Dalal Yamini, Wang Hongda, Lindsay Stuart, Henikoff Steven
Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America.
PLoS Biol. 2007 Aug;5(8):e218. doi: 10.1371/journal.pbio.0050218.
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.
着丝粒是在有丝分裂时负责染色体均等分离的特殊染色质结构，由着丝粒特异性组蛋白H3变体（CenH3）在表观遗传上维持。然而，着丝粒维持的机制基础尚不清楚。我们研究了来自黑腹果蝇细胞的CenH3核小体的生化特性。CenH3核小体的交联鉴定出包含一份CenH3、H2A、H2B和H4的异型四聚体。间期CenH3颗粒显示出约120个DNA碱基对的稳定结合。纯化的着丝粒核小体阵列通过电子显微镜呈现出典型的“串珠”外观，但在生理条件下似乎抵抗凝聚。原子力显微镜显示，天然含CenH3的核小体只有典型八聚体核小体高度的一半，证实交联检测到的四聚体结构包含整个间期核小体颗粒。体内稳定半核小体的这一证明为沉积在常染色质区域的着丝粒核小体的不稳定性提供了可能的基础，这可能有助于维持着丝粒身份。
