Modular aspects of kinesin force generation machinery

A kinesin-1 variant reveals motor-induced microtubule damage in cells

Kinesins drive the transport of cellular cargoes as they walk along microtubule tracks; however, recent work has suggested that the physical act of kinesins walking along microtubules can stress the microtubule lattice. Here, we describe a kinesin-1 KIF5C mutant with an increased ability to generate damage sites in the microtubule lattice as compared with the wild-type motor. The expression of the mutant motor in cultured cells resulted in microtubule breakage and fragmentation, suggesting that kinesin-1 variants with increased damage activity would have been selected against during evolution. The increased ability to damage microtubules is not due to the enhanced motility properties of the mutant motor, as the expression of the kinesin-3 motor KIF1A, which has similar single-motor motility properties, also caused increased microtubule pausing, bending, and buckling but not breakage. In cells, motor-induced microtubule breakage could not be prevented by increased α-tubulin K40 acetylation, a post-translational modification known to increase microtubule flexibility. In vitro, lattice damage induced by wild-type KIF5C was repaired by soluble tubulin and resulted in increased rescues and overall microtubule growth, whereas lattice damage induced by the KIF5C mutant resulted in larger repair sites that made the microtubule vulnerable to breakage and fragmentation when under mechanical stress. These results demonstrate that kinesin-1 motility causes defects in and damage to the microtubule lattice in cells. While cells have the capacity to repair lattice damage, conditions that exceed this capacity result in microtubule breakage and fragmentation and may contribute to human disease.

Materials for Smart Soft Actuator Systems

In contrast to conventional hard actuators, soft actuators offer many vivid advantages, such as improved flexibility, adaptability, and reconfigurability, which are intrinsic to living systems. These properties make them particularly promising for different applications, including soft electronics, surgery, drug delivery, artificial organs, or prosthesis. The additional degree of freedom for soft actuatoric devices can be provided through the use of intelligent materials, which are able to change their structure, macroscopic properties, and shape under the influence of external signals. The use of such intelligent materials allows a substantial reduction of a device's size, which enables a number of applications that cannot be realized by externally powered systems. This review aims to provide an overview of the properties of intelligent synthetic and living/natural materials used for the fabrication of soft robotic devices. We discuss basic physical/chemical properties of the main kinds of materials (elastomers, gels, shape memory polymers and gels, liquid crystalline elastomers, semicrystalline ferroelectric polymers, gels and hydrogels, other swelling polymers, materials with volume change during melting/crystallization, materials with tunable mechanical properties, and living and naturally derived materials), how they are related to actuation and soft robotic application, and effects of micro/macro structures on shape transformation, fabrication methods, and we highlight selected applications.

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.

Plasmodium berghei Kinesin-5 Associates With the Spindle Apparatus During Cell Division and Is Important for Efficient Production of Infectious Sporozoites

Kinesin-5 motors play essential roles in spindle apparatus assembly during cell division, by generating forces to establish and maintain the spindle bipolarity essential for proper chromosome segregation. Kinesin-5 is largely conserved structurally and functionally in model eukaryotes, but its role is unknown in the Plasmodium parasite, an evolutionarily divergent organism with several atypical features of both mitotic and meiotic cell division. We have investigated the function and subcellular location of kinesin-5 during cell division throughout the Plasmodium berghei life cycle. Deletion of kinesin-5 had little visible effect at any proliferative stage except sporozoite production in oocysts, resulting in a significant decrease in the number of motile sporozoites in mosquito salivary glands, which were able to infect a new vertebrate host. Live-cell imaging showed kinesin-5-GFP located on the spindle and at spindle poles during both atypical mitosis and meiosis. Fixed-cell immunofluorescence assays revealed kinesin-5 co-localized with α-tubulin and centrin-2 and a partial overlap with kinetochore marker NDC80 during early blood stage schizogony. Dual-color live-cell imaging showed that kinesin-5 is closely associated with NDC80 during male gametogony, but not with kinesin-8B, a marker of the basal body and axonemes of the forming flagella. Treatment of gametocytes with microtubule-specific inhibitors confirmed kinesin-5 association with nuclear spindles and not cytoplasmic axonemal microtubules. Altogether, our results demonstrate that kinesin-5 is associated with the spindle apparatus, expressed in proliferating parasite stages, and important for efficient production of infectious sporozoites.

How Kinesin-1 Utilize the Energy of Nucleotide: The Conformational Changes and Mechanochemical Coupling in the Unidirectional Motion of Kinesin-1

Kinesin-1 is a typical motile molecular motor and the founding member of the kinesin family. The most significant feature in the unidirectional motion of kinesin-1 is its processivity. To realize the fast and processive movement on the microtubule lattice, kinesin-1 efficiently transforms the chemical energy of nucleotide binding and hydrolysis to the energy of mechanical movement. The chemical and mechanical cycle of kinesin-1 are coupled to avoid futile nucleotide hydrolysis. In this paper, the research on the mechanical pathway of energy transition and the regulating mechanism of the mechanochemical cycle of kinesin-1 is reviewed.

The mechanochemistry of the kinesin-2 KIF3AC heterodimer is related to strain-dependent kinetic properties of KIF3A and KIF3C

KIF3AC is a mammalian neuron-specific kinesin-2 implicated in intracellular cargo transport. It is a heterodimer of KIF3A and KIF3C motor polypeptides which have distinct biochemical and motile properties as engineered homodimers. Single-molecule motility assays show that KIF3AC moves processively along microtubules at a rate faster than expected given the motility rates of the KIF3AA and much slower KIF3CC homodimers. To resolve the stepping kinetics of KIF3A and KIF3C motors in homo- and heterodimeric constructs and determine their transport potential under load, we assayed motor activity using interferometric scattering microscopy and optical trapping. The distribution of stepping durations of KIF3AC molecules is described by a rate (k ₁ = 11 s^-1) without apparent kinetic asymmetry. Asymmetry was also not apparent under hindering or assisting mechanical loads in the optical trap. KIF3AC shows increased force sensitivity relative to KIF3AA yet is more capable of stepping against mechanical load than KIF3CC. Interestingly, the behavior of KIF3C mirrors prior studies of kinesins with increased interhead compliance. Microtubule gliding assays containing 1:1 mixtures of KIF3AA and KIF3CC result in speeds similar to KIF3AC, suggesting the homodimers mechanically impact each other's motility to reproduce the behavior of the heterodimer. Our observations are consistent with a mechanism in which the stepping of KIF3C can be activated by KIF3A in a strain-dependent manner, similar to application of an assisting load. These results suggest that the mechanochemical properties of KIF3AC can be explained by the strain-dependent kinetics of KIF3A and KIF3C.

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.

Force production of human cytoplasmic dynein is limited by its processivity

Cytoplasmic dynein is a highly complex motor protein that generates forces toward the minus end of microtubules. Using optical tweezers, we demonstrate that the low processivity (ability to take multiple steps before dissociating) of human dynein limits its force generation due to premature microtubule dissociation. Using a high trap stiffness whereby the motor achieves greater force per step, we reveal that the motor's true maximal force ("stall force") is ~2 pN. Furthermore, an average force versus trap stiffness plot yields a hyperbolic curve that plateaus at the stall force. We derive an analytical equation that accurately describes this curve, predicting both stall force and zero-load processivity. This theoretical model describes the behavior of a kinesin motor under low-processivity conditions. Our work clarifies the true stall force and processivity of human dynein and provides a new paradigm for understanding and analyzing molecular motor force generation for weakly processive motors.

Structural basis for power stroke vs. Brownian ratchet mechanisms of motor proteins

Two mechanisms have been proposed for the function of motor proteins: The power stroke and the Brownian ratchet. The former refers to generation of a large downhill free energy gradient over which the motor protein moves nearly irreversibly in making a step, whereas the latter refers to biasing or rectifying the diffusive motion of the motor. Both mechanisms require input of free energy, which generally involves the processing of an ATP (adenosine 5'-triphosphate) molecule. Recent advances in experiments that reveal the details of the stepping motion of motor proteins, together with computer simulations of atomistic structures, have provided greater insights into the mechanisms. Here, we compare the various models of the power stroke and the Brownian ratchet that have been proposed. The 2 mechanisms are not mutually exclusive, and various motor proteins employ them to different extents to perform their biological function. As examples, we discuss linear motor proteins Kinesin-1 and myosin-V, and the rotary motor F₁-ATPase, all of which involve a power stroke as the essential element of their stepping mechanism.

Neck linker docking is critical for Kinesin-1 force generation in cells but at a cost to motor speed and processivity

Kinesin force generation involves ATP-induced docking of the neck linker (NL) along the motor core. However, the roles of the proposed steps of NL docking, cover-neck bundle (CNB) and asparagine latch (N-latch) formation, during force generation are unclear. Furthermore, the necessity of NL docking for transport of membrane-bound cargo in cells has not been tested. We generated kinesin-1 motors impaired in CNB and/or N-latch formation based on molecular dynamics simulations. The mutant motors displayed reduced force output and inability to stall in optical trap assays but exhibited increased speeds, run lengths, and landing rates under unloaded conditions. NL docking thus enhances force production but at a cost to speed and processivity. In cells, teams of mutant motors were hindered in their ability to drive transport of Golgi elements (high-load cargo) but not peroxisomes (low-load cargo). These results demonstrate that the NL serves as a mechanical element for kinesin-1 transport under physiological conditions.

High kinesin family member 11 expression predicts poor prognosis in patients with clear cell renal cell carcinoma

Aims

Kinesin family member 11 (Kif11) is a member of the kinesin family motor proteins, which is associated with spindle formation and tumour genesis. In this study, we investigated the relationship between Kif11 expression and clear cell renal cell carcinoma (CCRCC) development.

Methods

The relationship between Kif11 expression and CCRCC development was analysed by quantitative real-time (qRT)-PCR analyses, and tissue immunohistochemistry. The prognostic significance of Kif11 expression was explored by univariable and multivariable survival analyses of 143 included patients. Furthermore, SB743921 was used as a specific Kif11 inhibitor to treat 786-O cells with the epithelial to mesenchymal transition (EMT) process analysed by qRT-PCR, and cell survival rates analysed with Annexin V-FITC/PI staining followed by flow cytometric analyses. Disease-free survival curves of Kif11 with different cancers and the relationships between Kif11 and the von Hippel-Lindau disease tumour suppressor gene (VHL), and proliferating cell nuclear antigen (PCNA) in kidney cancer were further analysed using the GEPIA database.

Results

The levels of Kif11 mRNA were significantly higher in CCRCC tissues compared with corresponding non-cancerous tissues. The results of immunohistochemistry demonstrated that the expression of Kif11 protein was significantly associated with clinicopathologial parameters, including nuclear grade and TNM stage. The Kaplan-Meier survival curve indicated that high Kif11 expression, nuclear grade and TNM stage were independent factors to predict poor prognosis in patients with CCRCC. In addition, inhibition of Kif11 expression by SB743921 suppressed cell proliferation, migration and the EMT process with increased apoptosis rate.

Conclusions

These results combined with bioinformation analyses suggest that high Kif11 expression was associated with unfavourable prognosis in CCRCC and could be used as a potential prognostic marker in the clinical diagnosis of CCRCC.

Processive Kinesin-14 HSET Exhibits Directional Flexibility Depending on Motor Traffic

A common mitotic defect observed in cancer cells that possess supernumerary (more than two) centrosomes is multipolar spindle formation [1, 2]. Such structures are resolved into a bipolar geometry by minus-end-directed motor proteins, such as cytoplasmic dynein and the kinesin-14 HSET [3-8]. HSET is also thought to antagonize plus-end-directed kinesin-5 Eg5 to balance spindle forces [4, 5, 7, 9]. However, the biomechanics of this force opposition are unclear, as HSET has previously been defined as a non-processive motor [10-16]. Here, we use optical trapping to elucidate the mechanism of force generation by HSET. We show that a single HSET motor has a processive nature with the ability to complete multiple steps while trapped along a microtubule and when unloaded can move in both directions for microns. Compared to other kinesins, HSET has a relatively weak stall force of 1.1 pN [17, 18]. Moreover, HSET's tail domain and its interaction with the E-hook of tubulin are necessary for long-range motility. In vitro polarity-marked bundle assays revealed that HSET selectively generates force in anti-parallel bundles on the order of its stall force. When combined with varied ratios of Eg5, HSET adopts Eg5's directionality while acting as an antagonizing force brake, requiring at least a 10-fold higher Eg5 concentration to surpass HSET's sliding force. These results reveal HSET's ability to change roles within the spindle from acting as an adjustable microtubule slider and force regulator to a processive motor that aids in minus end focusing.

Dynamics of microtubules: highlights of recent computational and experimental investigations

Microtubules are found in most eukaryotic cells, with homologs in eubacteria and archea, and they have functional roles in mitosis, cell motility, intracellular transport, and the maintenance of cell shape. Numerous efforts have been expended over the last two decades to characterize the interactions between microtubules and the wide variety of microtubule associated proteins that control their dynamic behavior in cells resulting in microtubules being assembled and disassembled where and when they are required by the cell. We present the main findings regarding microtubule polymerization and depolymerization and review recent work about the molecular motors that modulate microtubule dynamics by inducing either microtubule depolymerization or severing. We also discuss the main experimental and computational approaches used to quantify the thermodynamics and mechanics of microtubule filaments.

Collective Force Regulation in Anti-parallel Microtubule Gliding by Dimeric Kif15 Kinesin Motors

During cell division, the mitotic kinesin-5 Eg5 generates most of the force required to separate centrosomes during spindle assembly. However, Kif15, another mitotic kinesin, can replace Eg5 function, permitting mammalian cells to acquire resistance to Eg5 poisons. Unlike Eg5, the mechanism by which Kif15 generates centrosome separation forces is unknown. Here we investigated the mechanical properties and force generation capacity of Kif15 at the single-molecule level using optical tweezers. We found that the non-motor microtubule-binding tail domain interacts with the microtubule's E-hook tail with a rupture force higher than the stall force of the motor. This allows Kif15 dimers to productively and efficiently generate forces that could potentially slide microtubules apart. Using an in vitro optical trapping and fluorescence assay, we found that Kif15 slides anti-parallel microtubules apart with gradual force buildup while parallel microtubule bundles remain stationary with a small amount of antagonizing force generated. A stochastic simulation shows the essential role of Kif15's tail domain for load storage within the Kif15-microtubule system. These results suggest a mechanism for how Kif15 rescues bipolar spindle assembly.

Exploiting the kinesin-1 molecular motor to generate a virus membrane penetration site

Viruses exploit cellular machineries to penetrate a host membrane and cause infection, a process that remains enigmatic for non-enveloped viruses. Here we probe how the non-enveloped polyomavirus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol, a crucial infection step. We find that the microtubule-based motor kinesin-1 is recruited to the ER membrane by binding to the transmembrane J-protein B14. Strikingly, this motor facilitates SV40 ER-to-cytosol transport by constructing a penetration site on the ER membrane called a ‘focus'. Neither kinesin-2, kinesin-3 nor kinesin-5 promotes foci formation or infection. The specific use of kinesin-1 is due to its unique ability to select posttranslationally modified microtubules for cargo transport and thereby spatially restrict focus formation to the perinucleus. These findings support the idea of a ‘tubulin code' for motor-dependent trafficking and establish a distinct kinesin-1 function in which a motor is exploited to create a viral membrane penetration site.

How non-enveloped viruses cross host membranes is incompletely understood. Here, Ravindran et al. show that polyomavirus SV40 recruits kinesin-1 to construct a penetration site on the ER membrane.

Evaluating the dynamics and electrostatic interactions of folded proteins in implicit solvents

Three implicit solvent models, namely GBMVII, FACTS, and SCPISM, were evaluated for their abilities to emulate an explicit solvent environment by comparing the simulated conformational ensembles, dynamics, and electrostatic interactions of the Src SH2 domain and the Lyn kinase domain. This assessment in terms of structural features in folded proteins expands upon the use of hydration energy as a metric for comparison. All-against-all rms coordinate deviation, average positional fluctuations, and ion-pair distance distribution were used to compare the implicit solvent models with the TIP3P explicit solvent model. Our study shows that the Src SH2 domains solvated with TIP3P, GBMVII, and FACTS sample similar global conformations. Additionally, the Src SH2 ion-pair distance distributions of solvent-exposed side chains corresponding to TIP3P, GBMVII, and FACTS do not differ substantially, indicating that GBMVII and FACTS are capable of modeling these electrostatic interactions. The ion-pair distance distributions of SCPISM are distinct from others, demonstrating that these electrostatic interactions are not adequately reproduced with the SCPISM model. On the other hand, for the Lyn kinase domain, a non-globular protein with bilobal structure and a large concavity on the surface, implicit solvent does not accurately model solvation to faithfully reproduce partially buried electrostatic interactions and lobe-lobe conformations. Our work reveals that local structure and dynamics of small, globular proteins are modeled well using FACTS and GBMVII. Nonetheless, global conformations and electrostatic interactions in concavities of multi-lobal proteins resulting from simulations with implicit solvent models do not match those obtained from explicit water simulations.

Kinesin-5 allosteric inhibitors uncouple the dynamics of nucleotide, microtubule, and neck-linker binding sites

Kinesin motor domains couple cycles of ATP hydrolysis to cycles of microtubule binding and conformational changes that result in directional force and movement on microtubules. The general principles of this mechanochemical coupling have been established; however, fundamental atomistic details of the underlying allosteric mechanisms remain unknown. This lack of knowledge hampers the development of new inhibitors and limits our understanding of how disease-associated mutations in distal sites can interfere with the fidelity of motor domain function. Here, we combine unbiased molecular-dynamics simulations, bioinformatics analysis, and mutational studies to elucidate the structural dynamic effects of nucleotide turnover and allosteric inhibition of the kinesin-5 motor. Multiple replica simulations of ATP-, ADP-, and inhibitor-bound states together with network analysis of correlated motions were used to create a dynamic protein structure network depicting the internal dynamic coordination of functional regions in each state. This analysis revealed the intervening residues involved in the dynamic coupling of nucleotide, microtubule, neck-linker, and inhibitor binding sites. The regions identified include the nucleotide binding switch regions, loop 5, loop 7, α4-α5-loop 13, α1, and β4-β6-β7. Also evident were nucleotide- and inhibitor-dependent shifts in the dynamic coupling paths linking functional sites. In particular, inhibitor binding to the loop 5 region affected β-sheet residues and α1, leading to a dynamic decoupling of nucleotide, microtubule, and neck-linker binding sites. Additional analyses of point mutations, including P131 (loop 5), Q78/I79 (α1), E166 (loop 7), and K272/I273 (β7) G325/G326 (loop 13), support their predicted role in mediating the dynamic coupling of distal functional surfaces. Collectively, our results and approach, which we make freely available to the community, provide a framework for explaining how binding events and point mutations can alter dynamic couplings that are critical for kinesin motor domain function.

New approach to capture and characterize synaptic proteome

Little is known regarding the identity of the population of proteins that are transported and localized to synapses. Here we describe a new approach that involves the isolation and systematic proteomic characterization of molecular motor kinesins to identify the populations of proteins transported to synapses. We used this approach to identify and compare proteins transported to synapses by kinesin (Kif) complexes Kif5C and Kif3A in the mouse hippocampus and prefrontal cortex. Approximately 40-50% of the protein cargos identified in our proteomics analysis of kinesin complexes are known synaptic proteins. We also found that the identity of kinesins and where they are expressed determine what proteins they transport. Our results reveal a previously unappreciated role of kinesins in regulating the composition of synaptic proteome.

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.

Mechanism and regulation of kinesin-5, an essential motor for the mitotic spindle

Mitotic cell division is the most fundamental task of all living cells. Cells have intricate and tightly regulated machinery to ensure that mitosis occurs with appropriate frequency and high fidelity. A core element of this machinery is the kinesin-5 motor protein, which plays essential roles in spindle formation and maintenance. In this review, we discuss how the structural and mechanical properties of kinesin-5 motors uniquely suit them to their mitotic role. We describe some of the small molecule inhibitors and regulatory proteins that act on kinesin-5, and discuss how these regulators may influence the process of cell division. Finally, we touch on some more recently described functions of kinesin-5 motors in non-dividing cells. Throughout, we highlight a number of open questions that impede our understanding of both this motor's function and the potential utility of kinesin-5 inhibitors.

Kar3Vik1 mechanochemistry is inhibited by mutation or deletion of the C terminus of the Vik1 subunit

Force production by kinesins has been linked to structural rearrangements of the N and C termini of their motor domain upon nucleotide binding. In recent crystal structures, the Kar3-associated protein Vik1 shows unexpected homology to these conformational states even though it lacks a nucleotide-binding site. This conservation infers a degree of commonality in the function of the N- and C-terminal regions during the mechanochemical cycle of all kinesins and kinesin-related proteins. We tested this inference by examining the functional effects on Kar3Vik1 of mutating or deleting residues in Vik1 that are involved in stabilizing the C terminus against the core and N terminus of the Vik1 motor homology domain (MHD). Point mutations at two moderately conserved residues near the Vik1 C terminus impaired microtubule gliding and microtubule-stimulated ATP turnover by Kar3Vik1. Deletion of the seven C-terminal residues inhibited Kar3Vik1 motility much more drastically. Interestingly, none of the point mutants seemed to perturb the ability of Kar3Vik1 to bind microtubules, whereas the C-terminal truncation mutant did. Molecular dynamics simulations of these C-terminal mutants showed distinct root mean square fluctuations in the N-terminal region of the Vik1 MHD that connects it to Kar3. Here, the degree of motion in the N-terminal portion of Vik1 highly correlated with that in the C terminus. These observations suggest that the N and C termini of the Vik1 MHD form a discrete folding motif that is part of a communication pathway to the nucleotide-binding site of Kar3.

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.