Negative regulation of NF-κB action by Set9-mediated lysine methylation of the RelA subunit

Proper regulation of NF-kappa B activity is critical to maintain and balance the inflammatory response. Inactivation of the NF-kappa B complex relies in part on the proteasome-mediated degradation of promoter-bound NF-kappa B, but the detailed molecular mechanism initiating this process remains elusive. Here, we show that the methylation of the RelA subunit of NF-kappa B has an important function in this process. Lysine methyltransferase Set9 physically associates with RelA in vitro and in vivo in response to TNF-alpha stimulation. Mutational and mass spectrometric analyses reveal that RelA is monomethylated by Set9 at lysine residues 314 and 315 in vitro and in vivo. Methylation of RelA inhibits NF-kappa B action by inducing the proteasome-mediated degradation of promoter-associated RelA. Depletion of Set9 by siRNA or mutation of the RelA methylation sites prolongs DNA binding of NF-kappa B and enhances TNF-alpha-induced expression of NF-kappa B target genes. Together, these findings unveil a novel mechanism by which methylation of RelA dictates the turnover of NF-kappa B and controls the NF-kappa B-mediated inflammatory response.