A Chemical Biology Approach to Reveal Sirt6-targeted Histone H3 Sites in Nucleosomes.

As a member of a highly conserved family of NAD(+)-dependent histone deacetylases, Sirt6 is a key regulator of mammalian genome stability, metabolism, and life span. Previous studies indicated that Sirt6 is hardwired to remove histone acetylation at H3K9 and H3K56. However, how Sirt6 recognizes its nucleosome substrates has been elusive due to the difficulty of accessing homogeneous acetyl-nucleosomes and the low activity of Sirt6 toward peptide substrates. Based on the fact that Sirt6 has an enhanced activity to remove long chain fatty acylation from lysine, we developed an approach to recombinantly synthesize histone H3 with a fatty acylated lysine, N(ε)-(7-octenoyl)-lysine (OcK), installed at a number of lysine sites and used these acyl-H3 proteins to assemble acyl-nucleosomes as active Sirt6 substrates. A chemical biology approach that visualizes OcK in nucleosomes and therefore allows direct sensitization of Sirt6 activities on its acyl-nucleosome substrates was also formulated. By combining these two approaches, we showed that Sirt6 actively removes acylation from H3K9, H3K18, and H3K27; has relatively low activities toward H3K4 and K3K23; but sluggishly removes acylation at H3K14, H3K36, H3K56, and H3K79. Overexpressing Sirt6 in 293T cells led to downregulated acetylation at H3K18 and K3K27, confirming these two novel Sirt6-targeted nucleosome lysine sites in cells. Given that downregulation of H3K18 acetylation is correlated with a poor prognosis of several cancer types and H3K27 acetylation antagonizes repressive gene regulation by di- and trimethylation at H3K27, our current study implies that Sirt6 may serve as a target for cancer intervention and regulatory pathway investigation in cells.