Structural and Single-Molecule Studies on the Assembly Mechanism of Histone H3-H4 by Fission Yeast AAA+ATPase Abo1

Abstract

Chromatin dynamics including nucleosome assembly and disassembly are crucial for genome maintenance, conservation of epigenetic information, and diverse DNA metabolic reactions. Sometimes, the nucleosome assembly or disassembly can be carried out via ATP-dependent pathway by ATPases. Human ATAD2 and its orthologs are bromodomain-containing AAA+ ATPases. ATAD2 and its budding yeast ortholog Yta7 are known to reduce nucleosome density to facilitate gene expression. Meanwhile, it was recently proposed that the fission yeast ortholog Abo1 contributes to increases nucleosome density and chromatin organization in vivo. In addition to this opposite biological function, the structure and molecular features of Abo1 remain poorly understood yet. Here we revealed the cryo-EM structures of Abo1. Interestingly, the ADP-bound state of Abo1 displays a symmetric hexameric ring structure, whereas the hexamer takes open spiral structure when it takes ATP. Such structural change of Abo1 according to ATP binding was observed in real time by high-speed AFM. The AFM results demonstrated that Abo1 subunits stochastically hydrolyze ATP. We also characterized the function of Abo1 using single-molecule DNA curtain and photobleaching assays. We manifested that Abo1 does not dislodge H3-H4 from DNA but assembles them onto DNA only when ATP hydrolysis is permitted. Furthermore, we found that assembly mode of H3-H4 by Abo1 is different from that by CAF-1 forming H3-H4 tetramers.