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Abstract
SummaryHistone chaperones physically interact with histones to direct proper assembly and disassembly of nucleosomes regulating diverse nuclear processes such as DNA replication, promoter remodeling, transcription elongation, DNA damage, and histone variant exchange. Currently, the best-characterized chaperone-histone interaction is that between the ubiquitous chaperone Asf1 and a dimer of H3 and H4. Nucleosome assembly proteins (Nap proteins) represent a distinct class of histone chaperone. Using pulsed electron double resonance (PELDOR) measurements and protein crosslinking, we show that two members of this class, Nap1 and Vps75, bind histones in the tetrameric conformation also observed when they are sequestered within the nucleosome. Furthermore, H3 and H4 trapped in their tetrameric state can be used as substrates in nucleosome assembly and chaperone-mediated lysine acetylation. This alternate mode of histone interaction provides a potential means of maintaining the integrity of the histone tetramer during cycles of nucleosome reassembly.
Highlights
Access to the genetic information of eukaryotic organisms is regulated in part by the assembly and disassembly of chromatin
There are at least seven major structural families of histone chaperones related to nucleoplasmin, chromatin assembly factor 1 (CAF-1), antisilencing factor 1 (Asf1), facilitates chromatin transcription (FACT), regulator of Ty1 transposition 106 (Rtt106), chaperone for Htz1/H2A-H2B dimer (Chz1), and nucleosome assembly protein 1 (Nap1) (Das et al, 2010; Hansen et al, 2010)
We show H3 and H4 remain as a tetramer during deposition onto DNA by Nap1 and that Vps75, but not Asf1, preferentially stimulates Rtt109 directed acetylation of H3 and H4 locked in their tetrameric conformation
Summary
Access to the genetic information of eukaryotic organisms is regulated in part by the assembly and disassembly of chromatin. This involves interplay between the action of histone chaperones, ATP-dependent remodeling enzymes, and histone-modifying enzymes. Asf has been cocrystallized with the histone fold domains of histones H3 and H4 (English et al, 2006; Natsume et al, 2007) This structure indicates that Asf makes contacts with both H3 and H4, and, consistent with previous biochemical investigations (English et al, 2005), these histones are present as a dimer rather than tetramer. Further support for the splitting of the tetramer has come from pull-down experiments suggesting that the Asf1-H3-H4 heterotrimer prevails in complex with CAF-1 and HIRA in mammalian cells (Tagami et al, 2004)
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