Centromeres are specialized nucleosomes playing an instrumental role in cell division as mitotic spindle attachment points. They have been assumed to exist as the standard octameric histone complex with one of the histones, H3, replaced by the centromere-specific variant cenP-A (or cenH3). Our recent studies and others'support the hypothesis that, at least during some part of the cell cycle, the centromere adopts a tetrameric form. Other studies point to possible centromere remodeling during the cell cycle and those findings, combined, motivate our investigation into possible variations in its composition at different stages of the cell cycle. For that purpose, centromeres are isolated from synchronized cells by immuno-precipitation and examined using biochemical and molecular biology tools. In addition we use atomic force microscopy to visualize and compare, at single-molecule scale, centromeres for their shapes, heights and volumes across the cell cycle. The results demonstrate that the centromere is a rather dynamic structure alternating between tetramer and octamer during the cycle. In particular, the centromere remains a tetramer for most of the cycle but changes its composition during the S-phase when DNA is replicated. The transformation probably begins during the transition from G1 to S-phase where we start seeing a mix of tetramers and octamers. The mechanism of such transformation is unknown, but the findings reveal novel processes that will help to further our understanding of the mechanics guiding the cell cycle.

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National Institute of Biomedical Imaging and Bioengineering
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Bui, Minh; Dimitriadis, Emilios K; Hoischen, Christian et al. (2012) Cell-cycle-dependent structural transitions in the human CENP-A nucleosome in vivo. Cell 150:317-26
Dimitriadis, Emilios K; Weber, Christian; Gill, Rajbir K et al. (2010) Tetrameric organization of vertebrate centromeric nucleosomes. Proc Natl Acad Sci U S A 107:20317-22