Actomyosin: law and order in motility

Novel Mutations in β-MYH7 Gene in Indian Patients With Dilated Cardiomyopathy

Background

Methods

Results

Conclusions

Dynamics of ATP-dependent and ATP-independent steppings of myosin-V on actin: catch-bond characteristics

An analytical theory is presented for the dynamics of myosin-V molecular motor, where both ATP-dependent and ATP-independent steppings are taken into account. Specifically, the dependences of velocity, run length and unbinding rate upon both forward and backward loads and ATP concentration are studied, explaining quantitatively the diverse available single-molecule data and providing predicted results. The results show that the unbinding rate increases with the increase of ATP concentration and levels off at both low and high ATP concentrations. More interestingly, at an ATP concentration that is not very low, the unbinding rate exhibits characteristics of a catch-slip bond under backward load, with the unbinding rate decreasing rapidly with the increase of the backward load in the range smaller than about 2.5 pN and then increasing slowly with the further increase of the backward load. By contrast, under forward load the unbinding rate exhibits a slip-bond characteristic.

A model for the chemomechanical coupling of myosin-V molecular motors

Herein, a model for the chemomechanical coupling of dimeric myosin-V motors is presented. Based on this model and the proposal that the rate constants of the ATPase activity of the two heads are independent of an external force in a range smaller than the stall force, we analytically studied the dynamics of the motor, such as the stepping ratio, dwell time between two mechanical steps, and velocity, under varying force and ATP concentrations. The theoretical results well reproduce the diverse available single-molecule experimental data. In particular, the experimental data showing that at a low ATP concentration, the dwell time and velocity have less force dependency than at a high ATP concentration is explained quantitatively. Moreover, the dependency of the chemomechanical coupling ratio on the force and ATP concentration was studied.

The paper presents a model of chemomechanical coupling of myosin-V motor, explaining the dynamics under varying force and ATP concentrations.

Comparative proteomics and expression analysis of five genes in Epicauta chinensis larvae from the first to fifth instar

Blister beetle is an important insect model for both medicinal and pure research. Previous research has mainly focused on its biology and biochemistry, but very little data is yet available in the molecular biology. This study uses differential proteomics technology to analyze the soluble proteins extracted from each of the 5 instars larvae of Epicauta chinensis. 42 of the differentially-expressed proteins were identified successfully by MALDI-TOF/TOF-MS. Some of these proteins' function and their expression profiles are analyzed. Our analysis revealed dynamics regulation of the following proteins: Axin-like protein pry-1 (APR-1), dihydrolipoyl dehydrogenase (DLD), vitellogenin (Vg) and lysozyme C (Lmz-S). APR-1 negatively regulates the Wnt signaling pathway. Its overexpression could result in embryo, leg, eye and ovary ectopica or malformation. DLD catalyzes the pyruvate into acetyl-CoA, the latter is the starting material of juvenile hormone (JH) and ipsdienol biosynthesis through the MVA pathway in insects. While Vg synthesis can be regulated by JH and stimulated by food factors. So DLD may affect the synthesis of JH, ipsdienol and Vg indirectly. The activity of lysozyme is an indicator of the immunity. Nutrition/food should be taken into account for its potential role during the development of larva in the future. Among the five genes and their corresponding proteins' expression, only hsc70 gene showed a good correspondence with the protein level. This reflects the fluctuating relationship between mRNA and protein levels.

Coherent Behavior and the Bound State of Water and K(+) Imply Another Model of Bioenergetics: Negative Entropy Instead of High-energy Bonds

Observations of coherent cellular behavior cannot be integrated into widely accepted membrane (pump) theory (MT) and its steady state energetics because of the thermal noise of assumed ordinary cell water and freely soluble cytoplasmic K(+). However, Ling disproved MT and proposed an alternative based on coherence, showing that rest (R) and action (A) are two different phases of protoplasm with different energy levels. The R-state is a coherent metastable low-entropy state as water and K(+) are bound to unfolded proteins. The A-state is the higher-entropy state because water and K(+) are free. The R-to-A phase transition is regarded as a mechanism to release energy for biological work, replacing the classical concept of high-energy bonds. Subsequent inactivation during the endergonic A-to-R phase transition needs an input of metabolic energy to restore the low entropy R-state. Matveev's native aggregation hypothesis allows to integrate the energetic details of globular proteins into this view.

A model for processive movement of single-headed myosin-IX

It is puzzling that in spite of its single-headed structure, myosin-IX can move processively along actin. Here, based on the experimental evidence that the strong binding of myosin to actin in rigor state induces structural changes to several local actin monomers, a Brownian ratchet model is proposed to describe this processive movement. In the model, the actin plays an active role in the motility of single-headed myosin, in contrast to the common belief that the actin acts only as a passive track for the motility of the myosin. The unidirectional movement is due to both the asymmetric periodic potential of the myosin interacting with actin and the forward Stokes force induced by the relative rotation of the neck domain to the motor domain, while the processivity is determined by the binding affinity of the myosin for actin in ATP state. This gives a good explanation to the high processivity of myosin-IX, which results from its high binding affinity for actin in ATP state due to the presence of unique loop 2 insertion or N-terminal extension. The experimental results on the motility of myosin-IX such as the step size, large forward/backward stepping ratio, run length, stall force, etc, are explained well.

Multiscale modeling of structural dynamics underlying force generation and product release in actomyosin complex

To decrypt the mechanistic basis of myosin motor function, it is essential to probe the conformational changes in actomyosin with high spatial and temporal resolutions. In a computational effort to meet this challenge, we have performed a multiscale modeling of the allosteric couplings and transition pathway of actomyosin complex by combining coarse-grained modeling of the entire complex with all-atom molecular dynamics simulations of the active site. Our modeling of allosteric couplings at the pre-powerstroke state has pinpointed key actin-activated couplings to distant myosin parts which are critical to force generation and the sequential release of phosphate and ADP. At the post-powerstroke state, we have identified isoform-dependent couplings which underlie the reciprocal coupling between actin binding and nucleotide binding in fast Myosin II, and load-dependent ADP release in Myosin V. Our modeling of transition pathway during powerstroke has outlined a clear sequence of structural events triggered by actin binding, which lead to subsequent force generation, twisting of central beta-sheet, and the sequential release of phosphate and ADP. Finally we have performed atomistic simulations of active-site dynamics based on an on-path "transition-state" myosin conformation, which has revealed significantly weakened coordination of phosphate by Switch II, and a disrupted key salt bridge between Switch I and II. Meanwhile, the coordination of MgADP by Switch I and P loop is less perturbed. As a result, the phosphate can be released prior to MgADP. This study has shed new lights on the controversy over the structural mechanism of actin-activated phosphate release and force generation in myosin motor.

Modulation of cell motility by spatial repositioning of enzymatic ATP/ADP exchange capacity

ATP is the "principal energy currency" in metabolism and the most versatile small molecular regulator of cellular activities. Although already much is known about the role of ATP in fundamental processes of living systems, data about its compartmentalization are rather scarce, and we still have only very limited understanding of whether patterns in the distribution of intracellular ATP concentration ("ATP inhomogeneity") do exist and have a regulatory role. Here we report on the analysis of coupling of local ATP supply to regulation of actomyosin behavior, a widespread and dynamic process with conspicuous high ATP dependence, which is central to cell shape changes and cell motility. As an experimental model, we use embryonic fibroblasts from knock-out mice without major ATP-ADP exchange enzymes, in which we (re)introduce the ATP/ADP exchange enzyme adenylate kinase-1 (AK1) and deliberately manipulate its spatial positioning by coupling to different artificial location tags. By transfection-complementation of AK1 variants and comparison with yellow fluorescent protein controls, we found that motility and spreading were enhanced in cells with AK1 with a focal contact guidance tag. Intermediary enhancement was observed in cells with membrane-targeted or cytosolic AK1. Use of a heterodimer-inducing approach for transient translocation of AK1 to focal contacts under conditions of constant global AK1 activity in the cell corroborated these results. Based on our findings with these model systems, we propose that local ATP supply in the cell periphery and "on site" fuelling of the actomyosin machinery, when maintained via enzymes involved in phosphoryl transfer, are codetermining factors in the control of cell motility.

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.

Model for unidirectional movement of axonemal and cytoplasmic dynein molecules

A model for the unidirectional movement of dynein is presented based on the structural observations and biochemical experimental results available. In this model, the binding affinity of dynein for microtubule (MT) is independent of its nucleotide state and the change between strong and weak MT-binding is determined naturally by the variation of relative orientation between the stalk and MT, as the stalk rotates following nucleotide-state transition. Thus the enigmatic communication from the adenosine triphosphate (ATP)-binding site in the globular domain to the far MT-binding site in the tip of the stalk, which is a prerequisite in conventional models, is not required. Using the present model, the previous experimental results such as the effect of ATP and adenosine diphosphate (ADP) bindings on dissociation of dynein from MT, the movement of single-headed axonemal dyneins at saturating ATP concentration, the load dependence of step-size for the movement of two-headed cytoplasmic dyneins and the dependence of stall force on ATP concentration can be well explained.

Orientation and mobility of actin in different intermediate states of the ATP hydrolysis cycle

Using polarization fluorimetry, we have investigated conformational changes of FITC-phalloidin-labeled F-actin in ghost muscle fibers. These changes were induced by myosin subfragment-1 (S1) in the absence and presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes of orientation and mobility of actin monomers during the ATPase cycle. The most pronounced differences in orientation (~4 degrees ) and in mobility (~43%) of actin were found between the actomyosin states induced by MgADP and MgATP.

Effect of nucleotides on the orientation and mobility of myosin subfragment-1 in ghost muscle fiber

Using polarization fluorimetry, the orientation and mobility of 1,5-IAEDANS specifically bound to Cys707 of myosin subfragment-1 (S1) were studied in ghost muscle tropomyosin-containing fibers in the absence and in the presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes in the structural state of the myosin head during the ATPase cycle. Maximal differences in the probe orientation by 4 degrees and its mobility by 30% were found between actomyosin states in the presence of MgADP and MgATP. It is suggested that interaction of S1 with F-actin induces nucleotide-dependent rotation of the whole motor domain of the myosin head or only the dye-binding site and also change in the head mobility.

Model for kinetics of myosin-V molecular motors

A hand-over-hand model is presented for the processive movement of myosin-V based on previous biochemical experimental results and structural observations of nucleotide-dependent conformational changes of single-headed myosins. The model shows that the ADP-release rate of the trailing head is much higher than that of the leading head, thus giving a 1:1 mechanochemical coupling for the processive movement of the motor. It explains well the previous finding that some 36-nm steps consist of two substeps, while other 36-nm steps consist of no substeps. Using the model, the calculated kinetic behaviors of myosin-V such as the main and intermediate dwell time distributions, the load dependence of the average main and intermediate dwell time and the load dependence of occurrence frequency of the intermediate state under various nucleotide conditions show good quantitative agreement with previous experimental results.

The structural basis of myosin V processive movement as revealed by electron cryomicroscopy

The processive motor myosin V has a relatively high affinity for actin in the presence of ATP and, thus, offers the unique opportunity to visualize some of the weaker, hitherto inaccessible, actin bound states of the ATPase cycle. Here, electron cryomicroscopy together with computer-based docking of crystal structures into three-dimensional (3D) reconstructions provide the atomic models of myosin V in both weak and strong actin bound states. One structure shows that ATP binding opens the long cleft dividing the actin binding region of the motor domain, thus destroying the strong binding actomyosin interface while rearranging loop 2 as a tether. Nucleotide analogs showed a second new state in which the lever arm points upward, in a prepower-stroke configuration (lever arm up) bound to actin before phosphate release. Our findings reveal how the structural elements of myosin V work together to allow myosin V to step along actin for multiple ATPase cycles without dissociating.

Interaction of myosin subfragment 1 with forms of monomeric actin

The ability of myosin subfragment 1 to interact with monomeric actin complexed to sequestering proteins was tested by a number of different techniques such as affinity absorption, chemical cross-linking, fluorescence titration, and competition procedures. For affinity absorption, actin was attached to agarose immobilized DNase I. Both chymotryptic subfragment 1 isoforms (S1A1 and S1A2) were retained by this affinity matrix. Fluorescence titration employing pyrenyl-actin in complex with deoxyribonuclease I (DNase I) or thymosin beta4 demonstrated S1 binding to these actin complexes. A K(D) of 5 x 10(-8) M for S1A1 binding to the actin-DNase I complex was determined. Fluorescence titration did not indicate binding of S1 to actin in complex with gelsolin segment 1 (G1) or vitamin D-binding protein (DBP). However, fluorescence competition experiments and analysis of tryptic cleavage patterns of S1 indicated its interaction with actin in complex with DBP or G1. Formation of the ternary DNase I-acto-S1 complex was directly demonstrated by sucrose density sedimentation. S1 binding to G-actin was found to be sensitive to ATP and an increase in ionic strength. Actin fixed in its monomeric state by DNase I was unable to significantly stimulate the Mg2+-dependent S1-ATPase activity. Both wild-type and a mutant of Dictyostelium discoideum myosin II subfragment 1 containing 12 additional lysine residues within an insertion of 20 residues into loop 2 (K12/20-Q532E) were found to also interact with actin-DNase I complex. Binding of the K12/20-Q532E mutant to the actin-DNase I complex occurred with higher affinity than wild-type S1 and was less sensitive to mono- and divalent cations.

Structural insights into actin-binding, branching and bundling proteins

Structural advances in our understanding of the functions of the actin cytoskeleton have come from diverse sources. On the one hand, the determination of the structure of a bacterial actin-like protein MreB reveals the prokaryotic origins of the actin cytoskeleton, whereas on the other, cryo-electron microscopy and crystallography have yielded reconstructions of many actin crosslinking, regulatory and binding proteins in complex with F-actin. Not least, a high-resolution structure of the Arp2/3 complex and a reconstruction with F-actin provides considerable insight into the eukaryotic machinery, vital for the formation of new F-actin barbed ends, a prerequisite for rapid actin polymerisation involved in cell shape change and motility.

The leukocyte cytoskeleton in cell migration and immune interactions

Leukocyte migration is crucial during the development of the immune system and in the responses to infection, inflammation, and tumor rejection. The migratory behavior of leukocytes under physiological and pathological conditions as well as the extracellular cues and intracellular machinery that control and guide migration have been studied thoroughly. The cytoskeleton of leukocytes is extremely versatile, bearing characteristic features that enable these cells to migrate under conditions of flow through narrow spaces and onto target tissues. What makes the cytoskeleton machinery so extraordinary is not so much its molecular composition, but its flexibility which allows it to display a unique combination of responses to the extracellular medium and a rapid regulation of the architecture of its components. This review focuses on the cytoskeleton of the leukocyte. Its molecular components and the regulation of their assembly and organization are discussed. Furthermore, it highlights aspects of the regulation of the leukocyte cytoskeleton that confer flexibility to these cells in order to perform their specific tasks. Finally, different subcellular structures such as the immunological synapse, the uropod of migrating leukocytes, and the phagosome displayed by phagocytic cells are discussed in detail. The relationship of the leukocyte with its environment occurs through different kinds of receptors that interact with ligands that are soluble, fixed on the membrane of other cells, or immobilized on the extracellular matrix. The impact of receptor-ligand binding on the functional responses and the rearrangement of the cytoskeleton is also examined.

Conformational change of the actomyosin complex drives the multiple stepping movement

Actin-myosin (actomyosin) generates mechanical force by consuming ATP molecules. We apply the energy landscape perspective to address a controversial issue as to whether the myosin head moves with multiple steps after a single ATP hydrolysis or only a single mechanical event of the lever-arm swinging follows a single ATP hydrolysis. Here we propose a theoretical model in which the refolding of the partially unfolded actomyosin complex and the movement of the myosin head along the actin filament are coupled. A single ATP hydrolysis is followed by the formation of a high free-energy partially unfolded actomyosin complex, which then gradually refolds with a concomitant multiple stepping movement on the way to the lowest free-energy rigor state. The model quantitatively explains the single-molecular observation of the multiple stepping movement and is consistent with structural observations of the disorder in the actomyosin-binding process. The model also explains the observed variety in dwell time before each step, which is not accounted for by previous models, such as the lever-arm or ratchet models.

Identification and phylogenetic analysis of Drosophila melanogaster myosins

Myosins constitute a superfamily of motor proteins that convert energy from ATP hydrolysis into mechanical movement along the actin filaments. Phylogenetic analysis currently places myosins into 17 classes based on class-specific features of their conserved motor domain. Traditionally, the myosins have been divided into two classes depending on whether they form monomers or dimers. The conventional myosin of muscle and nonmuscle cells forms class II myosins. They are complex molecules of four light chains bound to two heavy chains that form bipolar filaments via interactions between their coiled-coil tails (type II). Class I myosins are smaller monomeric myosins referred to as unconventional myosins. Now, at least 15 other classes of unconventional myosins are known. How many myosins are needed to ensure the proper development and function of eukaryotic organisms? Thus far, three types of myosins were found in budding yeast, six in the nematode Caenorhabditis elegans, and at least 12 in human. Here, we report on the identification and classification of Drosophila melanogaster myosins. Analysis of the Drosophila genome sequence identified 13 myosin genes. Phylogenetic analysis based on the sequence comparison of the myosin motor domains, as well as the presence of the class-specific domains, suggests that Drosophila myosins can be divided into nine major classes. Myosins belonging to previously described classes I, II, III, V, VI, and VII are present. Molecular and phylogenetic analysis indicates that the fruitfly genome contains at least five new myosins. Three of them fall into previously described myosin classes I, VII, and XV. Another myosin is a homolog of the mouse and human PDZ-containing myosins, forming the recently defined class XVIII myosins. PDZ domains are named after the postsynaptic density, disc-large, ZO-1 proteins in which they were first described. The fifth myosin shows a unique domain composition and a low homology to any of the existing classes. We propose that this is classified when similar myosins are identified in other species.

The regulatory domain of the myosin head behaves as a rigid lever

The regulatory domain of the myosin head is believed to serve as a lever arm that amplifies force generated in the catalytic domain and transmits this strain to the thick filament. The lever arm itself either can be passive or may have a more active role storing some of the energy created by hydrolysis of ATP. A structural correlate which might distinguish between these two possibilities (a passive or an active role) is the stiffness of the domain in question. To this effect we have examined the motion of the proximal (ELC) and distal (RLC) subdomains of the regulatory domain in reconstituted myosin filaments. Each subdomain was labeled with a spin label at a unique cysteine residue, Cys-136 of ELC or Cys-154 of mutant RLC, and its mobility was determined using saturation transfer electron paramagnetic resonance spectroscopy. The mobility of the two domains was similar; the effective correlation time (tau(eff)) for ELC was 17 micros and that for RLC was 22 micros. Additionally, following a 2-fold change of the global dynamics of the myosin head, effected by decreasing the interactions with the filament surface (or the other myosin head), the coupling of the intradomain dynamics remained unchanged. These data suggest that the regulatory domain of the myosin head acts as a single mechanically rigid body, consistent with the regulatory domain serving as a passive lever.

UNC-87 is an actin-bundling protein

The Caenorhabditis elegans unc-87 gene product is essential for the maintenance of the nematode body wall muscle where it is found colocalized with actin in the I band. The molecular domain structure of the protein reveals similarity to the C-terminal repeat region of the smooth muscle actin-binding protein calponin. In this study we investigated the in vitro function of UNC-87 using both the full-length recombinant molecule and several truncated mutants. According to analytical ultracentrifugation UNC-87 occurs as a monomer in solution. UNC-87 cosedimented with both smooth and skeletal muscle F-actin, but not with monomeric G-actin, and exhibited potent actin filament bundling activity. Actin binding was independent of the presence of tropomyosin and the actin cross-linking proteins filamin and alpha-actinin. Consistent with its actin bundling activity in vitro, UNC-87 tagged with green fluorescent protein associated with and promoted the formation of actin stress fiber bundles in living cells. These data identify UNC-87 as an actin-bundling protein and highlight the calponin-like repeats as a novel actin-binding module.

A new direction for myosin

Members of the myosin superfamily of actin-based motor proteins were previously thought to move only towards the barbed end of the actin filament. In an extraordinary reversal of this dogma, an abundant and widespread unconventional myosin known as myosin VI has recently been shown to move towards the pointed end of the actin filament - the opposite direction of all other characterized myosins. This discovery raises novel and intriguing questions about the molecular mechanisms of reversal and the biological roles of this 'backwards' myosin.