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

Binding Site Maturation Modulated by Molecular Density Underlies Ndc80 Binding to Kinetochore Receptor CENP-T

Macromolecular assembly depends on tightly regulated pairwise binding interactions that are selectively favored at assembly sites while being disfavored in the soluble phase. This selective control can arise due to molecular density-enhanced binding, as recently found for the kinetochore scaffold protein CENP-T. When clustered, CENP-T recruits markedly more Ndc80 complexes than its monomeric counterpart, but the underlying molecular basis remains elusive. Here, we use quantitative in vitro assays to reveal two distinct mechanisms driving this behavior. First, Ndc80 binding to CENP-T is a two-step process: initially, Ndc80 molecules rapidly associate and dissociate from disordered N-terminal binding sites on CENP-T. Over time, these sites undergo maturation, resulting in stronger Ndc80 retention. Second, we find that this maturation transition is regulated by a kinetic barrier that is sensitive to the molecular environment. In the soluble phase, binding site maturation is slow, but within CENP-T clusters, this process is markedly accelerated. Notably, the two Ndc80 binding sites in human CENP-T exhibit distinct maturation rates and environmental sensitivities, which correlate with their different amino-acid content and predicted binding conformations. This clustering-induced maturation is evident in dividing human cells, suggesting a distinct regulatory entry point for controlling kinetochore assembly. We propose that the tunable acceleration of binding site maturation by molecular crowding may represent a general mechanism for promoting the formation of macromolecular structures.

Kinesin-7 CENP-E mediates chromosome alignment and spindle assembly checkpoint in meiosis I

In eukaryotes, meiosis is the genetic basis for sexual reproduction, which is important for chromosome stability and species evolution. The defects in meiosis usually lead to chromosome aneuploidy, reduced gamete number, and genetic diseases, but the pathogenic mechanisms are not well clarified. Kinesin-7 CENP-E is a key regulator in chromosome alignment and spindle assembly checkpoint in cell division. However, the functions and mechanisms of CENP-E in male meiosis remain largely unknown. In this study, we have revealed that the CENP-E gene was highly expressed in the rat testis. CENP-E inhibition influences chromosome alignment and spindle organization in metaphase I spermatocytes. We have found that a portion of misaligned homologous chromosomes is located at the spindle poles after CENP-E inhibition, which further activates the spindle assembly checkpoint during the metaphase-to-anaphase transition in rat spermatocytes. Furthermore, CENP-E depletion leads to abnormal spermatogenesis, reduced sperm count, and abnormal sperm head structure. Our findings have elucidated that CENP-E is essential for homologous chromosome alignment and spindle assembly checkpoint in spermatocytes, which further contribute to chromosome stability and sperm cell quality during spermatogenesis.

Kinesin-7 CENP-E in tumorigenesis: Chromosome instability, spindle assembly checkpoint, and applications

Kinesin motors are a large family of molecular motors that walk along microtubules to fulfill many roles in intracellular transport, microtubule organization, and chromosome alignment. Kinesin-7 CENP-E (Centromere protein E) is a chromosome scaffold-associated protein that is located in the corona layer of centromeres, which participates in kinetochore-microtubule attachment, chromosome alignment, and spindle assembly checkpoint. Over the past 3 decades, CENP-E has attracted great interest as a promising new mitotic target for cancer therapy and drug development. In this review, we describe expression patterns of CENP-E in multiple tumors and highlight the functions of CENP-E in cancer cell proliferation. We summarize recent advances in structural domains, roles, and functions of CENP-E in cell division. Notably, we describe the dual functions of CENP-E in inhibiting and promoting tumorigenesis. We summarize the mechanisms by which CENP-E affects tumorigenesis through chromosome instability and spindle assembly checkpoints. Finally, we overview and summarize the CENP-E-specific inhibitors, mechanisms of drug resistances and their applications.

Determinant factors for residence time of kinesin motors at microtubule ends

Kinesins constitute a superfamily of microtubule (MT)-based motor proteins, which can perform diverse biological functions in cells such as transporting vesicle, regulating MT dynamics, and segregating chromosome. Some motors such as kinesin-1, kinesin-2, and kinesin-3 do the activity mainly on the MT lattice, while others such as kinesin-7 and kinesin-8 do the activity mainly at the MT plus end. To perform the different functions, it is required that the former motors can reside on the MT lattice for longer times than at the end, while the latter motors can reside at the MT plus end for long times. Here, a simple but general theory of the MT-end residence time of the kinesin motor is presented, with which the factors dictating the residence time are determined. The theory is further used to study specifically the MT-end residence times of Drosophila kinesin-1, kinesin-2/KIF3AB, kinesin-3/Unc104, kinesin-5/Eg5, kinesin-7/CENP-E, and kinesin-8/Kip3 motors, with the theoretical results being in agreement with the available experimental data.

CLASP2 recognizes tubulins exposed at the microtubule plus-end in a nucleotide state-sensitive manner

CLASPs (cytoplasmic linker-associated proteins) are ubiquitous stabilizers of microtubule dynamics, but their molecular targets at the microtubule plus-end are not understood. Using DNA origami-based reconstructions, we show that clusters of human CLASP2 form a load-bearing bond with terminal non-GTP tubulins at the stabilized microtubule tip. This activity relies on the unconventional TOG2 domain of CLASP2, which releases its high-affinity bond with non-GTP dimers upon their conversion into polymerization-competent GTP-tubulins. The ability of CLASP2 to recognize nucleotide-specific tubulin conformation and stabilize the catastrophe-promoting non-GTP tubulins intertwines with the previously underappreciated exchange between GDP and GTP at terminal tubulins. We propose that TOG2-dependent stabilization of sporadically occurring non-GTP tubulins represents a distinct molecular mechanism to suppress catastrophe at the freely assembling microtubule ends and to promote persistent tubulin assembly at the load-bearing tethered ends, such as at the kinetochores in dividing cells.

Modeling Study of the Dynamics of Kinesin-14 Molecular Motors

Kinesin-14s constitute a subfamily of the large superfamily of adenosine triphosphate-dependent microtubule-based motor proteins. Kinesin-14s have the motor domain at the C-terminal end of the peptide, playing key roles during spindle assembly and maintenance. Some of them are nonprocessive motors, whereas others can move processively on microtubules. Here, we take budding yeast Cik1-Kar3 and human HSET as examples to study theoretically the dynamics of the processive kinesin-14 motor moving on the single microtubule under load, the dynamics of the motor coupled with an Ndc80 protein moving on the single microtubule, the dynamics of the motor moving in microtubule arrays, and so on. The dynamics of the nonprocessive Drosophila Ncd motor is also discussed. The studies explain well the available experimental data and, moreover, provide predicted results. We show that the processive kinesin-14 motors can move efficiently in microtubule arrays toward the minus ends, and after reaching the minus ends, they can stay there stably, thus performing the function of organizing the microtubules in the bipolar spindle into polar arrays at the spindle poles.

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.

Ultrafast Force-Clamp Spectroscopy of Microtubule-Binding Proteins

Optical trapping has been instrumental for deciphering translocation mechanisms of the force-generating cytoskeletal proteins. However, studies of the dynamic interactions between microtubules (MTs) and MT-associated proteins (MAPs) with no motor activity are lagging. Investigating the motility of MAPs that can diffuse along MT walls is a particular challenge for optical-trapping assays because thermally driven motions rely on weak and highly transient interactions. Three-bead, ultrafast force-clamp (UFFC) spectroscopy has the potential to resolve static and diffusive translocations of different MAPs with sub-millisecond temporal resolution and sub-nanometer spatial precision. In this report, we present detailed procedures for implementing UFFC, including setup of the optical instrument and feedback control, immobilization and functionalization of pedestal beads, and preparation of MT dumbbells. Example results for strong static interactions were generated using the Kinesin-7 motor CENP-E in the presence of AMP-PNP. Time resolution for MAP-MT interactions in the UFFC assay is limited by the MT dumbbell relaxation time, which is significantly longer than reported for analogous experiments using actin filaments. UFFC, however, provides a unique opportunity for quantitative studies on MAPs that glide along MTs under a dragging force, as illustrated using the kinetochore-associated Ska complex.

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.

Kinesin-7 CENP-E regulates the formation and structural maintenance of the acrosome

The acrosome is a special organelle that develops from the Golgi apparatus and the endolysosomal compartment in the spermatids. Centromere protein E (CENP-E) is an essential kinesin motor in chromosome congression and alignment. This study is aimed at investigating the roles and mechanisms of kinesin-7 CENP-E in the formation of the acrosome during spermatogenesis. Male ICR mice are injected with GSK923295 for long-term inhibition of CENP-E. Chemical inhibition and siRNA-mediated knockdown of CENP-E are carried out in the GC-2 spd cells. The morphology of the acrosomes is determined by the HE staining, immunofluorescence, and transmission electron microscopy. We have identified CENP-E is a key factor in the formation and structural maintenance of the acrosome during acrosome biogenesis. Long-term inhibition of CENP-E by GSK923295 results in the asymmetric acrosome and the dispersed acrosome. CENP-E depletion leads to the malformation of the Golgi complex and abnormal targeting of the PICK1- and PIST-positive Golgi-associated vesicles. Our findings uncover an essential role of CENP-E in membrane trafficking and structural organization of the acrosome in the spermatids during spermatogenesis. Our results shed light on the molecular mechanisms involved in vesicle trafficking and architecture maintenance of the acrosome.

Microtubule end conversion mediated by motors and diffusing proteins with no intrinsic microtubule end-binding activity

Accurate chromosome segregation relies on microtubule end conversion, the ill-understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus-ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are elusive. We reconstituted end conversion in vitro using two kinetochore components: the plus end-directed kinesin CENP-E and microtubule-binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENP-E is to ensure close proximity between Ndc80 complexes and the microtubule plus-end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus-end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip-binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall-binding proteins with finely tuned force-dependent motility characteristics.

Intracellular cargo transport by single-headed kinesin motors

Kinesin motor proteins that drive intracellular transport share an overall architecture of two motor domain-containing subunits that dimerize through a coiled-coil stalk. Dimerization allows kinesins to be processive motors, taking many steps along the microtubule track before detaching. However, whether dimerization is required for intracellular transport remains unknown. Here, we address this issue using a combination of in vitro and cellular assays to directly compare dimeric motors across the kinesin-1, -2, and -3 families to their minimal monomeric forms. Surprisingly, we find that monomeric motors are able to work in teams to drive peroxisome dispersion in cells. However, peroxisome transport requires minimal force output, and we find that most monomeric motors are unable to disperse the Golgi complex, a high-load cargo. Strikingly, monomeric versions of the kinesin-2 family motors KIF3A and KIF3B are able to drive Golgi dispersion in cells, and teams of monomeric KIF3B motors can generate over 8 pN of force in an optical trap. We find that intracellular transport and force output by monomeric motors, but not dimeric motors, are significantly decreased by the addition of longer and more flexible motor-to-cargo linkers. Together, these results suggest that dimerization of kinesin motors is not required for intracellular transport; however, it enables motor-to-motor coordination and high force generation regardless of motor-to-cargo distance. Dimerization of kinesin motors is thus critical for cellular events that require an ability to generate or withstand high forces.

Mechanisms of kinesin-7 CENP-E in kinetochore-microtubule capture and chromosome alignment during cell division

Chromosome congression is essential for faithful chromosome segregation and genomic stability in cell division. Centromere-associated protein E (CENP-E), a plus-end-directed kinesin motor, is required for congression of pole-proximal chromosomes in metaphase. CENP-E accumulates at the outer plate of kinetochores and mediates the kinetochore-microtubule capture. CENP-E also transports the chromosomes along spindle microtubules towards the equatorial plate. CENP-E interacts with Bub1-related kinase, Aurora B and core kinetochore components during kinetochore-microtubule attachment. In this review, we introduce the structures and mechanochemistry of kinesin-7 CENP-E. We highlight the complicated interactions between CENP-E and partner proteins during chromosome congression. We summarise the detailed roles and mechanisms of CENP-E in mitosis and meiosis, including the kinetochore-microtubule capture, chromosome congression/alignment in metaphase and the regulation of spindle assembly checkpoint. We also shed a light on the roles of CENP-E in tumourigenesis and CENP-E's specific inhibitors.