Unconstrained steps of myosin VI appear longest among known molecular motors

Helical motion and torque generation by microtubule motors

Microtubule motors play key roles in cellular functions, such as transport, mitosis and cell motility. Fueled by ATP hydrolysis, they convert chemical energy into mechanical work, which enables their movement on microtubules. While their motion along the long axis of microtubules has been studied extensively, some motors display an off-axis component, which results in helical motion around microtubules and the generation of torque in addition to linear forces. Understanding these nuanced movements expands our comprehension of motor protein dynamics and their impact on cellular processes.

Effect of temperature on actin filament corkscrewing driven by nonprocessive myosin IC

Myosin family proteins are ATP-driven, actin filament-based motor proteins that generate force along actin filaments. In in vitro actin filament gliding assays, certain myosins generate rotation of gliding actin filaments around their long axes. In this study, we assessed the effects of temperature on the corkscrewing motion of actin filaments, including factors like gliding and rotational velocities and corkscrewing pitch. The corkscrewing motion was driven by a nonprocessive, full-length single-headed Drosophila myosin IC attached to an antibody adsorbed onto a cover glass. We performed an in vitro actin filament corkscrewing assay at temperatures ranging from 25 °C to 35 °C. We found that the gliding and rotational velocities and the pitch of corkscrewing actin filaments generated by myosin IC molecules increased with increasing temperature. Since the pitch is determined by dividing the gliding velocity by the rotational velocity, an increase in the pitch indicates that the gliding velocity increased faster than the rotational velocity with increasing temperature. These results suggest that temperature has distinct effects on the gliding and rotational forces produced by myosin IC, with implications for interpreting the temperature effect on torque-generation mechanisms driven by myosins on actin filaments at physiological temperatures.

Coordination, cooperation, competition, crowding and congestion of molecular motors: Theoretical models and computer simulations

Cytoskeletal motor proteins are biological nanomachines that convert chemical energy into mechanical work to carry out various functions such as cell division, cell motility, cargo transport, muscle contraction, beating of cilia and flagella, and ciliogenesis. Most of these processes are driven by the collective operation of several motors in the crowded viscous intracellular environment. Imaging and manipulation of the motors with powerful experimental probes have been complemented by mathematical analysis and computer simulations of the corresponding theoretical models. In this article, we illustrate some of the key theoretical approaches used to understand how coordination, cooperation and competition of multiple motors in the crowded intra-cellular environment drive the processes that are essential for biological function of a cell. In spite of the focus on theory, experimentalists will also find this article as an useful summary of the progress made so far in understanding multiple motor systems.

Membrane-bound myosin IC drives the chiral rotation of the gliding actin filament around its longitudinal axis

Myosin IC, a single-headed member of the myosin I family, specifically interacts with anionic phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) in the cell membrane via the pleckstrin homology domain located in the myosin IC tail. Myosin IC is widely expressed and physically links the cell membrane to the actin cytoskeleton; it plays various roles in membrane-associated physiological processes, including establishing cellular chirality, lipid transportation, and mechanosensing. In this study, we evaluated the motility of full-length myosin IC of Drosophila melanogaster via the three-dimensional tracking of quantum dots bound to actin filaments that glided over a membrane-bound myosin IC-coated surface. The results revealed that myosin IC drove a left-handed rotational motion in the gliding actin filament around its longitudinal axis, indicating that myosin IC generated a torque perpendicular to the gliding direction of the actin filament. The quantification of the rotational motion of actin filaments on fluid membranes containing different PI(4,5)P2 concentrations revealed that the rotational pitch was longer at lower PI(4,5)P2 concentrations. These results suggest that the torque generated by membrane-bound myosin IC molecules can be modulated based on the phospholipid composition of the cell membrane.

Landscape-Based View on the Stepping Movement of Myosin VI

Myosin VI dimer walks toward the minus end of the actin filament with a large and variable step size of 25–36 nm. Two competing models have been put forward to explain this large step size. The Spudich model assumes that the myosin VI dimer associates at a distal tail near the cargo-binding domain, which makes two full-length single α-helix (SAH) domains serve as long legs. In contrast, the Houdusse–Sweeney model assumes that the association occurs in the middle (between residues 913 and 940) of the SAH domain and that the three-helix bundles unfold to ensure the large step size. Their consistency with the observation of stepping motion with a large and variable step size has not been examined in detail. To compare the two proposed models of myosin VI, we computationally characterized the free energy landscape experienced by the leading head during the stepping movement along the actin filament using the elastic network model of two heads and an implicit model of the SAH domains. Our results showed that the Spudich model is more consistent with the 25–36 nm step size than the Houdusse–Sweeney model. The unfolding of the three-helix bundles gives rise to the free energy bias toward a shorter distance between two heads. Besides, the stiffness of the SAH domain is a key factor for giving strong energetic bias toward the longer distance of stepping. Free energy analysis of the stepping motion complements the visual inspection of static structures and enables a deeper understanding of underlying mechanisms of molecular motors.

Recent Advances in Biological Single-Molecule Applications of Optical Tweezers and Fluorescence Microscopy

Over the past two decades, single-molecule techniques have evolved into robust tools to study many fundamental biological processes. The combination of optical tweezers with fluorescence microscopy and microfluidics provides a powerful single-molecule manipulation and visualization technique that has found widespread application in biology. In this combined approach, the spatial (~nm) and temporal (~ms) resolution, as well as the force scale (~pN) accessible to optical tweezers is complemented with the power of fluorescence microscopy. Thereby, it provides information on the local presence, identity, spatial dynamics, and conformational dynamics of single biomolecules. Together, these techniques allow comprehensive studies of, among others, molecular motors, protein-protein and protein-DNA interactions, biomolecular conformational changes, and mechanotransduction pathways. In this chapter, recent applications of fluorescence microscopy in combination with optical trapping are discussed. After an introductory section, we provide a description of instrumentation together with the current capabilities and limitations of the approaches. Next we summarize recent studies that applied this combination of techniques in biological systems and highlight some representative biological assays to mark the exquisite opportunities that optical tweezers combined with fluorescence microscopy provide.

Measuring transport of motor cargos

In this chapter, we describe experimental techniques used in vitro to illuminate how small teams of motors can work to translocate cargos. We will focus on experiments utilizing in vitro reconstitution, artificial or ex vivo purified cargos, and fluorescence imaging. A number of studies have been able to recapitulate the activities of cargo transport driven by small teams of motors elucidating how multiple motors can work together to transport cargos within the cell. Here, we describe some of the methods employed and highlight important experimental details needed to perform these experiments.

Unitary step of gliding machinery in Mycoplasma mobile

Among the bacteria that glide on substrate surfaces, Mycoplasma mobile is one of the fastest, exhibiting smooth movement with a speed of 2.0-4.5 μm⋅s(-1) with a cycle of attachment to and detachment from sialylated oligosaccharides. To study the gliding mechanism at the molecular level, we applied an assay with a fluorescently labeled and membrane-permeabilized ghost model, and investigated the motility by high precision colocalization microscopy. Under conditions designed to reduce the number of motor interactions on a randomly oriented substrate, ghosts took unitary 70-nm steps in the direction of gliding. Although it remains possible that the stepping behavior is produced by multiple interactions, our data suggest that these steps are produced by a unitary gliding machine that need not move between sites arranged on a cytoskeletal lattice.

Coupling of lever arm swing and biased Brownian motion in actomyosin

An important unresolved problem associated with actomyosin motors is the role of Brownian motion in the process of force generation. On the basis of structural observations of myosins and actins, the widely held lever-arm hypothesis has been proposed, in which proteins are assumed to show sequential structural changes among observed and hypothesized structures to exert mechanical force. An alternative hypothesis, the Brownian motion hypothesis, has been supported by single-molecule experiments and emphasizes more on the roles of fluctuating protein movement. In this study, we address the long-standing controversy between the lever-arm hypothesis and the Brownian motion hypothesis through in silico observations of an actomyosin system. We study a system composed of myosin II and actin filament by calculating free-energy landscapes of actin-myosin interactions using the molecular dynamics method and by simulating transitions among dynamically changing free-energy landscapes using the Monte Carlo method. The results obtained by this combined multi-scale calculation show that myosin with inorganic phosphate (P_i) and ADP weakly binds to actin and that after releasing P_i and ADP, myosin moves along the actin filament toward the strong-binding site by exhibiting the biased Brownian motion, a behavior consistent with the observed single-molecular behavior of myosin. Conformational flexibility of loops at the actin-interface of myosin and the N-terminus of actin subunit is necessary for the distinct bias in the Brownian motion. Both the 5.5–11 nm displacement due to the biased Brownian motion and the 3–5 nm displacement due to lever-arm swing contribute to the net displacement of myosin. The calculated results further suggest that the recovery stroke of the lever arm plays an important role in enhancing the displacement of myosin through multiple cycles of ATP hydrolysis, suggesting a unified movement mechanism for various members of the myosin family.

Myosin II is a molecular motor that is fueled by ATP hydrolysis and generates mechanical force by interacting with actin filament. Comparison among various myosin structures obtained by X-ray and electron microscope analyses has led to the hypothesis that structural change of myosin in ATP hydrolysis cycle is the driving mechanism of force generation. However, single-molecule experiments have suggested an alternative mechanism in which myosin moves stochastically in a biased direction along actin filament. Computer simulation serves as a platform for assessing these hypotheses by revealing the prominent features of the dynamically changing landscape of actin-myosin interaction. The calculated results show that myosin binds to actin at different locations of actin filament in the weak- and strong-binding states and that the free energy has a global gradient from the weak-binding site to the strong-binding site. Myosin relaxing into the strong-binding state therefore necessarily shows the biased Brownian motion toward the strong-binding site. Lever-arm swing is induced during this relaxation process; therefore, lever-arm swing and the biased Brownian motion are coupled to contribute to the net displacement of myosin. This coupling should affect the dynamical behaviors of muscle and cardiac systems.

Spontaneous detachment of the leading head contributes to myosin VI backward steps

Myosin VI is an ATP driven molecular motor that normally takes forward and processive steps on actin filaments, but also on occasion stochastic backward steps. While a number of models have attempted to explain the backwards steps, none offer an acceptable mechanism for their existence. We therefore performed single molecule imaging of myosin VI and calculated the stepping rates of forward and backward steps at the single molecule level. The forward stepping rate was proportional to the ATP concentration, whereas the backward stepping rate was independent. Using these data, we proposed that spontaneous detachment of the leading head is uncoupled from ATP binding and is responsible for the backward steps of myosin VI.

Switching of myosin-V motion between the lever-arm swing and brownian search-and-catch

Motor proteins are force-generating nanomachines that are highly adaptable to their ever-changing biological environments and have a high energy conversion efficiency. Here we constructed an imaging system that uses optical tweezers and a DNA handle to visualize elementary mechanical processes of a nanomachine under load. We apply our system to myosin-V, a well-known motor protein that takes 72 nm 'hand-over-hand' steps composed of a 'lever-arm swing' and a 'brownian search-and-catch'. We find that the lever-arm swing generates a large proportion of the force at low load (<0.5 pN), resulting in 3 k(B)T of work. At high load (1.9 pN), however, the contribution of the brownian search-and-catch increases to dominate, reaching 13 k(B)T of work. We believe the ability to switch between these two force-generation modes facilitates myosin-V function at high efficiency while operating in a dynamic intracellular environment.

F(1)-ATPase: a prototypical rotary molecular motor

F(1)-ATPase, the soluble portion of ATP synthase, has been shown to be a rotary molecular motor in which the central γ subunit rotates inside the cylinder made of α(3)β(3) subunits. The rotation is powered by ATP hydrolysis in three catalytic sites, and reverse rotation of the γ subunit by an external force leads to ATP synthesis in the catalytic sites. Here I look back how our lab became involved in the study of this marvelous rotary machine, and discuss some aspects of its rotary mechanism while confessing we are far from understanding. This article is a very personal essay, not a scientific review, for this otherwise viral machines book.

How myosin motors power cellular functions: an exciting journey from structure to function: based on a lecture delivered at the 34th FEBS Congress in Prague, Czech Republic, July 2009

Molecular motors such as myosins are allosteric enzymes that power essential motility functions in the cell. Structural biology is an important tool for deciphering how these motors work. Myosins produce force upon the actin-driven conformational changes controlling the sequential release of the hydrolysis products of ATP (Pi followed by ADP). These conformational changes are amplified by a 'lever arm', which includes the region of the motor known as the converter and the adjacent elongated light chain binding region. Analysis of four structural states of the motor provides a detailed understanding of the rearrangements and pathways of communication in the motor that are necessary for detachment from the actin track and repriming of the motor. However, the important part of the cycle in which force is produced remains enigmatic and awaits new high-resolution structures. The value of a structural approach is particularly evident from clues provided by the structural states of the reverse myosin VI motor. Crystallographic structures have revealed that rearrangements within the converter subdomain occur, which explains why this myosin can produce a large stroke in the opposite direction to all other myosins, despite a very short lever arm. By providing a detailed understanding of the motor rearrangements, structural biology will continue to reveal essential information and help solve current enigma, such as how actin promotes force production, how motors are tuned for specific cellular roles or how motor/cargo interactions regulate the function of myosin in the cell.

Extension of a three-helix bundle domain of myosin VI and key role of calmodulins

The molecular motor protein myosin VI moves toward the minus-end of actin filaments with a step size of 30-36 nm. Such large step size either drastically limits the degree of complex formation between dimer subunits to leave enough length for the lever arms, or requires an extension of the lever arms' crystallographically observed structure. Recent experimental work proposed that myosin VI dimerization triggers the unfolding of the protein's proximal tail domain which could drive the needed lever-arm extension. Here, we demonstrate through steered molecular dynamics simulation the feasibility of sufficient extension arising from turning a three-helix bundle into a long α-helix. A key role is played by the known calmodulin binding that facilitates the extension by altering the strain path in myosin VI. Sequence analysis of the proximal tail domain suggests that further calmodulin binding sites open up when the domain's three-helix bundle is unfolded and that subsequent calmodulin binding stabilizes the extended lever arms.

Direct observation of the myosin Va recovery stroke that contributes to unidirectional stepping along actin

Myosins are ATP-driven linear molecular motors that work as cellular force generators, transporters, and force sensors. These functions are driven by large-scale nucleotide-dependent conformational changes, termed "strokes"; the "power stroke" is the force-generating swinging of the myosin light chain-binding "neck" domain relative to the motor domain "head" while bound to actin; the "recovery stroke" is the necessary initial motion that primes, or "cocks," myosin while detached from actin. Myosin Va is a processive dimer that steps unidirectionally along actin following a "hand over hand" mechanism in which the trailing head detaches and steps forward ∼72 nm. Despite large rotational Brownian motion of the detached head about a free joint adjoining the two necks, unidirectional stepping is achieved, in part by the power stroke of the attached head that moves the joint forward. However, the power stroke alone cannot fully account for preferential forward site binding since the orientation and angle stability of the detached head, which is determined by the properties of the recovery stroke, dictate actin binding site accessibility. Here, we directly observe the recovery stroke dynamics and fluctuations of myosin Va using a novel, transient caged ATP-controlling system that maintains constant ATP levels through stepwise UV-pulse sequences of varying intensity. We immobilized the neck of monomeric myosin Va on a surface and observed real time motions of bead(s) attached site-specifically to the head. ATP induces a transient swing of the neck to the post-recovery stroke conformation, where it remains for ∼40 s, until ATP hydrolysis products are released. Angle distributions indicate that the post-recovery stroke conformation is stabilized by ≥ 5 k(B)T of energy. The high kinetic and energetic stability of the post-recovery stroke conformation favors preferential binding of the detached head to a forward site 72 nm away. Thus, the recovery stroke contributes to unidirectional stepping of myosin Va.

The stepping pattern of myosin X is adapted for processive motility on bundled actin

Myosin X is a molecular motor that is adapted to select bundled actin filaments over single actin filaments for processive motility. Its unique form of motility suggests that myosin X's stepping mechanism takes advantage of the arrangement of actin filaments and the additional target binding sites found within a bundle. Here we use fluorescence imaging with one-nanometer accuracy to show that myosin X takes steps of ∼18 nm along a fascin-actin bundle. This step-size is well short of the 36-nm step-size observed in myosin V and myosin VI that corresponds to the actin pseudohelical repeat distance. Myosin X is able to walk along bundles with this step-size if it straddles two actin filaments, but would be quickly forced to spiral into the constrained interior of the bundle if it were to use only a single actin filament. We also demonstrate that myosin X takes many sideways steps as it walks along a bundle, suggesting that it can switch actin filament pairs within the bundle as it walks. Sideways steps to the left or the right occur on bundles with equal frequency, suggesting a degree of lateral flexibility such that the motor's working stroke does not bias it to the left or to the right. On single actin filaments, we find a broad mixture of 10-20-nm steps, which again falls short of the 36-nm actin repeat. Moreover, the motor leans to the right as it walks along single filaments, which may require myosin X to adopt strained configurations. As a control, we also tracked myosin V stepping along actin filaments and fascin-actin bundles. We find that myosin V follows a narrower path on both structures, walking primarily along one surface of an actin filament and following a single filament within a bundle while occasionally switching to neighboring filaments. Together, these results delineate some of the structural features of the motor and the track that allow myosin X to recognize actin filament bundles.

Switch between large hand-over-hand and small inchworm-like steps in myosin VI

Many biological motor molecules move within cells using stepsizes predictable from their structures. Myosin VI, however, has much larger and more broadly distributed stepsizes than those predicted from its short lever arms. We explain the discrepancy by monitoring Qdots and gold nanoparticles attached to the myosin-VI motor domains using high-sensitivity nanoimaging. The large stepsizes were attributed to an extended and relatively rigid lever arm; their variability to two stepsizes, one large (72 nm) and one small (44 nm). These results suggest that there exist two tilt angles during myosin-VI stepping, which correspond to the pre- and postpowerstroke states and regulate the leading head. The large steps are consistent with the previously reported hand-over-hand mechanism, while the small steps follow an inchworm-like mechanism and increase in frequency with ADP. Switching between these two mechanisms in a strain-sensitive, ADP-dependent manner allows myosin VI to fulfill its multiple cellular tasks including vesicle transport and membrane anchoring.

Unconventional processive mechanics of non-muscle myosin IIB

Proper tension maintenance in the cytoskeleton is essential for regulated cell polarity, cell motility, and division. Non-muscle myosin IIB (NMIIB) generates tension along actin filaments in many cell types, including neuronal, cardiac, and smooth muscle cells. Using a three-bead optical trapping assay, we recorded NMIIB interactions with actin filaments to determine if a NMIIB dimer cycles along an actin filament in a processive manner. Our results show that NMIIB is the first myosin II to exhibit evidence of processive stepping behavior. Analysis of these data reveals a forward displacement of 5.4 nm and, surprisingly, frequent backward steps of -5.9 nm. Processive stepping along the long pitch helix of actin may provide a mechanism for disassembly of fascin-actin bundles. Forward steps and detachment are weakly force-dependent at all forces, consistent with rate-limiting and force-dependent ADP release. However, backward steps are nearly force-independent. Our data support a model in which NMIIB can readily move in both directions at stall, which may be important for a general regulator of cytoskeleton tension.

The selective role of myosin VI in lymphoid leukemia cell migration

Several myosin isotypes are discussed to be involved in the migration of various cells ranging from tumor cells to leukocytes. We investigated the involvement of myosins II and VI in the lymphoid leukemia cells lines Jurkat, NB4, Dohh-2, and Molt-4 by a three-dimensional, collagen-based migration assay. Down-regulation of myosin VI by siRNA significantly reduced the migratory activity of all cells, whereas the pharmacological inhibition of non-muscle myosin II using blebbistatin had only marginal influence. Therefore, in contrast to differentiated leukocytes and cells from solid tumors, myosin VI plays a crucial role in the migration of leukemic cells.

Twirling motion of actin filaments in gliding assays with nonprocessive Myosin motors

We present a model study of gliding assays in which actin filaments are moved by nonprocessive myosin motors. We show that even if the power stroke of the motor protein has no lateral component, the filaments will rotate around their axis while moving over the surface. Notably, the handedness of this twirling motion is opposite from that of the actin filament structure. It stems from the fact that the gliding actin filament has target zones, where its subunits point toward the surface and are therefore more accessible for myosin heads. Each myosin head has a higher binding probability before it reaches the center of the target zone than afterwards, which results in a left-handed twirling. We present a stochastic simulation and an approximative analytical solution. The calculated pitch of the twirling motion depends on the filament velocity (ATP concentration). It reaches approximately 400 nm for low speeds and increases with higher speeds.

Myosin VI undergoes a 180 degrees power stroke implying an uncoupling of the front lever arm

We simultaneously measure both the step size, via FIONA, and the 3-D orientation, via DOPI, of the light-chain domain of individual dimeric myosin VIs. This allows for the correlation of the change in orientation of the light chain domain to the stepping of the motor. Three different pairs of positions were tested using a rigid bifunctional rhodamine on the calmodulin of the IQ domain. The data for all three labeling positions support the model that the light chain domain undergoes a significant rotation of approximately 180 degrees . Contrary to an earlier study [Sun, Y. et al. (2007) Mol Cell 28, 954-964], our data does not support a model of multiple angles of the lever arm of the lead head, nor "wiggly" walking on actin. Instead, we propose that for the two heads of myosin VI to coordinate their processive movement, the lever arm of the lead head must be uncoupled from the converter until the rear head detaches. More specifically, intramolecular strain causes the myosin VI lever arm of the lead head to uncouple from the motor domain, allowing the motor domain to go through its product-release (phosphate and ADP) steps at an unstrained rate. The lever arm of the lead head rebinds to the motor and attains a rigor conformation when the rear head detaches. By coupling the orientation and position information with previously described kinetics, this allows us to explain how myosin VI coordinates its heads processively while maintaining the ability to move under load with a (semi-) rigid lever arm.

Reverse conformational changes of the light chain-binding domain of myosin V and VI processive motor heads during and after hydrolysis of ATP by small-angle X-ray solution scattering

We used small-angle X-ray solution scattering (SAXS) technique to investigate the nucleotide-mediated conformational changes of the head domains [subfragment 1 (S1)] of myosin V and VI processive motors that govern their directional preference for motility on actin. Recombinant myosin V-S1 with two IQ motifs (MV-S1IQ2) and myosin VI-S1 (MVI-S1) were engineered from Sf9 cells using a baculovirus expression system. The radii of gyration (R(g)) of nucleotide-free MV-S1IQ2 and MVI-S1 were 48.6 and 48.8 A, respectively. In the presence of ATP, the R(g) value of MV-S1IQ2 decreased to 46.7 A, while that of MVI-S1 increased to 51.7 A, and the maximum chord length of the molecule decreased by ca 9% for MV-S1IQ2 and increased by ca 6% for MVI-S1. These opposite directional changes were consistent with those occurring in S1s with ADP and Vi or AlF(4)(-2) bound (i.e., in states mimicking the ADP/Pi-bound state of ATP hydrolysis). Binding of AMPPNP induced R(g) changes of both constructs similar to those in the presence of ATP, suggesting that the timing of the structural changes for their motion on actin is upon binding of ATP (the pre-hydrolysis state) during the ATPase cycle. Binding of ADP to MV-S1IQ2 and MVI-S1 caused their R(g) values to drop below those in the nucleotide-free state. Thus, upon the release of Pi, the reverse conformational change could occur, coupling to drive the directional motion on actin. The amount of R(g) change upon the release of Pi was ca 6.4 times greater in MVI-S1 than in MV-S1IQ2, relating to the production of the large stroke of the MVI motor during its translocation on actin. Atomic structural models for these S1s based upon the ab initio shape reconstruction from X-ray scattering data were constructed, showing that MVI-S1 has the light-chain-binding domain positioned in the opposite direction to MV-S1IQ2 in both the pre- and post-powerstroke transition. The angular change between the light chain-binding domains of MV-S1IQ2 in the pre- to post-powerstroke transition was approximately 50 degrees, comparable to that of MII-S1. On the other hand, that of MVI-S1 was approximately 100 degrees or approximately 130 degrees much less than the currently postulated changes to allow the maximal stroke size of myosin VI-S1 but still significantly larger than those of other myosins reported so far. The results suggest that some additional alterations or elements are required for MVI-S1 to take maximal working strokes along the actin filament.

Using electrical and optical tweezers to facilitate studies of molecular motors

Dielectrophoresis was used to stretch and suspend actin filaments across a trench etched between two electrodes patterned on a glass slide. Optical tweezers were used to bring a motor protein-coated bead into close proximity to a pre-selected, suspended actin filament, facilitating the attachment of the myosin-coated bead to the filament. The clearance beneath the filament allowed the bead to move freely along and around its filamentous track, unhindered by solid surfaces. Using defocused images, the three-dimensional position of the bead was tracked as a function of time to obtain its trajectory. Experiments were carried out with myosin V and myosin X. Both motor proteins followed left-handed helical paths with the myosin X motor exhibiting a shorter pitch than the myosin V. The combined use of electrostatic and optical tweezers facilitates the preparation of motility assays with suspended tracks. Variants of this technique will enable higher complexity experiments in vitro to better understand the behavior of motors in cells.

Twirling of actin by myosins II and V observed via polarized TIRF in a modified gliding assay

The force generated between actin and myosin acts predominantly along the direction of the actin filament, resulting in relative sliding of the thick and thin filaments in muscle or transport of myosin cargos along actin tracks. Previous studies have also detected lateral forces or torques that are generated between actin and myosin, but the origin and biological role of these sideways forces is not known. Here we adapt an actin gliding filament assay to measure the rotation of an actin filament about its axis ("twirling") as it is translocated by myosin. We quantify the rotation by determining the orientation of sparsely incorporated rhodamine-labeled actin monomers, using polarized total internal reflection microscopy. To determine the handedness of the filament rotation, linear incident polarizations in between the standard s- and p-polarizations were generated, decreasing the ambiguity of our probe orientation measurement fourfold. We found that whole myosin II and myosin V both twirl actin with a relatively long (approximately 1 microm), left-handed pitch that is insensitive to myosin concentration, filament length, and filament velocity.

Long single alpha-helical tail domains bridge the gap between structure and function of myosin VI

Myosin VI has challenged the lever arm hypothesis of myosin movement because of its ability to take approximately 36-nm steps along actin with a canonical lever arm that seems to be too short to allow such large steps. Here we demonstrate that the large step of dimeric myosin VI is primarily made possible by a medial tail in each monomer that forms a rare single alpha-helix of approximately 10 nm, which is anchored to the calmodulin-bound IQ domain by a globular proximal tail. With the medial tail contributing to the approximately 36-nm step, rather than dimerizing as previously proposed, we show that the cargo binding domain is the dimerization interface. Furthermore, the cargo binding domain seems to be folded back in the presence of the catalytic head, constituting a potential regulatory mechanism that inhibits dimerization.

Myosin VI walks "wiggly" on actin with large and variable tilting

Myosin VI is an unconventional motor protein with unusual motility properties such as its direction of motion and path on actin and a large stride relative to its short lever arms. To understand these features, the rotational dynamics of the lever arm were studied by single-molecule polarized total internal reflection fluorescence (polTIRF) microscopy during processive motility of myosin VI along actin. The axial angle is distributed in two peaks, consistent with the hand-over-hand model. The changes in lever arm angles during discrete steps suggest that it exhibits large and variable tilting in the plane of actin and to the sides. These motions imply that, in addition to the previously suggested flexible tail domain, there is a compliant region between the motor domain and lever arm that allows myosin VI to accommodate the helical position of binding sites while taking variable step sizes along the actin filament.

Myosin V walks by lever action and Brownian motion

Myosin V is a molecular motor that moves cargo along actin filaments. Its two heads, each attached to a long and relatively stiff neck, move alternately forward in a "hand-over-hand" fashion. To observe under a microscope how the necks move, we attached a micrometer-sized rod to one of the necks. The leading neck swings unidirectionally forward, whereas the trailing neck, once lifted, undergoes extensive Brownian rotation in all directions before landing on a site ahead of the leading head. The neck-neck joint is essentially free, and the neck motion supports a mechanism where the active swing of the leading neck biases the random motion of the lifted head to let it eventually land on a forward site.

Diffusion trajectory of an asymmetric object: information overlooked by the mean square displacement

Diffusion of an asymmetric object is characterized by its translational and rotational diffusion coefficients. Until now, anisotropic diffusion studies have been based on ensemble averages. Here we present a theoretical basis for the analysis of the trajectories of a single particle with anisotropic diffusion coefficients. We discuss the relevance of this method for motion of biomolecules in the membrane of living cells.

Application of peptide probe for evaluating affinity properties of proteins using quartz crystal microbalance

This study proved a possibility of a peptide probe for evaluating affinity properties of proteins. We have designed and synthesized three different peptide probes, H-Ala3-(Gly-Pro5)3-Gly-OH (peptide A), H-Ala3-(Gly-Pro5)-Gly-OH (peptide B) and H-Ala3-Gly-OH (peptide C) for testing their affinities to profilin. Each peptide probe was immobilized on a quartz crystal microbalance (QCM) sensor. The QCM sensor with the peptide A showed a 93 Hz decrease of resonant frequency which indicated profilin bound to the QCM sensor in a single layer. In a successive reaction with actin, the QCM analysis resulted in a 123 Hz decrease of resonant frequency which showed actin bound to the QCM sensor. A fluorescence microscope image of the sensor surface exhibited clear fluorescence after binding a rhodamine labeled actin on the sensor surface. These results supported stepwise reactions of profilin binding to the peptide A and actin binding to profilin. In the three peptide probes, the peptide A showed the highest affinity to profilin, i.e., sequence dependent affinity was confirmed.

What can myosin VI do in cells?

The recently solved structure of the myosin VI motor demonstrates that the unique insert at the end of the motor is responsible for the reversal of the normal myosin directionality. A second class-specific insert near the nucleotide-binding pocket contributes to myosin VI's unique kinetic tuning, allowing it to function either as an actin-based transporter or as an anchoring protein. Recent biochemical and biophysical studies have shown that the native molecule can form dimers upon clustering, and cell biological studies have demonstrated that it clearly does play both transport and anchoring roles in cells. These mechanistic insights allow us to speculate on how unusual aspects of myosin VI structure and function allow it to fill unique niches in cells.

Flexible light-chain and helical structure of F-actin explain the movement and step size of myosin-VI

Myosin-VI is a dimeric isoform of unconventional myosins. Single molecule experiments indicate that myosin-VI and myosin-V are processive molecular motors, but travel toward opposite ends of filamentous actin. Structural studies show several differences between myosin-V and VI, including a significant difference in the light-chain domain connecting the motor domains. Combining the measured kinetics of myosin-VI with the elasticity of the light chains, and the helical structure of F-actin, we compare and contrast the motility of myosin-VI with myosin-V. We show that the elastic properties of the light-chain domain control the stepping behavior of these motors. Simple models incorporating the motor elastic energy can quantitatively capture most of the observed data. Implications of our result for other processive motors are discussed.

Brownian molecular motors driven by rotation-translation coupling

We investigated three models of Brownian motors which convert rotational diffusion into directed translational motion by switching on and off a potential. In the first model a spatially asymmetric potential generates directed translational motion by rectifying rotational diffusion. It behaves much like a conventional flashing ratchet. The second model utilizes both rotational diffusion and drift to generate translational motion without spatial asymmetry in the potential. This second model can be driven by a combination of a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven) depending on the chosen parameters. In the third model, elements of both the Brownian motor and powerstroke mechanisms are combined by switching between three distinct states. Relevance of the model to biological motor proteins is discussed.

A hand-over-hand diffusing model for myosin-VI molecular motors

Single molecules of dimeric myosin-VI have been demonstrated to be able to move processively towards the pointed end of actin filament with a mean step size of approximately 36 nm. Here we present a hand-over-hand diffusing mechanism for this unidirectional movement. Based on this mechanism, its dynamical behaviors such as the step-size distribution, dwell-time distributions and mean dwell time at various ATP and ADP concentrations and under various loads are studied in detail. The calculated results show good agreement with previous experimental results. The processive movement of mutant myosin-V with its neck domains truncated to only one IQ motif can also be explained by using this hand-over-hand diffusing model.

Entropy rectifies the Brownian steps of kinesin

Kinesin is a stepping motor that successively produces forward and backward 8-nm steps along microtubules. Under physiological conditions, the steps powering kinesin's motility are biased in one direction and drive various biological motile processes. The physical mechanism underlying the unidirectional bias of the kinesin steps is not fully understood. Here we explored the mechanical kinetics and thermodynamics of forward and backward kinesin steps by analyzing their temperature and load dependence. Results show that the frequency asymmetry between forward and backward steps is produced by entropy. Furthermore, the magnitude of the entropic asymmetry is 6 k(B)T, more than three times greater than expected from a current model, in which a mechanical conformational change within the kinesin molecular structure directly biases the kinesin steps forward. We propose that the stepping direction of kinesin is preferably caused by an entropy asymmetry resulting from the compatibility between the kinesin and microtubule interaction based on their polar structures.

Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro

We have previously measured the process of displacement generation by a single head of muscle myosin (S1) using scanning probe nanometry. Given that the myosin head was rigidly attached to a fairly large scanning probe, it was assumed to stably interact with an underlying actin filament without diffusing away as would be the case in muscle. The myosin head has been shown to step back and forth stochastically along an actin filament with actin monomer repeats of 5.5 nm and to produce a net movement in the forward direction. The myosin head underwent 5 forward steps to produce a maximum displacement of 30 nm per ATP at low load (<1 pN). Here, we measured the steps over a wide range of forces up to 4 pN. The size of the steps (∼5.5 nm) did not change as the load increased whereas the number of steps per displacement and the stepping rate both decreased. The rate of the 5.5-nm steps at various force levels produced a force-velocity curve of individual actomyosin motors. The force-velocity curve from the individual myosin heads was comparable to that reported in muscle, suggesting that the fundamental mechanical properties in muscle are basically due to the intrinsic stochastic nature of individual actomyosin motors. In order to explain multiple stochastic steps, we propose a model arguing that the thermally-driven step of a myosin head is biased in the forward direction by a potential slope along the actin helical pitch resulting from steric compatibility between the binding sites of actin and a myosin head. Furthermore, computer simulations show that multiple cooperating heads undergoing stochastic steps generate a long (>60 nm) sliding distance per ATP between actin and myosin filaments, i.e., the movement is loosely coupled to the ATPase cycle as observed in muscle.

Characterization ofAmoeba proteus myosin VI immunoanalog

Amoeba proteus, the highly motile free-living unicellular organism, has been widely used as a model to study cell motility. However, molecular mechanisms underlying its unique locomotion and intracellular actin-based-only trafficking remain poorly understood. A search for myosin motors responsible for vesicular transport in these giant cells resulted in detection of 130-kDa protein interacting with several polyclonal antibodies against different tail regions of human and chicken myosin VI. This protein was binding to actin in the ATP-dependent manner, and immunoprecipitated with anti-myosin VI antibodies. In order to characterize its possible functions in vivo, its cellular distribution and colocalization with actin filaments and dynamin II during migration and pinocytosis were examined. In migrating amoebae, myosin VI immunoanalog localized to vesicular structures, particularly within the perinuclear and sub-plasma membrane areas, and colocalized with dynamin II immunoanalog and actin filaments. The colocalization was even more evident in pinocytotic cells as proteins concentrated within pinocytotic pseudopodia. Moreover, dynamin II and myosin VI immunoanalogs cosedimented with actin filaments, and were found on the same isolated vesicles. Blocking endogenous myosin VI immunoanalog with anti-myosin VI antibodies inhibited the rate of pseudopodia protrusion (about 19% decrease) and uroidal retraction (about 28% decrease) but did not affect cell morphology and the manner of cell migration. Treatment with anti-human dynamin II antibodies led to changes in directionality of amebae migration and affected the rate of only uroidal translocation (about 30% inhibition). These results indicate that myosin VI immunoanalog is expressed in protist Amoeba proteus and may be involved in vesicle translocation and cell locomotion.

A one-headed class V myosin molecule develops multiple large (approximately 32-nm) steps successively

Class V myosin (myosin-V) was first found as a processive motor that moves along an actin filament with large ( approximately 36-nm) successive steps and plays an important role in cargo transport in cells. Subsequently, several other myosins have also been found to move processively. Because myosin-V has two heads with ATP- and actin-binding sites, the mechanism of successive movement has been generally explained based on the two-headed structure. However, the fundamental problem of whether the two-headed structure is essential for the successive movement has not been solved. Here, we measure motility of engineered myosin-V having only one head by optical trapping nanometry. The results show that a single one-headed myosin-V undergoes multiple successive large (approximately 32-nm) steps, suggesting that a novel mechanism is operating for successive myosin movement.