Allosteric inhibition of kinesin-5 modulates its processive directional motility

Regulation of ciliary homeostasis by intraflagellar transport-independent kinesins

Cilia are highly conserved eukaryotic organelles that protrude from the cell surface and are involved in sensory perception, motility, and signaling. Their proper assembly and function rely on the bidirectional intraflagellar transport (IFT) system, which involves motor proteins, including antegrade kinesins and retrograde dynein. Although the role of IFT-mediated transport in cilia has been extensively studied, recent research has highlighted the contribution of IFT-independent kinesins in ciliary processes. The coordinated activities and interplay between IFT kinesins and IFT-independent kinesins are crucial for maintaining ciliary homeostasis. In this comprehensive review, we aim to delve into the specific contributions and mechanisms of action of the IFT-independent kinesins in cilia. By shedding light on their involvement, we hope to gain a more holistic perspective on ciliogenesis and ciliopathies.

A Brief Introduction to Single-Molecule Fluorescence Methods

One of the most popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which will be the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches will be addressed, we will first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we will review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We will end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, single-molecule localization super-resolution microscopy, to distance measurements with Förster resonance energy transfer and orientation measurements with fluorescence polarization.

Modeling nonsegmented negative-strand RNA virus (NNSV) transcription with ejective polymerase collisions and biased diffusion

Infections by non-segmented negative-strand RNA viruses (NNSV) are widely thought to entail gradient gene expression from the well-established existence of a single promoter at the 3' end of the viral genome and the assumption of constant transcriptional attenuation between genes. But multiple recent studies show viral mRNA levels in infections by respiratory syncytial virus (RSV), a major human pathogen and member of NNSV, that are inconsistent with a simple gradient. Here we integrate known and newly predicted phenomena into a biophysically reasonable model of NNSV transcription. Our model succeeds in capturing published observations of respiratory syncytial virus and vesicular stomatitis virus (VSV) mRNA levels. We therefore propose a novel understanding of NNSV transcription based on the possibility of ejective polymerase-polymerase collisions and, in the case of RSV, biased polymerase diffusion.

Drug resistance dependent on allostery: A P-loop rigor Eg5 mutant exhibits resistance to allosteric inhibition by STLC

The mitotic kinesin Eg5 has emerged as a potential anti-mitotic target for the purposes of cancer chemotherapy. Whether clinical resistance to these inhibitors can arise is unclear. We exploited HCT116 cancer cell line to select resistant clones to S-trityl-L-cysteine (STLC), an extensively studied Eg5 loop-L5 binding inhibitor. The STLC resistant clones differed in their resistance to other loop-L5 binding inhibitors but remained sensitive to the ATP class of competitive Eg5 specific inhibitors. Eg5 is still necessary for bipolar spindle formation in the resistant clones since the cells were sensitive to RNAi mediated depletion of Eg5. One clone expressing Eg5(T107N), a dominant point mutation in the P-loop of the ATP binding domain of the motor, appeared to be not only resistant but also dependent on the presence of STLC. Eg5(T107N) expression was associated also with resistance to the clinical relevant loop-L5 Eg5 inhibitors, Arry-520 and ispinesib. Ectopic expression of the Eg5(T107N) mutant in the absence of STLC was associated with strong non-exchangeable binding to microtubules causing them to bundle. Biochemical assays showed that in contrast to the wild type Eg5-STLC complex, the ATP binding site of the Eg5(T107N) is accessible for nucleotide exchange only when the inhibitor is present. We predict that resistance can be overcome by inhibitors that bind to other than the Eg5 loop-L5 binding site having different chemical scaffolds, and that allostery-dependent resistance to Eg5 inhibitors may also occur in cells and may have positive implications in chemotherapy since once diagnosed may be beneficial following cessation of the chemotherapeutic regimen.

Mechanisms by Which Kinesin-5 Motors Perform Their Multiple Intracellular Functions

Bipolar kinesin-5 motor proteins perform multiple intracellular functions, mainly during mitotic cell division. Their specialized structural characteristics enable these motors to perform their essential functions by crosslinking and sliding apart antiparallel microtubules (MTs). In this review, we discuss the specialized structural features of kinesin-5 motors, and the mechanisms by which these features relate to kinesin-5 functions and motile properties. In addition, we discuss the multiple roles of the kinesin-5 motors in dividing as well as in non-dividing cells, and examine their roles in pathogenetic conditions. We describe the recently discovered bidirectional motility in fungi kinesin-5 motors, and discuss its possible physiological relevance. Finally, we also focus on the multiple mechanisms of regulation of these unique motor proteins.

These motors were made for walking

Kinesins are a diverse group of adenosine triphosphate (ATP)-dependent motor proteins that transport cargos along microtubules (MTs) and change the organization of MT networks. Shared among all kinesins is a ~40 kDa motor domain that has evolved an impressive assortment of motility and MT remodeling mechanisms as a result of subtle tweaks and edits within its sequence. Several elegant studies of different kinesin isoforms have exposed the purpose of structural changes in the motor domain as it engages and leaves the MT. However, few studies have compared the sequences and MT contacts of these kinesins systematically. Along with clever strategies to trap kinesin-tubulin complexes for X-ray crystallography, new advancements in cryo-electron microscopy have produced a burst of high-resolution structures that show kinesin-MT interfaces more precisely than ever. This review considers the MT interactions of kinesin subfamilies that exhibit significant differences in speed, processivity, and MT remodeling activity. We show how their sequence variations relate to their tubulin footprint and, in turn, how this explains the molecular activities of previously characterized mutants. As more high-resolution structures become available, this type of assessment will quicken the pace toward establishing each kinesin's design-function relationship.

Protein Diffusion on Charged Biopolymers: DNA versus Microtubule

Protein diffusion in lower-dimensional spaces is used for various cellular functions. For example, sliding on DNA is essential for proteins searching for their target sites, and protein diffusion on microtubules is important for proper cell division and neuronal development. On the one hand, these linear diffusion processes are mediated by long-range electrostatic interactions between positively charged proteins and negatively charged biopolymers and have similar characteristic diffusion coefficients. On the other hand, DNA and microtubules have different structural properties. Here, using computational approaches, we studied the mechanism of protein diffusion along DNA and microtubules by exploring the diffusion of both protein types on both biopolymers. We found that DNA-binding and microtubule-binding proteins can diffuse on each other's substrates; however, the adopted diffusion mechanism depends on the molecular properties of the diffusing proteins and the biopolymers. On the protein side, only DNA-binding proteins can perform rotation-coupled diffusion along DNA, with this being due to their higher net charge and its spatial organization at the DNA recognition helix. By contrast, the lower net charge on microtubule-binding proteins enables them to diffuse more quickly than DNA-binding proteins on both biopolymers. On the biopolymer side, microtubules possess intrinsically disordered, negatively charged C-terminal tails that interact with microtubule-binding proteins, thus supporting their diffusion. Thus, although both DNA-binding and microtubule-binding proteins can diffuse on the negatively charged biopolymers, the unique molecular features of the biopolymers and of their natural substrates are essential for function.

Structure of Microtubule-Trapped Human Kinesin-5 and Its Mechanism of Inhibition Revealed Using Cryoelectron Microscopy

Kinesin-5 motors are vital mitotic spindle components, and disruption of their function perturbs cell division. We investigated the molecular mechanism of the human kinesin-5 inhibitor GSK-1, which allosterically promotes tight microtubule binding. GSK-1 inhibits monomeric human kinesin-5 ATPase and microtubule gliding activities, and promotes the motor's microtubule stabilization activity. Using cryoelectron microscopy, we determined the 3D structure of the microtubule-bound motor-GSK-1 at 3.8 Å overall resolution. The structure reveals that GSK-1 stabilizes the microtubule binding surface of the motor in an ATP-like conformation, while destabilizing regions of the motor around the empty nucleotide binding pocket. Density corresponding to GSK-1 is located between helix-α4 and helix-α6 in the motor domain at its interface with the microtubule. Using a combination of difference mapping and protein-ligand docking, we characterized the kinesin-5-GSK-1 interaction and further validated this binding site using mutagenesis. This work opens up new avenues of investigation of kinesin inhibition and spindle perturbation.

Tubulin tails and their modifications regulate protein diffusion on microtubules

Microtubules (MTs) are essential components of the eukaryotic cytoskeleton that serve as "highways" for intracellular trafficking. In addition to the well-known active transport of cargo by motor proteins, many MT-binding proteins seem to adopt diffusional motility as a transportation mechanism. However, because of the limited spatial resolution of current experimental techniques, the detailed mechanism of protein diffusion has not been elucidated. In particular, the precise role of tubulin tails and tail modifications in the diffusion process is unclear. Here, using coarse-grained molecular dynamics simulations validated against atomistic simulations, we explore the molecular mechanism of protein diffusion along MTs. We found that electrostatic interactions play a central role in protein diffusion; the disordered tubulin tails enhance affinity but slow down diffusion, and diffusion occurs in discrete steps. While diffusion along wild-type MT is performed in steps of dimeric tubulin, the removal of the tails results in a step of monomeric tubulin. We found that the energy barrier for diffusion is larger when diffusion on MTs is mediated primarily by the MT tails rather than the MT body. In addition, globular proteins (EB1 and PRC1) diffuse more slowly than an intrinsically disordered protein (Tau) on MTs. Finally, we found that polyglutamylation and polyglycylation of tubulin tails lead to slower protein diffusion along MTs, although polyglycylation leads to faster diffusion across MT protofilaments. Taken together, our results explain experimentally observed data and shed light on the roles played by disordered tubulin tails and tail modifications in the molecular mechanism of protein diffusion along MTs.

The kinesin-5 tail domain directly modulates the mechanochemical cycle of the motor domain for anti-parallel microtubule sliding

Kinesin-5 motors organize mitotic spindles by sliding apart microtubules. They are homotetramers with dimeric motor and tail domains at both ends of a bipolar minifilament. Here, we describe a regulatory mechanism involving direct binding between tail and motor domains and its fundamental role in microtubule sliding. Kinesin-5 tails decrease microtubule-stimulated ATP-hydrolysis by specifically engaging motor domains in the nucleotide-free or ADP states. Cryo-EM reveals that tail binding stabilizes an open motor domain ATP-active site. Full-length motors undergo slow motility and cluster together along microtubules, while tail-deleted motors exhibit rapid motility without clustering. The tail is critical for motors to zipper together two microtubules by generating substantial sliding forces. The tail is essential for mitotic spindle localization, which becomes severely reduced in tail-deleted motors. Our studies suggest a revised microtubule-sliding model, in which kinesin-5 tails stabilize motor domains in the microtubule-bound state by slowing ATP-binding, resulting in high-force production at both homotetramer ends.

Small molecule allosteric uncoupling of microtubule depolymerase activity from motility in human Kinesin-5 during mitotic spindle assembly

Human Kinesin-5 (Eg5) has a large number of known allosteric inhibitors that disrupt its mitotic function. Small-molecule inhibitors of Eg5 are candidate anti-cancer agents and important probes for understanding the cellular function. Here we show that Eg5 is capable of more than one type of microtubule interaction, and these activities can be controlled by allosteric agents. While both monastrol and S-trityl-L-cysteine inhibit Eg5 motility, our data reveal an unexpected ability of these loop5 targeting inhibitors to differentially control a novel Eg5 microtubule depolymerizing activity. Remarkably, small molecule loop5 effectors are able to independently modulate discrete functional interactions between the motor and microtubule track. We establish that motility can be uncoupled from the microtubule depolymerase activity and argue that loop5-targeting inhibitors of Kinesin-5 should not all be considered functionally synonymous. Also, the depolymerizing activity of the motor does not contribute to the genesis of monopolar spindles during allosteric inhibition of motility, but instead reveals a new function. We propose that, in addition to its canonical role in participating in the construction of the three-dimensional mitotic spindle structure, Eg5 also plays a distinct role in regulating the dynamics of individual microtubules, and thereby impacts the density of the mitotic spindle.

Phragmoplast Orienting Kinesin 2 Is a Weak Motor Switching between Processive and Diffusive Modes

Plant development and morphology relies on the accurate insertion of new cell walls during cytokinesis. However, how a plant cell correctly orients a new wall is poorly understood. Two kinesin class-12 members, phragmoplast orienting kinesin 1 (POK1) and POK2, are involved in the process, but how these molecular machines work is not known. Here, we used in vivo and single-molecule in vitro measurements to determine how Arabidopsis thaliana POK2 motors function mechanically. We found that POK2 is a very weak, on average plus-end-directed, moderately fast kinesin. Interestingly, POK2 switches between processive and diffusive modes characterized by an exclusive-state mean-squared-displacement analysis. Our results support a model that POK motors push against peripheral microtubules of the phragmoplast for its guidance. This pushing model may mechanically explain the conspicuous narrowing of the division site. Together, our findings provide mechanical insight into how active motors accurately position new cell walls in plants.

Exploring a potential allosteric inhibition mechanism in the motor domain of human Eg-5

Kinesin-5 (Eg-5), microtubule motor protein, is one of the emerging drug targets in cancer research. Several inhibitors have been reported to bind the hEg-5 "motor domain" in two different locations that are potentially allosteric. Interestingly, the crystal structure of Eg-5 bound to benzimidazole unveils two chemically different allosteric pockets (PDB ID: 3ZCW). The allosteric modulators inhibit Eg-5 activity by causing conformational changes that affect nucleotide turnover rate. In the present work, three allosteric inhibitors were simulated along with the substrate nucleotides (ADP and ATP) to capture conformation changes induced by the allosteric inhibitors. To analyze the allosteric inhibition mechanism, we used dynamics cross-correlation, principal component analysis (PCA), and enthalpic calculations. The loop L5 interaction is determined by the type of substrate bind at the nucleotide binding site. The SW-II flexibility increased upon dual allosteric inhibition by SB-743921 and 6a. The ionic interaction between R221-E116 is observed only in the presence of two allosteric inhibitors. Also, we noticed that the α2/α3 helical orientation is responsible for the SW-1 loop position and substrate binding. Our simulation data suggest the critical chemical features required to block the motor domain by the allosteric inhibitors. The results summarized in this work will help the researchers to design better therapeutic agents targeting hEg-5. Communicated by Ramaswamy H. Sarma.

Cancer drug therapy and stochastic modeling of "nano-motors"

Background

Controlled inhibition of kinesin motor proteins is highly desired in the field of oncology. Among other interventions, there exists “targeted chemotherapeutic regime/options” of selective Eg5 competitive and allosteric inhibitors, inducing cancer cell apoptosis and tumor regression with improved safety profiles.

Research question

Though promising, such studies are still under clinical trials, for the discovery of efficient and least harmful Eg5 inhibitors. The aim of this research was to bridge the computational modeling approach with drug design and therapy of cancer cells.

Methods

A computational model, interfaced with the clinical data of “Eg5 dynamics” and “inhibitors” via special functions, is presented in this article. Comparisons are made for the drug efficacy, and the threshold values are predicted through numerical simulations.

Results

Results are obtained to depict the dynamics induced by ispinesib, when used as an inhibitor of kinesin Eg5, on cancer cell lines.

Cooperative Accumulation of Dynein-Dynactin at Microtubule Minus-Ends Drives Microtubule Network Reorganization

Cytoplasmic dynein-1 is a minus-end-directed motor protein that transports cargo over long distances and organizes the intracellular microtubule (MT) network. How dynein motor activity is harnessed for these diverse functions remains unknown. Here, we have uncovered a mechanism for how processive dynein-dynactin complexes drive MT-MT sliding, reorganization, and focusing, activities required for mitotic spindle assembly. We find that motors cooperatively accumulate, in limited numbers, at MT minus-ends. Minus-end accumulations drive MT-MT sliding, independent of MT orientation, resulting in the clustering of MT minus-ends. At a mesoscale level, activated dynein-dynactin drives the formation and coalescence of MT asters. Macroscopically, dynein-dynactin activity leads to bulk contraction of millimeter-scale MT networks, suggesting that minus-end accumulations of motors produce network-scale contractile stresses. Our data provide a model for how localized dynein activity is harnessed by cells to produce contractile stresses within the cytoskeleton, for example, during mitotic spindle assembly.

A Brief Introduction to Single-Molecule Fluorescence Methods

One of the more popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which will be the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches will be addressed, we will first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we will review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We will end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, single-molecule localization super-resolution microscopy, to distance measurements with Förster Resonance Energy Transfer and orientation measurements with fluorescence polarization.

Molecular mechanisms of kinesin-14 motors in spindle assembly and chromosome segregation

During eukaryote cell division, molecular motors are crucial regulators of microtubule organization, spindle assembly, chromosome segregation and intracellular transport. The kinesin-14 motors are evolutionarily conserved minus-end-directed kinesin motors that occur in diverse organisms from simple yeasts to higher eukaryotes. Members of the kinesin-14 motor family can bind to, crosslink or slide microtubules and, thus, regulate microtubule organization and spindle assembly. In this Commentary, we present the common subthemes that have emerged from studies of the molecular kinetics and mechanics of kinesin-14 motors, particularly with regard to their non-processive movement, their ability to crosslink microtubules and interact with the minus- and plus-ends of microtubules, and with microtubule-organizing center proteins. In particular, counteracting forces between minus-end-directed kinesin-14 and plus-end-directed kinesin-5 motors have recently been implicated in the regulation of microtubule nucleation. We also discuss recent progress in our current understanding of the multiple and fundamental functions that kinesin-14 motors family members have in important aspects of cell division, including the spindle pole, spindle organization and chromosome segregation.

Mechanical properties of spindle poles are symmetrically balanced

The metaphase spindle is organized for accurate chromosome segregation. One of the fundamental features of the spindle across the species is its symmetrical shape; the spindle consists of two polar arrays of microtubules at both ends. Although it has been suggested that the formation of the bipolar shape requires force balance coordination by molecular motors, i.e., kinesins and dyneins, quantitative analysis for the pole mechanics has not been conducted. Here, we demonstrate that it is not only the shape but also the stiffness and microtubule density of the pairs of pole regions are symmetrically balanced in single spindles self-assembled in Xenopus egg extracts. We found that the inhibition of dynein functions dramatically reduced the stiffness and microtubule density in the pole region. By contrast, the inhibition of one of the kinesins, Eg5, which is the antagonistic motor protein of dynein, increased the value of these parameters. Moreover, the inhibition of both dynein and Eg5 recovered these parameter values to those of non-treated spindle poles. We also found that, when one pole structure was held widened with the use of two glass microneedles, the opposite pole structure spontaneously widened, resulting in the formation of the barrel-like shaped spindle. The values of stiffness and microtubule density in the manipulated pole region decreased, following the spontaneous decrement of those in the paired unmanipulated pole region. These results suggest that the spindle possesses a mechanism to dynamically maintain its symmetry in mechanical properties.

Embedding dual function into molecular motors through collective motion

Protein motors, such as kinesins and dyneins, bind to a microtubule and travel along it in a specific direction. Previously, it was thought that the directionality for a given motor was constant in the absence of an external force. However, the directionality of the kinesin-5 Cin8 was recently found to change as the number of motors that bind to the same microtubule is increased. Here, we introduce a simple mechanical model of a microtubule-sliding assay in which multiple motors interact with the filament. We show that, due to the collective phenomenon, the directionality of the motor changes (e.g., from minus- to plus- end directionality), depending on the number of motors. This is induced by a large diffusive component in the directional walk and by the subsequent frustrated motor configuration, in which multiple motors pull the filament in opposite directions, similar to a game of tug-of-war. A possible role of the dual-directional motors for the mitotic spindle formation is also discussed. Our framework provides a general mechanism to embed two conflicting tasks into a single molecular machine, which works context-dependently.

Metaphase Spindle Assembly

A microtubule-based bipolar spindle is required for error-free chromosome segregation during cell division. In this review I discuss the molecular mechanisms required for the assembly of this dynamic micrometer-scale structure in animal cells.

Distinct Interaction Modes of the Kinesin-13 Motor Domain with the Microtubule

Kinesins-13s are members of the kinesin superfamily of motor proteins that depolymerize microtubules (MTs) and have no motile activity. Instead of generating unidirectional movement over the MT lattice, like most other kinesins, kinesins-13s undergo one-dimensional diffusion (ODD) and induce depolymerization at the MT ends. To understand the mechanism of ODD and the origin of the distinct kinesin-13 functionality, we used ensemble and single-molecule fluorescence polarization microscopy to analyze the behavior and conformation of Drosophila melanogaster kinesin-13 KLP10A protein constructs bound to the MT lattice. We found that KLP10A interacts with the MT in two coexisting modes: one in which the motor domain binds with a specific orientation to the MT lattice and another where the motor domain is very mobile and able to undergo ODD. By comparing the orientation and dynamic behavior of mutated and deletion constructs we conclude that 1) the Kinesin-13 class specific neck domain and loop-2 help orienting the motor domain relative to the MT. 2) During ODD the KLP10A motor-domain changes orientation rapidly (rocks or tumbles). 3) The motor domain alone is capable of undergoing ODD. 4) A second tubulin binding site in the KLP10A motor domain is not critical for ODD. 5) The neck domain is not the element preventing KLP10A from binding to the MT lattice like motile kinesins.

Measuring Pushing and Braking Forces Generated by Ensembles of Kinesin-5 Crosslinking Two Microtubules

The proper organization of the microtubule-based mitotic spindle is proposed to depend on nanometer-sized motor proteins generating forces that scale with a micron-sized geometric feature, such as microtubule overlap length. However, it is unclear whether such regulation can be achieved by any mitotic motor protein. Here, we employ an optical-trap- and total internal reflection fluorescence (TIRF)-based assay to show that ensembles of kinesin-5, a conserved mitotic motor protein, can push apart overlapping antiparallel microtubules to generate a force whose magnitude scales with filament overlap length. We also find that kinesin-5 can produce overlap-length-dependent "brake-like" resistance against relative microtubule sliding in both parallel and antiparallel geometries, an activity that has been suggested by cell biological studies but had not been directly measured. Together, these findings, along with numerical simulations, reveal how a motor protein can function as an analog converter, "reading" simple geometric and dynamic features in cytoskeletal networks to produce regulated force outputs.

Biased Brownian motion as a mechanism to facilitate nanometer-scale exploration of the microtubule plus end by a kinesin-8

Kinesin-8s are plus-end-directed motors that negatively regulate microtubule (MT) length. Well-characterized members of this subfamily (Kip3, Kif18A) exhibit two important properties: (i) They are "ultraprocessive," a feature enabled by a second MT-binding site that tethers the motors to a MT track, and (ii) they dissociate infrequently from the plus end. Together, these characteristics combined with their plus-end motility cause Kip3 and Kif18A to enrich preferentially at the plus ends of long MTs, promoting MT catastrophes or pausing. Kif18B, an understudied human kinesin-8, also limits MT growth during mitosis. In contrast to Kif18A and Kip3, localization of Kif18B to plus ends relies on binding to the plus-end tracking protein EB1, making the relationship between its potential plus-end-directed motility and plus-end accumulation unclear. Using single-molecule assays, we show that Kif18B is only modestly processive and that the motor switches frequently between directed and diffusive modes of motility. Diffusion is promoted by the tail domain, which also contains a second MT-binding site that decreases the off rate of the motor from the MT lattice. In cells, Kif18B concentrates at the extreme tip of a subset of MTs, superseding EB1. Our data demonstrate that kinesin-8 motors use diverse design principles to target MT plus ends, which likely target them to the plus ends of distinct MT subpopulations in the mitotic spindle.

TPX2 Inhibits Eg5 by Interactions with Both Motor and Microtubule

The microtubule-associated protein, TPX2, regulates the activity of the mitotic kinesin, Eg5, but the mechanism of regulation is not established. Using total internal reflection fluorescence microscopy, we observed that Eg5, in extracts of mammalian cells expressing Eg5-EGFP, moved processively toward the microtubule plus-end at an average velocity of 14 nm/s. TPX2 bound to microtubules with an apparent dissociation constant of ∼ 200 nm, and microtubule binding was not dependent on the C-terminal tails of tubulin. Using single molecule assays, we found that full-length TPX2 dramatically reduced Eg5 velocity, whereas truncated TPX2, which lacks the domain that is required for the interaction with Eg5, was a less effective inhibitor at the same concentration. To determine the region(s) of Eg5 that is required for interaction with TPX2, we performed microtubule gliding assays. Dimeric, but not monomeric, Eg5 was differentially inhibited by full-length and truncated TPX2, demonstrating that dimerization or residues in the neck region are important for the interaction of TPX2 with Eg5. These results show that both microtubule binding and interaction with Eg5 contribute to motor inhibition by TPX2 and demonstrate the utility of mammalian cell extracts for biophysical assays.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

Processive cytoskeletal motors studied with single-molecule fluorescence techniques

Processive cytoskeletal motors from the myosin, kinesin, and dynein families walk on actin filaments and microtubules to drive cellular transport and organization in eukaryotic cells. These remarkable molecular machines are able to take hundreds of successive steps at speeds of up to several microns per second, allowing them to effectively move vesicles and organelles throughout the cytoplasm. Here, we focus on single-molecule fluorescence techniques and discuss their wide-ranging applications to the field of cytoskeletal motor research. We cover both traditional fluorescence and sub-diffraction imaging of motors, providing examples of how fluorescence data can be used to measure biophysical parameters of motors such as coordination, stepping mechanism, gating, and processivity. We also outline some remaining challenges in the field and suggest future directions.

Prime movers: the mechanochemistry of mitotic kinesins

Mitotic spindles are self-organizing protein machines that harness teams of multiple force generators to drive chromosome segregation. Kinesins are key members of these force-generating teams. Different kinesins walk directionally along dynamic microtubules, anchor, crosslink, align and sort microtubules into polarized bundles, and influence microtubule dynamics by interacting with microtubule tips. The mechanochemical mechanisms of these kinesins are specialized to enable each type to make a specific contribution to spindle self-organization and chromosome segregation.

The Kinesin-12 Kif15 is a processive track-switching tetramer

Kinesin-12 motors are a little studied branch of the kinesin superfamily with the human protein (Kif15) implicated in spindle mechanics and chromosome movement. In this study, we reconstitute full-length hKif15 and its microtubule-targeting factor hTpx2 in vitro to gain insight into the motors mode of operation. We reveal that hKif15 is a plus-end-directed processive homotetramer that can step against loads of up to 3.5 pN. We further show that hKif15 is the first kinesin that effectively switches microtubule tracks at intersections, enabling it to navigate microtubule networks, such as the spindle. hKif15 tetramers are also capable of cross-linking microtubules, but unexpectedly, this does not depend on hTpx2. Instead, we find that hTpx2 inhibits hKif15 stepping when microtubule-bound. Our data reveal that hKif15 is a second tetrameric spindle motor in addition to the kinesin-5 Eg5 and provides insight into the mechanisms by which hKif15 and its inhibitor hTpx2 modulate spindle microtubule architecture.

DOI:http://dx.doi.org/10.7554/eLife.01724.001

Before a cell can divide, it produces an extra copy of all its chromosomes, and it must then ensure that each daughter cell ends up with one copy of each chromosome. During the division process, a structure called the spindle forms in the cell. This spindle is made of thread-like extensions called microtubules that grow from two poles at opposite ends of the cell. These microtubules are responsible for getting the chromosomes to line up in the middle of the cell, and then pulling half of the chromosomes to one end of the cell, and half to the other end. The cell then divides into two daughter cells.

Two motor proteins—so-called because they consume chemical energy to ‘walk’ along the microtubules—have important roles in this process: Kif11 motor proteins mainly drive the formation of the spindle and thus division of the chromosomes. A cell that does not contain Kif11 can only divide if it contains extra copies of a second motor protein called Kif15: this suggests that Kif15 can serve as some sort of back up for Kif11.

Normal cells only divide when new cells are needed for growth or to replace old cells that have died. Cancer cells, on the other hand, divide in a way that is not controlled. Drugs that interfere with Kif11 have been developed in the hope that they will stop cancer cells dividing, but these drugs have not been very effective in clinical tests, possibly due to the Kif15 back up. Scientists hope, therefore, that a better understanding of the role of Kif15 may lead to improved cancer treatments.

Drechsler et al. have isolated individual Kif15 motor proteins and used advanced microscopy techniques to study them in action. These experiments showed that Kif15 motor proteins can travel long distances along a single microtubule, and can also switch to a different microtubule at intersections. This movement of Kif15 is stopped when they bump into Tpx2 proteins, which are sitting on the microtubules. Together, these proteins can also form links between microtubules that can withstand high forces. These properties provide a starting point to understand how Kif15 can act as a back up for Kif11 in cells. In the future, it will be important to work out how Kif11 and Kif15 motor proteins work together in teams to build the spindle.

DOI:http://dx.doi.org/10.7554/eLife.01724.002

Comprehensive structural model of the mechanochemical cycle of a mitotic motor highlights molecular adaptations in the kinesin family

Kinesins are responsible for a wide variety of microtubule-based, ATP-dependent functions. Their motor domain drives these activities, but the molecular adaptations that specify these diverse and essential cellular activities are poorly understood. It has been assumed that the first identified kinesin--the transport motor kinesin-1--is the mechanistic paradigm for the entire superfamily, but accumulating evidence suggests otherwise. To address the deficits in our understanding of the molecular basis of functional divergence within the kinesin superfamily, we studied kinesin-5s, which are essential mitotic motors whose inhibition blocks cell division. Using cryo-electron microscopy and determination of structure at subnanometer resolution, we have visualized conformations of microtubule-bound human kinesin-5 motor domain at successive steps in its ATPase cycle. After ATP hydrolysis, nucleotide-dependent conformational changes in the active site are allosterically propagated into rotations of the motor domain and uncurling of the drug-binding loop L5. In addition, the mechanical neck-linker element that is crucial for motor stepping undergoes discrete, ordered displacements. We also observed large reorientations of the motor N terminus that indicate its importance for kinesin-5 function through control of neck-linker conformation. A kinesin-5 mutant lacking this N terminus is enzymatically active, and ATP-dependent neck-linker movement and motility are defective, although not ablated. All these aspects of kinesin-5 mechanochemistry are distinct from kinesin-1. Our findings directly demonstrate the regulatory role of the kinesin-5 N terminus in collaboration with the motor's structured neck-linker and highlight the multiple adaptations within kinesin motor domains that tune their mechanochemistries according to distinct functional requirements.

Kinesin-8 is a low-force motor protein with a weakly bound slip state

During the cell cycle, kinesin-8s control the length of microtubules by interacting with their plus ends. To reach these ends, the motors have to be able to take many steps without dissociating. However, the underlying mechanism for this high processivity and how stepping is affected by force are unclear. Here, we tracked the motion of yeast (Kip3) and human (Kif18A) kinesin-8s with high precision under varying loads using optical tweezers. Surprisingly, both kinesin-8 motors were much weaker compared with other kinesins. Furthermore, we discovered a force-induced stick-slip motion: the motor frequently slipped, recovered from this state, and then resumed normal stepping motility without detaching from the microtubule. The low forces are consistent with kinesin-8s being regulators of microtubule dynamics rather than cargo transporters. The weakly bound slip state, reminiscent of a molecular safety leash, may be an adaptation for high processivity.

Kinesin-5: cross-bridging mechanism to targeted clinical therapy

Kinesin motor proteins comprise an ATPase superfamily that works hand in hand with microtubules in every eukaryote. The mitotic kinesins, by virtue of their potential therapeutic role in cancerous cells, have been a major focus of research for the past 28 years since the discovery of the canonical Kinesin-1 heavy chain. Perhaps the simplest player in mitotic spindle assembly, Kinesin-5 (also known as Kif11, Eg5, or kinesin spindle protein, KSP) is a plus-end-directed motor localized to interpolar spindle microtubules and to the spindle poles. Comprised of a homotetramer complex, its function primarily is to slide anti-parallel microtubules apart from one another. Based on multi-faceted analyses of this motor from numerous laboratories over the years, we have learned a great deal about the function of this motor at the atomic level for catalysis and as an integrated element of the cytoskeleton. These data have, in turn, informed the function of motile kinesins on the whole, as well as spearheaded integrative models of the mitotic apparatus in particular and regulation of the microtubule cytoskeleton in general. We review what is known about how this nanomotor works, its place inside the cytoskeleton of cells, and its small-molecule inhibitors that provide a toolbox for understanding motor function and for anticancer treatment in the clinic.

A chimeric kinesin-1 head/kinesin-5 tail motor switches between diffusive and processive motility

Homotetrameric kinesin-5 motors are essential for chromosome separation and assembly of the mitotic spindle. These kinesins bind between two microtubules (MTs) and slide them apart, toward the spindle poles. This process must be tightly regulated in mitosis. In in vitro assays, Eg5 moves diffusively on single MTs and switches to a directed mode between MTs. How allosteric communication between opposing motor domains works remains unclear, but kinesin-5 tail domains may be involved. Here we present a single-molecule fluorescence study of a tetrameric kinesin-1 head/kinesin-5 tail chimera, DK4mer. This motor exhibited fast processive motility on single MTs interrupted by pauses. Like Eg5, DK4mer diffused along MTs with ADP, and slid antiparallel MTs apart with ATP. In contrast to Eg5, diffusive and processive periods were clearly distinguishable. This allowed us to measure transition rates among states and for unbinding as a function of buffer ionic strength. These data, together with results from controls using tail-less dimers, indicate that there are two modes of interaction with MTs, separated by an energy barrier. This result suggests a scheme of motor regulation that involves switching between two bound states, possibly allosterically controlled by the opposing tetramer end. Such a scheme is likely to be relevant for the regulation of native kinesin-5 motors.

"Snapshots" of ispinesib-induced conformational changes in the mitotic kinesin Eg5

Kinesins comprise a superfamily of molecular motors that drive a wide variety of cellular physiologies, from cytoplasmic transport to formation of the bipolar spindle in mitosis. These differing roles are reflected in corresponding polymorphisms in key kinesin structural elements. One of these is a unique loop and stem motif found in all kinesins and referred to as loop 5 (L5). This loop is longest in the mitotic kinesin Eg5 and is the target for a number of small molecule inhibitors, including ispinesib, which is being used in clinical trials in patients with cancer. In this study, we have used x-ray crystallography to identify a new structure of an Eg5-ispinesib complex and have combined this with transient state kinetics to identify a plausible sequence of conformational changes that occur in response to ispinesib binding. Our results demonstrate that ispinesib-induced structural changes in L5 from Eg5 lead to subsequent changes in the conformation of the switch II loop and helix and in the neck linker. We conclude that L5 in Eg5 simultaneously regulates the structure of both the ATP binding site and the motor's mechanical elements that generate force.

New insights into the mechanism of force generation by kinesin-5 molecular motors

Kinesin-5 motors are members of a superfamily of microtubule-dependent ATPases and are widely conserved among eukaryotes. Kinesin-5s typically form homotetramers with pairs of motor domains located at either end of a dumbbell-shaped molecule. This quaternary structure enables cross-linking and ATP-driven sliding of pairs of microtubules, although the exact molecular mechanism of this activity is still unclear. Kinesin-5 function has been characterized in greatest detail in cell division, although a number of interphase roles have also been defined. The kinesin-5 ATPase is tuned for slow microtubule sliding rather than cellular transport and-in vertebrates-can be inhibited specifically by allosteric small molecules currently in cancer clinical trials. The biophysical and structural basis of kinesin-5 mechanochemistry is being elucidated and has provided further insight into kinesin-5 activities. However, it is likely that the precise mechanism of these important motors has evolved according to functional context and regulation in individual organisms.

The structural basis of force generation by the mitotic motor kinesin-5

Kinesin-5 is required for forming the bipolar spindle during mitosis. Its motor domain, which contains nucleotide and microtubule binding sites and mechanical elements to generate force, has evolved distinct properties for its spindle-based functions. In this study, we report subnanometer resolution cryoelectron microscopy reconstructions of microtubule-bound human kinesin-5 before and after nucleotide binding and combine this information with studies of the kinetics of nucleotide-induced neck linker and cover strand movement. These studies reveal coupled, nucleotide-dependent conformational changes that explain many of this motor's properties. We find that ATP binding induces a ratchet-like docking of the neck linker and simultaneous, parallel docking of the N-terminal cover strand. Loop L5, the binding site for allosteric inhibitors of kinesin-5, also undergoes a dramatic reorientation when ATP binds, suggesting that it is directly involved in controlling nucleotide binding. Our structures indicate that allosteric inhibitors of human kinesin-5, which are being developed as anti-cancer therapeutics, bind to a motor conformation that occurs in the course of normal function. However, due to evolutionarily defined sequence variations in L5, this conformation is not adopted by invertebrate kinesin-5s, explaining their resistance to drug inhibition. Together, our data reveal the precision with which the molecular mechanism of kinesin-5 motors has evolved for force generation.

Neck-linker length dependence of processive Kinesin-5 motility

Processive motility of individual molecules is essential for the function of many kinesin motors. Processivity for kinesins relies on communication between the two heads of a dimeric molecule, such that binding strictly alternates. The main communicating elements are believed to be the two neck linkers connecting the motors' stalks and heads. A proposed mechanism for coordination is the transmission of stress through the neck linkers. It is believed that the efficiency of gating depends on the length of the neck linker. Recent studies have presented support for a simple model in which the length of the neck linker directly controls the degree of processivity. Based on a previously published Kinesin-1/Kinesin-5 chimera, Eg5Kin, we have analyzed the motility of 12 motor constructs: we have varied the length of the neck linker in the range between 9 and 21 amino acids using the corresponding native Kinesin-5 sequence (Xenopus laevis Eg5). We found, surprisingly, that neither velocity nor force generation depended on neck-linker length. We also found that constructs with short neck linkers, down to 12 amino acids, were still highly processive, while processivity was lost at a length of 9 amino acids. Run lengths were maximal with neck linkers close to the native Kinesin-5 length and decreased beyond that length. This finding generally confirms the coordinating role of the neck linker for kinesin motility but challenges the simplest model postulating a motor-type-independent optimal length. Instead, our results suggest that different kinesins might be optimized for different neck-linker lengths.

Reconstructing the free-energy landscape associated to molecular motors processivity

We propose a biochemical model providing the kinetic and energetic descriptions of the processivity dynamics of kinesin and dinein molecular motors. Our approach is a modified version of a well known model describing kinesin dynamics and considers the presence of a competitive inhibition reaction by ADP. We first reconstruct a continuous free-energy landscape of the cycle catalyst process that allows us to calculate the number of steps given by a single molecular motor. Then, we calculate an analytical expression associated to the translational velocity and the stopping time of the molecular motor in terms of time and ATP concentration. An energetic interpretation of motor processivity is discussed in quantitative form by using experimental data. We also predict a time duration of collective processes that agrees with experimental reports.

The loop 5 element structurally and kinetically coordinates dimers of the human kinesin-5, Eg5

Eg5 is a homotetrameric kinesin-5 motor protein that generates outward force on the overlapping, antiparallel microtubules (MTs) of the mitotic spindle. Upon binding an MT, an Eg5 dimer releases one ADP molecule, undergoes a slow (∼0.5 s(-1)) isomerization, and finally releases a second ADP, adopting a tightly MT-bound, nucleotide-free (APO) conformation. This conformation precedes ATP binding and stepping. Here, we use mutagenesis, steady-state and pre-steady-state kinetics, motility assays, and electron paramagnetic resonance spectroscopy to examine Eg5 monomers and dimers as they bind MTs and initiate stepping. We demonstrate that a critical element of Eg5, loop 5 (L5), accelerates ADP release during the initial MT-binding event. Furthermore, our electron paramagnetic resonance data show that L5 mediates the slow isomerization by preventing Eg5 dimer heads from binding the MT until they release ADP. Finally, we find that Eg5 having a seven-residue deletion within L5 can still hydrolyze ATP and move along MTs, suggesting that L5 is not required to accelerate subsequent steps of the motor along the MT. Taken together, these properties of L5 explain the kinetic effects of L5-directed inhibition on Eg5 activity and may direct further interventions targeting Eg5 activity.

Interference with kinesin-based anterograde neurofilament axonal transport increases neurofilament-neurofilament bundling

Neurofilaments (NFs) associate with each other and with other cytoskeletal elements to form a lattice that supports the mature axon. Phosphorylation contributes to formation of this stationary population of NFs by fostering cation-dependent interactions among NF sidearms. Association of NFs with the stationary phase indirectly competes with NF axonal transport by withdrawing NFs from kinesin-dependent motility along microtubules. We therefore hypothesized that inhibition of anterograde NF transport may increase incorporation into the stationary phase. To test this hypothesis, we treated differentiated NB2a/d1 cells expressing GFP-tagged NF subunits with monastrol, a specific inhibitor of kinesin-5. Monastrol significantly inhibited anterograde axonal transport of NF-H but not NF-M, and increased the incorporation of newly-transported NF subunits into axonal NF bundles. These findings support the notion that NF transport and bundling exert opposing forces on axonal NF dynamics, and that inhibition of anterograde transport of NFs can increase their incorporation into the stationary phase.

Dynamic reorganization of Eg5 in the mammalian spindle throughout mitosis requires dynein and TPX2

Kinesin-5 is an essential mitotic motor. However, how its spatial-temporal distribution is regulated in mitosis remains poorly understood. We expressed localization and affinity purification-tagged Eg5 from a mouse bacterial artificial chromosome (this construct was called mEg5) and found its distribution to be tightly regulated throughout mitosis. Fluorescence recovery after photobleaching analysis showed rapid Eg5 turnover throughout mitosis, which cannot be accounted for by microtubule turnover. Total internal reflection fluorescence microscopy and high-resolution, single-particle tracking revealed that mEg5 punctae on both astral and midzone microtubules rapidly bind and unbind. mEg5 punctae on midzone microtubules moved transiently both toward and away from spindle poles. In contrast, mEg5 punctae on astral microtubules moved transiently toward microtubule minus ends during early mitosis but switched to plus end-directed motion during anaphase. These observations explain the poleward accumulation of Eg5 in early mitosis and its redistribution in anaphase. Inhibition of dynein blocked mEg5 movement on astral microtubules, whereas depletion of the Eg5-binding protein TPX2 resulted in plus end-directed mEg5 movement. However, motion of Eg5 on midzone microtubules was not altered. Our results reveal differential and precise spatial and temporal regulation of Eg5 in the spindle mediated by dynein and TPX2.

Directionality of individual kinesin-5 Cin8 motors is modulated by loop 8, ionic strength and microtubule geometry

Kinesin-5 motors fulfil essential roles in mitotic spindle morphogenesis and dynamics as slow, processive microtubule (MT) plus-end directed motors. The Saccharomyces cerevisiae kinesin-5 Cin8 was found, surprisingly, to switch directionality. Here, we have examined directionality using single-molecule fluorescence motility assays and live-cell microscopy. On spindles, Cin8 motors mostly moved slowly (∼25 nm/s) towards the midzone, but occasionally also faster (∼55 nm/s) towards the spindle poles. In vitro, individual Cin8 motors could be switched by ionic conditions from rapid (380 nm/s) and processive minus-end to slow plus-end motion on single MTs. At high ionic strength, Cin8 motors rapidly alternated directionalities between antiparallel MTs, while driving steady plus-end relative sliding. Between parallel MTs, plus-end motion was only occasionally observed. Deletion of the uniquely large insert in loop 8 of Cin8 induced bias towards minus-end motility and affected the ionic strength-dependent directional switching of Cin8 in vitro. The deletion mutant cells exhibited reduced midzone-directed motility and efficiency to support spindle elongation, indicating the importance of directionality control for the anaphase function of Cin8.

Interhead tension determines processivity across diverse N-terminal kinesins

Consistent with their diverse intracellular roles, the processivity of N-terminal kinesin motors varies considerably between different families. Kinetics experiments on isolated motor domains suggest that differences in processivity result from differences in the underlying biochemistry of the catalytic heads. However, the length of the flexible neck linker domain also varies from 14 to 18 residues between families. Because the neck linker acts as a mechanical element that transmits interhead tension, altering its mechanical properties is expected to affect both front and rear head gating, mechanisms that underlie processive walking. To test the hypothesis that processivity differences result from family-specific differences in neck linker mechanics, we systematically altered the neck linker length in kinesin-1, -2, -3, -5, and -7 motors and measured run length and velocity in a single-molecule fluorescence assay. Shortening the neck linkers of kinesin-3 (Unc104/KIF1A) and kinesin-5 (Eg5/KSP) to 14 residues enhanced processivity to match kinesin-1, which has a 14-residue neck linker. After substituting a single residue in the last alpha helix of the catalytic core, kinesin-7 (CENP-E) exhibited this same behavior. This convergence of processivity was observed even though motor speeds varied over a 25-fold range. These results suggest that differences in unloaded processivity between diverse kinesins is primarily due to differences in the lengths of their neck linker domains rather than specific tuning of rate constants in their ATP hydrolysis cycles.

Spindle pole mechanics studied in mitotic asters: dynamic distribution of spindle forces through compliant linkages

During cell division, chromosomes must faithfully segregate to maintain genome integrity, and this dynamic mechanical process is driven by the macromolecular machinery of the mitotic spindle. However, little is known about spindle mechanics. For example, spindle microtubules are organized by numerous cross-linking proteins yet the mechanical properties of those cross-links remain unexplored. To examine the mechanical properties of microtubule cross-links we applied optical trapping to mitotic asters that form in mammalian mitotic extracts. These asters are foci of microtubules, motors, and microtubule-associated proteins that reflect many of the functional properties of spindle poles and represent centrosome-independent spindle-pole analogs. We observed bidirectional motor-driven microtubule movements, showing that microtubule linkages within asters are remarkably compliant (mean stiffness 0.025 pN/nm) and mediated by only a handful of cross-links. Depleting the motor Eg5 reduced this stiffness, indicating that Eg5 contributes to the mechanical properties of microtubule asters in a manner consistent with its localization to spindle poles in cells. We propose that compliant linkages among microtubules provide a mechanical architecture capable of accommodating microtubule movements and distributing force among microtubules without loss of pole integrity-a mechanical paradigm that may be important throughout the spindle.

Directional switching of the kinesin Cin8 through motor coupling

Kinesin motor proteins are thought to move exclusively in either one or the other direction along microtubules. Proteins of the kinesin-5 family are tetrameric microtubule cross-linking motors important for cell division and differentiation in various organisms. Kinesin-5 motors are considered to be plus-end-directed. However, here we found that purified kinesin-5 Cin8 from budding yeast could behave as a bidirectional kinesin. On individual microtubules, single Cin8 motors were minus-end-directed motors, whereas they switched to plus-end-directed motility when working in a team of motors sliding antiparallel microtubules apart. This kinesin can thus change directionality of movement depending on whether it acts alone or in an ensemble.

Moving into the cell: single-molecule studies of molecular motors in complex environments

Much has been learned in the past decades about molecular force generation. Single-molecule techniques, such as atomic force microscopy, single-molecule fluorescence microscopy and optical tweezers, have been key in resolving the mechanisms behind the power strokes, 'processive' steps and forces of cytoskeletal motors. However, it remains unclear how single force generators are integrated into composite mechanical machines in cells to generate complex functions such as mitosis, locomotion, intracellular transport or mechanical sensory transduction. Using dynamic single-molecule techniques to track, manipulate and probe cytoskeletal motor proteins will be crucial in providing new insights.

A nonmotor microtubule binding site in kinesin-5 is required for filament crosslinking and sliding

Kinesin-5, a widely conserved motor protein required for assembly of the bipolar mitotic spindle in eukaryotes, forms homotetramers with two pairs of motor domains positioned at opposite ends of a dumbbell-shaped molecule [1-3]. It has long been assumed that this configuration of motor domains is the basis of kinesin-5's ability to drive relative sliding of microtubules [2, 4, 5]. Recently, it was suggested that in addition to the N-terminal motor domain, kinesin-5 also has a nonmotor microtubule binding site in its C terminus [6]. However, it is not known how the nonmotor domain contributes to motor activity, or how a kinesin-5 tetramer utilizes a combination of four motor and four nonmotor microtubule binding sites for its microtubule organizing functions. Here we show, in single molecule assays, that kinesin-5 homotetramers require the nonmotor C terminus for crosslinking and relative sliding of two microtubules. Remarkably, this domain enhances kinesin-5's microtubule binding without substantially reducing motor activity. Our results suggest that tetramerization of kinesin-5's low-processivity motor domains is not sufficient for microtubule sliding because the motor domains alone are unlikely to maintain persistent microtubule crosslinks. Rather, kinesin-5 utilizes nonmotor microtubule binding sites to tune its microtubule attachment dynamics, enabling it to efficiently align and sort microtubules during metaphase spindle assembly and function.

A brief introduction to single-molecule fluorescence methods

One of the more popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which is the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches are addressed, we first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, to distance measurements with Förster Resonance Energy Transfer and orientation measurements with fluorescence polarization.

Microtubule organization by the antagonistic mitotic motors kinesin-5 and kinesin-14

During cell division, different molecular motors act synergistically to rearrange microtubules. Minus end-directed motors are thought to have a dual role: focusing microtubule ends to poles and establishing together with plus end-directed motors a balance of force between antiparallel microtubules in the spindle. We study here the competing action of Xenopus laevis kinesin-14 and -5 in vitro in situations in which these motors with opposite directionality cross-link and slide microtubules. We find that full-length kinesin-14 can form microtubule asters without additional factors, whereas kinesin-5 does not, likely reflecting an adaptation to mitotic function. A stable balance of force is not established between two antiparallel microtubules with these motors. Instead, directional instability is generated, promoting efficient motor and microtubule sorting. A nonmotor microtubule cross-linker can suppress directional instability but also impedes microtubule sorting, illustrating a conflict between stability and dynamicity of organization. These results establish the basic organizational properties of these antagonistic mitotic motors and a microtubule bundler.

NSC 622124 inhibits human Eg5 and other kinesins via interaction with the conserved microtubule-binding site

Kinesin-5 proteins are essential for formation of a bipolar mitotic spindle in most and, perhaps all, eukaryotic cells. Several Kinesin-5 proteins, notably the human version, HsEg5, are targets of a constantly expanding group of small-molecule inhibitors, which hold promise both as tools for probing mechanochemical transduction and as anticancer agents. Although most such compounds are selective for HsEg5 and closely related Kinesin-5 proteins, some, such as NSC 622124, exhibit activity against at least one kinesin from outside the Kinesin-5 family. Here we show NSC 622124, despite identification in a screen that yielded inhibitors now known to target the HsEg5 monastrol-binding site, does not compete with [(14)C]monastrol for binding to HsEg5 and is able to inhibit the basal and microtubule-stimulated ATPase activity of the monastrol-insensitive Kinesin-5, KLP61F. NSC 622124 competes with microtubules, but not ATP, for interaction with HsEg5 and disrupts the microtubule binding of HsEg5, KLP61F, and Kinesin-1. Proteolytic degradation of an HsEg5.NSC622124 complex revealed that segments of the alpha3 and alpha5 helices map to the inhibitor-binding site. Overall, our results demonstrate that NSC 622124 targets the conserved microtubule-binding site of kinesin proteins. Further, unlike compounds previously reported to target the kinesin microtubule-binding site, NSC 622124 does not produce any enhancement of basal ATPase activity and thus acts solely as a negative regulator through interaction with a site traditionally viewed as a binding region for positive regulators (i.e., microtubules). Our work emphasizes the concept that microtubule-dependent motor proteins may be controlled at multiple sites by both positive and negative effectors.