Kinase and Phosphatase Cross-Talk at the Kinetochore

Abstract

Multiple kinases and phosphatases act on the kinetochore to control chromosome segregation: Aurora B, Mps1, Bub1, Plk1, Cdk1, PP1, and PP2A-B56, have all been shown to regulate both kinetochore-microtubule attachments and the spindle assembly checkpoint. Given that so many kinases and phosphatases converge onto two key mitotic processes, it is perhaps not surprising to learn that they are, quite literally, entangled in cross-talk. Inhibition of any one of these enzymes produces secondary effects on all the others, which results in a complicated picture that is very difficult to interpret. This review aims to clarify this picture by first collating the direct effects of each enzyme into one overarching schematic of regulation at the Knl1/Mis12/Ndc80 (KMN) network (a major signaling hub at the outer kinetochore). This schematic will then be used to discuss the implications of the cross-talk that connects these enzymes; both in terms of why it may be needed to produce the right type of kinetochore signals and why it nevertheless complicates our interpretations about which enzymes control what processes. Finally, some general experimental approaches will be discussed that could help to characterize kinetochore signaling by dissociating the direct from indirect effect of kinase or phosphatase inhibition in vivo. Together, this review should provide a framework to help understand how a network of kinases and phosphatases cooperate to regulate two key mitotic processes.

Triggering mitosis

“…These two phosphatases are known to play a critical role in providing localised phosphatase activity at the kinetochore to regulate microtubule attachment, SAC signalling and sister chromatid separation . Thus, a fine‐tuned balance of various kinase and phosphatase activities is critical for the coordination of mitotic progression.…”

Regulating a key mitotic regulator, polo‐like kinase 1 (PLK1)

“…PLK1’s localization and function has been heavily studied at kinetochores, a complex of proteins associated with the centromere of a chromosome, where microtubules attach during cell division (reviewed in Saurin, ). PLK1‐localization at kinetochores is highest during prometaphase, where it is recruited initially by Bub1 (Qi et al, ), NudC (Nishino et al, ), and BubR1 (Elowe et al, ; Suijkerbuijk, Vleugel, Teixeira, & Kops, ).…”

Kinetochore phosphatases suppress autonomous kinase activity to control the spindle assembly checkpoint

“…The spindle assembly checkpoint (SAC) prevents mitotic exit until chromosomes have attached to microtubules via the kinetochore 1,2 . MPS1 kinase initiates SAC signalling by localising to unattached kinetochores and phosphorylating the SAC scaffold KNL1 on repeat motifs known as 'MELT repeats' (for the amino acid consensus Met-Glu-Leu-Thr) [3][4][5] .…”

“…MPS1 kinase initiates SAC signalling by localising to unattached kinetochores and phosphorylating the SAC scaffold KNL1 on repeat motifs known as 'MELT repeats' (for the amino acid consensus Met-Glu-Leu-Thr) [3][4][5] . Once phosphorylated, these MELT motifs recruit the heterotetrameric BUB1-BUB3-BUB3-BUBR1 complex (hereafter BUB complex) to kinetochores [6][7][8][9] , which, directly or indirectly, recruits all other proteins needed to activate the SAC and block mitotic exit 1,2 . Once kinetochores attach to microtubules, the local SAC signal must be rapidly extinguished by at least three different mechanisms: 1) localised MPS1 activity is inhibited [10][11][12] , 2) key phosphorylation sites, such as the MELT repeats, are dephosphorylated by KNL1-localised phosphatases [13][14][15][16][17] , and 3) dynein motors physically transport SAC components away from kinetochores down microtubules 18 .…”