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

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 stimulator(2). 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 TFIID3-5. Intensive studies have revealed that CIA/ASF1 mediates nucleosome assembly by forming a complex with another histone chaperone in human cells 6 and yeast 7, and is involved in DNA replication 1,2, transcription 4,8 - 10, DNA repair 1,2,11,12 and silencing/ anti-silencing(1,2,8,13-15) in yeast. CIA/ASF1 was shown as a major storage chaperone for soluble histones in proliferating human cells(6,16). 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 angstrom 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.