Kinetochore: Structure, Function and Evolution

Abstract

Duplicated eukaryotic chromosomes are segregated into daughter cells through cell division. Faithful chromosome segregation depends on kinetochores, which are specialized macromolecular structures built upon centromeric chromatin. The dynamic kinetochore structures connect chromosomes with spindle microtubules, power chromosome movement, and signal the activation and silencing of the spindle assembly checkpoint (SAC). Molecular analyses of the components and architecture of kinetochores have advanced rapidly in recent years. A human kinetochore contains approximately 200 proteins, many of which are evolutionarily conserved in other organisms. A histone H3 variant, CENP‐A and associated constitutive centromere proteins lay the foundation for kinetochore build‐up. Multiple kinetochore‐localised microtubule‐binding proteins including the Ndc80 complex help regulate chromosome movement. The SAC signalling originates from kinetochores and contributes to the fidelity of chromosome segregation. Many fascinating properties remain to be elucidated about the kinetochore as a fundamental machinery to maintain genomic stability.

Key Concepts:

  • Chromosome segregation in eukaryotic cells depends upon connecting spindle microtubules with special macromolecular structures on chromosomes called kinetochores.

  • The centromere is the chromosomal locus where a kinetochore is built.

  • Laying the foundation for kinetochore assembly at centromeres are CENP‐A (a histone H3 variant) containing nucleosomes and a group of CENP‐A associated proteins (termed constitutive centromere proteins).

  • There are multiple microtubule motors and nonmotor microtubule‐binding proteins localised at kinetochores to coordinate chromosome movement.

  • A 10 protein complex called KMN network is currently thought to provide the primary end‐on microtubule‐binding activity.

  • The spindle assembly checkpoint (SAC) monitors the kinetochore–microtubule attachment and signals the delay of the metaphase‐to‐anaphase transition when defects are detected.

  • Conformational change of MAD2 and assembly of the mitotic checkpoint complex (MCC) are the key events to activate the SAC.

  • Comparative studies of similar and distinct kinetochore composition, structure and function in different species and during mitosis or meiosis have provided evolutionary perspectives on mechanisms regulating chromosome segregation.

Keywords: kinetochore; centromere; mitosis; chromosome segregation; spindle assembly checkpoint; microtubule‐binding proteins

Figure 1.

Molecular build‐up of the human centromere–kinetochore complex. Shown is a schematic diagram of relative positioning of CCAN proteins (marked by single letters), several microtubule‐binding proteins and checkpoint proteins at a human centromere–kinetochore complex. The dependency relationship and position measurement were not meant to be exhaustive and based on results from many labs including (Wan et al., ) as recently reviewed by (Westhorpe and Straight, ). CENP‐A‐containing nucleosomes constitute the predominant epigenetic mark of centromeric chromatin, which may also contain canonical histone H3 nucleosomes, and the nucleosome‐like CENP‐S/T/W/X heterotetramer. CENP‐C and CENP‐N directly interacts with CENP‐A. CENP‐C N‐terminus binds to the Mis12 complex, which then recruits the Ndc80 complex and the KNL1‐Zwint1 complex to form the KMN network. The Ndc80 complex can be independently recruited to kinetochores through interaction with the CENP‐T N‐terminus. The interaction of the Ndc80 complex with microtubules may be buttressed by the Ska complex. CENP‐N forms a complex with CENP‐L/M, which then recruits CENP‐H/I/K complex and CENP‐O/P/Q/R/U complex. Note CENP‐U, CENP‐Q and KNL1 also bind to microtubules at least in vitro. The N‐terminus of KNL1 may also serve as a scaffold to bind spindle assembly checkpoint proteins, placing the checkpoint proteins in proximity to microtubule‐binding activities at kinetochores.

Figure 2.

The centromere/kinetochore under light and electron microscopy. (A) Immunofluorescence of a prometaphase HeLa cell showing centromeres (red) together with microtubules (green) and chromosomes (blue). (B) Comparison of kinetochores with (a–d) or without (e–h) microtubules attached in rat kangaroo PtK1 cells. (a and e) Classical views of the kinetochore after conventional chemical fixation methods. (b and f) The kinetochore is less distinct after improved structural preservation using high pressure freezing and freeze substitution (HPF/FS), demonstrating that the structure is more open and loosely organised than originally thought. (c–d, g–h) Structural details are revealed using electron tomography to obtain 3D images of kinetochores preserved by HPF/FS. Reproduced with permission from O'Connell et al. (). © Elsevier. (C) (top) Kinetochore fibrils (arrowhead) and flared microtubule ends (arrow) as revealed by electron tomography in a mitotic PtK1 cell. (middle and bottom) More examples of fibrillar materials associated with microtubule ends (arrows). Adapted from McIntosh et al. () with permission from Elsevier Ltd.

Figure 3.

The spindle assembly checkpoint. (a) Shown is a diagram to view the spindle assembly checkpoint as a signal transduction pathway. Unattached or tensionless kinetochores enrich some or all checkpoint proteins to initiate ‘wait anaphase’ signalling cascade. One of the major known signal transducer and signal amplifier is C‐MAD2. C‐MAD2 accumulation leads to assembly of the MCC as the checkpoint effector. The MCC then binds to APC/C to inhibit the anaphase onset. (b) The O‐C MAD2 conversion model. The MAD1:C‐MAD2 heterotetramer recruited to defective kinetochores functions as a catalyst to bind O‐MAD2 through O:C heterodimerization and converts it into C‐MAD2, probably through a transition state (empty circles). (c) Models of APC/C core, APC/CCDC20, APC/CMCC based on negative stained electron microscopy images. CDC20 is indicated in purple and the MCC in red. Adapted from Herzog et al. () with permission from AAAS.

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Liu, Song‐Tao(Aug 2014) Kinetochore: Structure, Function and Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0006237.pub2]