Tetrameric Structure of Centromeric Nucleosomes in Interphase Drosophila Cells |
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Authors: | Yamini Dalal Yamini Dalal Yamini Dalal Yamini Dalal |
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Institution: | 1 Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America, 2 Howard Hughes Medical Institute, Seattle, Washington, United States of America, 3 Biodesign Institute, Arizona State University, Tempe, Arizona, United States of America |
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Abstract: | Centromeres, the specialized chromatin structures that are responsible for equal segregation of chromosomes at mitosis, are epigenetically maintained by a centromere-specific histone H3 variant (CenH3). However, the mechanistic basis for centromere maintenance is unknown. We investigated biochemical properties of CenH3 nucleosomes from Drosophila melanogaster cells. Cross-linking of CenH3 nucleosomes identifies heterotypic tetramers containing one copy of CenH3, H2A, H2B, and H4 each. Interphase CenH3 particles display a stable association of approximately 120 DNA base pairs. Purified centromeric nucleosomal arrays have typical “beads-on-a-string” appearance by electron microscopy but appear to resist condensation under physiological conditions. Atomic force microscopy reveals that native CenH3-containing nucleosomes are only half as high as canonical octameric nucleosomes are, confirming that the tetrameric structure detected by cross-linking comprises the entire interphase nucleosome particle. This demonstration of stable half-nucleosomes in vivo provides a possible basis for the instability of centromeric nucleosomes that are deposited in euchromatic regions, which might help maintain centromere identity. |
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