A non-motor microtubule binding site is essential for the high processivity and mitotic function of kinesin-8 Kif18A

A model of microtubule depolymerization by kinesin-8 motor proteins

The dimeric kinesin-8 motors have the biological function of depolymerizing microtubules (MTs) from the plus end. However, the molecular mechanism of the depolymerization promoted by the kinesin-8 motors is still undetermined. Here, a model is proposed for the MT depolymerization by the kinesin-8 motors. Based on the model, the dynamics of depolymerization in the presence of the single motor at the MT plus end under no load and under load on the motor is studied theoretically. The dynamics of depolymerization in the presence of multiple motors at the MT plus end is also analyzed. The theoretical results explain well the available experimental data. The studies can also be applicable to other families of kinesin motors such as kinesin-13 mitotic centromere-associated kinesin motors that have the ability to depolymerize MTs.

Control of motor landing and processivity by the CAP-Gly domain in the KIF13B tail

Microtubules are major components of the eukaryotic cytoskeleton. Posttranslational modifications (PTMs) of tubulin regulates interactions with microtubule-associated proteins (MAPs). One unique PTM is the cyclical removal and re-addition of the C-terminal tyrosine of α-tubulin and MAPs containing CAP-Gly domains specifically recognize tyrosinated microtubules. KIF13B, a long-distance transport kinesin, contains a conserved CAP-Gly domain, but the role of the CAP-Gly domain in KIF13B's motility along microtubules remains unknown. To address this, we investigate the interaction between KIF13B's CAP-Gly domain, and tyrosinated microtubules. We find that KIF13B's CAP-Gly domain influences the initial motor-microtubule interaction, as well as processive motility along microtubules. The effect of the CAP-Gly domain is enhanced when the motor domain is in the ADP state, suggesting an interplay between the N-terminal motor domain and C-terminal CAP-Gly domain. These results reveal that specialized kinesin tail domains play active roles in the initiation and continuation of motor movement.

Reconstitution of an active human CENP-E motor

CENP-E is a large kinesin motor protein which plays pivotal roles in mitosis by facilitating chromosome capture and alignment, and promoting microtubule flux in the spindle. So far, it has not been possible to obtain active human CENP-E to study its molecular properties. Xenopus CENP-E motor has been characterized in vitro and is used as a model motor; however, its protein sequence differs significantly from human CENP-E. Here, we characterize human CENP-E motility in vitro. Full-length CENP-E exhibits an increase in run length and longer residency times on microtubules when compared to CENP-E motor truncations, indicating that the C-terminal microtubule-binding site enhances the processivity when the full-length motor is active. In contrast with constitutively active human CENP-E truncations, full-length human CENP-E has a reduced microtubule landing rate in vitro, suggesting that the non-motor coiled-coil regions self-regulate motor activity. Together, we demonstrate that human CENP-E is a processive motor, providing a useful tool to study the mechanistic basis for how human CENP-E drives chromosome congression and spindle organization during human cell division.

Single depolymerizing and transport kinesins stabilize microtubule ends

Microtubules are highly dynamic cellular filaments and an accurate control of their length is important for many intracellular processes like cell division. Among other factors, microtubule length is actively modulated by motors from the kinesin superfamily. For example, yeast kinesin-8, Kip3, motors depolymerize microtubules by a cooperative, force- and length-dependent mechanism. However, whether single motors can also depolymerize microtubules is unclear. Here, we measured how single kinesin motors influenced the stability of microtubules in an in vitro assay. Using label-free interference reflection microscopy, we determined the spontaneous microtubule depolymerization rate of stabilized microtubules in the presence of kinesins. Surprisingly, we found that both single Kip3 and nondepolymerizing kinesin-1 transport motors, used as a control, stabilized microtubules further. For Kip3, this behavior is contrary to the collective force-dependent depolymerization activity of multiple motors. Because of the control measurement, the finding may hint at a more general stabilization mechanism. The complex, concentration-dependent interaction with microtubule ends provides new insights into the molecular mechanism of kinesin-8 and its regulatory function of microtubule length.

Leaving no-one behind: how CENP-E facilitates chromosome alignment

Chromosome alignment and biorientation is essential for mitotic progression and genomic stability. Most chromosomes align at the spindle equator in a motor-independent manner. However, a subset of polar kinetochores fail to bi-orient and require a microtubule motor-based transport mechanism to move to the cell equator. Centromere Protein E (CENP-E/KIF10) is a kinesin motor from the Kinesin-7 family, which localizes to unattached kinetochores during mitosis and utilizes plus-end directed microtubule motility to slide mono-oriented chromosomes to the spindle equator. Recent work has revealed how CENP-E cooperates with chromokinesins and dynein to mediate chromosome congression and highlighted its role at aligned chromosomes. Additionally, we have gained new mechanistic insights into the targeting and regulation of CENP-E motor activity at the kinetochore. Here, we will review the function of CENP-E in chromosome congression, the pathways that contribute to CENP-E loading at the kinetochore, and how CENP-E activity is regulated during mitosis.

Intrinsically Disordered Domain of Kinesin-3 Kif14 Enables Unique Functional Diversity

In addition to their force-generating motor domains, kinesin motor proteins feature various accessory domains enabling them to fulfill a variety of functions in the cell. Human kinesin-3, Kif14, localizes to the midbody of the mitotic spindle and is involved in the progression of cytokinesis. The specific motor properties enabling Kif14's cellular functions, however, remain unknown. Here, we show in vitro that the intrinsically disordered N-terminal domain of Kif14 enables unique functional diversity of the kinesin. Using single molecule TIRF microscopy, we found that Kif14 exists either as a diffusible monomer or as processive dimer and that the disordered domain (1) enables diffusibility of the monomeric Kif14, (2) renders the dimeric Kif14 super-processive and enables the kinesin to pass through highly crowded areas, (3) enables robust, autonomous Kif14 tracking of growing microtubule tips, independent of microtubule end-binding (EB) proteins, and (4) is sufficient to enable crosslinking of parallel microtubules and necessary to enable Kif14-driven sliding of antiparallel ones. We explain these features of Kif14 by the observed diffusible interaction of the disordered domain with the microtubule lattice and the observed increased affinity of the disordered domain for GTP-bound tubulin. We suggest that the disordered domain tethers the motor domain to the microtubule providing a diffusible foothold and a regulatory hub, tuning the kinesin's interaction with microtubules. Our findings thus exemplify pliable protein tethering as a fundamental mechanism of molecular motor regulation.

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.

Kinesin-8 motors: regulation of microtubule dynamics and chromosome movements

Microtubules are essential for intracellular transport, cell motility, spindle assembly, and chromosome segregation during cell division. Microtubule dynamics regulate the proper spindle organization and thus contribute to chromosome congression and segregation. Accumulating studies suggest that kinesin-8 motors are emerging regulators of microtubule dynamics and organizations. In this review, we provide an overview of the studies focused on kinesin-8 motors in cell division. We discuss the structures and molecular kinetics of kinesin-8 motors. We highlight the essential roles and mechanisms of kinesin-8 in the regulation of microtubule dynamics and spindle organization. We also shed light on the functions of kinesin-8 motors in chromosome movement and the spindle assembly checkpoint during the cell cycle.

Bioenergetics of the Dictyostelium Kinesin-8 Motor Isoform

The functional organization of microtubules in eukaryotic cells requires a combination of their inherent dynamic properties, interactions with motor machineries, and interactions with accessory proteins to affect growth, shrinkage, stability, and architecture. In most organisms, the Kinesin-8 family of motors play an integral role in these organizations, well known for their mitotic activities in microtubule (MT) length control and kinetochore interactions. In Dictyostelium discoideum, the function of Kinesin-8 remains elusive. We present here some biochemical properties and localization data that indicate that this motor (DdKif10) shares some motility properties with other Kinesin-8s but also illustrates differences in microtubule localization and depolymerase action that highlight functional diversity.

Selective and ATP-competitive kinesin KIF18A inhibitor suppresses the replication of influenza A virus

The influenza virus is one of the major public health threats. However, the development of efficient vaccines and therapeutic drugs to combat this virus is greatly limited by its frequent genetic mutations. Because of this, targeting the host factors required for influenza virus replication may be a more effective strategy for inhibiting a broader spectrum of variants. Here, we demonstrated that inhibition of a motor protein kinesin family member 18A (KIF18A) suppresses the replication of the influenza A virus (IAV). The expression of KIF18A in host cells was increased following IAV infection. Intriguingly, treatment with the selective and ATP‐competitive mitotic kinesin KIF18A inhibitor BTB‐1 substantially decreased the expression of viral RNAs and proteins, and the production of infectious viral particles, while overexpression of KIF18A enhanced the replication of IAV. Importantly, BTB‐1 treatment attenuated the activation of AKT, p38 MAPK, SAPK and Ran‐binding protein 3 (RanBP3), which led to the prevention of the nuclear export of viral ribonucleoprotein complexes. Notably, administration of BTB‐1 greatly improved the viability of IAV‐infected mice. Collectively, our results unveiled a beneficial role of KIF18A in IAV replication, and thus, KIF18A could be a potential therapeutic target for the control of IAV infection.

Meiosis-Specific Functions of Kinesin Motors in Cohesin Removal and Maintenance of Chromosome Integrity in Budding Yeast

Kinesin motors provide the molecular forces at the kinetochore-microtubule interface and along the spindle to control chromosome segregation. During meiosis with two rounds of microtubule assembly-disassembly, the roles of motor proteins remain unexplored. We observed that in contrast to mitosis, Cin8 and Kip3 together are indispensable for meiosis. While examining meiosis in cin8Δ kip3Δ cells, we detected chromosome breakage in the meiosis II cells. The double mutant exhibits a delay in cohesin removal during anaphase I. Consequently, some cells fail to undergo meiosis II and form dyads, while some, as they progress through meiosis II, cause a defect in chromosome integrity. We believe that in the latter cells, an imbalance of spindle-mediated force and the simultaneous persistence of cohesin on chromosomes cause their breakage. We provide evidence that tension generated by Cin8 and Kip3 through microtubule cross-linking is essential for signaling efficient cohesin removal and the maintenance of chromosome integrity during meiosis.

Kinesin family member KIF18A is a critical cellular factor that regulates the differentiation and activation of dendritic cells

BACKGROUND

KIF18A is a kinesin family member that is involved in various cellular processes including cell division, cell transformation, and carcinogenesis. However, its possible role in the regulation of host immunity has not been examined.

OBJECTIVE

The aim of this study is to investigate the functional role of KIF18A in the differentiation and activation of dendritic cells (DCs) that are the most efficient antigen-presenting cells.

METHODS

A bioinformatic analysis of the KIF18A gene family was performed to understand its sequence variability and evolutionary history. To inhibit KIF18A activity, a highly specific small molecule inhibitor for KIF18A, BTB-1 was used. DCs were differentiated from mouse bone marrow (BM) cells from 6 to 7 week old C57BL/6 mice with recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF). Expression of KIF18A was measured by Western blotting. The surface expression of differentiation and activation markers on DCs were analyzed by flow cytometry.

RESULTS

The phylogenetic analysis revealed that the KIF18A gene family is remarkably conserved across vertebrates. Interestingly, the expression of KIF18A was increased as BM precursor cells differentiated into DCs. BTB-1 treatment strongly inhibited the differentiation of BM cells into DCs in a dose-dependent manner. Furthermore, treatment of immature DCs with BTB-1 significantly impaired the expression of activation markers on DCs including MHC class I, CD80, and CD86 upon TLR4 or TLR7 treatment.

CONCLUSION

Our results reveal that KIF18A is a critical DC differentiation and activation regulator. Therefore, KIF18A could be a potential therapeutic target for immune-mediated disorders.

Kinesin-8 and Dis1/TOG collaborate to limit spindle elongation from prophase to anaphase A for proper chromosome segregation in fission yeast

High-fidelity chromosome segregation relies on proper microtubule regulation. Kinesin-8 has been shown to destabilise microtubules to reduce metaphase spindle length and chromosome movements in multiple species. XMAP215/chTOG polymerases catalyse microtubule growth for spindle assembly, elongation and kinetochore-microtubule attachment. Understanding of their biochemical activity has advanced, but little work directly addresses the functionality and interplay of these conserved factors. We utilised the synthetic lethality of fission yeast kinesin-8 (Klp5-Klp6) and XMAP215/chTOG (Dis1) to study their individual and overlapping roles. We found that the non-motor kinesin-8 tailbox is essential for mitotic function; mutation compromises plus-end-directed processivity. Klp5-Klp6 induces catastrophes to control microtubule length and, surprisingly, Dis1 collaborates with kinesin-8 to slow spindle elongation. Together, they enforce a maximum spindle length for a viable metaphase-anaphase transition and limit elongation during anaphase A to prevent lagging chromatids. Our work provides mechanistic insight into how kinesin-8 negatively regulates microtubules and how this functionally overlaps with Dis1 and highlights the importance of spindle length control in mitosis.

Expression of KIF18A Is Associated with Increased Tumor Stage and Cell Proliferation in Prostate Cancer

Background

The role of KIF18A in tumorigenesis and tumor development has been well studied in several cancers, but not in prostate cancer. In this study, we investigated the potential prognostic utility of KIF18A and its role in prostate cancer progression.

Material/Methods

We collected prostate cancer and paracancerous tissue samples from the same patient. Immunohistochemical staining was performed to investigate the KIF18A expression levels in the clinical sample. The Cancer Genome Atlas (TCGA) database was analyzed via a bioinformatics approach to gain insight into the relationship between KIF18A expression and prognosis. We examined the effect of KIF18A knockdown on PC-3 cell proliferation via colony formation and MTT assays. Flow cytometry was used to assess the effect of KIF18A knockdown on PC-3 cell apoptosis. Transwell invasion assay was performed to assess whether KIF18A affects the invasion ability of PC-3 cells.

Results

The KIF18A protein level was higher in PCa tissue than in paracancerous tissue. The In addition, upregulated KIF18A suggested a poor tumor stage and prognosis for prostate cancer patients. Our in vitro experiments demonstrated that KIF18A knockdown in PC-3 cells significantly inhibited proliferation and metastasis.

Conclusions

High KIF18A expression in prostate cancer patients predicts a poor prognosis. KIF18A knockdown inhibits prostate cell proliferation and metastasis. Therefore, this study confirms the usefulness of KIF18A as an oncological prognostic indicator and a potential therapeutic target for prostate cancer.

Probing Mitotic CENP-E Kinesin with the Tethered Cargo Motion Assay and Laser Tweezers

Coiled-coil stalks of various kinesins differ significantly in predicted length and structure; this is an adaption that helps these motors carry out their specialized functions. However, little is known about the dynamic stalk configuration in moving motors. To gain insight into the conformational properties of the transporting motors, we developed a theoretical model to predict Brownian motion of a microbead tethered to the tail of a single, freely walking molecule. This approach, which we call the tethered cargo motion (TCM) assay, provides an accurate measure of the mechanical properties of motor-cargo tethering, verified using kinesin-1 conjugated to a microbead via DNA links in vitro. Applying the TCM assay to the mitotic kinesin CENP-E unexpectedly revealed that when walking along a microtubule track, this highly elongated molecule with a contour length of 230 nm formed a 20-nm-long tether. The stalk of a walking CENP-E could not be extended fully by application of sideways force with optical tweezers (up to 4 pN), implying that CENP-E carries its cargo in a compact configuration. Assisting force applied along the microtubule track accelerates CENP-E walking, but this increase does not depend on the presence of the CENP-E stalk. Our results suggest that the unusually large stalk of CENP-E has little role in regulating its function as a transporter. The adjustable stalk configuration may represent a regulatory mechanism for controlling the physical reach between kinetochore-bound CENP-E and spindle microtubules, or it may assist localizing various kinetochore regulators in the immediate vicinity of the kinetochore-embedded microtubule ends. The TCM assay and underlying theoretical framework will provide a general guide for determining the dynamic configurations of various molecular motors moving along their tracks, freely or under force.

High kinesin family member 18A expression correlates with poor prognosis in primary lung adenocarcinoma

Background

Lung adenocarcinoma (LUAD) is the most prevalent pathological subtype of lung cancer. Kinesin family member 18A (KIF18A) plays an important role in tumorigenesis. Its roles in breast cancer, colorectal cancer, and other tumors have been demonstrated; however, studies of KIF18A in LUAD are limited. This study aimed to determine the role of KIF18A in LUAD progression and prognostic prediction.

Methods

KIF18A expression was examined in LUAD cells and tissues by immunohistochemistry and Western blotting. Cell proliferation assay was performed to study the role of KIF18A in LUAD cells. Correlations between KIF18A expression and clinicopathological features were analyzed. The role of KIF18A in LUAD prognosis was evaluated using data from The Cancer Genome Atlas (TCGA).

Results

KIF18A expression was increased in tumor cells and tissues. Downregulation of KIF18A expression resulted in the suppression of cancer cell proliferation in in vitro assays, and was particularly related to poor tumor differentiation, big tumor size, lymph node metastasis, and more advanced tumor stage. In the TCGA dataset, high KIF18A messenger RNA expression was associated with poor disease‐free and overall survival in patients with LUAD. In addition, multivariate analysis indicated that KIF18A is an independent prognostic factor of disease‐free and overall survival in LUAD.

Conclusions

Collectively, our results demonstrate that KIFl8A is highly expressed in LUAD. KIFl8A plays an important role in LUAD cell proliferation, but is a poor prognostic factor.

The Orphan Kinesin PAKRP2 Achieves Processive Motility via a Noncanonical Stepping Mechanism

Phragmoplast-associated kinesin-related protein 2 (PAKRP2) is an orphan kinesin in Arabidopsis thaliana that is thought to transport vesicles along phragmoplast microtubules for cell plate formation. Here, using single-molecule fluorescence microscopy, we show that PAKRP2 is the first orphan kinesin to exhibit processive plus-end-directed motility on single microtubules as individual homodimers. Our results show that PAKRP2 processivity is achieved despite having an exceptionally long (32 residues) neck linker. Furthermore, using high-resolution nanoparticle tracking, we find that PAKRP2 steps via a hand-over-hand mechanism that includes frequent side steps, a prolonged diffusional search of the tethered head, and tight coupling of the ATP hydrolysis cycle to the forward-stepping cycle. Interestingly, truncating the PAKRP2 neck linker to 14 residues decreases the run length of PAKRP2; thus, the long neck linker enhances the processive behavior. Based on the canonical model of kinesin stepping, such a long neck linker is expected to decrease the processivity and disrupt the coupling of ATP hydrolysis to forward stepping. Therefore, we conclude that PAKRP2 employs a noncanonical strategy for processive motility, wherein a long neck linker is coupled with a slow ATP hydrolysis rate to allow for an extended diffusional search during each step without sacrificing processivity or efficiency.

Kinesin-binding protein ensures accurate chromosome segregation by buffering KIF18A and KIF15

Kinesin-binding protein (KBP) is identified as a regulator of the kinesins KIF18A and KIF15 during mitosis. KBP buffers the activity of these motors to control chromosome alignment and spindle integrity in metaphase and prevent lagging chromosomes in anaphase.

Mitotic kinesins must be regulated to ensure a precise balance of spindle forces and accurate segregation of chromosomes into daughter cells. Here, we demonstrate that kinesin-binding protein (KBP) reduces the activity of KIF18A and KIF15 during metaphase. Overexpression of KBP disrupts the movement and alignment of mitotic chromosomes and decreases spindle length, a combination of phenotypes observed in cells deficient for KIF18A and KIF15, respectively. We show through gliding filament and microtubule co-pelleting assays that KBP directly inhibits KIF18A and KIF15 motor activity by preventing microtubule binding. Consistent with these effects, the mitotic localizations of KIF18A and KIF15 are altered by overexpression of KBP. Cells depleted of KBP exhibit lagging chromosomes in anaphase, an effect that is recapitulated by KIF15 and KIF18A overexpression. Based on these data, we propose a model in which KBP acts as a protein buffer in mitosis, protecting cells from excessive KIF18A and KIF15 activity to promote accurate chromosome segregation.

KIF18A's neck linker permits navigation of microtubule-bound obstacles within the mitotic spindle

KIF18A (kinesin-8) is required for mammalian mitotic chromosome alignment. KIF18A confines chromosome movement to the mitotic spindle equator by accumulating at the plus-ends of kinetochore microtubule bundles (K-fibers), where it functions to suppress K-fiber dynamics. It is not understood how the motor accumulates at K-fiber plus-ends, a difficult feat requiring the motor to navigate protein dense microtubule tracks. Our data indicate that KIF18A's relatively long neck linker is required for the motor's accumulation at K-fiber plus-ends. Shorter neck linker (sNL) variants of KIF18A display a deficiency in accumulation at the ends of K-fibers at the center of the spindle. Depletion of K-fiber-binding proteins reduces the KIF18A sNL localization defect, whereas their overexpression reduces wild-type KIF18A's ability to accumulate on this same K-fiber subset. Furthermore, single-molecule assays indicate that KIF18A sNL motors are less proficient in navigating microtubules coated with microtubule-associated proteins. Taken together, these results support a model in which KIF18A's neck linker length permits efficient navigation of obstacles to reach K-fiber ends during mitosis.

Emerging Insights into the Function of Kinesin-8 Proteins in Microtubule Length Regulation

Proper regulation of microtubules (MTs) is critical for the execution of diverse cellular processes, including mitotic spindle assembly and chromosome segregation. There are a multitude of cellular factors that regulate the dynamicity of MTs and play critical roles in mitosis. Members of the Kinesin-8 family of motor proteins act as MT-destabilizing factors to control MT length in a spatially and temporally regulated manner. In this review, we focus on recent advances in our understanding of the structure and function of the Kinesin-8 motor domain, and the emerging contributions of the C-terminal tail of Kinesin-8 proteins to regulate motor activity and localization.

Drosophila kinesin-8 stabilizes the kinetochore-microtubule interaction

Kinesin-8 motor proteins control chromosome alignment in a variety of species, but the specific biochemical activity responsible is unclear. Edzuka and Goshima find that Drosophila kinesin-8 (Klp67A) exhibits both microtubule plus end–stabilizing and –destabilizing activities in vitro. In cells, Klp67A, and likely human kinesin-8 (KIF18A) as well, stabilize the kinetochore–microtubule attachment during mitosis.

Kinesin-8 is required for proper chromosome alignment in a variety of animal and yeast cell types. However, it is unclear how this motor protein family controls chromosome alignment, as multiple biochemical activities, including inconsistent ones between studies, have been identified. Here, we find that Drosophila kinesin-8 (Klp67A) possesses both microtubule (MT) plus end–stabilizing and –destabilizing activity, in addition to kinesin-8's commonly observed MT plus end–directed motility and tubulin-binding activity in vitro. We further show that Klp67A is required for stable kinetochore–MT attachment during prometaphase in S2 cells. In the absence of Klp67A, abnormally long MTs interact in an “end-on” fashion with kinetochores at normal frequency. However, the interaction is unstable, and MTs frequently become detached. This phenotype is rescued by ectopic expression of the MT plus end–stabilizing factor CLASP, but not by artificial shortening of MTs. We show that human kinesin-8 (KIF18A) is also important to ensure proper MT attachment. Overall, these results suggest that the MT-stabilizing activity of kinesin-8 is critical for stable kinetochore–MT attachment.

Opposing kinesin complexes queue at plus tips to ensure microtubule catastrophe at cell ends

In fission yeast, the lengths of interphase microtubule (iMT) arrays are adapted to cell length to maintain cell polarity and to help centre the nucleus and cell division ring. Here, we show that length regulation of iMTs is dictated by spatially regulated competition between MT‐stabilising Tea2/Tip1/Mal3 (Kinesin‐7) and MT‐destabilising Klp5/Klp6/Mcp1 (Kinesin‐8) complexes at iMT plus ends. During MT growth, the Tea2/Tip1/Mal3 complex remains bound to the plus ends of iMT bundles, thereby restricting access to the plus ends by Klp5/Klp6/Mcp1, which accumulate behind it. At cell ends, Klp5/Klp6/Mcp1 invades the space occupied by the Tea2/Tip1/Tea1 kinesin complex triggering its displacement from iMT plus ends and MT catastrophe. These data show that in vivo, whilst an iMT length‐dependent model for catastrophe factor accumulation has validity, length control of iMTs is an emergent property reflecting spatially regulated competition between distinct kinesin complexes at the MT plus tip.

KIF22 coordinates CAR and EGFR dynamics to promote cancer cell proliferation

The coxsackievirus and adenovirus receptor (CAR) is a transmembrane receptor that plays a key role in cell-cell adhesion. CAR is found in normal epithelial cells and is increased in abundance in various human tumors, including lung carcinomas. We investigated the potential mechanisms by which CAR contributes to cancer cell growth and found that depletion of CAR in human lung cancer cells reduced anchorage-independent growth, epidermal growth factor (EGF)-dependent proliferation, and tumor growth in vivo. EGF induced the phosphorylation of CAR and its subsequent relocalization to cell junctions through the activation of the kinase PKCδ. EGF promoted the binding of CAR to the chromokinesin KIF22. KIF22-dependent regulation of microtubule dynamics led to delayed EGFR internalization, enhanced EGFR signaling, and coordination of CAR dynamics at cell-cell junctions. These data suggest a role for KIF22 in the coordination of membrane receptors and provide potential new therapeutic strategies to combat lung tumor growth.

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.

Structural basis of human kinesin-8 function and inhibition

Kinesins are a superfamily of ATP-dependent motors important for many microtubule-based functions, including multiple roles in mitosis. Small-molecule inhibitors of mitotic kinesins disrupt cell division and are being developed as antimitotic therapies. We investigated the molecular mechanism of the multitasking human mitotic kinesin Kif18A and its inhibition by the small molecule BTB-1. We used cryo-electron microscopy to visualize nucleotide-dependent conformational changes in microtubule-bound Kif18A, and the conformation of microtubule-bound, BTB-1-bound Kif18A. We calculated a putative BTB-1–binding site and validated this site experimentally to reveal the BTB-1 inhibition mechanism. Our work points to a general mechanism of kinesin inhibition, with wide implications for a targeted blockade of these motors in both dividing and interphase cells.

Kinesin motors play diverse roles in mitosis and are targets for antimitotic drugs. The clinical significance of these motors emphasizes the importance of understanding the molecular basis of their function. Equally important, investigations into the modes of inhibition of these motors provide crucial information about their molecular mechanisms. Kif18A regulates spindle microtubules through its dual functionality, with microtubule-based stepping and regulation of microtubule dynamics. We investigated the mechanism of Kif18A and its inhibition by the small molecule BTB-1. The Kif18A motor domain drives ATP-dependent plus-end microtubule gliding, and undergoes conformational changes consistent with canonical mechanisms of plus-end–directed motility. The Kif18A motor domain also depolymerizes microtubule plus and minus ends. BTB-1 inhibits both of these microtubule-based Kif18A activities. A reconstruction of BTB-1–bound, microtubule-bound Kif18A, in combination with computational modeling, identified an allosteric BTB-1–binding site near loop5, where it blocks the ATP-dependent conformational changes that we characterized. Strikingly, BTB-1 binding is close to that of well-characterized Kif11 inhibitors that block tight microtubule binding, whereas BTB-1 traps Kif18A on the microtubule. Our work highlights a general mechanism of kinesin inhibition in which small-molecule binding near loop5 prevents a range of conformational changes, blocking motor function.

Xenopus laevis Kif18A is a highly processive kinesin required for meiotic spindle integrity

The assembly and functionality of the mitotic spindle depends on the coordinated activities of microtubule-associated motor proteins of the dynein and kinesin superfamily. Our current understanding of the function of motor proteins is significantly shaped by studies using Xenopus laevis egg extract as its open structure allows complex experimental manipulations hardly feasible in other model systems. Yet, the Kinesin-8 orthologue of human Kif18A has not been described in Xenopus laevis so far. Here, we report the cloning and characterization of Xenopus laevis (Xl) Kif18A. Xenopus Kif18A is expressed during oocyte maturation and its depletion from meiotic egg extract results in severe spindle defects. These defects can be rescued by wild-type Kif18A, but not Kif18A lacking motor activity or the C-terminus. Single-molecule microscopy assays revealed that Xl_Kif18A possesses high processivity, which depends on an additional C-terminal microtubule-binding site. Human tissue culture cells depleted of endogenous Kif18A display mitotic defects, which can be rescued by wild-type, but not tail-less Xl_Kif18A. Thus, Xl_Kif18A is the functional orthologue of human Kif18A whose activity is essential for the correct function of meiotic spindles in Xenopus oocytes.

Summary: The highly processive kinesin Kif18A, which is expressed during oocyte maturation in Xenopus laevis, is required for correct spindle formation in meiotic egg extracts and can functionally complement human Kif18A in tissue culture cells.

Biophysics of Microtubule End Coupling at the Kinetochore

The main physiological function of mitotic kinetochores is to provide durable attachment to spindle microtubules, which segregate chromosomes in order to partition them equally between the two daughter cells. Numerous kinetochore components that can bind directly to microtubules have been identified, including ATP-dependent motors and various microtubule-associated proteins with no motor activity. A major challenge facing the field is to explain chromosome motions based on the biochemical and structural properties of these individual kinetochore components and their assemblies. This chapter reviews the molecular mechanisms responsible for the motions associated with dynamic microtubule tips at the single-molecule level, as well as the activities of multimolecular ensembles called couplers. These couplers enable persistent kinetochore motion even under load, but their exact composition and structure remain unknown. Because no natural or artificial macro-machines function in an analogous manner to these molecular nano-devices, understanding their underlying biophysical mechanisms will require conceptual advances.

Activity-Dependent Regulation of Distinct Transport and Cytoskeletal Remodeling Functions of the Dendritic Kinesin KIF21B

The dendritic arbor is subject to continual activity-dependent remodeling, requiring a balance between directed cargo trafficking and dynamic restructuring of the underlying microtubule tracks. How cytoskeletal components are able to dynamically regulate these processes to maintain this balance remains largely unknown. By combining single-molecule assays and live imaging in rat hippocampal neurons, we have identified the kinesin-4 KIF21B as a molecular regulator of activity-dependent trafficking and microtubule dynamicity in dendrites. We find that KIF21B contributes to the retrograde trafficking of brain-derived neurotrophic factor (BDNF)-TrkB complexes and also regulates microtubule dynamics through a separable, non-motor microtubule-binding domain. Neuronal activity enhances the motility of KIF21B at the expense of its role in cytoskeletal remodeling, the first example of a kinesin whose function is directly tuned to neuronal activity state. These studies suggest a model in which KIF21B navigates the complex cytoskeletal environment of dendrites by compartmentalizing functions in an activity-dependent manner.

Motility and microtubule depolymerization mechanisms of the Kinesin-8 motor, KIF19A

The kinesin-8 motor, KIF19A, accumulates at cilia tips and controls cilium length. Defective KIF19A leads to hydrocephalus and female infertility because of abnormally elongated cilia. Uniquely among kinesins, KIF19A possesses the dual functions of motility along ciliary microtubules and depolymerization of microtubules. To elucidate the molecular mechanisms of these functions we solved the crystal structure of its motor domain and determined its cryo-electron microscopy structure complexed with a microtubule. The features of KIF19A that enable its dual function are clustered on its microtubule-binding side. Unexpectedly, a destabilized switch II coordinates with a destabilized L8 to enable KIF19A to adjust to both straight and curved microtubule protofilaments. The basic clusters of L2 and L12 tether the microtubule. The long L2 with a characteristic acidic-hydrophobic-basic sequence effectively stabilizes the curved conformation of microtubule ends. Hence, KIF19A utilizes multiple strategies to accomplish the dual functions of motility and microtubule depolymerization by ATP hydrolysis.

DOI:http://dx.doi.org/10.7554/eLife.18101.001

The cells that line the airways and other passages in the body have hair-like structures called cilia on their surface. Maintaining the cilia at an appropriate length is key to allowing fluid to flow efficiently in these passages. A protein called tubulin forms scaffolds known as microtubules that give each cilium its shape and allow it to change length.

Motor proteins are also found in cilia, and travel along the microtubules to transport substances. One of these microtubule-based motors, referred to as KIF19A, accumulates at the tip of cilia and controls their length. It does so by combining two actions: it moves along the microtubule to the tip of the cilium, and then removes tubulin molecules from the end. Microtubules are straight along their length and curved at the end, and it is thought that kinesin recognizes both of these shapes in order to carry out these roles. A single region of the KIF19A protein appears to be able to accomplish both roles, but the molecular changes that the protein undergoes to do so are not known.

Wang et al. have now investigated these changes by determining the structure of the motor domain of KIF19A from mice using two experimental approaches: X-ray crystallography and cryo-electron microscopy. These structures showed that the specific structural features responsible for the protein's dual roles are indeed clustered on the side of the protein that binds to the microtubule. Wang et al. also identified the regions that make KIF19A flexible enough to fit this interface with both straight and curved microtubules.

Next, Wang et al. found that other regions of KIF19A stop it detaching from the microtubule and allow it to stabilize the curved shape of microtubule ends; this stimulates the microtubule to disassemble, or “depolymerize”. The findings show that KIF19A uses multiple strategies to enable it to carry out its roles. To understand better how KIF19A depolymerizes the microtubule, a more detailed structure of KIF19A together with tubulin will be needed. Structural studies of KIF19A in cilia will also be useful to understand how the protein controls the length of microtubules.

DOI:http://dx.doi.org/10.7554/eLife.18101.002

Elevated expression of KIF18A enhances cell proliferation and predicts poor survival in human clear cell renal carcinoma

The function of kinesin family member 18A (KIF18A) in human renal cell carcinoma (RCC) is unclear. The purpose of the current study was to determine the expression and prognostic significance of KIF18A in RCC. Specimens from 273 RCC patients undergoing nephrectomies were studied. Expression of KIF18A mRNA was examined by reverse transcription-polymerase chain reaction (RT-PCR) or quantitative PCR, and the expression of KIF18A protein was examined by immunohistochemistry and western blotting. The expression of KIF18A in clear-cell RCC cell lines was decreased using small interfering RNA targeting KIF18A, and increased by transfection with KIF18A cDNA. The proliferative ability of RCC cells in vitro and in vivo was detected by WST-1 assay and an animal xenograft model with BALB/c nude mice, respectively. The association between KIF18A expression and overall survival was calculated using Kaplan-Meier analysis. The results showed that KIF18A expression was significantly increased in RCC tissues compared with normal kidney tissues. The level of KIF18A expression was significantly associated with tumor stage, histological grade, metastasis and tumor size. Moreover, KIF18A increased the proliferation of RCC cells in vitro and in vivo. KIF18A expression was upregulated in RCC and enhanced the proliferation of RCC cells. Therefore, it appears that KIF18A plays a key role in the carcinogenesis and progression of RCC, and is a novel candidate prognostic marker for RCC patients. Furthermore, silencing KIF18A expression may serve as a new therapeutic strategy against RCC.

Kinesin-8 effects on mitotic microtubule dynamics contribute to spindle function in fission yeast

Kinesin-8 motor proteins destabilize microtubules and increase chromosome loss in mitosis. In fission yeast, aberrant microtubule-driven kinetochore pushing movements, tripolar mitotic spindles, and fluctuations in metaphase spindle length occurred in kinesin-8–deletion mutants. A mathematical model can explain these results.

Kinesin-8 motor proteins destabilize microtubules. Their absence during cell division is associated with disorganized mitotic chromosome movements and chromosome loss. Despite recent work studying effects of kinesin-8s on microtubule dynamics, it remains unclear whether the kinesin-8 mitotic phenotypes are consequences of their effect on microtubule dynamics, their well-established motor activity, or additional, unknown functions. To better understand the role of kinesin-8 proteins in mitosis, we studied the effects of deletion of the fission yeast kinesin-8 proteins Klp5 and Klp6 on chromosome movements and spindle length dynamics. Aberrant microtubule-driven kinetochore pushing movements and tripolar mitotic spindles occurred in cells lacking Klp5 but not Klp6. Kinesin-8–deletion strains showed large fluctuations in metaphase spindle length, suggesting a disruption of spindle length stabilization. Comparison of our results from light microscopy with a mathematical model suggests that kinesin-8–induced effects on microtubule dynamics, kinetochore attachment stability, and sliding force in the spindle can explain the aberrant chromosome movements and spindle length fluctuations seen.

Kinesin-5 is a microtubule polymerase

Kinesin-5 slides antiparallel microtubules during spindle assembly, and regulates the branching of growing axons. Besides the mechanical activities enabled by its tetrameric configuration, the specific motor properties of kinesin-5 that underlie its cellular function remain unclear. Here by engineering a stable kinesin-5 dimer and reconstituting microtubule dynamics in vitro, we demonstrate that kinesin-5 promotes microtubule polymerization by increasing the growth rate and decreasing the catastrophe frequency. Strikingly, microtubules growing in the presence of kinesin-5 have curved plus ends, suggesting that the motor stabilizes growing protofilaments. Single-molecule fluorescence experiments reveal that kinesin-5 remains bound to the plus ends of static microtubules for 7 s, and tracks growing microtubule plus ends in a manner dependent on its processivity. We propose that kinesin-5 pauses at microtubule plus ends and enhances polymerization by stabilizing longitudinal tubulin-tubulin interactions, and that these activities underlie the ability kinesin-5 to slide and stabilize microtubule bundles in cells.

Impact-Free Measurement of Microtubule Rotations on Kinesin and Cytoplasmic-Dynein Coated Surfaces

Knowledge about the three-dimensional stepping of motor proteins on the surface of microtubules (MTs) as well as the torsional components in their power strokes can be inferred from longitudinal MT rotations in gliding motility assays. In previous studies, optical detection of these rotations relied on the tracking of rather large optical probes present on the outer MT surface. However, these probes may act as obstacles for motor stepping and may prevent the unhindered rotation of the gliding MTs. To overcome these limitations, we devised a novel, impact-free method to detect MT rotations based on fluorescent speckles within the MT structure in combination with fluorescence-interference contrast microscopy. We (i) confirmed the rotational pitches of MTs gliding on surfaces coated by kinesin-1 and kinesin-8 motors, (ii) demonstrated the superiority of our method over previous approaches on kinesin-8 coated surfaces at low ATP concentration, and (iii) identified MT rotations driven by mammalian cytoplasmic dynein, indicating that during collective motion cytoplasmic dynein side-steps with a bias in one direction. Our novel method is easy to implement on any state-of-the-art fluorescence microscope and allows for high-throughput experiments.

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.

Probing microtubule polymerisation state at single kinetochores during metaphase chromosome motion

Kinetochores regulate the dynamics of attached microtubule bundles (kinetochore-fibres, K-fibres) to generate the forces necessary for chromosome movements in mitosis. Current models suggest that poleward-moving kinetochores are attached to depolymerising K-fibres and anti-poleward-moving kinetochores to polymerising K-fibres. How the dynamics of individual microtubules within the K-fibre relate to poleward and anti-poleward movements is poorly understood. To investigate this, we developed a live-cell imaging assay combined with computational image analysis that allows eGFP-tagged EB3 (also known as MAPRE3) to be quantified at thousands of individual metaphase kinetochores as they undergo poleward and anti-poleward motion. Surprisingly, we found that K-fibres are incoherent, containing both polymerising and depolymerising microtubules – with a small polymerisation bias for anti-poleward-moving kinetochores. K-fibres also display bursts of EB3 intensity, predominantly on anti-poleward-moving kinetochores, equivalent to more coherent polymerisation, and this was associated with more regular oscillations. The frequency of bursts and the polymerisation bias decreased upon loss of kinesin-13, whereas loss of kinesin-8 elevated polymerisation bias. Thus, kinetochores actively set the balance of microtubule polymerisation dynamics in the K-fibre while remaining largely robust to fluctuations in microtubule polymerisation.

Synthesis and biological evaluation of optimized inhibitors of the mitotic kinesin Kif18A

The mitotic spindle, a highly dynamic structure composed of microtubules, mediates the segregation of the previously duplicated genome into the two nascent daughter cells. Errors in this process contribute to pathology including tumor formation. Key for the shape and function of the mitotic spindle are kinesins, molecular motor proteins that convert chemical energy into mechanical work. Due to their fast mode of action, small molecules are valuable tools to dissect the dynamic functions of kinesins during mitosis. In this study, we report the identification of optimized small molecule inhibitors of the mitotic kinesin Kif18A. Using BTB-1, the first identified Kif18A inhibitor, as a lead compound, we synthesized a collection of derivatives. We demonstrate that some of the synthesized derivatives potently inhibited the ATPase activity of Kif18A with a half maximal inhibitory concentration (IC50) value in the low micromolar range. In vitro analysis of a panel of Kif18A-related kinesins revealed that the two most potent compounds show improved selectivity compared to BTB-1. Structure-activity relationship studies identified substituents mediating undesired inhibitory effects on microtubule polymerization. In summary, our study provides key insights into the mechanism of action of BTB-1 and its analogs, which will have a great impact on the further development of highly selective and bioactive Kif18A inhibitors. Since Kif18A is frequently overexpressed in solid tumors, such compounds are not only of great interest for basic research but also have the potential to open up new strategies for the treatment of human diseases.

Kinesin Motor Enzymology: Chemistry, Structure, and Physics of Nanoscale Molecular Machines

Molecular motors are enzymes that convert chemical potential energy into controlled kinetic energy for mechanical work inside cells. Understanding the biophysics of these motors is essential for appreciating life as well as apprehending diseases that arise from motor malfunction. This review focuses on kinesin motor enzymology with special emphasis on the literature that reports the chemistry, structure and physics of several different kinesin superfamily members.

The contribution of αβ-tubulin curvature to microtubule dynamics

Microtubules are dynamic polymers of αβ-tubulin that form diverse cellular structures, such as the mitotic spindle for cell division, the backbone of neurons, and axonemes. To control the architecture of microtubule networks, microtubule-associated proteins (MAPs) and motor proteins regulate microtubule growth, shrinkage, and the transitions between these states. Recent evidence shows that many MAPs exert their effects by selectively binding to distinct conformations of polymerized or unpolymerized αβ-tubulin. The ability of αβ-tubulin to adopt distinct conformations contributes to the intrinsic polymerization dynamics of microtubules. αβ-Tubulin conformation is a fundamental property that MAPs monitor and control to build proper microtubule networks.

Molecular mechanisms for microtubule length regulation by kinesin-8 and XMAP215 proteins

The cytoskeleton is regulated by a plethora of enzymes that influence the stability and dynamics of cytoskeletal filaments. How microtubules (MTs) are controlled is of particular importance for mitosis, during which dynamic MTs are responsible for proper segregation of chromosomes. Molecular motors of the kinesin-8 protein family have been shown to depolymerize MTs in a length-dependent manner, and recent experimental and theoretical evidence suggests a possible role for kinesin-8 in the dynamic regulation of MTs. However, so far the detailed molecular mechanisms of how these molecular motors interact with the growing MT tip remain elusive. Here we show that two distinct scenarios for the interactions of kinesin-8 with the MT tip lead to qualitatively different MT dynamics, including accurate length control as well as intermittent dynamics. We give a comprehensive analysis of the regimes where length regulation is possible and characterize how the stationary length depends on the biochemical rates and the bulk concentrations of the various proteins. For a neutral scenario, where MTs grow irrespective of whether the MT tip is occupied by a molecular motor, length regulation is possible only for a narrow range of biochemical rates, and, in particular, limited to small polymerization rates. By contrast, for an inhibition scenario, where the presence of a motor at the MT tip inhibits MT growth, the regime where length regulation is possible is extremely broad and includes high growth rates. These results also apply to situations where a polymerizing enzyme like XMAP215 and kinesin-8 mutually exclude each other from the MT tip. Moreover, we characterize the differences in the stochastic length dynamics between the two scenarios. While for the neutral scenario length is tightly controlled, length dynamics is intermittent for the inhibition scenario and exhibits extended periods of MT growth and shrinkage. On a broader perspective, the set of models established in this work quite generally suggest that mutual exclusion of molecules at the ends of cytoskeletal filaments is an important factor for filament dynamics and regulation.

Kinesin-12 Kif15 targets kinetochore fibers through an intrinsic two-step mechanism

Proteins that recognize and act on specific subsets of microtubules (MTs) enable the varied functions of the MT cytoskeleton. We recently discovered that Kif15 localizes exclusively to kinetochore fibers (K-fibers) or bundles of kinetochore-MTs within the mitotic spindle. It is currently speculated that the MT-associated protein TPX2 loads Kif15 onto spindle MTs, but this model has not been rigorously tested. Here, we show that Kif15 accumulates on MT bundles as a consequence of two inherent biochemical properties. First, Kif15 is self-repressed by its C terminus. Second, Kif15 harbors a nonmotor MT-binding site, enabling dimeric Kif15 to crosslink and slide MTs. Two-MT binding activates Kif15, resulting in its accumulation on and motility within MT bundles but not on individual MTs. We propose that Kif15 targets K-fibers via an intrinsic two-step mechanism involving molecular unfolding and two-MT binding. This work challenges the current model of Kif15 regulation and provides the first account of a kinesin that specifically recognizes a higher-order MT array.

A unique kinesin-8 surface loop provides specificity for chromosome alignment

Kif18A and Kif4A display a similar ability to attenuate the dynamics of microtubules but function to control the lengths of distinct subsets of spindle microtubules during mitosis. Kif18A and Kif4A are not functionally equivalent for chromosome alignment, and Kif18A's function in this process depends on its loop2 region.

Microtubule length control is essential for the assembly and function of the mitotic spindle. Kinesin-like motor proteins that directly attenuate microtubule dynamics make key contributions to this control, but the specificity of these motors for different subpopulations of spindle microtubules is not understood. Kif18A (kinesin-8) localizes to the plus ends of the relatively slowly growing kinetochore fibers (K-fibers) and attenuates their dynamics, whereas Kif4A (kinesin-4) localizes to mitotic chromatin and suppresses the growth of highly dynamic, nonkinetochore microtubules. Although Kif18A and Kif4A similarly suppress microtubule growth in vitro, it remains unclear whether microtubule-attenuating motors control the lengths of K-fibers and nonkinetochore microtubules through a common mechanism. To address this question, we engineered chimeric kinesins that contain the Kif4A, Kif18B (kinesin-8), or Kif5B (kinesin-1) motor domain fused to the C-terminal tail of Kif18A. Each of these chimeric kinesins localizes to K-fibers; however, K-fiber length control requires an activity specific to kinesin-8s. Mutational studies of Kif18A indicate that this control depends on both its C-terminus and a unique, positively charged surface loop, called loop2, within the motor domain. These data support a model in which microtubule-attenuating kinesins are molecularly “tuned” to control the dynamics of specific subsets of spindle microtubules.

The microtubule plus-end tracking protein ARMADILLO-REPEAT KINESIN1 promotes microtubule catastrophe in Arabidopsis

Microtubule dynamics are critically important for plant cell development. Here, we show that Arabidopsis thaliana ARMADILLO-REPEAT KINESIN1 (ARK1) plays a key role in root hair tip growth by promoting microtubule catastrophe events. This destabilizing activity appears to maintain adequate free tubulin concentrations in order to permit rapid microtubule growth, which in turn is correlated with uniform tip growth. Microtubules in ark1-1 root hairs exhibited reduced catastrophe frequency and slower growth velocities, both of which were restored by low concentrations of the microtubule-destabilizing drug oryzalin. An ARK1-GFP (green fluorescent protein) fusion protein expressed under its endogenous promoter localized to growing microtubule plus ends and rescued the ark1-1 root hair phenotype. Transient overexpression of ARK1-RFP (red fluorescent protein) increased microtubule catastrophe frequency. ARK1-fusion protein constructs lacking the N-terminal motor domain still labeled microtubules, suggesting the existence of a second microtubule binding domain at the C terminus of ARK1. ARK1-GFP was broadly expressed in seedlings, but mutant phenotypes were restricted to root hairs, indicating that ARK1's function is redundant in cells other than those forming root hairs.

Pre-anaphase chromosome oscillations are regulated by the antagonistic activities of Cdk1 and PP1 on Kif18A

Upon congression at the spindle equator, vertebrate chromosomes display oscillatory movements which typically decline as cells progress towards anaphase. Kinesin-8 Kif18A has been identified as a suppressor of chromosome movements, but how its activity is temporally regulated to dampen chromosome oscillations before anaphase onset remained mysterious. Here, we identify a regulatory network composed of cyclin-dependent kinase-1 (Cdk1) and protein phosphatase-1 (PP1) that antagonistically regulate Kif18A. Cdk1-mediated inhibitory phosphorylation of Kif18A promotes chromosome oscillations in early metaphase. PP1 induces metaphase plate thinning by directly dephosphorylating Kif18A. Chromosome attachment induces Cdk1 inactivation and kinetochore recruitment of PP1α/γ. Thus, we propose that chromosome biorientation mediates the alignment of chromosomes at the metaphase plate by tipping the balance in favour of dephosphorylated Kif18A capable of suppressing the oscillatory movements of chromosomes. Notably, interfering with chromosome oscillations severely impairs the fidelity of sister chromatid segregation demonstrating the importance of timely controlled chromosome dynamics for the maintenance of genome integrity.

The family-specific K-loop influences the microtubule on-rate but not the superprocessivity of kinesin-3 motors

The kinesin-3 family–specific, positively charged insert, the K-loop, in loop 12 of the motor domain plays a critical role in cargo transport by enhancing the initial interaction of cargo-bound dimeric motors with the microtubule. The replacement of the K-loop, however, does not abolish the superprocessive motion of this class of kinesin motors.

The kinesin-3 family (KIF) is one of the largest among the kinesin superfamily and an important driver of a variety of cellular transport events. Whereas all kinesins contain the highly conserved kinesin motor domain, different families have evolved unique motor features that enable different mechanical and functional outputs. A defining feature of kinesin-3 motors is the presence of a positively charged insert, the K-loop, in loop 12 of their motor domains. However, the mechanical and functional output of the K-loop with respect to processive motility of dimeric kinesin-3 motors is unknown. We find that, surprisingly, the K-loop plays no role in generating the superprocessive motion of dimeric kinesin-3 motors (KIF1, KIF13, and KIF16). Instead, we find that the K-loop provides kinesin-3 motors with a high microtubule affinity in the motor's ADP-bound state, a state that for other kinesins binds only weakly to the microtubule surface. A high microtubule affinity results in a high landing rate of processive kinesin-3 motors on the microtubule surface. We propose that the family-specific K-loop contributes to efficient kinesin-3 cargo transport by enhancing the initial interaction of dimeric motors with the microtubule track.

Dimerization of mammalian kinesin-3 motors results in superprocessive motion

The kinesin-3 family is one of the largest among the kinesin superfamily and its members play important roles in a wide range of cellular transport activities, yet the molecular mechanisms of kinesin-3 regulation and cargo transport are largely unknown. We performed a comprehensive analysis of mammalian kinesin-3 motors from three different subfamilies (KIF1, KIF13, and KIF16). Using Forster resonance energy transfer microscopy in live cells, we show for the first time to our knowledge that KIF16B motors undergo cargo-mediated dimerization. The molecular mechanisms that regulate the monomer-to-dimer transition center around the neck coil (NC) segment and its ability to undergo intramolecular interactions in the monomer state versus intermolecular interactions in the dimer state. Regulation of NC dimerization is unique to the kinesin-3 family and in the case of KIF13A and KIF13B requires the release of a proline-induced kink between the NC and subsequent coiled-coil 1 segments. We show that dimerization of kinesin-3 motors results in superprocessive motion, with average run lengths of ∼10 μm, and that this property is intrinsic to the dimeric kinesin-3 motor domain. This finding opens up studies on the mechanistic basis of motor processivity. Such high processivity has not been observed for any other motor protein and suggests that kinesin-3 motors are evolutionarily adapted to serve as the marathon runners of the cellular world.

Role and regulation of kinesin-8 motors through the cell cycle

Members of the kinesin-8 motor family play a central role in controlling microtubule length throughout the eukaryotic cell cycle. Inactivation of kinesin-8 causes defects in cell polarity during interphase and astral and mitotic spindle length, metaphase chromosome alignment, timing of anaphase onset and accuracy of chromosome segregation. Although the biophysical mechanism by which kinesin-8 molecules influence microtubule dynamics has been studied extensively in a variety of species, a consensus view has yet to emerge. One reason for this might be that some members of the kinesin-8 family can associate to other microtubule-associated proteins, cell cycle regulatory proteins and other kinesin family members. In this review we consider how cell cycle specific modification and its association to other regulatory proteins may modulate the function of kinesin-8 to enable it to function as a master regulator of microtubule dynamics.

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.

The molecular basis for kinesin functional specificity during mitosis

Microtubule-based motor proteins play key roles during mitosis to assemble the bipolar spindle, define the cell division axis, and align and segregate the chromosomes. The majority of mitotic motors are members of the kinesin superfamily. Despite sharing a conserved catalytic core, each kinesin has distinct functions and localization, and is uniquely regulated in time and space. These distinct behaviors and functional specificity are generated by variations in the enzymatic domain as well as the non-conserved regions outside of the kinesin motor domain and the stalk. These flanking regions can directly modulate the properties of the kinesin motor through dimerization or self-interactions, and can associate with extrinsic factors, such as microtubule or DNA binding proteins, to provide additional functional properties. This review discusses the recently identified molecular mechanisms that explain how the control and functional specification of mitotic kinesins is achieved. © 2013 Wiley Periodicals, Inc.

Kinetochore kinesin CENP-E is a processive bi-directional tracker of dynamic microtubule tips

During vertebrate mitosis, the centromere-associated kinesin CENP-E (centromere protein E) transports misaligned chromosomes to the plus ends of spindle microtubules. Subsequently, the kinetochores that form at the centromeres establish stable associations with microtubule ends, which assemble and disassemble dynamically. Here we provide evidence that after chromosomes have congressed and bi-oriented, the CENP-E motor continues to play an active role at kinetochores, enhancing their links with dynamic microtubule ends. Using a combination of single-molecule approaches and laser trapping in vitro, we demonstrate that once reaching microtubule ends, CENP-E converts from a lateral transporter into a microtubule tip-tracker that maintains association with both assembling and disassembling microtubule tips. Computational modelling of this behaviour supports our proposal that CENP-E tip-tracks bi-directionally through a tethered motor mechanism, which relies on both the motor and tail domains of CENP-E. Our results provide a molecular framework for the contribution of CENP-E to the stability of attachments between kinetochores and dynamic microtubule ends.