3.2 Phosphoregulation of Kif18A by Cdk1 and Plk1
3.2.3 Regulation of Kif18A by Plk1 phosphorylation .1 Kif18A and Plk1 localization at the kinetochore
Thus, it is worth to test whether cells expressing GFP‐Kif18AS674A display an increased cold‐sensitivity indicative of impaired kinetochore‐microtubule interactions compared to cells expressing GFP‐Kif18AWT and if so, to test whether components of the spindle assembly checkpoint localize to these kinetochores. It would be also of interst to investigate whether the phosphorylation of Kif18A at S674 facilitates the interaction to other plus tip binding factors like CENP‐E or CENP‐F.
To conclude, of particular interest is to investigate whether Cdk1 phosphorylation does directly affect the intrinsic kinetic properties of Kif18A e.g. increase in the affinity for the plus tips or indirect by regulating the activity of Kif18A by facilitating the interaction to other proteins which in turn regulate its activity.
For example, recent studies demonstrate that the depolymerase activity of MCAK (kinesin‐13) is coregulated by Aurora B (mitotic kinase) and inner centromere Kin‐I stimulator ICIS126.
3.2.3 Regulation of Kif18A by Plk1 phosphorylation 3.2.3.1 Kif18A and Plk1 localization at the kinetochore
Co‐immunostaining with Plk1 antibodies and GFP‐Kif18A revealed that in a subset of sister kinetochores pairs, Kif18A colocalized with Plk1 at kMT plus‐ends. At other kinetochores GFP‐Kif18A localized in a gradient along spindle MTs close to the kMTs plus‐ends and thus did not show co‐localization with Plk1.
This observation raises the interesting question at which kinetochores (mono‐or biorientated, attached or unattached, lagging or laeding) Kif18A and Plk1 co‐
localize and at which not.
Recent studies indicate that Kif18A localizes asymmetrically to sister kinetochores24, however experimental evidence to which of the both sister kinetochores is lacking.
Given the length dependent accumulation of kinesin‐8 at the microtubule plus end revealed in in vitro studies, it is assumed that the motor preferentially targets to the longer kMTs24; 66. Recent reports demonstrate that the kinase activity of Plk1 is asymmetrically distributed, high kinase levels of Plk1 were observed at kinetochores lacking stable attachment to spindle microtubules127. Hence, careful analysis of kinetochores costained with Kif18A, Plk1 and a markerprotein which e.g. only localizes to kinetochores which are under tension (pBubR1) could gain insights into the underlying mechanism of the asymmetric localization of Kif18A and its regulation e.g. by Plk1.
3.2.3.2 Plk1 phosphorylation impairs Kif18A localization
Our immunofluorescence analyses revealed that in Kif18A RNAi‐background, GFP‐
Kif18AWT and GFPKif18AS859A but not GFPKif18AS859E efficiently accumulated at the plus tips of kinetochore microtubules (Fig 2.3.5B). The localization of the non‐
phosphorylatable Plk1 phosphorylation mutant GFP‐Kif18AS859A to the plus tips of kMTs is consistent with our data demonstrating that Kif18A localizes to the ends of spindle microtubules in the absence of Plk1 kinase activity.
Thus, it is likely that the phosphorylation at S859 by Plk1 negatively regulates either the targeting of Kif18A to or maintaining at the kMTs plus ends. Given the high kinase activity of Plk1 at the kinetochores which have not yet made stable attachments raises the interesting question whether the phosphorylation of Plk1 inhibits Kif18A from specifically targeting to the plus ends of kinetochore microtubules which are not under tension.
Or does this phosphorylation directly or indirectly prevent the accumulation of Kif18A at the plus ends of kMTs? Does this phosphorylation event may act in concert with other factors (e.g. the presence of more dynamic microtubules at kinetochores which have not established stable attachments) to ensure that Kif18A is not localized to these kinetochores.
In trying to find answers to these questions one may has to re‐consider what functions Plk1 and Kif18A fulfill at the plus‐ends of kMTs. Plk1 is thought to establish and to regulate kinetochore – microtubule interactions thereby promoting stable attachments between kinetochores and spindle microtubules86; 128. Consistent with this function of Plk1 is the particular high kinase activity on the kinetochores of those chromosomes that have not yet been attached to both poles of the mitotic spindle and are not under tension86; 127. Kif18A is thought to play an essential role in reducing the oscillatory speeds of biorientated chromosomes to control their alignment in the spindle equator. Consistent with this function are localization studies of Kif18A indicating that the motor is found along spindle microtubules in prometaphase and concentrates at the kMTs plus‐ends of more or less aligned chromosomes (Fig 2.1.2 and24).
Therefore is is likely that the high kinase activity of Plk1 at the unattached kinetochore, inhibits the motor from accumulating at kinetochores which have not yet made stable attachments to spindle microtubules. What is the physiological relevance of this phosphorylation? It might be that with this phosphorylation event the cell ensures that Kif18A is only localized to biorientated kinetochores with stable attachments thereby promoting the selective suppression of kMTs dynamics within their k‐fibers. Thus, this phosphorylation event may either allows the selective enrichment of Kif18A at attached kinetochores (low Plk1 kinase acivity, hence accumulation of Kif18A) thereby controlling the efficient alignment of these sisterchromatids and the metaphase plate and moreover allows the selective inhibition of Kif18A association to unattached kinetochores allowing high microtubule dynamics and/or turnover to either generate stable attachments or to correct improper ones.
3.2.3.3 Plk1 phosphorylation of Kif18A and mitotic progression
Our functional analysis of Kif18A‐RNAi cells expressing Kif18AS859E were characterized by an increase in the duration from NEBD to anaphase onset compared to Kif18A‐RNAi cells expressing Kif18AWT or Kif18AS859A (Fig 2.3.5B). Our immunofluorescene studies indicate that cells stably expressing GFP‐Kif18AS859E are characterized by defects in chromosome alignment (Fig 2.3.5B). Thus, these studies indicate that the phosphorylation of S859 is critical for timely progression through mitosis.
Therefore it is interesting to consider if this site is dephosphorylated by e.g. protein phosphatase 1 PP1 or 2A PP2A to dynamically regulate Kif18A localization and/or activity for either the leading or lagging kinetochore. Interstingly, PP1 is localized to the outer kinetochore and both phosphatases are implicated in regulating microtubule dynamics during mitosis129‐130.
Thus, a dynamic balance of phosphorylation of Kif18A mediated by Plk1 in prometaphase (and probably other kinases) and may phosphatases regulate the association of Kif18A with kMTs plus‐ends and hence control for the dynamic movements of chromosomes to the spindle equator.
3.2.4 Cdk1 and Plk1 phosphorylation and the effects on basic kinetic