Kinesin swivels to permit microtubule movement in any direction

Twist response of actin filaments

Actin cytoskeleton force generation, sensing, and adaptation are dictated by the bending and twisting mechanics of filaments. Here, we use magnetic tweezers and microfluidics to twist and pull individual actin filaments and evaluate their response to applied loads. Twisted filaments bend and dissipate torsional strain by adopting a supercoiled plectoneme. Pulling prevents plectoneme formation, which causes twisted filaments to sever. Analysis over a range of twisting and pulling forces and direct visualization of filament and single subunit twisting fluctuations yield an actin filament torsional persistence length of ~10 µm, similar to the bending persistence length. Filament severing by cofilin is driven by local twist strain at boundaries between bare and decorated segments and is accelerated by low pN pulling forces. This work explains how contractile forces generated by myosin motors accelerate filament severing by cofilin and establishes a role for filament twisting in the regulation of actin filament stability and assembly dynamics.

Crowd Control: Effects of Physical Crowding on Cargo Movement in Healthy and Diseased Neurons

High concentration of cytoskeletal filaments, organelles, and proteins along with the space constraints due to the axon's narrow geometry lead inevitably to intracellular physical crowding along the axon of a neuron. Local cargo movement is essential for maintaining steady cargo transport in the axon, and this may be impeded by physical crowding. Molecular motors that mediate active transport share movement mechanisms that allow them to bypass physical crowding present on microtubule tracks. Many neurodegenerative diseases, irrespective of how they are initiated, show increased physical crowding owing to the greater number of stalled organelles and structural changes associated with the cytoskeleton. Increased physical crowding may be a significant factor in slowing cargo transport to synapses, contributing to disease progression and culminating in the dying back of the neuronal process. This review explores the idea that physical crowding can impede cargo movement along the neuronal process. We examine the sources of physical crowding and strategies used by molecular motors that might enable cargo to circumvent physically crowded locations. Finally, we describe sub-cellular changes in neurodegenerative diseases that may alter physical crowding and discuss the implications of such changes on cargo movement.

In vitro reconstitution of lateral to end-on conversion of kinetochore-microtubule attachments

During mitosis, kinetochores often bind to the walls of spindle microtubules, but these lateral interactions are then converted into a different binding mode in which microtubule plus-ends are embedded at kinetochores, forming dynamic "end-on" attachments. This remarkable configuration allows continuous addition or loss of tubulin subunits from the kinetochore-bound microtubule ends, concomitant with movement of the chromosomes. Here, we describe novel experimental assays for investigating this phenomenon using a well-defined in vitro reconstitution system visualized by fluorescence microscopy. Our assays take advantage of the kinetochore kinesin CENP-E, which assists in microtubule end conversion in vertebrate cells. In the experimental setup, CENP-E is conjugated to coverslip-immobilized microbeads coated with selected kinetochore components, creating conditions suitable for microtubule gliding and formation of either static or dynamic end-on microtubule attachment. This system makes it possible to analyze, in a systematic and rigorous manner, the molecular friction generated by the microtubule wall-binding proteins during lateral transport, as well as the ability of these proteins to establish and maintain association with microtubule plus-end, providing unique insights into the specific activities of various kinetochore components.

Kinesin rotates unidirectionally and generates torque while walking on microtubules

Cytoskeletal motors drive many essential cellular processes. For example, kinesin-1 transports cargo in a step-wise manner along microtubules. To resolve rotations during stepping, we used optical tweezers combined with an optical microprotractor and torsion balance using highly birefringent microspheres to directly and simultaneously measure the translocation, rotation, force, and torque generated by individual kinesin-1 motors. While, at low adenosine 5'-triphosphate (ATP) concentrations, motors did not generate torque, we found that motors translocating along microtubules at saturating ATP concentrations rotated unidirectionally, producing significant torque on the probes. Accounting for the rotational work makes kinesin a highly efficient machine. These results imply that the motor's gait follows a rotary hand-over-hand mechanism. Our method is generally applicable to study rotational and linear motion of molecular machines, and our findings have implications for kinesin-driven cellular processes.

High-Resolution Imaging of a Single Gliding Protofilament of Tubulins by HS-AFM

In vitro gliding assay of microtubules (MTs) on kinesins has provided us with valuable biophysical and chemo-mechanical insights of this biomolecular motor system. Visualization of MTs in an in vitro gliding assay has been mainly dependent on optical microscopes, limited resolution of which often render them insufficient sources of desired information. In this work, using high speed atomic force microscopy (HS-AFM), which allows imaging with higher resolution, we monitored MTs and protofilaments (PFs) of tubulins while gliding on kinesins. Moreover, under the HS-AFM, we also observed splitting of gliding MTs into single PFs at their leading ends. The split single PFs interacted with kinesins and exhibited translational motion, but with a slower velocity than the MTs. Our investigation at the molecular level, using the HS-AFM, would provide new insights to the mechanics of MTs in dynamic systems and their interaction with motor proteins.

Label-free Imaging of Microtubules with Sub-nm Precision Using Interferometric Scattering Microscopy

Current in vitro optical studies of microtubule dynamics tend to rely on fluorescent labeling of tubulin, with tracking accuracy thereby limited by the quantum yield of fluorophores and by photobleaching. Here, we demonstrate label-free tracking of microtubules with nanometer precision at kilohertz frame rates using interferometric scattering microscopy (iSCAT). With microtubules tethered to a glass substrate using low-density kinesin, we readily detect sequential 8 nm steps in the microtubule center of mass, characteristic of a single kinesin molecule moving a microtubule. iSCAT also permits dynamic changes in filament length to be measured with <5 nm precision. Using the arbitrarily long observation time enabled by label-free iSCAT imaging, we demonstrate continuous monitoring of microtubule disassembly over a 30 min period. The ability of iSCAT to track microtubules with nm precision together with its potential for label-free single protein detection and simultaneous single molecule fluorescence imaging represent a unique platform for novel approaches to studying microtubule dynamics.

A Restrictive Cardiomyopathy Mutation in an Invariant Proline at the Myosin Head/Rod Junction Enhances Head Flexibility and Function, Yielding Muscle Defects in Drosophila

An "invariant proline" separates the myosin S1 head from its S2 tail and is proposed to be critical for orienting S1 during its interaction with actin, a process that leads to muscle contraction. Mutation of the invariant proline to leucine (P838L) caused dominant restrictive cardiomyopathy in a pediatric patient (Karam et al., Congenit. Heart Dis. 3:138-43, 2008). Here, we use Drosophila melanogaster to model this mutation and dissect its effects on the biochemical and biophysical properties of myosin, as well as on the structure and physiology of skeletal and cardiac muscles. P838L mutant myosin isolated from indirect flight muscles of transgenic Drosophila showed elevated ATPase and actin sliding velocity in vitro. Furthermore, the mutant heads exhibited increased rotational flexibility, and there was an increase in the average angle between the two heads. Indirect flight muscle myofibril assembly was minimally affected in mutant homozygotes, and isolated fibers displayed normal mechanical properties. However, myofibrils degraded during aging, correlating with reduced flight abilities. In contrast, hearts from homozygotes and heterozygotes showed normal morphology, myofibrillar arrays, and contractile parameters. When P838L was placed in trans to Mhc(5), an allele known to cause cardiac restriction in flies, it did not yield the constricted phenotype. Overall, our studies suggest that increased rotational flexibility of myosin S1 enhances myosin ATPase and actin sliding. Moreover, instability of P838L myofibrils leads to decreased function during aging of Drosophila skeletal muscle, but not cardiac muscle, despite the strong evolutionary conservation of the P838 residue.

The non-processive rice kinesin-14 OsKCH1 transports actin filaments along microtubules with two distinct velocities

Microtubules and actin filaments function coordinately in many cellular processes(1-3). Although much of this coordination is mediated by proteins that statically bridge the two cytoskeletal networks(4-6), kinesin-14 motors with an actin binding calponin homology domain (KCHs) have been discovered as putatively dynamic crosslinkers in plants(7,8). OsKCH1, a KCH from rice, interacts with both microtubules and actin filaments in vivo and in vitro(9). However, it has remained unclear whether this interaction is dynamic or if actin binding reduces or even abolishes the motor's motility on microtubules(10,11). Here, we directly show in vitro that OsKCH1 is a non-processive, minus-end-directed motor that transports actin filaments along microtubules. Interestingly, we observe two distinct transport velocities dependent on the relative orientation of the actin filaments with respect to the microtubules. In addition, torsional compliance measurements on individual molecules reveal low flexibility in OsKCH1. We suggest that the orientation-dependent transport velocities emerge from OsKCH1's low torsional compliance combined with an inherently oriented binding to the actin filament. Together, our results imply a central role of OsKCH1 in the polar orientation of actin filaments along microtubules, and thus a contribution to the organization of the cytoskeletal architecture.

Understanding the guiding of kinesin/microtubule-based microtransporters in microfabricated tracks

Microtransporters using cargo-laden microtubules propelled by kinesin motors are attractive for numerous applications in nanotechnology. To improve the efficiency of transport, the movement of microtubules must be guided by microfabricated tracks. However, the mechanisms of the guiding methods used are not fully understood. Here, using computer simulation, we systematically studied the guiding of such microtransporters by three different types of guiding methods: a chemical boundary, a physical barrier, and their combination. The simulation reproduced the probabilities of guiding previously observed experimentally for the three methods. Moreover, the simulation provided further insight into the mechanisms of guiding, which overturn previous assumptions and models.

Molecular motor-powered shuttles along multi-walled carbon nanotube tracks

As a complementary tool to nanofluidics, biomolecular-based transport is envisioned for nanotechnological devices. We report a new method for guiding microtubule shuttles on multi-walled carbon nanotube tracks, aligned by dielectrophoresis on a functionalized surface. In the absence of electric field and in fluid flow, alignment is maintained. The directed translocation of kinesin propelled microtubules has been investigated using fluorescence microscopy. To our knowledge, this is the first demonstration of microtubules gliding along carbon nanotubes.

Kinesin backsteps

Kinesin-1 is a walking machine that takes ~8 nm steps along microtubules. Some aspects of the molecular mechanism of walking are now clear, but many are not. In the present paper, we discuss currently controversial points, focusing on the pathways by which kinesin takes occasional backsteps. Backsteps represent failures of the forwards-biasing mechanism. By studying the mechanochemistry of backstepping, one can learn much about the underlying molecular mechanisms responsible for forwards directional bias in the walking action.

The mechanical properties of kinesin-1: a holistic approach

During the last 25 years, a vast amount of research has gone into understanding the mechanochemical cycle of kinesin-1 and similar processive motor proteins. An experimental method that has been widely used to this effect is the in vitro study of kinesin-1 molecules moving along microtubules while pulling a bead, the position of which is monitored optically while trapped in a laser focus. Analysing results from such experiments, in which thermally excited water molecules are violently buffeting the system components, can be quite difficult. At low loads, the effect of the mechanical properties of the entire molecule must be taken into account, as stalk compliance means the bead position recorded is only weakly coupled to the movement of the motor domains, the sites of ATP hydrolysis and microtubule binding. In the present review, findings on the mechanical and functional properties of the various domains of full-length kinesin-1 molecules are summarized and a computer model is presented that uses this information to simulate the motion of a bead carried by a kinesin molecule along a microtubule, with and without a weak optical trap present. A video sequence made from individual steps of the simulation gives a three-dimensional visual insight into these types of experiment at the molecular level.

S. pombe kinesins-8 promote both nucleation and catastrophe of microtubules

The kinesins-8 were originally thought to be microtubule depolymerases, but are now emerging as more versatile catalysts of microtubule dynamics. We show here that S. pombe Klp5-436 and Klp6-440 are non-processive plus-end-directed motors whose in vitro velocities on S. pombe microtubules at 7 and 23 nm s(-1) are too slow to keep pace with the growing tips of dynamic interphase microtubules in living S. pombe. In vitro, Klp5 and 6 dimers exhibit a hitherto-undescribed combination of strong enhancement of microtubule nucleation with no effect on growth rate or catastrophe frequency. By contrast in vivo, both Klp5 and Klp6 promote microtubule catastrophe at cell ends whilst Klp6 also increases the number of interphase microtubule arrays (IMAs). Our data support a model in which Klp5/6 bind tightly to free tubulin heterodimers, strongly promoting the nucleation of new microtubules, and then continue to land as a tubulin-motor complex on the tips of growing microtubules, with the motors then dissociating after a few seconds residence on the lattice. In vivo, we predict that only at cell ends, when growing microtubule tips become lodged and their growth slows down, will Klp5/6 motor activity succeed in tracking growing microtubule tips. This mechanism would allow Klp5/6 to detect the arrival of microtubule tips at cells ends and to amplify the intrinsic tendency for microtubules to catastrophise in compression at cell ends. Our evidence identifies Klp5 and 6 as spatial regulators of microtubule dynamics that enhance both microtubule nucleation at the cell centre and microtubule catastrophe at the cell ends.

Processive behaviour of kinesin observed using micro-fabricated cantilevers

The mechanical characterization of biomolecular motors requires force sensors with sub-piconewton resolution. The coupling of a nanoscale motor to this type of microscale sensors introduces structural deformations in the motor according to the thermally activated degrees of freedom of the sensor. At present, no simple solution is available to reduce these effects. Here, we exploit the advantages of micro-fabricated cantilevers to produce a force sensor with essentially one degree of freedom and a spring constant of 0.03 pN nm(-1) for the study of the molecular motor protein kinesin-1. During processive runs, the cantilever constrains the movement of the cargo binding domain of kinesin in a straight line, parallel to the microtubule track, and excludes specific reaction coordinates such as cargo rotation. In these conditions, we measured a step size of 8.0 ± 0.4 nm and a maximal unloaded velocity of 820 ± 80 nm s(-1) at saturated adenosine triphosphate (ATP) concentration. We concluded that the motor does not need to rotate its tail as it moves through consecutive stepping cycles.

Predicting the stochastic guiding of kinesin-driven microtubules in microfabricated tracks: a statistical-mechanics-based modeling approach

Directional control of microtubule shuttles via microfabricated tracks is key to the development of controlled nanoscale mass transport by kinesin motor molecules. Here we develop and test a model to quantitatively predict the stochastic behavior of microtubule guiding when they mechanically collide with the sidewalls of lithographically patterned tracks. By taking into account appropriate probability distributions of microscopic states of the microtubule system, the model allows us to theoretically analyze the roles of collision conditions and kinesin surface densities in determining how the motion of microtubule shuttles is controlled. In addition, we experimentally observe the statistics of microtubule collision events and compare our theoretical prediction with experimental data to validate our model. The model will direct the design of future hybrid nanotechnology devices that integrate nanoscale transport systems powered by kinesin-driven molecular shuttles.

Direct measurements of kinesin torsional properties reveal flexible domains and occasional stalk reversals during stepping

Kinesin is a homodimeric motor with two catalytic heads joined to a stalk via short neck linkers (NLs). We measured the torsional properties of single recombinant molecules by tracking the thermal angular motions of fluorescently labeled beads bound to the C terminus of the stalk. When kinesin heads were immobilized on microtubules (MTs) under varied nucleotide conditions, we observed bounded or unbounded angular diffusion, depending on whether one or both heads were attached to the MT. Free rotation implies that NLs act as swivels. From data on constrained diffusion, we conclude that the coiled-coil stalk domains are approximately 30-fold stiffer than its flexible "hinge" regions. Surprisingly, while tracking processive kinesin motion at low ATP concentrations, we observed occasional abrupt reversals in the directional orientations of the stalk. Our results impose constraints on kinesin walking models and suggest a role for rotational freedom in cargo transport.

Biomolecular motor-driven molecular sorter

We have developed a novel, microfabricated, stand-alone microfluidic device that can efficiently sort and concentrate (bio-)analyte molecules by using kinesin motors and microtubules as a chemo-mechanical transduction machine. The device removes hundreds of targeted molecules per second from an analyte stream by translocating functionalized microtubules with kinesin across the stream and concentrating them at a horseshoe-shaped collector. Target biomolecule concentrations increase up to three orders of magnitude within one hour of operation.

Models of protein linear molecular motors for dynamic nanodevices

Protein molecular motors are natural nano-machines that convert the chemical energy from the hydrolysis of adenosine triphosphate into mechanical work. These efficient machines are central to many biological processes, including cellular motion, muscle contraction and cell division. The remarkable energetic efficiency of the protein molecular motors coupled with their nano-scale has prompted an increasing number of studies focusing on their integration in hybrid micro- and nanodevices, in particular using linear molecular motors. The translation of these tentative devices into technologically and economically feasible ones requires an engineering, design-orientated approach based on a structured formalism, preferably mathematical. This contribution reviews the present state of the art in the modelling of protein linear molecular motors, as relevant to the future design-orientated development of hybrid dynamic nanodevices.

Direction and speed of microtubule movements driven by kinesin motors arranged on catchin thick filaments

Conventional kinesin (Kinesin-1) is a microtubule-based molecular motor that supports intracellular vesicle/organelle transport in various eukaryotic cells. To arrange kinesin motors similarly to myosin motors on thick filaments in muscles, the motor domain of rat conventional kinesin (amino acid residues 1-430) fused to the C-terminal 829 amino acid residues of catchin (KHC430Cat) was bacterially expressed and attached to catchin filaments that can attach to and arrange myosin molecules in a bipolar manner on their surface. Unlike the case of myosin where actin filaments move toward the center much faster than in the opposite direction along the catchin filaments, microtubules moved at the same speed in both directions. In addition, many microtubules moved across the filaments at the same speed with various angles between the axes of the microtubule and catchin filament. Kinesin/catchin chimera proteins with a shorter kinesin neck domain were also prepared. Those without the whole hinge 1 domain and the C-terminal part of the neck helix moved microtubules toward the center of the catchin filaments significantly, but only slightly, faster than in the opposite direction, although the movements in both directions were slower than those of the KHC430Cat construct. The results suggest that kinesin has substantial mechanical flexibility within the motor domain, possibly within the neck linker, enabling its interaction with microtubules having any orientation.

The role of casein in supporting the operation of surface bound kinesin

Microtubules and associated motor proteins such as kinesin are envisioned for applications such as bioseparation and molecular sorting to powering hybrid synthetic mechanical devices. One of the challenges in realizing such systems is retaining motor functionality on device surfaces. Kinesin motors adsorbed onto glass surfaces lose their functionality or ability to interact with microtubules if not adsorbed with other supporting proteins. Casein, a milk protein, is commonly used in microtubule motility assays to preserve kinesin functionality. However, the mechanism responsible for this preservation of motor function is unknown. To study casein and kinesin interaction, a series of microtubule motility assays were performed where whole milk casein, or its alphas1 and alphas2, beta or kappa subunits, were introduced or omitted at various steps of the motility assay. In addition, a series of epifluorescence and total internal reflection microscopy (TIRF) experiments were conducted where fluorescently labeled casein was introduced at various steps of the motility assay to assess casein-casein and casein-glass binding dynamics. From these experiments it is concluded that casein forms a bi-layer which supports the operation of kinesin. The first tightly bound layer of casein mainly performs the function of anchoring the kinesin while the second more loosely bound layer of casein positions the head domain of the kinesin to more optimally interact with microtubules. Studies on individual casein subunits indicate that beta casein was most effective in supporting kinesin functionality while kappa casein was found to be least effective.

Processive kinesins require loose mechanical coupling for efficient collective motility

Processive motor proteins are stochastic steppers that perform actual mechanical steps for only a minor fraction of the time they are bound to the filament track. Motors usually work in teams and therefore the question arises whether the stochasticity of stepping can cause mutual interference when motors are mechanically coupled. We used biocompatible surfaces to immobilize processive kinesin-1 motors at controlled surface densities in a mechanically well-defined way. This helped us to study quantitatively how mechanical coupling between motors affects the efficiency of collective microtubule transport. We found that kinesin-1 constructs that lack most of the non-motor sequence slow each other down when collectively transporting a microtubule, depending on the number of interacting motors. This negative interference observed for a motor ensemble can be explained quantitatively by a mathematical model using the known physical properties of individual molecules of kinesin-1. The non-motor extension of kinesin-1 reduces this mutual interference, indicating that loose mechanical coupling between motors is required for efficient transport by ensembles of processive motors.

Secondary structure and compliance of a predicted flexible domain in kinesin-1 necessary for cooperation of motors

Although the mechanism by which a kinesin-1 molecule moves individually along a microtubule is quite well-understood, the way that many kinesin-1 motor proteins bound to the same cargo move together along a microtubule is not. We identified a 60-amino-acid-long domain, termed Hinge 1, in kinesin-1 from Drosophila melanogaster that is located between the coiled coils of the neck and stalk domains. Its deletion reduces microtubule gliding speed in multiple-motor assays but not single-motor assays. Hinge 1 thus facilitates the cooperation of motors by preventing them from impeding each other. We addressed the structural basis for this phenomenon. Video-microscopy of single microtubule-bound full-length motors reveals the sporadic occurrence of high-compliance states alternating with longer-lived, low-compliance states. The deletion of Hinge 1 abolishes transitions to the high-compliance state. Based on Fourier transform infrared, circular dichroism, and fluorescence spectroscopy of Hinge 1 peptides, we propose that low-compliance states correspond to an unexpected structured organization of the central Hinge 1 region, whereas high-compliance states correspond to the loss of that structure. We hypothesize that strain accumulated during multiple-kinesin motility populates the high-compliance state by unfolding helical secondary structure in the central Hinge 1 domain flanked by unordered regions, thereby preventing the motors from interfering with each other in multiple-motor situations.

Harnessing biological motors to engineer systems for nanoscale transport and assembly

Living systems use biological nanomotors to build life's essential molecules--such as DNA and proteins--as well as to transport cargo inside cells with both spatial and temporal precision. Each motor is highly specialized and carries out a distinct function within the cell. Some have even evolved sophisticated mechanisms to ensure quality control during nanomanufacturing processes, whether to correct errors in biosynthesis or to detect and permit the repair of damaged transport highways. In general, these nanomotors consume chemical energy in order to undergo a series of shape changes that let them interact sequentially with other molecules. Here we review some of the many tasks that biomotors perform and analyse their underlying design principles from an engineering perspective. We also discuss experiments and strategies to integrate biomotors into synthetic environments for applications such as sensing, transport and assembly.

Nanomechanical model of microtubule translocation in the presence of electric fields

Research efforts in recent years have been directed toward actively controlling the direction of translocation of microtubules on a kinesin-coated glass surface with E-fields (electric fields), opening up the possibility of engineering controllable nanodevices that integrate microtubules and motor proteins into their function. Here, we present a detailed, biophysical model that quantitatively describes our observations on the steering of microtubules by electric fields. A sudden application of an electric field parallel to the surface and normal to the translocation direction of a microtubule bends the leading end toward the anode, because Coulombic (electrophoretic) forces are dominant on negatively charged microtubules. Modeling this bending as a cantilever deflection with uniform loading requires accurate mechanical and electrical properties of microtubules, including their charge density, viscous drag, and flexural rigidity. We determined the charge density of microtubules from measurements of the electrophoretic mobility in a "zero flow" capillary electrophoresis column and estimate it to be 256 e(-) per micron of length. Viscous drag forces on deflecting microtubules in electroosmotic flows were studied theoretically and experimentally by directly characterizing flows using a caged dye imaging method. The flexural rigidity of microtubules was measured by applying E-fields to microtubules with biotinylated segments that were bound to streptavidin-coated surfaces. From the calculated loading, and the Bernoulli-Euler curvature and moment equation, we find that the flexural rigidity of microtubules depends on their length, suggesting microtubules are anisotropic. Finally, our model accurately predicts the biophysical properties and behavior of microtubules directed by E-fields, which opens new avenues for the design of biomolecular nanotransport systems.

Microtubule curvatures under perpendicular electric forces reveal a low persistence length

The mechanics of microtubules, cylindrical protein filaments that constitute the cytoskeleton, have been well characterized on long length scales. Here, we investigate the persistence length of short (approximately 0.1 microm) ends of microtubules by measuring the trajectories of kinesin-propelled microtubules under perpendicular electric forces. We relate the measured trajectory curvatures to the biased thermal fluctuations of the leading microtubule end, and upon including all electrohydrodynamic forces, we find that the persistence length of the microtubule ends is only 0.08 +/- 0.02 mm. This is significantly shorter than the well established value of approximately 4-8 mm that is measured for long microtubules. Our data are in good agreement with recent theoretical predictions that microtubules mechanically behave as a loose assembly of independent protofilaments on these short length scales.

Effect of spastic paraplegia mutations in KIF5A kinesin on transport activity

Hereditary spastic paraplegia (HSP) is a neurodegenerative disease caused by motoneuron degeneration. It is linked to at least 30 loci, among them SPG10, which causes dominant forms and originates in point mutations in the neuronal Kinesin-1 gene (KIF5A). Here, we investigate the motility of KIF5A and four HSP mutants. All mutations are single amino-acid exchanges and located in kinesin's motor or neck domain. The mutation in the neck (A361V) did not change the gliding properties in vitro, the others either reduced microtubule affinity or gliding velocity or both. In laser-trapping assays, none of the mutants moved more than a few steps along microtubules. Motility assays with mixtures of homodimeric wild-type, homodimeric mutant and heterodimeric wild-type/mutant motors revealed that only one mutant (N256S) reduces the gliding velocity at ratios present in heterozygous patients, whereas the others (K253N, R280C) do not. Attached to quantum dots as artificial cargo, mixtures involving N256S mutants produced slower cargo populations lagging behind in transport, whereas mixtures with the other mutants led to populations of quantum dots that rarely bound to microtubules. These differences indicate that the dominant inheritance of SPG10 is caused by two different mechanisms that both reduce the gross cargo flux, leading to deficient supply of the synapse.

Myosin-V makes two brownian 90 degrees rotations per 36-nm step

Myosin-V processively walks on actin filaments in a hand-over-hand fashion. The identical structures of the heads predict a symmetric hand-over-hand mechanism where regular, unidirectional rotation occurs during a 36-nm step. We investigated this by observing how fixed myosin-V rotates actin filaments. Actin filaments randomly rotated 90 degrees both clockwise and counter-clockwise during each step. Furthermore, ATP-dependent rotations were regularly followed by ATP-independent ones. Kinetic analysis indicated that the two 90 degrees rotations relate to the coordinated unbinding and rebinding of the heads with actin. We propose a 'brownian rotation hand-over-hand' model, in which myosin-V randomly rotates by thermally twisting its elastic neck domains during the 36-nm step. The brownian rotation may be advantageous for cargo transport through a crowded actin meshwork and for carrying cargoes reliably via multiple myosin-V molecules in the cell.

Biomolecular motor-driven microtubule translocation in the presence of shear flow: modeling microtubule deflection due to shear

We have previously demonstrated that shear flow aligns microtubules moving on kinesin-coated microchannels with the flow direction, and statistically analyzed the rate of microtubule alignment under different concentrations of kinesin as well as strengths of shear flow. These data qualitatively support the hypothesis that the alignment results from the leading ends of translocating microtubules bending into the direction of the flow due to viscous drag force. Here, we present a cantilever-beam model that quantitatively shows agreement between this hypothesis and observation. Specifically, the model couples the exact nonlinear solution for cantilever-beam deflection with drag coefficients determined by numerical simulations of microtubules in the presence of shear flow near a wall. Coupled with flexural rigidity results of our previous study (which used electric fields), the established model successfully predicts new experimental data for microtubule bending in response to shear flow, further supporting our hypothesis for the mechanism of microtubule alignment. We expect that the newly-calculated drag coefficients and beam-bending model may be useful for biophysical studies as well as interpretation of in vivo data and the design of kinesin/microtubule-based devices.

Detection of fractional steps in cargo movement by the collective operation of kinesin-1 motors

The stepping behavior of single kinesin-1 motor proteins has been studied in great detail. However, in cells, these motors often do not work alone but rather function in small groups when they transport cellular cargo. Until now, the cooperative interactions between motors in such groups were poorly understood. A fundamental question is whether two or more motors that move the same cargo step in synchrony, producing the same step size as a single motor, or whether the step size of the cargo movement varies. To answer this question, we performed in vitro gliding motility assays, where microtubules coated with quantum dots were driven over a glass surface by a known number of kinesin-1 motors. The motion of individual microtubules was then tracked with nanometer precision. In the case of transport by two kinesin-1 motors, we found successive 4-nm steps, corresponding to half the step size of a single motor. Dwell-time analysis did not reveal any coordination, in the sense of alternate stepping, between the motors. When three motors interacted in collective transport, we identified distinct forward and backward jumps on the order of 10 nm. The existence of the fractional steps as well as the distinct jumps illustrate a lack of synchronization and has implications for the analysis of motor-driven organelle movement investigated in vivo.

Cargo Transport: Two Motors Are Sometimes Better Than One

Molecular motor proteins are crucial for the proper distribution of organelles and vesicles in cells. Much of our current understanding of how motors function stems from studies of single motors moving cargos in vitro. More recently, however, there has been mounting evidence that the cooperation of multiple motors in moving cargos and the regulation of motor-filament affinity could be key mechanisms that cells utilize to regulate cargo transport. Here, we review these recent advances and present a picture of how the different mechanisms of regulating the number of motors moving a cargo could facilitate cellular functions.

Microtubule transport, concentration and alignment in enclosed microfluidic channels

The kinesin-microtubule system has emerged as a versatile model system for biologically-derived microscale transport. While kinesin motors in cells transport cargo along static microtubule tracks, for in vitro transport applications it is preferable to invert the system and transport cargo-functionalized microtubules along immobilized kinesin motors. However, for efficient cargo transport and to enable this novel transport system to be interfaced with traditional microfluidics, it is important to fabricate enclosed microchannels that are compatible with kinesin motors and microtubules, that enable fluorescence imaging of microtubule movement, and that provide fluidic connections for sample introduction. Here we construct a three-tier hierarchical system of microfluidic channels that links microscale transport channels to macroscopic fluid connections. Shallow microchannels (5 microm wide and 1 microm deep) are etched in a glass substrate and bonded to a cover glass using PMMA as an adhesive, while intermediate channels (approximately 100 microm wide) serve as reservoirs and connect to 250 microm deep microchannels that hold fine gauge tubing for fluid injection. To demonstrate the utility of this device, we first show the performance of a directional rectifier that redirects 96% of moving microtubules and, because any microtubules that detach rapidly rebind to the motor-coated surface, suffers no microtubule loss over time. Second, we develop an approach, using a headless kinesin construct, to eliminate gradients in motor adsorption and microtubule binding in the enclosed channels, which enables precise control of kinesin density in the microchannels. Finally, we show that a 60 microm diameter circular ring functionalized with motors concentrates and aligns bundles of approximately 3000 uniformly oriented microtubules, while suffering negligible ATP depletion. These aligned isopolar microtubules are an important tool for microscale transport applications and can be employed as a model in vitro system for studying kinesin-driven microtubule organization in cells.

Hydrodynamics of isotropic and liquid crystalline active polymer solutions

We describe the large-scale collective behavior of solutions of polar biofilaments and stationary and mobile crosslinkers. Both mobile and stationary crosslinkers induce filament alignment promoting either polar or nematic order. In addition, mobile crosslinkers, such as clusters of motor proteins, exchange forces and torques among the filaments and render the homogeneous states unstable via filament bundling. We start from a Smoluchowski equation for rigid filaments in solutions, where pairwise crosslink-mediated interactions among the filaments yield translational and rotational currents. The large-scale properties of the system are described in terms of continuum equations for filament and motor densities, polarization, and alignment tensor obtained by coarse-graining the Smoluchovski equation. The possible homogeneous and inhomogeneous states of the systems are obtained as stable solutions of the dynamical equations and are characterized in terms of experimentally accessible parameters. We make contact with work by other authors and show that our model allows for an estimate of the various parameters in the hydrodynamic equations in terms of physical properties of the crosslinkers.

The distance that kinesin-1 holds its cargo from the microtubule surface measured by fluorescence interference contrast microscopy

Kinesin-1 is a motor protein that carries cellular cargo such as membrane-bounded organelles along microtubules (MTs). The homodimeric motor molecule contains two N-terminal motor domains (the motor "heads"), a long coiled-coil domain (the "rod" or "stalk"), and two small globular "tail" domains. Much has been learned about how kinesin's heads step along a MT and how the tail is involved in cargo binding and autoinhibition. However, little is known about the role of the rod. Here, we investigate the extension of the rod during active transport by measuring the height at which MTs glide over a kinesin-coated surface in the presence of ATP. To perform height measurements with nanometer precision, we used fluorescence interference contrast microscopy, which is based on the self-interference of fluorescent light from objects near a reflecting surface. Using an in situ calibrating method, we determined that kinesin-1 molecules elevate gliding MTs 17 +/- 2 nm (mean +/- SEM) above the surface. When varying the composition of the surrounding nucleotides or removing the negatively charged -COOH termini of the MTs by subtilisin digestion, we found no significant changes in the measured distance. Even though this distance is significantly shorter than the contour length of the motor molecule ( approximately 60 nm), it may be sufficient to prevent proteins bound to the MTs or prevent the organelles from interfering with transport.

Calcium regulates scallop muscle by changing myosin flexibility

Muscle myosins are molecular motors that convert the chemical free energy available from ATP hydrolysis into mechanical displacement of actin filaments, bringing about muscle contraction. Myosin cross-bridges exert force on actin filaments during a cycle of attached and detached states that are coupled to each round of ATP hydrolysis. Contraction and ATPase activity of the striated adductor muscle of scallop is controlled by calcium ion binding to myosin. This mechanism of the so-called "thick filament regulation" is quite different to vertebrate striated muscle which is switched on and off via "thin filament regulation" whereby calcium ions bind to regulatory proteins associated with the actin filaments. We have used an optically based single molecule technique to measure the angular disposition adopted by the two myosin heads whilst bound to actin in the presence and absence of calcium ions. This has allowed us to directly observe the movement of individual myosin heads in aqueous solution at room temperature in real time. We address the issue of how scallop striated muscle myosin might be regulated by calcium and have interpreted our results in terms of the structures of smooth muscle myosin that also exhibit thick filament regulation.

The complex interplay between the neck and hinge domains in kinesin-1 dimerization and motor activity

Kinesin-1 dimerizes via the coiled-coil neck domain. In contrast to animal kinesins, neck dimerization of the fungal kinesin-1 NcKin requires additional residues from the hinge. Using chimeric constructs containing or lacking fungal-specific elements, the proximal part of the hinge was shown to stabilize the neck coiled-coil conformation in a complex manner. The conserved fungal kinesin hinge residue W384 caused neck coiled-coil formation in a chimeric NcKin construct, including parts of the human kinesin-1 stalk. The stabilizing effect was retained in a NcKinW384F mutant, suggesting important pi-stacking interactions. Without the stalk, W384 was not sufficient to induce coiled-coil formation, indicating that W384 is part of a cluster of several residues required for neck coiled-coil folding. A W384-less chimera of NcKin and human kinesin possessed a non-coiled-coil neck conformation and showed inhibited activity that could be reactivated when artificial interstrand disulfide bonds were used to stabilize the neck coiled-coil conformation. On the basis of yeast two-hybrid data, we propose that the proximal hinge can bind kinesin's cargo-free tail domain and causes inactivation of kinesin by disrupting the neck coiled-coil conformation.

High flexibility of the actomyosin crossbridge resides in skeletal muscle myosin subfragment-2 as demonstrated by a new single molecule assay

Popular views of force generation in muscle indicate that a lever arm in the myosin head initiates displacement of the thin filament. However, this lever arm is attached to the thick filament backbone by a flexible combination of coiled coils and hinges in the myosin subfragment-2 (S2); therefore, efficient force generation depends on tension development in this linking structure. Herein, a single molecule assay is developed to examine the flexibility of the intact S2 relative to that of the myosin head. Fluorescently labeled myosin rod is polymerized onto a single myosin molecule that is bound to actin, and the resulting Brownian motion of the rod is analyzed at video rates by digital image processing. Complete rotations of the rod suggest significant amounts of random coil in the linking structure. The close similarity of twist rates for double-headed and single-headed myosin indicates that most of the flexibility originates at or beyond the first pitch of coiled coil in S2 and most likely at the hinge connecting S2 and the light meromyosin. The myosin head has a smaller but still detectable impact on this flexibility, since the addition of ADP to the rigor crossbridge produces differential effects on the torsional characteristics of double-headed versus single-headed myosin.

Does axonemal dynein push, pull, or oscillate?

Dynein is the molecular motor that provides motive force in cilia and flagella. Dynein is anchored to the A-subtubule of the outer doublets by a club-shaped extension called the stem, which supports the large globular head of the molecule. Dynein forms an attachment or cross-bridge to the B-subtubule of the adjacent outer doublet through a slender appendage extending from the head that is called the stalk or alternately the B-link. It is generally thought that the B-link mediates the interdoublet transfer of force that bends the flagellum. This requires that energy released at the site of ATP hydrolysis, located in the globular head, be transferred as mechanical work to the microtubule binding site at the tip of the B-link. It has been proposed that this is accomplished by a sideways or rotational translocation of the B-link caused by a rotation of the globular head. An estimate of the stiffness of the B-link and stem derived from the recently published data of Burgess et al. [2003: Nature 421:715-718] yields a maximum stiffness of 0.47 pN/nm for the B-link and 0.1 pN/nm for the stem. The B-link stiffness would allow transfer of 3.8 pN of force in response to an 8-nm displacement of the B-link tip. However, if as proposed the globular head of the dynein heavy chain is supported by the stem, the B-link and stem elasticity are in series. Thus, the flexibility of the stem would limit the force that can be transferred laterally by the entire dynein heavy chain to 0.6 pN at 8 nm displacement. This force is insufficient to support flagellar motility. So, if the stem were the only support for the globular head, then force would have to be transmitted linearly along the axis defined by the stem and B-link. Because this configuration is never observed, the hypothesis that dynein generates force by lateral displacement of the B-link is more attractive, but requires that the globular head of the dynein is stabilized by an additional means of support during the power stroke. We propose that the microtubule affinity of the tip of the B-link is independent of the ATP-dependent powerstroke, and that detachment from the B-subtubule is regulated by tension. A dynein cross-bridge cycle that incorporates an anchored head, together with a ratchet-like mechanism for microtubule translocation by the B-link, would have distinct advantages. This mechanism may reconcile dynein oscillation and interdoublet sliding within one cross-bridge mechanism.

Single fungal kinesin motor molecules move processively along microtubules

Conventional kinesins are two-headed molecular motors that move as single molecules micrometer-long distances on microtubules by using energy derived from ATP hydrolysis. The presence of two heads is a prerequisite for this processive motility, but other interacting domains, like the neck and K-loop, influence the processivity and are implicated in allowing some single-headed kinesins to move processively. Neurospora kinesin (NKin) is a phylogenetically distant, dimeric kinesin from Neurospora crassa with high gliding speed and an unusual neck domain. We quantified the processivity of NKin and compared it to human kinesin, HKin, using gliding and fluorescence-based processivity assays. Our data show that NKin is a processive motor. Single NKin molecules translocated microtubules in gliding assays on average 2.14 micro m (N = 46). When we tracked single, fluorescently labeled NKin motors, they moved on average 1.75 micro m (N = 182) before detaching from the microtubule, whereas HKin motors moved shorter distances (0.83 micro m, N = 229) under identical conditions. NKin is therefore at least twice as processive as HKin. These studies, together with biochemical work, provide a basis for experiments to dissect the molecular mechanisms of processive movement.

The kinesin-related protein MCAK is a microtubule depolymerase that forms an ATP-hydrolyzing complex at microtubule ends

MCAK belongs to the Kin I subfamily of kinesin-related proteins, a unique group of motor proteins that are not motile but instead destabilize microtubules. We show that MCAK is an ATPase that catalytically depolymerizes microtubules by accelerating, 100-fold, the rate of dissociation of tubulin from microtubule ends. MCAK has one high-affinity binding site per protofilament end, which, when occupied, has both the depolymerase and ATPase activities. MCAK targets protofilament ends very rapidly (on-rate 54 micro M(-1).s(-1)), perhaps by diffusion along the microtubule lattice, and, once there, removes approximately 20 tubulin dimers at a rate of 1 s(-1). We propose that up to 14 MCAK dimers assemble at the end of a microtubule to form an ATP-hydrolyzing complex that processively depolymerizes the microtubule.

Coordination of kinesin's two heads studied with mutant heterodimers

A conventional kinesin molecule has two identical catalytic domains (heads) and is thought to use them alternately to move processively, with 8-nm steps. To clarify how each head contributes to the observed steps, we have constructed heterodimeric kinesins that consist of two distinct heads. The heterodimers in which one of the heads is mutated in a microtubule-binding loop moved processively, even when the parent mutant homodimers bound too weakly to retain microtubules in microtubule-gliding assays. The velocities of the heterodimers were only slightly higher than those of the mutant homodimers, although mixtures of these weak-binding mutant homodimers and the WT dimers moved microtubules at a velocity similar to the WT. Thus, the mutant head affects the motility of the WT head only when they are in the same molecule. The maximum force a single heterodimer produced in optical trapping nanometry was intermediate between the WT and mutant homodimers, indicating that both heads contribute to the maximum force at the same time. These results demonstrate close collaboration of kinesin's two heads in producing force and motility.

Distinguishing inchworm and hand-over-hand processive kinesin movement by neck rotation measurements

The motor enzyme kinesin makes hundreds of unidirectional 8-nanometer steps without detaching from or freely sliding along the microtubule on which it moves. We investigated the kinesin stepping mechanism by immobilizing a Drosophila kinesin derivative through the carboxyl-terminal end of the neck coiled-coil domain and measuring orientations of microtubules moved by single enzyme molecules at submicromolar adenosine triphosphate concentrations. The kinesin-mediated microtubule-surface linkage was sufficiently torsionally stiff (>/=2.0 +/- 0.9 x 10(-20) Newton meters per radian2) that stepping by the hypothesized symmetric hand-over-hand mechanism would produce 180 degree rotations of the microtubule relative to the immobilized kinesin neck. In fact, there were no rotations, a finding that is inconsistent with symmetric hand-over-hand movement. An alternative "inchworm" mechanism is consistent with our experimental results.

Molecular shuttles based on motor proteins: active transport in synthetic environments

Active transport in cells, utilizing molecular motors like kinesin and myosin, provides the inspiration for the integration of active transport into synthetic devices. Hybrid devices, employing motor proteins in a synthetic environment, are the first prototypes of molecular shuttles. Here the basic characteristics of motor proteins are discussed from an engineering point of view, and the experiments aimed at incorporating motor proteins, such as myosins and kinesins, into devices are reviewed. The key problems for the construction of a molecular shuttle are: guiding the direction of motion, controlling the speed, and loading and unloading of cargo. Various techniques, relying on surface topography and chemistry as well as flow fields and electric fields, have been developed to guide the movement of molecular shuttles on surfaces. The control of ATP concentration, acting as a fuel supply, can serve as a means to control the speed of movement. The loading process requires the coupling of cargo to the shuttle, ideally by a strong and specific link. Applications of molecular shuttles can be envisioned, e.g. in the field of nano-electro-mechanical systems (NEMS), where scaling laws favor active transport over fluid flow, and in the bottom-up assembly of novel materials.

A dynamical model of kinesin-microtubule motility assays

A two-dimensional stochastic model for the dynamics of microtubules in gliding-assay experiments is presented here, which includes the viscous drag acting on the moving fiber and the interaction with the kinesins. For this purpose, we model kinesin as a spring, and explicitly use parameter values to characterize the model from experimental data. We numerically compute the mean attachment lifetimes of all motors, the total force exerted on the microtubules at all times, the effects of a distribution in the motor speeds, and also the mean velocity of a microtubule in a gliding assay. We find quantitative agreement with the results of J. Howard, A. J. Hudspeth, and R. D. Vale, Nature. 342:154-158. We perform additional numerical analysis of the individual motors, and show how cancellation of the forces exerted by the many motors creates a resultant longitudinal force much smaller than the maximum force that could be exerted by a single motor. We also examine the effects of inhomogeneities in the motor-speeds. Finally, we present a simple theoretical model for microtubules dynamics in gliding assays. We show that the model can be analytically solved in the limit of few motors attached to the microtubule and in the opposite limit of high motor density. We find that the speed of the microtubule goes like the mean speed of the motors in good quantitative agreement with the experimental and numerical results.

Purification and characterization of native conventional kinesin, HSET, and CENP-E from mitotic hela cells

We have developed a strategy for the purification of native microtubule motor proteins from mitotic HeLa cells and describe here the purification and characterization of human conventional kinesin and two human kinesin-related proteins, HSET and CENP-E. We found that the 120-kDa HeLa cell conventional kinesin is an active motor that induces microtubule gliding at approximately 30 microm/min at room temperature. This active form of HeLa cell kinesin does not contain light chains, although light chains were detected in other fractions. HSET, a member of the C-terminal kinesin subfamily, was also purified in native form for the first time, and the protein migrates as a single band at approximately 75 kDa. The purified HSET is an active motor that induces microtubule gliding at a rate of approximately 5 microm/min, and microtubules glide for an average of 3 microm before ceasing movement. Finally, we purified native CENP-E, a kinesin-related protein that has been implicated in chromosome congression during mitosis, and we found that this form of CENP-E does not induce microtubule gliding but is able to bind to microtubules.