Faithful chromosome segregation is mandatory for cell and organismal viability. Kinetochores, large protein assemblies embedded in centromeric chromatin, establish a mechanical link between chromosomes and spindle microtubules. The KMN network, a conserved 10-subunit kinetochore complex, harbors the microtubule-binding interface. RWD domains in the KMN subunits Spc24 and Spc25 mediate kinetochore targeting of the microtubule-binding subunits by interacting with the Mis12 complex, a KMN subcomplex that tethers directly onto the underlying chromatin layer. Here, we show that Knl1, a KMN subunit involved in mitotic checkpoint signaling, also contains RWD domains that bind the Mis12 complex and that mediate kinetochore targeting of Knl1. By reporting the first 3D electron microscopy structure of the KMN network, we provide a comprehensive framework to interpret how interactions of RWD-containing proteins with the Mis12 complex shape KMN network topology. Our observations unveil a regular pattern in the construction of the outer kinetochore.

Posttranslational modifications of core histones contribute to driving changes in chromatin conformation and compaction. Herein, we investigated the role of histone deacetylation on the mitotic process by inhibiting histone deacetylases shortly before mitosis in human primary fibroblasts. Cells entering mitosis with hyperacetylated histones displayed altered chromatin conformation associated with decreased reactivity to the anti-Ser 10 phospho H3 antibody, increased recruitment of protein phosphatase 1-δ on mitotic chromosomes, and depletion of heterochromatin protein 1 from the centromeric heterochromatin. Inhibition of histone deacetylation before mitosis produced defective chromosome condensation and impaired mitotic progression in living cells, suggesting that improper chromosome condensation may induce mitotic checkpoint activation. In situ hybridization analysis on anaphase cells demonstrated the presence of chromatin bridges, which were caused by persisting cohesion along sister chromatid arms after centromere separation. Thus, the presence of hyperacetylated chromatin during mitosis impairs proper chromosome condensation during the pre-anaphase stages, resulting in poor sister chromatid resolution. Lagging chromosomes consisting of single or paired sisters were also induced by the presence of hyperacetylated histones, indicating that the less constrained centromeric organization associated with heterochromatin protein 1 depletion may promote the attachment of kinetochores to microtubules coming from both poles

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The aneuploidy-inducing activity of alachlor and dichlorvos, two pesticides representing an important source of human exposure to potential carcinogens, has been evaluated in a cytokinesis block micronucleus assay combined with anti-kinetochore (CREST) staining to detect chromosome loss and in situ hybridization with chromosome-specific centromeric probes for the analysis of non-disjunction. Cytofluorimetric analysis to assess potential interference of the chemicals with cell cycle progression and TUNEL assay to detect apoptosis were also performed. The results obtained show that both environmental compounds induced significant and dose-related increases of total micronuclei (MN) and CREST-positive MN as compared with the concurrent solvent control. The chemicals were also capable of promoting chromosome non-disjunction. However, the two pesticides differed in their mode of action: alachlor induced both chromosomal aberrations and aneuploidy, while the genotoxic activity of dichlorvos was only related to aneuploidy induction. Cytofluorimetric analyses showed that dichlorvos caused a marked accumulation of cells in the G2/M phase of cell cycle and indicate a potential for this chemical to interfere with mitosis. Furthermore, dichlorvos induced CREST-positive MN at a concentration lower than the one producing apoptosis, suggesting that dichlorvos-induced aneuploid cells may persist in the growing cell population.