Making an effective switch at the kinetochore by phosphorylation and dephosphorylation

Abstract

The kinetochore, the proteinaceous structure on the mitotic centromere, functions as a mechanical latch that hooks onto microtubules to support directional movement of chromosomes. The structure also brings in a number of signaling molecules, such as kinases and phosphatases, which regulate microtubule dynamics and cell cycle progression. Erroneous microtubule attachment is destabilized by Aurora B-mediated phosphorylation of multiple microtubule-binding protein complexes at the kinetochore, such as the KMN network proteins and the Ska/Dam1 complex, while Plk-dependent phosphorylation of BubR1 stabilizes kinetochore-microtubule attachment by recruiting PP2A-B56. Spindle assembly checkpoint (SAC) signaling, which is activated by unattached kinetochores and inhibits the metaphase-to-anaphase transition, depends on kinetochore recruitment of the kinase Bub1 through Mps1-mediated phosphorylation of the kinetochore protein KNL1 (also known as Blinkin in mammals, Spc105 in budding yeast, and Spc7 in fission yeast). Recruitment of protein phosphatase 1 to KNL1 is necessary to silence the SAC upon bioriented microtubule attachment. One of the key unsolved questions in the mitosis field is how a mechanical change at the kinetochore upon microtubule attachment is converted to these and other chemical signals that control microtubule attachment and the SAC. Rapid progress in the field is revealing the existence of an intricate signaling network created right on the kinetochore. Here we review the current understanding of phosphorylation-mediated regulation of kinetochore functions and discuss how this signaling network generates an accurate switch that turns on and off the signaling output in response to kinetochore-microtubule attachment.

Figure 1. Schematic architecture of the vertebrate kinetochore

Kinetochore components are arranged to highlight the overall geometry of the kinetochore, with more chromosome-proximal components considered inner and the microtubule-proximal components considered outer.

Figure 2. Phosphorylation sites at the kinetochore

A comparison of known phosphorylations present at the kinetochore of a chromosome before (top panel) and after (bottom panel) achieving bioriented microtubule attachments. The unattached kinetochore is generating the spindle assembly checkpoint (SAC) cell cycle arrest via formation of the mitotic checkpoint complex (MCC). The color and letter within each phosphorylation (star shapes) correspond to the kinase responsible (see legend, top). Question marks in the lower panel denote phosphorylations that may be retained upon borientation but for which more information is required.

Figure 3. Lateral attachments

Diagram of the kinetochore components important for establishing initial, lateral attachments to microtubules early in prometaphase. The dual roles of Aurora B dependent phosphorylation in recruiting the microtubule-interacting motor proteins dynein and CENP-E to kinetochores and destabilizing the interactions between microtubules and the Ndc80 complex are shown.

Figure 4. Phospho-regulation of kinetochore functions

Schematic illustrations of kinase and phosphatase signaling networks controlling kinetochore function. Colored arrows and inhibition symbols show the activity of kinases and phosphatases of the corresponding color. Large black arrows and inhibition symbols denote the functional consequences and feedback loops generated by this phospho-signaling, with small black arrows representing subprocesses within each signaling network.