Earnshaw Lab - Cell AuthorsSacristan, C., Samejima,K., Ruiz, L.A., Deb, M., Lambers, M.L.A., Buckle, A., Brackley, C.A., Robertson, D., Hori, T., Webb, S., Kiewisz, R., Bepler, T., van Kwawegen, E., Risteski, P., Vukušić, K., Tolić, I.M., Müller-Reichert, T., Fukagawa, T., Gilbert, N., Marenduzzo, D., Earnshaw, W.C., and Kops, G.J.P.L. Summary of Paper by Natalia Kochanova, Earnshaw Lab Image Researchers in Edinburgh and at the Hubrecht Institute have used genomics and modelling (see above) and microscopy, respectively, to demonstrate for the first time that kinetochores, the regions that attach mitotic chromosomes to spindle microtubules and direct chromosome segregation, are bipartite structures that can bind two microtubule bundles. Genomic mapping (above) clearly reveals this bipartite structure (blue bars show regions that make contacts to the left of the kinetochore - white region; red bars make contacts to the right). This structure may help to promote efficient chromosome attachment to microtubules, but poses a risk for incorrect attachments that can lead to cancer. Centromeres are marked by histone H3 variant CENP-A, with CENP-A nucleosomes being interspersed with H3 nucleosomes and this whole domain being flanked by the pericentromeric chromatin. CENP-A containing nucleosomes are the foundation for the kinetochore, which is important for microtubule attachment and chromosome segregation.Classic models suggested that one microtubule bundle is attached to a button-like compact kinetochore. However, in recent work from the Earnshaw and the Kops labs, published in Cell, the team discovered that the kinetochore domain has a bipartite sub-structure.Indeed, expansion microscopy in cells from several organisms showed two kinetochore domains frequently present on single mitotic chromatids (i.e. four domains per mitotic chromosome). This was further confirmed by Kumiko Samejima’s Capture-C experiments, which mapped the directionality of 3D organisation of centromeres in interphase and mitosis and ChIP-seq from Moonmoon Deb, who found out a characteristic “cat’s ears” ChIP-seq pattern of condensin II on the non-repetitive centromere of the Z chromosome. Capture-C experiments on chicken cell lines acutely depleted of both condensin I and II demonstrated that condensin, but not cohesin, is required for centromere folding into this bipartite structure.Interestingly, expansion microscopy and electron tomography revealed that this bipartite kinetochore can bind two discrete microtubule bundles, and these can form merotelic attachments (one kinetochore attached to opposite spindle poles) in lagging chromosomes in cancer cell lines. Cohesin is required to prevent the merotelic attachments, which are greatly increased following the depletion of Sororin, a protein that helps stablilise cohesin on chromatin. Thus, the centromeric kinetochore domain structure is shaped by condensin and stabilized by cohesin. It is bipartite, with two bundles of microtubules attached to it. Future understanding of how this conformation can lead to formation of merotelic attachments may contribute to prevention of chromosomal instability in diseases.Related LinksJournal URLEarnshaw Lab WebsiteDOI This article was published on 2024-06-17
