Structure and function of the histone chaperone CIA/ASF1 complexed with histones H3 and H4

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Abstract
Structural conversion of the nucleosome, a minimum unit of chromatin structure made up of histones and DNA, has critical effects on DNA-mediated reactions. The mechanism of nucleosome assembly and disassembly has been elusive but now the crystal structure of a histone chaperone in complex with histones H3 and H4 has been determined. 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)1 by yeast genetic screening. Shortly after that, the CIA–histone-H3–H4 complex was isolated from Drosophila as a histone chaperone CAF-1 stimulator2. 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 TFIID3,4,5. Intensive studies have revealed that CIA/ASF1 mediates nucleosome assembly by forming a complex with another histone chaperone in human cells6 and yeast7, and is involved in DNA replication1,2, transcription4,8,9,10, DNA repair1,2,11,12 and silencing/anti-silencing1,2,8,13,14,15 in yeast. CIA/ASF1 was shown as a major storage chaperone for soluble histones in proliferating human cells6,16. 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 Å 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 β-strand of histone H4 changes its partner from the β-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.