KIF1D is a fast non-processive kinesin that demonstrates novel K-loop-dependent mechanochemistry

Reconstitution of Organelle Transport Along Microtubules In Vitro

In this chapter, we describe methods for reconstituting and analyzing the transport of isolated endogenous cargoes in vitro. Intracellular cargoes are transported along microtubules by teams of kinesin and dynein motors and their cargo-specific adaptor proteins. Observations from living cells show that organelles and vesicular cargoes exhibit diverse motility characteristics. Yet, our knowledge of the molecular mechanisms by which intracellular transport is regulated is not well understood. Here, we describe step-by-step protocols for the extraction of phagosomes from cells at different stages of maturation, and reconstitution of their motility along microtubules in vitro. Quantitative immunofluorescence and photobleaching techniques are also described to measure the number of motors and adaptor proteins on these isolated cargoes. In addition, we describe techniques for tracking the motility of isolated cargoes along microtubules using TIRF microscopy and quantitative force measurements using an optical trap. These methods enable us to study how the sets of motors and adaptors that drive the transport of endogenous cargoes regulate their trafficking in cells.

Force generation of KIF1C is impaired by pathogenic mutations

Intracellular transport is essential for neuronal function and survival. The most effective plus-end-directed neuronal transporter is the kinesin-3 KIF1C, which transports large secretory vesicles and endosomes.1-4 Mutations in KIF1C cause hereditary spastic paraplegia and cerebellar dysfunction in human patients.5-8 In contrast to other kinesin-3s, KIF1C is a stable dimer and a highly processive motor in its native state.9,10 Here, we establish a baseline for the single-molecule mechanics of Kif1C. We show that full-length KIF1C molecules can processively step against the load of an optical trap and reach average stall forces of 3.7 pN. Compared with kinesin-1, KIF1C has a higher propensity to slip backward under load, which results in a lower maximal single-molecule force. However, KIF1C remains attached to the microtubule while slipping backward and re-engages quickly, consistent with its super processivity. Two pathogenic mutations, P176L and R169W, that cause hereditary spastic paraplegia in humans7,8 maintain fast, processive single-molecule motility in vitro but with decreased run length and slightly increased unloaded velocity compared with the wild-type motor. Under load in an optical trap, force generation by these mutants is severely reduced. In cells, the same mutants are impaired in producing sufficient force to efficiently relocate organelles. Our results show how its mechanics supports KIF1C's role as an intracellular transporter and explain how pathogenic mutations at the microtubule-binding interface of KIF1C impair the cellular function of these long-distance transporters and result in neuronal disease.

The force required to remove tubulin from the microtubule lattice by pulling on its α-tubulin C-terminal tail

Severing enzymes and molecular motors extract tubulin from the walls of microtubules by exerting mechanical force on subunits buried in the lattice. However, how much force is needed to remove tubulin from microtubules is not known, nor is the pathway by which subunits are removed. Using a site-specific functionalization method, we applied forces to the C-terminus of α-tubulin with an optical tweezer and found that a force of ~30 pN is required to extract tubulin from the microtubule wall. Additionally, we discovered that partial unfolding is an intermediate step in tubulin removal. The unfolding and extraction forces are similar to those generated by AAA-unfoldases. Lastly, we show that three kinesin-1 motor proteins can also extract tubulin from the microtubule lattice. Our results provide the first experimental investigation of how tubulin responds to mechanical forces exerted on its α-tubulin C-terminal tail and have implications for the mechanisms of severing enzymes and microtubule stability.

Tubulin, the building blocks of microtubules, can be removed from the microtubule wall by mechanical forces. Using single-molecule methods, the authors show that tubulin partially unfolds prior to its removal and determined the tubulin-extraction force.

Motor usage imprints microtubule stability along the shaft

Tubulin dimers assemble into dynamic microtubules, which are used by molecular motors as tracks for intracellular transport. Organization and dynamics of the microtubule network are commonly thought to be regulated at the polymer ends, where tubulin dimers can be added or removed. Here, we show that molecular motors running on microtubules cause exchange of dimers along the shaft in vitro and in cells. These sites of dimer exchange act as rescue sites where depolymerizing microtubules stop shrinking and start re-growing. Consequently, the average length of microtubules increases depending on how frequently they are used as motor tracks. An increase of motor activity densifies the cellular microtubule network and enhances cell polarity. Running motors leave marks in the shaft, serving as traces of microtubule usage to organize the polarity landscape of the cell.

α1A and α1C form microtubules to display distinct properties mainly mediated by their C-terminal tails

Microtubules consisting of α/β-tubulin dimers play critical roles in cells. More than seven genes encode α-tubulin in vertebrates. However, the property of microtubules composed of different α-tubulin isotypes is largely unknown. Here, we purified recombinant tubulin heterodimers of mouse α-tubulin isotypes including α1A and α1C with β-tubulin isotype β2A. In vitro microtubule reconstitution assay detected that α1C/β2A microtubules grew faster and underwent catastrophe less frequently than α1A/β2A microtubules. Generation of chimeric tail-swapped and point-mutation tubulins revealed that the carboxyl-terminal (C-terminal) tails of α-tubulin isotypes largely accounted for the differences in polymerization dynamics of α1A/β2A and α1C/β2A microtubules. Kinetics analysis showed that in comparison to α1A/β2A microtubules, α1C/β2A microtubules displayed higher on-rate, lower off-rate, and similar GTP hydrolysis rate at the plus-end, suggesting a contribution of higher plus-end affinity to faster growth and less frequent catastrophe of α1C/β2A microtubules. Furthermore, EB1 had a higher binding ability to α1C/β2A microtubules than to α1A/β2A ones, which could also be attributed to the difference in the C-terminal tails of these two α-tubulin isotypes. Thus, α-tubulin isotypes diversify microtubule properties, which, to a great extent, could be accounted by their C-terminal tails.

Pathogenic mutations in the kinesin-3 motor KIF1A diminish force generation and movement through allosteric mechanisms

The kinesin-3 motor KIF1A functions in neurons, where its fast and superprocessive motility facilitates long-distance transport, but little is known about its force-generating properties. Using optical tweezers, we demonstrate that KIF1A stalls at an opposing load of ~3 pN but more frequently detaches at lower forces. KIF1A rapidly reattaches to the microtubule to resume motion due to its class-specific K-loop, resulting in a unique clustering of force generation events. To test the importance of neck linker docking in KIF1A force generation, we introduced mutations linked to human neurodevelopmental disorders. Molecular dynamics simulations predict that V8M and Y89D mutations impair neck linker docking. Indeed, both mutations dramatically reduce the force generation of KIF1A but not the motor’s ability to rapidly reattach to the microtubule. Although both mutations relieve autoinhibition of the full-length motor, the mutant motors display decreased velocities, run lengths, and landing rates and delayed cargo transport in cells. These results advance our understanding of how mutations in KIF1A can manifest in disease.

Kinesin-Recruiting Microtubules Exhibit Collective Gliding Motion while Forming Motor Trails

Microtubules gliding on surfaces coated with kinesin motors are minimalist experimental systems for studying collective behavior. Collective behavior in these systems arises from interactions between filaments, for example, from steric interactions, depletion forces, or cross-links. To maximize the utilization of system components and the production of work, it is desirable to achieve mutualistic interactions leading to the congregations of both types of agents, that is, cytoskeletal filaments and molecular motors. To this end, we used a microtubule-kinesin system, where motors reversibly bind to the surface via an interaction between a hexahistidine (His₆) tag on the motor and a Ni(II)-nitrilotriacetic acid (Ni-NTA) moiety on the surface. The surface density of binding sites for kinesin motors was increased relative to our earlier work, driving the motors from the solution to the surface. Characterization of the motor-surface interactions in the absence of microtubules yielded kinetic parameters consistent with previous data and revealed the capacity of the surface to support two-dimensional motor diffusion. The motor density gradually fell over 2 h, presumably due to the stripping of Ni(II) from the NTA moieties on the surface. Microtubules gliding on these reversibly bound motors were unable to cross each other and at high enough densities began to align and form long, dense bundles. The kinesin motors accumulated in trails surrounding the microtubule bundles and participated in microtubule transport.

Identification of key residues that regulate the interaction of kinesins with microtubule ends

Kinesins are molecular motors that use energy derived from ATP turnover to walk along microtubules, or when at the microtubule end, regulate growth or shrinkage. All kinesins that regulate microtubule dynamics have long residence times at microtubule ends, whereas those that only walk have short end‐residence times. Here, we identify key amino acids involved in end binding by showing that when critical residues from Kinesin‐13, which depolymerises microtubules, are introduced into Kinesin‐1, a walking kinesin with no effect on microtubule dynamics, the end‐residence time is increased up to several‐fold. This indicates that the interface between the kinesin motor domain and the microtubule is malleable and can be tuned to favour either lattice or end binding.

Multivalent electrostatic microtubule interactions of synthetic peptides are sufficient to mimic advanced MAP-like behavior

Microtubule-associated proteins (MAPs) are a functionally highly diverse class of proteins that help to adjust the shape and function of the microtubule cytoskeleton in space and time. For this purpose, MAPs structurally support microtubules, modulate their dynamic instability, or regulate the activity of associated molecular motors. The microtubule-binding domains of MAPs are structurally divergent, but often depend on electrostatic interactions with the negatively charged surface of the microtubule. This suggests that the surface exposure of positive charges rather than a certain structural fold is sufficient for a protein to associate with microtubules. Consistently, positively charged artificial objects have been shown to associate with microtubules and to diffuse along their lattice. Natural MAPs, however, show a more sophisticated functionality beyond lattice-diffusion. Here, we asked whether basic electrostatic interactions are sufficient to also support advanced MAP functionality. To test this hypothesis, we studied simple positively charged peptide sequences for the occurrence of typical MAP-like behavior. We found that a multivalent peptide construct featuring four lysine-alanine heptarepeats (starPEG-(KA7)₄)-but not its monovalent KA7-subunits-show advanced, biologically relevant MAP-like behavior: starPEG-(KA7)₄ binds microtubules in the low nanomolar range, diffuses along their lattice with the ability to switch between intersecting microtubules, and tracks depolymerizing microtubule ends. Further, starPEG-(KA7)₄ promotes microtubule nucleation and growth, mediates depolymerization coupled pulling at plus ends, and bundles microtubules without significantly interfering with other proteins on the microtubule lattice (as exemplified by the motor kinesin-1). Our results show that positive charges and multivalency are sufficient to mimic advanced MAP-like behavior.

Hook3 is a scaffold for the opposite-polarity microtubule-based motors cytoplasmic dynein-1 and KIF1C

The unidirectional and opposite-polarity microtubule-based motors, dynein and kinesin, drive long-distance intracellular cargo transport. Cellular observations suggest that opposite-polarity motors may be coupled. We recently identified an interaction between the cytoplasmic dynein-1 activating adaptor Hook3 and the kinesin-3 KIF1C. Here, using in vitro reconstitutions with purified components, we show that KIF1C and dynein/dynactin can exist in a complex scaffolded by Hook3. Full-length Hook3 binds to and activates dynein/dynactin motility. Hook3 also binds to a short region in the "tail" of KIF1C, but unlike dynein/dynactin, this interaction does not activate KIF1C. Hook3 scaffolding allows dynein to transport KIF1C toward the microtubule minus end, and KIF1C to transport dynein toward the microtubule plus end. In cells, KIF1C can recruit Hook3 to the cell periphery, although the cellular role of the complex containing both motors remains unknown. We propose that Hook3's ability to scaffold dynein/dynactin and KIF1C may regulate bidirectional motility, promote motor recycling, or sequester the pool of available dynein/dynactin activating adaptors.

Three-Dimensional Optical Tweezers Tracking Resolves Random Sideward Steps of the Kinesin-8 Kip3

The budding yeast kinesin-8 Kip3 is a highly processive motor protein that walks to the ends of cytoskeletal microtubules and shortens them in a collective manner. However, how exactly Kip3 reaches the microtubule end is unclear. Although rotations of microtubules in multimotored Kip3 gliding assays implied directed sideward switching between microtubule protofilaments, two-dimensional, single-molecule, optical-tweezers assays indicated that Kip3 randomly switched protofilaments. Here, we topographically suspended microtubules such that Kip3 motors could freely access the microtubules in three dimensions. Tracking single-motor-driven microspheres with a three-dimensional, zero-load, optical-tweezers-based force clamp showed that Kip3 switched protofilaments in discrete steps equally frequent in both directions. A statistical analysis confirmed the diffusive sideward motion of Kip3, consistent with the two-dimensional single-molecule results. Furthermore, we found that motors were in one of three states: either not switching protofilaments or switching between them with a slow or fast sideward-stepping rate. Interestingly, this sideward diffusion was limited to one turn, suggesting that motors could not cross the microtubule seam. The diffusive protofilament switching may enable Kip3 to efficiently bypass obstacles and reach the microtubule end for length regulation.

Reversibly Bound Kinesin-1 Motor Proteins Propelling Microtubules Demonstrate Dynamic Recruitment of Active Building Blocks

Biological materials and systems often dynamically self-assemble and disassemble, forming temporary structures as needed and allowing for dynamic responses to stimuli and changing environmental conditions. However, this dynamic interplay of localized component recruitment and release has been difficult to achieve in artificial molecular-scale systems, which are usually designed to have long-lasting, stable bonds. Here, we report the experimental realization of a molecular-scale system that dynamically assembles and disassembles its building blocks while retaining functionality. In our system, filaments (microtubules) recruit biomolecular motors (kinesins) to a surface engineered to allow for the reversible binding of the kinesin-1 motors. These recruited motors work to propel the cytoskeletal filaments along the surface. After the microtubules leave the motors behind, the trail of motors disassembles, releasing the motors back into solution. Engineering such dynamic systems may allow us to create materials that mimic the way in which biological systems achieve self-healing and adaptation.

Challenges in Estimating the Motility Parameters of Single Processive Motor Proteins

Cytoskeletal motor proteins are essential to the function of a wide range of intracellular mechano-systems. The biophysical characterization of their movement along their filamentous tracks is therefore of large importance. Toward this end, single-molecule, in vitro stepping-motility assays are commonly used to determine motor velocity and run length. However, comparing results from such experiments has proved difficult due to influences from variations in the experimental conditions and the data analysis methods. Here, we investigate the movement of fluorescently labeled, processive, dimeric motor proteins and propose a unified algorithm to correct the measurements for finite filament length as well as photobleaching. Particular emphasis is put on estimating the statistical errors associated with the proposed evaluation method, as knowledge of these values is crucial when comparing measurements from different experiments. Testing our approach with simulated and experimental data from GFP-labeled kinesin-1 motors stepping along immobilized microtubules, we show 1) that velocity distributions should be fitted by a t location-scale probability density function rather than by a normal distribution; 2) that the impossibility to measure events shorter than the image acquisition time needs to be taken into account; 3) that the interaction time and run length of the motors can be estimated independent of the filament length distribution; and 4) that the dimeric nature of the motors needs to be considered when correcting for photobleaching. Moreover, our analysis reveals that controlling the temperature during the experiments with a precision below 1 K is of importance. We believe our method will not only improve the evaluation of experimental data, but also allow for better statistical comparisons between different populations of motor proteins (e.g., with distinct mutations or linked to different cargos) and filaments (e.g., in distinct nucleotide states or with different posttranslational modifications). Therefore, we include a detailed workflow for image processing and analysis (including MATLAB code), serving as a tutorial for the estimation of motility parameters in stepping-motility assays.

Intracellular Cargo Transport by Kinesin-3 Motors

Intracellular transport along microtubules enables cellular cargoes to efficiently reach the extremities of large, eukaryotic cells. While it would take more than 200 years for a small vesicle to diffuse from the cell body to the growing tip of a one-meter long axon, transport by a kinesin allows delivery in one week. It is clear from this example that the evolution of intracellular transport was tightly linked to the development of complex and macroscopic life forms. The human genome encodes 45 kinesins, 8 of those belonging to the family of kinesin-3 organelle transporters that are known to transport a variety of cargoes towards the plus end of microtubules. However, their mode of action, their tertiary structure, and regulation are controversial. In this review, we summarize the latest developments in our understanding of these fascinating molecular motors.

Dynactin has two antagonistic regulatory domains and exerts opposing effects on dynein motility

Dynactin is a dynein-regulating protein that increases the processivity of dynein movement on microtubules. Recent studies have shown that a tripartite complex of dynein–dynactin–Bicaudal D2 is essential for highly processive movement. To elucidate the regulation of dynein motility by dynactin, we focused on two isoforms (A and B) of dynactin 1 (DCTN1), the largest subunit of dynactin that contains both microtubule- and dynein-binding domains. The only difference between the primary structures of the two isoforms is that DCTN1B lacks the K-rich domain, a cluster of basic residues. We measured dynein motility by single molecule observation of recombinant dynein and dynactin. Whereas the tripartite complex containing DCTN1A exhibited highly processive movement, the complex containing DCTN1B dissociated from microtubules with no apparent processive movement. This inhibitory effect of DCTN1B was caused by reductions of the microtubule-binding affinities of both dynein and dynactin, which was attributed to the coiled-coil 1 domain of DCTN1. In DCTN1A, the K-rich domain antagonized these inhibitory effects. Therefore, dynactin has two antagonistic domains and promotes or suppresses dynein motility to accomplish correct localization and functions of dynein within a cell.

Transport efficiency of membrane-anchored kinesin-1 motors depends on motor density and diffusivity

In eukaryotic cells, membranous vesicles and organelles are transported by ensembles of motor proteins. These motors, such as kinesin-1, have been well characterized in vitro as single molecules or as ensembles rigidly attached to nonbiological substrates. However, the collective transport by membrane-anchored motors, that is, motors attached to a fluid lipid bilayer, is poorly understood. Here, we investigate the influence of motors' anchorage to a lipid bilayer on the collective transport characteristics. We reconstituted "membrane-anchored" gliding motility assays using truncated kinesin-1 motors with a streptavidin-binding peptide tag that can attach to streptavidin-loaded, supported lipid bilayers. We found that the diffusing kinesin-1 motors propelled the microtubules in the presence of ATP. Notably, we found the gliding velocity of the microtubules to be strongly dependent on the number of motors and their diffusivity in the lipid bilayer. The microtubule gliding velocity increased with increasing motor density and membrane viscosity, reaching up to the stepping velocity of single motors. This finding is in contrast to conventional gliding motility assays where the density of surface-immobilized kinesin-1 motors does not influence the microtubule velocity over a wide range. We reason that the transport efficiency of membrane-anchored motors is reduced because of their slippage in the lipid bilayer, an effect that we directly observed using single-molecule fluorescence microscopy. Our results illustrate the importance of motor-cargo coupling, which potentially provides cells with an additional means of regulating the efficiency of cargo transport.

Kinesin-1 motors can circumvent permanent roadblocks by side-shifting to neighboring protofilaments

Obstacles on the surface of microtubules can lead to defective cargo transport, proposed to play a role in neurological diseases such as Alzheimer's. However, little is known about how motor proteins, which follow individual microtubule protofilaments (such as kinesin-1), deal with obstacles on the molecular level. Here, we used rigor-binding mutants of kinesin-1 as roadblocks to permanently obstruct individual microtubule binding sites and studied the movement of individual kinesin-1 motors by single-molecule fluorescence and dark-field scattering microscopy in vitro. In the presence of roadblocks, kinesin-1 often stopped for ∼ 0.4 s before either detaching or continuing to move, whereby the latter circumvention events occurred in >30% after a stopping event. Consequently, and in agreement with numerical simulations, the mean velocity, mean run length, and mean dwell time of the kinesin-1 motors decreased upon increasing the roadblock density. Tracking individual kinesin-1 motors labeled by 40 nm gold particles with 6 nm spatial and 1 ms temporal precision revealed that ∼ 70% of the circumvention events were associated with significant transverse shifts perpendicular to the axis of the microtubule. These side-shifts, which occurred with equal likelihood to the left and right, were accompanied by a range of longitudinal shifts suggesting that roadblock circumvention involves the unbinding and rebinding of the motors. Thus, processive motors, which commonly follow individual protofilaments in the absence of obstacles, appear to possess intrinsic circumvention mechanisms. These mechanisms were potentially optimized by evolution for the motor's specific intracellular tasks and environments.

The Kinesin-8 Kip3 switches protofilaments in a sideward random walk asymmetrically biased by force

Molecular motors translocate along cytoskeletal filaments, as in the case of kinesin motors on microtubules. Although conventional kinesin-1 tracks a single microtubule protofilament, other kinesins, akin to dyneins, switch protofilaments. However, the molecular trajectory-whether protofilament switching occurs in a directed or stochastic manner-is unclear. Here, we used high-resolution optical tweezers to track the path of single budding yeast kinesin-8, Kip3, motor proteins. Under applied sideward loads, we found that individual motors stepped sideward in both directions, with and against loads, with a broad distribution in measured step sizes. Interestingly, the force response depended on the direction. Based on a statistical analysis and simulations accounting for the geometry, we propose a diffusive sideward stepping motion of Kip3 on the microtubule lattice, asymmetrically biased by force. This finding is consistent with previous multimotor gliding assays and sheds light on the molecular switching mechanism. For kinesin-8, the diffusive switching mechanism may enable the motor to bypass obstacles and reach the microtubule end for length regulation. For other motors, such a mechanism may have implications for torque generation around the filament axis.

Small Crowders Slow Down Kinesin-1 Stepping by Hindering Motor Domain Diffusion

The dimeric motor protein kinesin-1 moves processively along microtubules against forces of up to 7 pN. However, the mechanism of force generation is still debated. Here, we point to the crucial importance of diffusion of the tethered motor domain for the stepping of kinesin-1: small crowders stop the motor at a viscosity of 5 mPa·s-corresponding to a hydrodynamic load in the sub-fN (~10^{-4} pN) range-whereas large crowders have no impact even at viscosities above 100 mPa·s. This indicates that the scale-dependent, effective viscosity experienced by the tethered motor domain is a key factor determining kinesin's functionality. Our results emphasize the role of diffusion in the kinesin-1 stepping mechanism and the general importance of the viscosity scaling paradigm in nanomechanics.

Doublecortin recognizes the longitudinal curvature of the microtubule end and lattice

BACKGROUND

Microtubule ends have distinct biochemical and structural features from those of the lattice. Several proteins that control microtubule behavior can distinguish the end of a microtubule from the lattice. The end-binding protein EB1, for example, recognizes the nucleotide state of microtubule ends, which are enriched in GTP-tubulin. EB1 shares its binding site with Doublecortin (DCX), a protein expressed in developing neurons. We showed recently that DCX binds with highest affinity to microtubule ends.

RESULTS

Here we show that DCX recognizes microtubule ends by a novel mechanism based on lattice curvature. Using single-molecule microscopy, we show that DCX "comets" do not elongate at faster microtubule growth rates and DCX does not recognize two out of three GTP analogs. We demonstrate that DCX binds with higher affinity to curved microtubule lattices than to straight ones. We find that curvature recognition is a property of single DCX molecules. Straightening of protofilaments (pfs) at microtubule ends with paclitaxel significantly attenuates end-recognition by DCX, but not EB1. Mutations in DCX found in patients with double cortex syndrome disrupted curvature recognition.

CONCLUSIONS

We propose a model in which DCX recognizes microtubule ends through specific interactions with their structure. We conclude that microtubule ends have two distinct features that proteins can recognize independently, namely a structural feature related to curvature and nucleotide state.

Directional persistence of migrating cells requires Kif1C-mediated stabilization of trailing adhesions

Directional cell migration requires the establishment and maintenance of long-term differences in structure and function between the front and back of a cell. Here, we show that the microtubule motor Kif1C contributes to persistent cell migration primarily through stabilization of an extended cell rear. Kif1C-mediated transport of α5β1-integrins is required for the proper maturation of trailing focal adhesions and resistance to tail retraction. Tail retraction precedes and induces changes in migration direction. Stabilization of cell tails through inhibition of myosin II activity suppresses the Kif1C depletion phenotype and results in longer-lived tails and higher directional stability of migrating cells. Taken together, these findings indicate that the maintenance of an extended, tense cell tail facilitates directional migration. We propose a rear drag mechanism for directional persistence of migration whereby the counterforce originating from a well-anchored tail serves to maintain directionality of the force-generating leading edge of the cell.

Using a quartz paraboloid for versatile wide-field TIR microscopy with sub-nanometer localization accuracy

Illumination based on objective-type total internal reflection (TIR) is nowadays widely used in high-performance fluorescence microscopy. However, the desirable application of such setups for dark-field imaging of scattering entities is cumbersome due to the spatial overlap of illumination and detection light, which cannot be separated spectrally. Here, we report a novel TIR approach based on a parabolically shaped quartz prism that allows for the detection of single-molecule fluorescence as well as single-particle scattering with high signal-to-noise ratios. We demonstrate homogeneous and spatially invariant illumination profiles in combination with a convenient control over a wide range of illumination angles. Moreover, we quantitatively compare the fluorescence performance of our setup to objective-type TIR and demonstrate sub-nanometer localization accuracies for the scattering of 40 nm gold nanoparticles (AuNPs). When bound to individual kinesin-1 motors, the AuNPs reliably report on the characteristic 8 nm stepping along microtubules.

Tubulin acetylation alone does not affect kinesin-1 velocity and run length in vitro

Kinesin-1 plays a major role in anterograde transport of intracellular cargo along microtubules. Currently, there is an ongoing debate of whether α-tubulin K40 acetylation directly enhances the velocity of kinesin-1 and its affinity to the microtubule track. We compared motor motility on microtubules reconstituted from acetylated and deacetylated tubulin. For both, single- and multi-motor in vitro motility assays, we demonstrate that tubulin acetylation alone does not affect kinesin-1 velocity and run length.

Doublecortin recognizes the 13-protofilament microtubule cooperatively and tracks microtubule ends

Neurons, like all cells, face the problem that tubulin forms microtubules with too many or too few protofilaments (pfs). Cells overcome this heterogeneity with the γ-tubulin ring complex, which provides a nucleation template for 13-pf microtubules. Doublecortin (DCX), a protein that stabilizes microtubules in developing neurons, also nucleates 13-pf microtubules in vitro. Using fluorescence microscopy assays, we show that the binding of DCX to microtubules is optimized for the lateral curvature of the 13-pf lattice. This sensitivity depends on a cooperative interaction wherein DCX molecules decrease the dissociation rate of their neighbors. Mutations in DCX found in patients with subcortical band heterotopia weaken these cooperative interactions. Using assays with dynamic microtubules, we discovered that DCX binds to polymerization intermediates at growing microtubule ends. These results support a mechanism for stabilizing 13-pf microtubules that allows DCX to template new 13-pf microtubules through associations with the sides of the microtubule lattice.

Long-range transport of giant vesicles along microtubule networks

We report on a minimal system to mimic intracellular transport of membrane-bounded, vesicular cargo. In a cell-free assay, purified kinesin-1 motor proteins were directly anchored to the membrane of giant unilamellar vesicles, and their movement studied along two-dimensional microtubule networks. Motion-tracking of vesicles with diameters of 1-3 μm revealed traveling distances up to the millimeter range. The transport velocities were identical to velocities of cargo-free motors. Using total internal reflection fluorescence (TIRF) microscopy, we were able to estimate the number of GFP-labeled motors involved in the transport of a single vesicle. We found that the vesicles were transported by the cooperative activity of typically 5-10 motor molecules. The presented assay is expected to open up further applications in the field of synthetic biology, aiming at the in vitro reconstitution of sub-cellular multi-motor transport systems. It may also find applications in bionanotechnology, where the controlled long-range transport of artificial cargo is a promising means to advance current lab-on-a-chip systems.

Tracking single particles and elongated filaments with nanometer precision

Recent developments in image processing have greatly advanced our understanding of biomolecular processes in vitro and in vivo. In particular, using Gaussian models to fit the intensity profiles of nanometer-sized objects have enabled their two-dimensional localization with a precision in the one-nanometer range. Here, we present an algorithm to precisely localize curved filaments whose structures are characterized by subresolution diameters and micrometer lengths. Using surface-immobilized microtubules, fluorescently labeled with rhodamine, we demonstrate positional precisions of ∼2 nm when determining the filament centerline and ∼9 nm when localizing the filament tips. Combined with state-of-the-art single particle tracking we apply the algorithm 1), to motor-proteins stepping on immobilized microtubules, 2), to depolymerizing microtubules, and 3), to microtubules gliding over motor-coated surfaces.

Kinesin assembly and movement in cells

Long-distance transport in eukaryotic cells is driven by molecular motors that move along microtubule tracks. Molecular motors of the kinesin superfamily contain a kinesin motor domain attached to family-specific sequences for cargo binding, regulation, and oligomerization. The biochemical and biophysical properties of the kinesin motor domain have been widely studied, yet little is known about how kinesin motors work in the complex cellular environment. We discuss recent studies on the three major families involved in intracellular transport (kinesin-1, kinesin-2, and kinesin-3) that have begun to bridge the gap in knowledge between the in vitro and in vivo behaviors of kinesin motors. These studies have increased our understanding of how kinesin subunits assemble to produce a functional motor, how kinesin motors are affected by biochemical cues and obstacles present on cellular microtubules, and how multiple motors on a cargo surface can work collectively for increased force production and travel distance.

Fluorescence imaging of single Kinesin motors on immobilized microtubules

Recent developments in optical microscopy and nanometer tracking have greatly improved our understanding of cytoskeletal motor proteins. Using fluorescence microscopy, dynamic interactions are now routinely observed in vitro on the level of single molecules mainly using a geometry, where fluorescently labeled motors move on surface-immobilized filaments. In this chapter, we review recent methods related to single-molecule kinesin motility assays. In particular, we aim to provide practical advice on: how to set up the assays, how to acquire high-precision data from fluorescently labeled kinesin motors and attached quantum dots, and how to analyze data by nanometer tracking.

Properties of the kinesin-3 NcKin3 motor domain and implications for neck function

Members of the Kinesin-3 family are microtubule motors involved in the transport of membranous cargo. NcKin3 from the fungus Neurospora crassa is dimeric but inactivates one of its motor heads to generate nonprocessive motility. To determine how one of the heads is inactivated, we investigated truncated monomeric constructs. None of the constructs generated processive single-molecule motility, and multimotor velocities depended linearly on the number of residues remaining in the neck. The kinetic analysis suggests futile ATP hydrolysis cycles, because a representative monomer showed a faster ATP turnover than the dimer while supporting slower motility. The K(0.5,MT) was 70-fold lower, the microtubule-bound portion of the kinetic cycle eight-fold longer and the microtubule detachment rate almost 15-fold slower than that of the dimer. Moreover, the monomer's microtubule-dependent ADP release occurred three-fold to four-fold faster than k(cat) (125 versus 34 s(-1)), whereas phosphate release was approximately equally fast (29 s(-1)). A dimeric construct containing a structure-breaking insert between motor head and neck showed a similar behaviour. These data suggest that the heads of the wild-type NcKin3 motor are strictly coupled via the neck domain, and that the dimeric structure is required for proper detachment after one ATPase cycle. This is the first direct comparison of a monomeric Kinesin-3 with its dimeric full-length counterpart, and the kinetic changes observed here may also apply to other Kinesin-3 motors.

Diffusion and directed movement: in vitro motile properties of fission yeast kinesin-14 Pkl1

Fission yeast Pkl1 is a kinesin-14A family member that is known to be localized at the cellular spindle and is capable of hydrolyzing ATP. However, its motility has not been detected. Here, we show that Pkl1 is a slow, minus end-directed microtubule motor with a maximum velocity of 33+/-9 nm/s. The Km,MT value of steady-state ATPase activity of Pkl1 was as low as 6.4+/-1.1 nM, which is 20-30 times smaller than that of kinesin-1 and another kinesin-14A family member, Ncd, indicating a high affinity of Pkl1 for microtubules. However, the duty ratio of 0.05 indicates that Pkl1 spends only a small fraction of the ATPase cycle strongly associated with a microtubule. By using total internal reflection fluorescence microscopy, we demonstrated that single molecules of Pkl1 were not highly processive but only exhibited biased one-dimensional diffusion along microtubules, whereas several molecules of Pkl1, probably fewer than 10 molecules, cooperatively moved along microtubules and substantially reduced the diffusive component in the movement. Our results suggest that Pkl1 molecules work in groups to move and generate forces in a cooperative manner for their mitotic functions.

Setting up roadblocks for kinesin-1: mechanism for the selective speed control of cargo carrying microtubules

Motor-driven cytoskeletal filaments are versatile transport platforms for nanosized cargo in molecular sorting and nano-assembly devices. However, because cargo and motors share the filament lattice as a common substrate for their activity, it is important to understand the influence of cargo-loading on transport properties. By performing single-molecule stepping assays on biotinylated microtubules we found that individual kinesin-1 motors frequently stopped upon encounters with attached streptavidin molecules. Consequently, we attribute the deceleration of cargo-laden microtubules in gliding assays to an obstruction of kinesin-1 paths on the microtubule lattice rather than to 'frictional' cargo-surface interactions. We propose to apply this obstacle-caused slow-down of gliding microtubules in a novel molecular detection scheme: Using a mixture of two distinct microtubule populations that each bind a different kind of protein, the presence of these proteins can be detected via speed changes in the respective microtubule populations.

The mitotic kinesin CENP-E is a processive transport motor

In vivo studies suggest that centromeric protein E (CENP-E), a kinesin-7 family member, plays a key role in the movement of chromosomes toward the metaphase plate during mitosis. How CENP-E accomplishes this crucial task, however, is not clear. Here we present single-molecule measurements of CENP-E that demonstrate that this motor moves processively toward the plus end of microtubules, with an average run length of 2.6 +/- 0.2 mum, in a hand-over-hand fashion, taking 8-nm steps with a stall force of 6 +/- 0.1 pN. The ATP dependence of motor velocity obeys Michaelis-Menten kinetics with K(M,ATP) = 35 +/- 5 muM. All of these features are remarkably similar to those for kinesin-1-a highly processive transport motor. We, therefore, propose that CENP-E transports chromosomes in a manner analogous to how kinesin-1 transports cytoplasmic vesicles.

Subunits interactions in kinesin motors

Kinesins form a large and diverse superfamily of proteins involved in numerous important cellular processes. The majority of them are molecular motors moving along microtubules. Conversion of chemical energy into mechanical work is accomplished in a sequence of events involving both biochemical and conformational alternation of the motor structure called the mechanochemical cycle. Different members of the kinesin superfamily can either perform their function in large groups or act as single molecules. Conventional kinesin, a member of the kinesin-1 subfamily, exemplifies the second type of motor which requires tight coordination of the mechanochemical cycle in two identical subunits to accomplish processive movement toward the microtubule plus end. Recent results strongly support an asymmetric hand-over-hand model of "walking" for this protein. Conformational strain between two subunits at the stage of the cycle where both heads are attached to the microtubule seems to be a major factor in intersubunit coordination, although molecular and kinetic details of this phenomenon are not yet deciphered. We discuss also current knowledge concerning intersubunit coordination in other kinesin subfamilies. Members of the kinesin-3 class use at least three different mechanisms of movement and can translocate in monomeric or dimeric forms. It is not known to what extent intersubunit coordination takes place in Ncd, a dimeric member of the kinesin-14 subfamily which, unlike conventional kinesin, exercises a power-stroke toward the microtubule minus end. Eg5, a member of the kinesin-5 subfamily is a homotetrameric protein with two kinesin-1-like dimeric halves controlled by their relative orientation on two microtubules. It seems that diversity of subunit organization, quaternary structures and cellular functions in the kinesin superfamily are reflected also by the divergent extent and mechanism of intersubunit coordination during kinesin movement along microtubules.

Analysis of the weak interactions of ADP-Unc104 and ADP-kinesin with microtubules and their inhibition by MAP2c

Microtubule based motors like conventional kinesin (Kinesin-1) and Unc104 (Kinesin-3), and classical microtubule associated proteins (MAPs), including MAP2, are intimately involved in neurite formation and organelle transport. The processive motility of both these kinesins involves weak microtubule interactions in the ADP-bound states. Using cosedimentation assays, we have investigated these weak interactions and characterized their inhibition by MAP2c. We show that Unc104 binds microtubules with five-fold weaker affinity and two-fold higher stoichiometry compared with conventional kinesin. Unc104 and conventional kinesin binding affinities are primarily dependent on positively charged residues in the Unc104 K-loop and conventional kinesin neck coiled-coil and removal of these residues affects Unc104 and conventional kinesin differently. We observed that MAP2c acts primarily as a competitive inhibitor of Unc104 but a mixed inhibitor of conventional kinesin. Our data suggest a specific model in which MAP2c differentially interferes with each kinesin motor by inhibiting its weak attachment to the tubulin C-termini. This is reminiscent of the defects we have observed in Unc104 and kinesin mutants in which the positively charged residues in K-loop and neck coiled-coil domains were removed.

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.

Kinetic and mechanistic basis of the nonprocessive Kinesin-3 motor NcKin3

Kinesin-3 motors have been shown to transport cellular cargo along microtubules and to function according to mechanisms that differ from the conventional hand-over-hand mechanism. To find out whether the mechanisms described for Kif1A and CeUnc104 cover the full spectrum of Kinesin-3 motors, we characterize here NcKin3, a novel member of the Kinesin-3 family that localizes to mitochondria of ascomycetes. We show that NcKin3 does not move in a K-loop-dependent way as Kif1A or in a cluster-dependent way as CeUnc104. Its in vitro gliding velocity ranges between 0.30 and 0.64 mum/s and correlates positively with motor density. The processivity index (k(bi,ratio)) of approximately 3 reveals that not more than three ATP molecules are hydrolyzed per productive microtubule encounter. The NcKin3 duty ratio of 0.03 indicates that the motor spends only a minute fraction of the ATPase cycle attached to the filament. Unlike other Kinesin-3 family members, NcKin3 forms stable dimers, but only one subunit releases ADP in a microtubule-dependent fashion. Together, these data exclude a processive hand-over-hand mechanism of movement and suggest a power-stroke mechanism where nucleotide-dependent structural changes in a single motor domain lead to displacement of the motor along the filament. Thus, NcKin3 is the first plus end-directed kinesin motor that is dimeric but moves in a nonprocessive fashion to its destination.

The novel protein KBP regulates mitochondria localization by interaction with a kinesin-like protein

Background

Members of the Kinesin-3 family of kinesin-like proteins mediate transport of axonal vesicles (KIF1A, KIF1Bβ), distribution of mitochondria (KIF1Bα) and anterograde Golgi to ER vesicle transport (KIF1C). Until now, little is known about the regulation of kinesin-like proteins. Several proteins interact with members of this protein family. Here we report on a novel, KIF1 binding protein (KBP) that was identified in yeast two-hybrid screens.

Results

KBP was identified by using the yeast-two-hybrid system with an amino-terminal fragment of KIF1C as a bait that is strongly homologous to KIF1B. Here we investigated the interaction of KBP and KIF1B. The full length proteins coimmunoprecipitated after overexpression and in untransfected 293 cells. Immunofluorescence experiments revealed that KBP was mainly localized to mitochondria, as has been described for KIF1Bα. Overexpression of a deletion mutant or reduction of the KBP protein level using an anti-sense construct led to an aggregation of mitochondria. Such an effect is probably due to the lower activity of KIF1Bα in the absence of KBP, as was revealed in motility assays.

Conclusion

KBP is a new binding partner for KIF1Bα that is a regulator of its transport function and thus represents a new type of kinesin interacting protein.

Fractionation and characterization of kinesin II species in vertebrate brain

Recent research on kinesin motors has outlined the diversity of the superfamily and defined specific cargoes moved by kinesin family (KIF) members. Owing to the difficulty of purifying large amounts of native motors, much of this work has relied on recombinant proteins expressed in vitro. This approach does not allow ready determination of the complement of kinesin motors present in a given tissue, the relative amounts of different motors, or comparison of their native activities. To address these questions, we isolated nucleotide-dependent, microtubule-binding proteins from 13-day chick embryo brain. Proteins were enriched by microtubule affinity purification, then subjected to velocity sedimentation to separate the 20S dynein/dynactin pool from a slower sedimenting KIF containing pool. Analysis of the latter pool by anion exchange chromatography revealed three KIF species: kinesin I (KIF5), kinesin II (KIF3), and KIF1C (Unc104/KIF1). The most abundant species, kinesin I, exhibited the expected long range microtubule gliding activity. By contrast, KIF1C did not move microtubules. Kinesin II, the second most abundant KIF, could be fractionated into two pools, one containing predominantly A/B isoforms and the other containing A/C isoforms. The two motor species had similar activities, powering microtubule gliding at slower speeds and over shorter distances than kinesin I.

The lipid binding pleckstrin homology domain in UNC-104 kinesin is necessary for synaptic vesicle transport in Caenorhabditis elegans

UNC-104 (KIF1A) is a kinesin motor that transports synaptic vesicles from the neuronal cell body to the terminal. Previous in vitro studies have shown that a Dictyostelium relative of UNC-104 transports liposomes containing acidic phospholipids, but whether this interaction is needed for the recognition and transport of synaptic vesicles in metazoans remains unexplored. Here, we have introduced mutations in the nonmotor domain of UNC-104 and examined whether these mutant motors can rescue an unc-104 Caenorhabditis elegans strain. We show that a pleckstrin homology (PH) domain in UNC-104 is essential for membrane transport in living C. elegans, that this PH domain binds specifically to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)), and that point mutants in the PH domain that interfere with PI(4,5)P(2) binding in vitro also interfere with UNC-104 function in vivo. Several other lipid-binding modules could not effectively substitute for the UNC-104 PH domain in this in vivo assay. Real time imaging also revealed that a lipid-binding point mutation in the PH domain reduced movement velocity and processivity of individual UNC-104::GFP punctae in neurites. These results reveal a critical role for PI(4,5)P(2) binding in UNC-104-mediated axonal transport and shows that the cargo-binding properties of the distal PH domain can affect motor output.

Regulation of KinI kinesin ATPase activity by binding to the microtubule lattice

KinI kinesins are important in regulating the complex dynamics of the microtubule cytoskeleton. They are unusual in that they depolymerize, rather than move along microtubules. To determine the attributes of KinIs that distinguish them from translocating kinesins, we examined the ATPase activity, microtubule affinity, and three-dimensional microtubule-bound structure of a minimal KinI motor domain. Together, the kinetic, affinity, and structural data lead to the conclusion that on binding to the microtubule lattice, KinIs release ADP and enter a stable, low-affinity, regulated state, from which they do not readily progress through the ATPase cycle. This state may favor detachment, or diffusion of the KinI to its site of action, the microtubule ends. Unlike conventional translocating kinesins, which are microtubule lattice–stimulated ATPases, it seems that with KinIs, nucleotide-mediated modulation of tubulin affinity is only possible when it is coupled to protofilament deformation. This provides an elegant mechanistic basis for their unique depolymerizing activity.

What kinesin does at roadblocks: the coordination mechanism for molecular walking

Competing models for the coordination of processive stepping in kinesin can be tested by introducing a roadblock to prevent lead head attachment. We used T93N, an irreversibly binding mutant monomer, as a roadblock, and measured the rates of nucleotide-induced detachment of kinesin monomers or dimers with and without the T93N roadblock using microflash photolysis combined with stopped flow. Control nucleotide-induced monomer (rK340) unbinding was 73.6 s(-1) for ATP and 40.5 s(-1) for ADP. Control ADP-induced dimer (rK430) unbinding was 18.6 s(-1). Added 20 mM Pi slowed both monomer and dimer unbinding. With the roadblock in place, lead head attachment of dimers is prevented and ATP-induced trail head unbinding was then 42 s(-1). This is less than two-fold slower than the stepping rate of unimpeded rK430 dimers (50-70 s(-1)), indicating that during walking, lead head attachment induces at most only a slight (less than two-fold) acceleration of trail head detachment. As we discuss, this implies a coordination model having very fast (>2000 s(-1)) ATP-induced attachment of the lead head, followed by slower, strain-sensitive ADP release from the lead head.

The kinesin family member BimC contains a second microtubule binding region attached to the N terminus of the motor domain

The kinesin family member BimC has a highly positively charged domain of approximately 70 amino acids at the N terminus of the motor domain. Motor domain constructs of BimC were prepared with and without this extra domain to determine its influence. The level of microtubules needed for half saturation of the ATPase of BimC motor domain constructs is reduced by approximately 7000-fold at low ionic strength upon addition of this extra N-terminal extension. Although the change in microtubule affinity is less at higher salt, addition of the N-terminal domain still produces a 20-fold increase in affinity for microtubules in 200 mm potassium acetate. A fusion protein of the N-terminal domain and thioredoxin binds tightly to MTs at low salt, consistent with the increased affinity of motor domain constructs (which contain the N-terminal domain) being due to the additional binding of the N-terminal domain to the microtubule. Hydrodynamic analysis indicates that the N-terminal extension is in a highly extended conformation, suggesting that it may be intrinsically disordered. Fusion of the N-terminal extension of BimC onto the motor domain of conventional kinesin produces a similar large increase in microtubule affinity without significant reduction in kcat or velocity in an in vitro motility assay, suggesting that the N-terminal extension can act in a modular manner to increase the microtubule affinity of kinesin motor domains without a decrease in velocity.

Processive and nonprocessive models of kinesin movement

Conventional kinesin is the prototypic member of a family of diverse proteins that use the chemical energy of ATP hydrolysis to generate force and move along microtubules. These proteins, which are involved in a wide range of cellular functions, have been identified in protozoa, fungi, plants, and animals and possess a high degree of sequence conservation among species in their motor domains. The biochemical properties of kinesin and its homologues, in conjunction with the recently solved three-dimensional structures of several kinesin motors, have contributed to our understanding of the mechanism of kinesin movement along microtubules. We discuss several models for movement, including the hand-over-hand, inchworm, and biased diffusion models of processive movement, as well as models of nonprocessive movement.

Molecular motors

Life implies movement. Most forms of movement in the living world are powered by tiny protein machines known as molecular motors. Among the best known are motors that use sophisticated intramolecular amplification mechanisms to take nanometre steps along protein tracks in the cytoplasm. These motors transport a wide variety of cargo, power cell locomotion, drive cell division and, when combined in large ensembles, allow organisms to move. Motor defects can lead to severe diseases or may even be lethal. Basic principles of motor design and mechanism have now been derived, and an understanding of their complex cellular roles is emerging.

The human chromokinesin Kid is a plus end-directed microtubule-based motor

Kid is a kinesin-like DNA-binding protein known to be involved in chromosome movement during mitosis, although its actual motor function has not been demonstrated. Here, we describe the initial characterization of Kid as a microtubule-based motor using optical trapping microscopy. A bacterially expressed fusion protein consisting of a truncated Kid fragment (amino acids 1-388 or 1-439) is indeed an active microtubule motor with an average speed of approximately 160 nm/s, and the polarity of movement is plus end directed. We could not detect processive movement of either monomeric Kid or dimerizing chimeric Kid; however, low levels of processivity (a few steps) cannot be detected with our method. These results are consistent with Kid having a role in chromosome congression in vivo, where it would be responsible for the polar ejection forces acting on the chromosome arms.

K-loop insertion restores microtubule depolymerizing activity of a "neckless" MCAK mutant

Unlike most kinesins, mitotic centromere-associated kinesin (MCAK) does not translocate along the surface of microtubules (MTs), but instead depolymerizes them. Among the motile kinesins, refinements that are unique for specific cellular functions, such as directionality and processivity, are under the control of a "neck" domain adjacent to the ATP-hydrolyzing motor domain. Despite its apparent lack of motility, MCAK also contains a neck domain. We found that deletions and alanine substitutions of highly conserved positively charged residues in the MCAK neck domain significantly reduced MT depolymerization activity. Furthermore, substitution of MCAK's neck domain with either the positively charged KIF1A K-loop or poly-lysine rescues the loss of MT-depolymerizing activity observed in the neckless MCAK mutant. We propose that the neck, analogously to the K-loop, interacts electrostatically with the tubulin COOH terminus to permit diffusional translocation of MCAK along the surface of MTs. This weak-binding interaction may also play an important role in processivity of MCAK-induced MT depolymerization.

Chemomechanical coupling of the forward and backward steps of single kinesin molecules

The molecular motor kinesin travels processively along a microtubule in a stepwise manner. Here we have studied the chemomechanical coupling of the hydrolysis of ATP to the mechanical work of kinesin by analysing the individual stepwise movements according to the directionality of the movements. Kinesin molecules move primarily in the forward direction and only occasionally in the backward direction. The hydrolysis of a single ATP molecule is coupled to either the forward or the backward movement. This bidirectional movement is well described by a model of Brownian motion assuming an asymmetric potential of activation energy. Thus, the stepwise movement along the microtubule is most probably due to Brownian motion that is biased towards the forward direction by chemical energy stored in ATP molecules.