Abstract
How eukaryotic chromosomes are compacted during mitosis has been a leading question in cell biology since the nineteenth century. Non-histone proteins such as condensin complexes contribute to chromosome shaping, but appear not to be necessary for mitotic chromatin compaction. Histone modifications are known to affect chromatin structure. As histones undergo major changes in their post-translational modifications during mitotic entry, we speculated that the spectrum of cell-cycle-specific histone modifications might contribute to chromosome compaction during mitosis. To test this hypothesis, we isolated core histones from interphase and mitotic cells and reconstituted chromatin with them. We used mass spectrometry to show that key post-translational modifications remained intact during our isolation procedure. Light, atomic force and transmission electron microscopy analysis showed that chromatin assembled from mitotic histones has a much greater tendency to aggregate than chromatin assembled from interphase histones, even under low magnesium conditions where interphase chromatin remains as separate beads-on-a-string structures. These observations are consistent with the hypothesis that mitotic chromosome formation is a two-stage process with changes in the spectrum of histone post-translational modifications driving mitotic chromatin compaction, while the action of non-histone proteins such as condensin may then shape the condensed chromosomes into their classic mitotic morphology.
Highlights
Dividing eukaryotic cells must partition their genomes between daughter cells
To understand the role of histone post-translational modifications (PTMs) in mitotic chromatin condensation, we adopted an in vitro approach, whereby we could detect differences in the compaction of nucleosome arrays reconstituted with histones bearing cell-cycle-regulated PTMs
We previously reported that mitotic chromosomes can form and segregate in chicken DT-40 cells depleted of at least 95% of their condensin complexes, provided that Repo-Man was prevented from targeting protein phosphatase 1 to anaphase chromatin [19]
Summary
Dividing eukaryotic cells must partition their genomes between daughter cells. Interphase chromosomes undergo a dramatic transformation into condensed linear bodies that can be partitioned by the mitotic spindle without becoming tangled or trapped at the division plane. How this transition occurs has been an important question since Flemming’s initial description of mitosis [1]. Hi-C experiments showed that each chromosome is divided into ‘compartments’—regions of high or low gene expression [2]. These compartments are further subdivided into topologically associated domains (TADs), which bring distal enhancer sites into close proximity to their cognate promoters. Upon entry into mitosis this hierarchical structure is lost, and the chromosomes instead exhibit global, locus-independent interactions of genome sites less than 10 Mb apart [3]
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