title: Structure and function of the histone chaperone CIA/ASF1 complexed with histones H3 and H4.
abstract: CIA (CCG1-interacting factor A)/ASF1, which is the most conserved histone chaperone among the eukaryotes, was genetically identified as a factor for an anti-silencing function (Asf1) by yeast genetic screening. Shortly after that, the CIA-histone-H3-H4 complex was isolated from Drosophila as a histone chaperone CAF-1 stimulator. Human CIA-I/II (ASF1a/b) was identified as a histone chaperone that interacts with the bromodomain-an acetylated-histone-recognizing domain-of CCG1, in the general transcription initiation factor TFIID. Intensive studies have revealed that CIA/ASF1 mediates nucleosome assembly by forming a complex with another histone chaperone in human cells and yeast, and is involved in DNA replication, transcription, DNA repair and silencing/anti-silencing in yeast. CIA/ASF1 was shown as a major storage chaperone for soluble histones in proliferating human cells. Despite all these biochemical and biological functional analyses, the structure-function relationship of the nucleosome assembly/disassembly activity of CIA/ASF1 has remained elusive. Here we report the crystal structure, at 2.7 A resolution, of CIA-I in complex with histones H3 and H4. The structure shows the histone H3-H4 dimer's mutually exclusive interactions with another histone H3-H4 dimer and CIA-I. The carboxy-terminal beta-strand of histone H4 changes its partner from the beta-strand in histone H2A to that of CIA-I through large conformational change. In vitro functional analysis demonstrated that CIA-I has a histone H3-H4 tetramer-disrupting activity. Mutants with weak histone H3-H4 dimer binding activity showed critical functional effects on cellular processes related to transcription. The histone H3-H4 tetramer-disrupting activity of CIA/ASF1 and the crystal structure of the CIA/ASF1-histone-H3-H4 dimer complex should give insights into mechanisms of both nucleosome assembly/disassembly and nucleosome semi-conservative replication.
authors.affiliation: Japan Biological Information Research Centre (JBIRC), Japan Biological Informatics Consortium (JBIC), 2-42 Aomi, Koto-ku, Tokyo 135-0064, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Natsume
authors.forename: Masamitsu
authors.initials: M
authors.lastname: Eitoku
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Akai
authors.forename: Norihiko
authors.initials: N
authors.lastname: Sano
authors.forename: Masami
authors.initials: M
authors.lastname: Horikoshi
authors.forename: Toshiya
authors.initials: T
authors.lastname: Senda
mesh_terms: D000818:Animals; D001665:Binding Sites; D018797:Cell Cycle Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D019281:Dimerization; D006657:Histones / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D018832:Molecular Chaperones; D009154:Mutation; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D014982:Xenopus laevis
chemical_list: C491641:ASF1A protein, human; D018797:Cell Cycle Proteins; D006657:Histones; D018832:Molecular Chaperones