Stabilization of Sir3 interactions by an epigenetic metabolic small molecule, O-acetyl-ADP-ribose, on yeast SIR-nucleosome silent heterochromatin

Abstract In Saccharomyces cerevisiae , Sir proteinsmediate heterochromatinepigenetic gene silencing. The assembly of silent heterochromatinrequires histone deacetylation by Sir2, conformational change of SIR complexes, and followed by spreading of SIR complexes along the chromatin fiber to form extended silent heterochromatindomains. Sir2 couples histone deacetylation and NAD hydrolysis to generate an epigenetic metabolic small molecule, O-acetyl-ADP-ribose (AAR). Here, we demonstrate that AAR physically associates with Sir3 and that polySir3-AAR formation has a specific and essential role in the assembly of silent SIR- nucleosomepre- heterochromatinfilaments. Furthermore, we show that AAR is capable of stabilizing binding of the Sir3 BAH domainto the Sir3 carboxyl-terminal region. Our data suggests that for the assembly of SIR- nucleosomepre- heterochromatinfilament, the structural rearrangement of SIR- nucleosomeis important and result in creating more stable interactions of Sir3, such as the inter-molecule Sir3-Sir3 interaction, and the Sir3- nucleosomeinteraction within the filaments. In conclusion, our results reveal the importance of AAR, indicating that it not only affects the conformational rearrangement of SIR complexes but also might function as a critical fine-tuning modulatory component of yeast silent SIR- nucleosomepre- heterochromatinby stabilizing the intermolecular interaction between Sir3 N- and C-terminal regions.