3.1 Characterization of Kif18A – a dual functional kinesin required for mitotic chromosome alignment
3.1.8 Kif18A controlls chromosome alignment by suppressing chromosome oscillations
Length dependent depolymerases like kinesin‐8s could define the set length of microtubules at which growth is exactly balanced by shrinkage. In this case a length‐dependent depolymerase, by antagonizing such polymerases (a MT stimulating factor like CENP‐E) , would lead to a precisely defined microtubule length at which the end concentration of the depolymerase equals a threshold, such that depolymerization exactly balances growth. Thus, a length‐dependent depolymerase, together with growth‐promoting proteins, can lead to the precise control of microtubule length.
Therefore, and may in line with these ideas, is the most recent finding that Kif18A interacts with CENP‐E, a microtubule elongation factor114. Thus, these two proteins could provide the cellular machinery implicated in the coordinated, cooperative length dependent control with the cell ensures the setting of length and position of the spindle microtubules. How is this system regulated and controlled? Is this a self feedback system?
3.1.8 Kif18A controlls chromosome alignment by suppressing chromosome oscillations
Our siRNA‐based studies indicate that Kif18A is required for chromosome congression (Fig 2.1.5). In line with our results are genetic and siRNA based studies from other organism demonstrating that kinesin‐8s are implicated in mitotic chromosome alignment21‐22; 24; 28; 31; 64.
3.1.8.1 How do kinesin‐8 motors functionally contribute to chromosome congression?
Studies from Stumpff et al. using high‐resolution live cell imaging combined with quantitative measurements of kinetochore movements revealed that Kif18A controls the persistent movement of chromosomes by both increasing the rate at which they make directional switches and slowing the velocity of their movements24.
Furthermore, Kif18A’s accumulation on kMTs and its ability to suppress oscillatory movements are dependent on its motor activity and vary within the spindle. Based on this data, Stumpff and colleagues proposed a model in which Kif18A utilizes a combination of length‐dependent plus‐end accumulation and concentration‐
dependent modulation of kMT plus‐end dynamics to affect kinetochore velocity leading to controlled mitotic chromosome positioning24. A more recent report by Jaqaman using an automated 4D live cell assay for systematic probing of HeLa kinetochore dynamics demontrated that, consistent with Stumpff et al., depletion of Kif18A increased the oscillation speed of aligned kinetochores while depletion of MCAK increased it24; 66. However, in disagreement with Stumpff et al. Kif18A depletion did not significantly changed the average oscillation and breathing periods (the bundles of kinetochore‐bound microtubules alternate between phases of growth and shrinkage, making chromosomes undergo regular oscillatory movements along the spindle axis. These movements are, in general, accompanied by changes in the distance between sister‐kinetochores, which are commonly referred to as breathing). The discrepancy may arise from manual selection of sisters as opposed to a comprehensive tracking of a large set of sister kinetochore pairs in 4D.
Jaqaman and coauthors suggested that Kif18A together with MCAK primarily set the speed of kinetochore oscillations and breathing with possibly other regulators of MT stability and turnover. Their data led them propose the following model in which MCAK and Kif18A have opposing effects on oscillation speed suggesting that these two kinetochore‐associated MT depolymerases are mutual antagonists in regulating the speed of sister kinetochore oscillations.
The speed decrease and increase induced by abrogation of MCAK and Kif18A, respectively, suggest that MCAK preferentially promotes depolymerization of MTs at the leading sister, whereas Kif18A preferentially promotes depolymerization at the lagging sister, generating resistance to the sister pair movement, leading to processive thinning of the metaphase plate. (as cells progress towards anaphase, their metaphase plates get thinner, due to a progressive decrease in oscillation speed).
Consistent with this conclusion are previous localization studies in fixed spindles showing unequal distributions of Kif18A localizationand MCAK phosphorylation between sister kinetochore pairs. Both proteins could affect oscillation speed by controlling the number of MTs in a k‐fiber that undergo a catastrophe event and/or the rate of MT depolymerization.
3.1.8.2 How do the biochemical properties of the kinesin‐8 motor enable them to regulate mitotic chromosome alignment?
Consistent with the biochemical properties of Kip3p, Gupta et al. demonstrated that in vivo, loss of Kip3p reduces the catastrophe frequency. However, loss of Kip3 also
reduces the rescue frequency of cytoplasmatic and kinetochore microtubules. Thus, Kip3p can promote both catastrophes and, unexpectedly, rescues of microtubules in cells. Moreover, Kip3p slows the disassembly rate of live microtubules in S.
cerevesiae.
Similarly, in mammalian cells, loss of the kinesin‐8 motor Kif18A unexpectedly increases both the speed of chromosome movement and reduces the frequency with which chromosomes switch directions (an activity that simultaneously requires rescue and catastrophe of kinetochore fiber microtubules attached to sister‐
kinetochores)24. To conclude, kinesin‐8 from yeast and human suppress microtubule dynamics and reduce the shortening velocity in vivo.
These acitivities contrast with the kinetochore‐coupled depolymerase, MCAK (kinesin‐13), which promotes catastrophes and, not unexpectedly, reduces rescues in pure solutions of dynamic microtubules117. As mentioned above, expectedly, MCAK increases the speed of chromosome movement. Thus, it appears that kinesin‐8 and kinesin‐13 can both promote catastrophes and in addition to that activity, kinesin‐8 are able to promote rescues. If kinesin‐8s can promote both catastrophes and rescues then this would be consistent with the observed suppression of chromosome movement shown by Stumpff et al.
Thus, it appears that kinesin‐8s in vivo possess besides to their well established plus end specific depolymerase activity, rescue promoting activity and thereby distinguish them from kinesin‐13. Based on these observations some interesting questions arise.
How could we explain the antagonistic behaviour on oscillation speeds of MCAK and Kif18A in vivo, considering that both proteins are microtubule depolymerases in vitro?
In conclusion, our studies and recent work from other labs have established that Kif18A is a keyplayer in chromosome alignment by precisely regulating kMTs plus end dynamics.
3.1.8.3 How do kinesin‐8s provide the dynamic linkage at the plus ends of kMTs for force generation to control chromosome alignment?
Our studies and recent work from other labs have established that Kif18A is a keyplayer in chromosome alignment by precisely regulating kinetochore microtubule plus end dynamics. However, do kinesin‐8s provide the dynamic linkage at the plus ends of kMTs for force generation coupled to microtubule depolymerization and polymerization to control their alignment or do they just regulate the dynamics of kMTs while other proteins act to link and to stabilize these connections?
Attempts to answer this question may arise from studies of kinesin‐8, Klp5/6 from fission yeast demonstrating that they are plus‐end directed motors that can couple microtubule depolymerization to cargo movement by harnessing the energy of tubulin depolymerization118. In this study Klp5/6 were capable of tubulin‐
depolymerisation dependent, ATP independent motion in the minus direction118. In line with this study is a report demonstrating that Klp5/6 and the Dam1 complex fulfil essential overlapping functions in coordinating controlled bipolar chromosome attachment119.
Given the overlapping function of kinesin‐8 and Dam1 in fission yeast, it is tempting to speculate that Kif18A exerts its function by a similar mechanism as the Dam1 complex in human cells. (human cells lack the Dam1 complex)
3.1.8.3.1 Do kinesin‐8s in humans functionally counterpart the Dam1 complex in yeast?
In general, the Dam1 complex (complex consists of 10 subunits) assembles into rings to form dynamic linkages to disassembling kinetochore microtubules.120 Moreover, it is assumed that the Dam1 ring formation allows processive movement on depolymerising microtubule ends, this processive movement could help chromosomes to stay attached to disassembling kinetochore microtubules121. In addition, the ring is needed as a force‐coupling device that translates the mechanical energy of the powerstroke (outward peeling of protofilaments) into a sustained movement along the lattice121‐122. In conclusion, the Dam1 complex assembles into rings thereby providing dynamic linkages to maintain connections with dynamic microtubule polymers and couple microtubule disassembly to chromosome movement.
Further support for the above mentioned idea that Kif18A may resembles the function of the Dam1 complex comes from electron microscopy studies. Moores et al. observed that Kif18A, similar to MCAK, can induce protofilament bending.
This curled form (tubulin rings) of the microtubule protofilament is evident in the electron microscope104; 110. Moreover, this study demonstrated that the rings are composed of two bands of protein density: an outer band of curved tubulin dimers and an inner band of kinesin‐8‐MD. A subset of these rings was composed of 14 kinesin‐8‐MD–tubulin dimer complexes. Thus, kinesin‐8 and the Dam1 share common functional and biochemical similarities. First, kinesin‐8 and the Dam1 show dual localization to the plus‐ends of kinetochore microtubules and spindle microtubules24; 32; 123. Second, the localization of the Dam1 and Kinesin‐8 (Kif18A) to the plus tips of kMTs is microtubule dependent24; 32; 124. Third, loss of function of the Dam1 complex results in a checkpoint dependent arrest and causes severe chromosome congression and segregation defects24; 32; 123. Fourth, both the Dam1 and kinesin‐8, Klp5/6, from fission yeast can move cargo along a microtubule in a depolymerization‐coupled manner118; 121.
Finally, both form cooperative assemblies. Thus, the cooperative mechanism shown to be the underlying mechanism for kinesin‐8 mediated microtubule disassembly in vitro35 could allow kinesin‐8 to assemble in rings104 to either induce depolymerisation (force generation) and to dynamically stay attached to the disassembling microtubules to move chromosomes (during congression and anaphase). From these ideas new questions arise: Are kinesin‐8 from humans able to move cargo coupled to microtubule depolymerisation in vitro? What are the linker molecules at the kinetochore (CENP‐E and/or CENP‐F)? How do kinesin‐8s transmit the forces generated by the dynamic microtubule plus end to the inner kinetochore to allow centromere oscillations during metaphase and poleward chromosome movement during anaphase? Does the ring formation induced by kinesin‐8 and the interaction with Cenp‐E, a plus end microtubule elongation factor, explain the observed suppression of kinetochore microtubule dynamics mediated by kinesin‐8?
3.1.9 Proposed model of Kif18A function at the plus tips of kinetochore