3.1 Characterization of Kif18A – a dual functional kinesin required for mitotic chromosome alignment
3.1.6 Kif18A localizes to the plus ends of mitotic spindle microtubules
In vivo, kinesin‐8s only localize to stabilized microtubules e.g. kinetochore microtubules. These k‐fibers are bundles of 15‐20 MTs which are connected to a kinetochore on a mitotic chromosome. These stabilized microtubules (still undergoing cycles of polymerization and depolymerization) and may other factors/signals could allow kinesin‐8 to accumulate and to form a gradient or a comet‐like structure as observed for kinesin‐8 member Kif18A24‐25; 32 and finally to act cooperatively to mediate microtubule disassembly.
However, some open questions remain: What properties do plus ends of kinetochore microtubules have for kinesin‐8 to remain bound? Or in turn which intrinsic properties share the kineisn‐8 to reside for a long time at the plus tips? Do the plus ends or the kinesin‐8 molecules have molecular signals (specific posttranslational modification of tubulin or Kif18A) or has Kif18A a consensus motif to both target and stay attached at the plus tips like the EB proteins which target growing microtubules plus tips via the SKIP motif?
3.1.6 Kif18A localizes to the plus ends of mitotic spindle microtubules
Consistent with our report, Stumpff et al. found that endogeous Kif18A exhibits a dynamic localization to the plus‐ends of kinetochore microtubules. In prometaphase cells, the motor is found along spindle MTs and localizes near the outer‐kinetochore marker Hec1 at only a subset of kinetochores (Fig 2.1.2). In metaphase cells with aligned chromosomes, Kif18A localizes as a comet‐like gradient along most if not all kMTs, and this localization requires Kif18A’s motor activity24. During anaphase, Kif18A is still seen at kinetochores and, additionally, begins to accumulate in the spindle midzone. Kif18A concentrates at the midbody during telophase and cytokinesis. In addition, co‐immunostaining of metaphase sister kinetochore pairs with Hec1 and Kif18A antibodies revealed that the peak of Kif18A fluorescence is distal to the peak of Hec1 fluorescence in respect to the chromatin consistent with localization of Kif18A at kMT plus‐ends.
However, unlike Hec1 fluorescence, which is equal on both sister kinetochores, the peak intensity of Kif18A is significantly greater on one sister kinetochore relative to the other. Moreover, analysis of optical sections through metaphase spindles revealed that the concentration of Kif18A is greater on kMTs at the periphery of the spindle compared to those at the spindle interior closer to the pole‐to‐pole axis24. Thus, the accumulation of Kif18A on kMT plus‐ends varies within the spindle.
Moreover, previous studies found that the localization of Kif18A to kinetochores was also dependent on MTs, as Kif18A was not detected at kinetochores after depolymerization of MTs with nocodazole24.
Taken together, these data suggest that Kif18A utilizes its plus‐end directed motility to form a gradient along kMTs during metaphase and that the accumulation is greater at the outer periphery of the spindle.
Studies from other organism including yeast and drosophila revealed that, like the human homolog, Kip3p and Klp67A, respectively, localize to the tips of kinetochore microtubules25; 28. Fluorescent speckle microscopy (FSM) demonstrated that Kip3 moved to the microtubule plus end by directional motility in vivo25. In addition, Kip3p was observed to be strongly concentrated at the tips of growing microtubules25. Thus, the plus‐end localization observed from different kinesin‐8 organsim may reflect a common, intrinsic biomechanical property: to translocate towards the plus tips of microtubules.
These in vivo observations are entirely consistent with the biochemical properties of kinesin‐8: highly processive plus‐end directed motors. These features allow kinesin‐
8 toefficiently target the plus ends in vivo as observed.
3.1.6.1 How do kinesin‐8s manage to accumulate at the plus tips of dynamic microtubules?
The mechanism of kinesin‐8 accumulation at microtubule tips in vivo may be due to either an intrinsic affinity for the plus end or be mediated through other plus end‐
binding factors. kMTs are characterized by their stability and attachment to the kinetochore, the multiprotein complex that attaches MTs to the centromere region.
Therfore it is likely that kinesin‐8 interact with proteins that stably link the plus ends of k‐MTs with the kinetochore. In line with this idea are reports demonstrating that Kif18A interacts with CENP‐E112. CENP‐E was shown to stabilize the connection between kMT plus‐ends and the kinetochore113. Interestingly, a most recent report demonstrates that CENP‐E, as part of its microtubule–kinetochore function, steps to the microtubule plus end, where it stabilizes a straight‐end conformation that favors α,β‐tubulin addition114.
Interestingly, previous localization studies in fixed spindles have shown unequal concentrations of Kif18A between sister kinetochore pairs24. This may suggests that Kif18A accumulates more at the longer k‐fiber. However, this is a speculative model and thus some open questions emerge: Does Kif18A has a variable affinity for different states of the kinetochore ‐ as a leading or lagging sister ‐ ? How is this affinity of the motor for either state regulated? Does this asymmetric localization depend on an interacting protein which localizes preferred to one of the both sisters? Or is this asymmetric distribution of Kif18A based on intrinsic properties of the motor itself ‐ to enrich length dependent at the plus tips of microtubules, as a consequence Kif18A should localize to the lagging sister. At which time or phase during mitotic progession does Kif18A localize asymmetrically?