stimulated by a cGMP-activated myosin light chain kinase and inhibited by myosin heavy chain kinases (MHCKs) that block bipolar filament assembly. Known MHCKs include MHCK A and MHCK B, which have a novel type of kinase catalytic domain joined to a WD repeat domain, and MHC-protein kinase C (PKC), which contains both diacylglycerol kinase and PKC-related protein kinase catalytic domains. A Dictyostelium PAK (PAKa) acts indirectly to promote myosin II filament formation, suggesting that the MHCKs may be indirectly regulated by Rac GTPases.

Insertion or deletion of a single residue in the strut sequence of Dictyostelium myosin II abolishes strong binding to actin

The strut loop, one of the three loops that connects the upper and lower 50K subdomains of myosin, plays a role as a "strut" to keep the relative disposition of the two subdomains. A single residue was either inserted into or deleted from this loop. The insertion or deletion mutation abolished the in vivo motor functions of myosin, as revealed by the fact that the mutant myosins did not complement the phenotypic defects of the myosin-null cells. In vitro studies of purified full-length myosins and their subfragment-1s (S1s) revealed that the insertion mutants virtually lost the strong binding to actin although their motor functions in the absence of actin remained almost normal, showing that only the hydrophobic actin-myosin association was selectively affected by the insertion mutations. Unlike the insertion mutants, the deletion mutant showed defects both in the strong-binding state and the rate-limiting step of ATPase cycle. These results indicate the functional importance of the strut loop in establishing the strong-binding state of myosin and thereby achieving successful power strokes.