Structural and single-molecule studies reveal the molecular function underlying histone loading of Schizosaccharomyces Pombe AAA+ ATPase Abo1

Abstract

Chromatin dynamics is important for genomic stability, gene regulation, and epigenetic inheritance. Chromatin assembly proteins, which form nucleosomes, are crucial factors for chromatin dynamics. Especially, bromodomain-containing AAA+ ATPase family can mediate chromatin assembly-disassembly mechanism. In Saccharomyces cerevisiae, Yta7 is known to disrupt chromatin organization to facilitate transcriptions. Meanwhile, it was recently reported that AboI, a fission yeast homolog of Yta7, contributes to chromatin assembly. However, the biological role and molecular features of AboI remain poorly understood yet. Here we characterize the molecular function of AboI in the structural and biophysical perspectives. The cryo-EM and high speed AFM reveals the structures of AboI. In apo- or ADP-bound state, Abo1 has a symmetric hexameric ring structure whereas ATP-bound Abo1 has open spiral structure. On the other hand, using single-molecule DNA curtain assay, we demonstrate that AboI does not dislodge H3-H4 histones from DNA but deposits the histones onto DNA only when ATP hydrolysis is allowed. In addition, we show that Abo1 does not load H2A-H2B dimer but histone octamer. We also examine the Abo1 and DNA interaction. Interestingly, Abo1 does not bind DNA regardless of ATP and neither histone nor DNA stimulate ATP hydrolysis activity of Abo1. Moreover, we investigate the details about histone deposition onto DNA using single-molecule photobleaching assay. we find that Abo1 requires at least 80 bp long DNA for H3-H4 deposition and deposits more histones as DNA length increases. Based on these findings, we propose allosteric communication model for H3-H4 deposition by AboI, in which ATP hydrolysis changes the configuration of histones to facilitate their deposition to DNA.