Figure 6

Model of H3/H4 dimer binding by Asf1N. (A) Asf1 binds to a dimer of histones H3/H4. The structure of Asf1N-H3α3 was modeled onto the structure of the yeast nucleosome (PDB ID 1ID3) by aligning the α-carbons of H3α3 with the respective α-carbons of the nucleosome structure. Asf1 is shown in orange and is represented in cartoon form. Nucleosomal histones H3 (yellow) and H4 (green) are shown in cartoon form with the H3a3-helix represented as a Cα trace for the nucleosomal (yellow) and complex crystal structure (blue). (B) Asf1 occludes binding of a second H3/H4 dimer while leaving critical decoding residues surface exposed. Asf1 is shown with a transparent surface representation. An H3/H4 dimer that would be occluded by Asf1 binding is shown as a grey cartoon. Yeast (light blue) and metazoan (red) H3 residues that differ among the H3.1/H3.2 and H3.3 isoforms are shown as sticks with a transparent surface representation, and labeled "ID" for "Isoform Determinants". The H3-K56 residue (dark blue) that is acetylated on newly synthesized histones and the region responsible for HIRA binding (green, labeled "HIRA BD"), are also shown.