Crystal structure of the human centromeric nucleosome containing CENP-A

The centromeric nucleosome Centromeres are epigenetically marked by the assembly of nucleosomes containing CENP-A, a centromere-specific histone H3 variant. Tachiwana et al . report the crystal structure of the human centromeric nucleosome bound to DNA. They find a canonical arrangement of an octamer of histone proteins with DNA wrapped in a left-handed superhelix. There is flexibility in the DNA regions at the entrance and exit of the nucleosome, and a loop in CENP-A may help to stabilize its incorporation into centromeric chromatin. As the first view of the CENP-A-containing nucleosome, the structure helps to clarify various models that have been debated in the literature. In eukaryotes, accurate chromosome segregation during mitosis and meiosis is coordinated by kinetochores, which are unique chromosomal sites for microtubule attachment 1 , 2 . Centromeres specify the kinetochore formation sites on individual chromosomes, and are epigenetically marked by the assembly of nucleosomes containing the centromere-specific histone H3 variant, CENP-A 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . Although the underlying mechanism is unclear, centromere inheritance is probably dictated by the architecture of the centromeric nucleosome. Here we report the crystal structure of the human centromeric nucleosome containing CENP-A and its cognate α-satellite DNA derivative (147 base pairs). In the human CENP-A nucleosome, the DNA is wrapped around the histone octamer, consisting of two each of histones H2A, H2B, H4 and CENP-A, in a left-handed orientation. However, unlike the canonical H3 nucleosome, only the central 121 base pairs of the DNA are visible. The thirteen base pairs from both ends of the DNA are invisible in the crystal structure, and the αN helix of CENP-A is shorter than that of H3, which is known to be important for the orientation of the DNA ends in the canonical H3 nucleosome 13 . A structural comparison of the CENP-A and H3 nucleosomes revealed that CENP-A contains two extra amino acid residues (Arg 80 and Gly 81) in the loop 1 region, which is completely exposed to the solvent. Mutations of the CENP-A loop 1 residues reduced CENP-A retention at the centromeres in human cells. Therefore, the CENP-A loop 1 may function in stabilizing the centromeric chromatin containing CENP-A, possibly by providing a binding site for trans -acting factors. The structure provides the first atomic-resolution picture of the centromere-specific nucleosome.