Insights into the process of EB1-dependent tip-tracking of kinesin-14 Ncd. The role of the microtubule

The Kinesin-14 tail: Dual microtubule binding domains drive spindle morphogenesis through tight microtubule cross-linking and robust sliding

Proper spindle assembly requires the Kinesin-14 (K-14) family of motors to organize microtubules (MT) into the bipolar spindle by cross-linking and sliding antiparallel and parallel MTs through their motor and tail domains. How they mediate these different activities is unclear. We identified two MT-binding domains (MBD1 and MBD2) within the Xenopus K-14 XCTK2 tail and found that MBD1 MT affinity was weaker than MBD2. Comparable with full-length GFP-XCTK2 wild-type protein (GX-WT), GFP-XCTK2 containing the MBD1 mutations (GX-MBD1mut) stimulated spindle assembly, localized moderately on the spindle, and formed narrow spindles. In contrast, GX-MBD2mut only partially stimulated spindle assembly, localized weakly on the spindle, and formed shorter spindles. Biochemical reconstitution of MT cross-linking and sliding demonstrated that GX-MBD2mut slid antiparallel MTs faster than GX-WT and GX-MBD1mut. However, GX-WT and GX-MBD1mut statically cross-linked the majority of parallel MTs, whereas GX-MBD2mut equally slid and statically cross-linked parallel MTs without affecting their sliding velocity. These results provide a mechanism by which the two different MBDs in the K-14 tail balance antiparallel MT sliding velocity (MBD1) and tight parallel MT cross-linking (MBD2), which are important for spindle assembly and localization, and provide a basis for characterizing how molecular motors organize MTs within the spindle.

The Kinesin-14 Tail: Dual microtubule binding domains drive spindle morphogenesis through tight microtubule cross-linking and robust sliding

Proper spindle assembly requires the Kinesin-14 family of motors to organize microtubules (MTs) into the bipolar spindle by cross-linking and sliding anti-parallel and parallel MTs through their motor and tail domains. How they mediate these different activities is unclear. We identified two MT binding domains (MBD1 and MBD2) within the Xenopus Kinesin-14 XCTK2 tail and found that MBD1 MT affinity was weaker than MBD2. Comparable to full-length GFP-XCTK2 wild-type protein (GX-WT), GFP-XCTK2 containing the MBD1 mutations (GX-MBD1mut) stimulated spindle assembly, localized moderately on the spindle, and formed narrow spindles. In contrast, GX-MBD2mut only partially stimulated spindle assembly, localized weakly on the spindle, and formed shorter spindles. Biochemical reconstitution of MT cross-linking and sliding demonstrated that GX-MBD2mut slid anti-parallel MTs faster than GX-WT and GX-MBD1mut. However, GX-WT and GX-MBD1mut statically cross-linked the majority of parallel MTs, whereas GX-MBD2mut equally slid and statically cross-linked parallel MTs without affecting their sliding velocity. These results provide a mechanism by which the two different MT binding domains in the Kinesin-14 tail balance anti-parallel MT sliding velocity (MBD1) and tight parallel MT cross-linking (MBD2), which are important for spindle assembly and localization, and provide a basis for characterizing how molecular motors organize MTs within the spindle.

Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate dynamic instability

Kinesin-14 motors represent an essential class of molecular motors that bind to microtubules and then walk toward the microtubule minus-end. However, whether these motors can interact with growing plus-ends of microtubules to impact the lengthening of microtubules remains unknown. We found that Kinesin-14 motors could bind to a protein that resides at growing microtubule plus-ends and then pull this protein away from the growing end. This interaction acted to disrupt microtubule growth and decrease microtubule lengths in cells, likely by exerting minus-end–directed forces at the microtubule tip to alter the configuration of the growing microtubule plus-end. This work demonstrates general principles for the diverse roles that force-generating molecular motors can play in regulating cellular processes.

Kinesin-14 molecular motors represent an essential class of proteins that bind microtubules and walk toward their minus-ends. Previous studies have described important roles for Kinesin-14 motors at microtubule minus-ends, but their role in regulating plus-end dynamics remains controversial. Kinesin-14 motors have been shown to bind the EB family of microtubule plus-end binding proteins, suggesting that these minus-end–directed motors could interact with growing microtubule plus-ends. In this work, we explored the role of minus-end–directed Kinesin-14 motor forces in controlling plus-end microtubule dynamics. In cells, a Kinesin-14 mutant with reduced affinity to EB proteins led to increased microtubule lengths. Cell-free biophysical microscopy assays were performed using Kinesin-14 motors and an EB family marker of growing microtubule plus-ends, Mal3, which revealed that when Kinesin-14 motors bound to Mal3 at growing microtubule plus-ends, the motors subsequently walked toward the minus-end, and Mal3 was pulled away from the growing microtubule tip. Strikingly, these interactions resulted in an approximately twofold decrease in the expected postinteraction microtubule lifetime. Furthermore, generic minus-end–directed tension forces, generated by tethering growing plus-ends to the coverslip using λ-DNA, led to an approximately sevenfold decrease in the expected postinteraction microtubule growth length. In contrast, the inhibition of Kinesin-14 minus-end–directed motility led to extended tip interactions and to an increase in the expected postinteraction microtubule lifetime, indicating that plus-ends were stabilized by nonmotile Kinesin-14 motors. Together, we find that Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate microtubule lengths in cells.

The Putative RNA-Binding Protein Dri1 Promotes the Loading of Kinesin-14/Klp2 to the Mitotic Spindle and Is Sequestered into Heat-Induced Protein Aggregates in Fission Yeast

Cells form a bipolar spindle during mitosis to ensure accurate chromosome segregation. Proper spindle architecture is established by a set of kinesin motors and microtubule-associated proteins. In most eukaryotes, kinesin-5 motors are essential for this process, and genetic or chemical inhibition of their activity leads to the emergence of monopolar spindles and cell death. However, these deficiencies can be rescued by simultaneous inactivation of kinesin-14 motors, as they counteract kinesin-5. We conducted detailed genetic analyses in fission yeast to understand the mechanisms driving spindle assembly in the absence of kinesin-5. Here, we show that deletion of the dri1 gene, which encodes a putative RNA-binding protein, can rescue temperature sensitivity caused by cut7-22, a fission yeast kinesin-5 mutant. Interestingly, kinesin-14/Klp2 levels on the spindles in the cut7 mutants were significantly reduced by the dri1 deletion, although the total levels of Klp2 and the stability of spindle microtubules remained unaffected. Moreover, RNA-binding motifs of Dri1 are essential for its cytoplasmic localization and function. We have also found that a portion of Dri1 is spatially and functionally sequestered by chaperone-based protein aggregates upon mild heat stress and limits cell division at high temperatures. We propose that Dri1 might be involved in post-transcriptional regulation through its RNA-binding ability to promote the loading of Klp2 on the spindle microtubules.

A simple and reproducible protocol of glass surface silanization for TIRF microscopy imaging

We describe a simple and reproducible protocol for the preparation of microscope glass slides for in vitro motility assays that use total internal reflection fluorescence microscopy. The developed method utilizes trimethylchlorosilane (TMCS) as a silanizing reagent, which in the presence of imidazole as a catalyst and under optimized conditions enables reproducible preparation of high-quality hydrophobic glass surfaces. This method presents a simplification and improvement in reproducibility over the commonly applied protocol utilizing dichlorodimethylsilane (DDS) as a silanizing agent. We demonstrate the applicability of the new method by performing the analysis of the interactions of a molecular motor, kinesin-1 with microtubules.

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

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