3.2 Phosphoregulation of Kif18A by Cdk1 and Plk1
3.2.2 Regulation of Kif18A by Cdk1 phosphorylation .1 Cdk1 phosphorylates Kif18A at S674 in vitro and in vivo
There is increasing evidence that molecular motors are temporally and spatially regulated, for example by phosphorylations, to ensure their specific tasks at the correct time and at the correct location in the spindle87‐89. The essential role of Kif18A in chromosome congression suggests that the activity of Kif18A has to be controlled in a precisely spatiotemporal manner.
As the timing of Kif18A phosphorylation correlates with the activity of Cdk1‐
cyclinB1 and of Plk1, these data suggest that Kif18A is a phosphotarget of Cdk1 and Plk1 in vivo and suggest that the acitivity of Kif18A is regulated by phosphorylation during mitosis.
3.2.1.1 Cdk1 and Plk1 phosphorylate the C‐terminus of Kif18A in vitro
Consistent with the in vivo studies, our in vitro kinase assays, demonstrate that His‐
Kif18AFL and His‐Kif18AC593‐898 are substrates of Cdk1 and Plk1 in vitro (Fig 2.3.4A).
These in vitro kinase assays revealed that the C‐terminus of Kif18A is stongly phosphorylated by Cdk1‐cyclinB1 (Fig 2.2.4) and by Plk1 (Fig 2.3.4B).
Taken together, based on our data it is likely that the activity of Kif18A is regulated by Cdk1 and Plk1 via C‐terminal phosphorylation events. Notably, it has been published that basic kinetic properties (e.g. motility or binding) of CENP‐E and of EG5 are regulated by Cdk1 phosphorylation of their C‐termini87‐88. (discussed in more detail under 3.2.4)
3.2.2 Regulation of Kif18A by Cdk1 phosphorylation 3.2.2.1 Cdk1 phosphorylates Kif18A at S674 in vitro and in vivo
Our data revealed that Kif18A is phosphorylated in vitro at its C‐terminus by Cdk1.
Moreover, serin to alanine substitutions at S674 and S684 indicate that these both sites are the major Cdk1 phosphorylation sites in vitro.
Most importantly, in extracts prepared from HeLa cells stably expressing GFP‐
Kif18A harbouring a serin to alanine substitution at S674 the characteristic mitotic upshift of Kif18A was also significantly abrogated compared to cells stably expressing GFP‐Kif18AWT. Additionally, sequence analyses revealed that S674 is an evolutionary conserved phosphorylation site matching the preferred Cdk1 consensus motif. In summary, these data suggest that S674 is one of the major Cdk1 phosphorylation sites at Kif18A and may Cdk1 phosphorylation regulates Kif18A at S674 in vitro and in vivo.
3.2.2.2 Cdk1 phosphorylation of Kif18A and chromosome dynamics
Our immunofluorescence analyses revealed that in Kif18A RNAi‐background, GFP‐
Kif18AWT and GFPKif18AS674D but not GFPKif18AS674A efficiently accumulated at the plus tips of kinetochore microtubules. Functional analysis of Kif18A‐RNAi HeLa cells expressing Kif18AS674A revealed that these cells were characterized by a slight increase in the duration from NEBD to anaphase onset compared to Kif18A‐RNAi cells expressing Kif18AWT or Kif18AS674D (Fig 2.2.6B). Moreover, the stable co‐
expression of HeLa cells with GFP‐Kif18AS674A and mcherry‐histon2B revealed that in a subset of these cells, chromosomes moved occasionally out and back into the metaphase plates (Fig2.2.6A). In addition, cells expressing GFP‐Kif18AS674A displayed broader metaphase plates compared to cells expressing GFP‐Kif18AWT or GFP‐Kif18AS674D (Fig 2.2.6E). Analysis of the interkinetochore distance (IKD) – a measurement of the distance between kinetochores of the two sisterchromatids – revealed that cells expressing GFP‐Kif18AS674A displayed reduced distances compared to cells expressing GFP‐Kif18AWT (data not shown).
In conclusion, Cdk1 phosphorylation appears to be involved in promoting the proper localization of Kif18A to the plus tips of kMTs and hence in controlling mitotic chromosome alignment.
Is the phosphorylation of S674 by Cdk1 involved in the asymmetric localization of Kif18A? Does the phosphorylation of Cdk1 increases the affinity of Kif18A for either the lagging or the leading kinetochore? Attempts to answer these questions may come from previous localization studies which have shown that Kif18A distributes asymmetrically to kinetochores24, however experimental evidence for the selective targeting of Kif18A to either the lagging or the leading kinetochore is lacking. Based on the in vitro findings it is assumed that the kinesin‐8 motors preferentially target to the plus tips of longer kMTs. Support for this assumption comes from studies from Jaqaman et al. who postulated that MCAK depolymerizes kMTs at the leading kinetochore, while Kif18A promotes depolymerization at the lagging one66. It is tempting to conclude that Cdk1, by phosphorylating Kif18A, may promote the selective targeting and accumulation of the motor to the outer kinetochore of sisterchromatids stably attached to spindle microtubules.
It is also interesting to consider whether Cdk1 phosphorylation at S674 is involved in the dynamic linkage of Kif18A to disassembling kMTs to generate force.
Cdk1 phosphorylation may promote the association of Kif18A with other plus tip binding proteins and/or linkerproteins which may stabilizes kinetochore‐
microtubule interactions to ensure the stable but dynamic linkage of Kif18A to kinetochores. This idea is might be in line with this report indicating that the Cdk1‐
cyclinB1 complex functions in efficient attachment of microtubules to kinetochores125. Thus, in consistence with this idea are our time lapse microscopy studies which indicate that cells expressing GFP‐Kif18AS674A are partially impaired to maintain stable attachments allowing a subset of these chromosomes to escape from the metaphase plate (Fig 2.2.6A). Consistent with the impaired KT‐MT interactions are the reduced IKD of cells expressing GFP‐Kif18AS674A as the IKD reflects the tension acting on attached, bioriented sisterchromatids.
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