Mobilization of hyperacetylated mononucleosomes by purified yeast ISW2 in vitro

• Acetylation of nucleosomal H4 can potentially abolish ISWI catalytic activity.
• ‘Natively’ hyperacetylated core histones were purified from mammalian cells.
• Highly acetylated nucleosomes were reconstituted and assayed with purified ISW2.
• Histone acetylation had no apparent effects on ISW2 nucleosome mobilization.
• This suggests that histone acetylation does not necessarily abolish ISWI functions.

Catalytic activity of ISWI chromatin remodelers, which regulate nucleosome positioning on the DNA, depends on interactions of the putative acidic patch in ISWI helicase domain with the N-termini of nucleosomal H4 – such, that removal of H4 termini abolishes ISWI remodeling. Acetylation of H4 termini is also known to disrupt H4 interactions with acidic protein surfaces, and thus, histone acetylation could potentially impede ISWI functions. Since active chromatin in vivo is hyperacetylated, it is important to clarify if ISWI activities can function on the in vivo hyperacetylated nucleosomes. We evaluated if purified yeast ISW2 can act on mononucleosomes in which all four core histones are highly acetylated. Mononucleosomes were assembled using purified histones from mammalian CV1 cells grown in the presence of deacetylase inhibitor Trichostatin A (TSA). The CV1 cell line is characterized by fast kinetic of accumulation of highly acetylated histone isoforms in response to TSA treatment. However, such ‘native’ histone hyperacetylation had no apparent effects on the nucleosome remodeling propensities, suggesting that histone hyperacetylation does not necessarily block ISWI functions and that ISWI enzymes can function on active chromatin as well.