The ubiquitous inducible transcription factor NF-κB plays central roles in immune and inflammatory responses and in tumorigenesis. Complex posttranslational modifications of the p65 subunit (RelA) are a major aspect of the extremely flexible regulation of NF-κB activity. Although phosphorylation, acetylation, ubiquitination, and lysine methylation of NF-κB have been well described, arginine methylation has not yet been found. We now report that, in response to IL-1β, the p65 subunit of NF-κB is dimethylated on arginine 30 (R30) by protein-arginine methyltransferase 5 (PRMT5). Expression of the R30A and R30K mutants of p65 substantially decreased the ability of NF-κB to bind to κB elements and to drive gene expression. A model in which dimethyl R30 is placed into the crystal structure of p65 predicts new van der Waals contacts that stabilize intraprotein interactions and indirectly increase the affinity of p65 for DNA. PRMT5 was the only arginine methyltransferase that coprecipitated with p65, and its overexpression increased NF-κB activity, whereas PRMT5 knockdown had the opposite effect. Microarray analysis revealed that ∼85% of the NF-κB-inducible genes that are down-regulated by the R30A mutation are similarly down-regulated by knocking PRMT5 down. Many cytokine and chemokine genes are among these, and conditioned media from cells expressing the R30A mutant of p65 had much less NF-κB-inducing activity than media from cells expressing the wild-type protein. PRMT5 is overexpressed in many types of cancer, often to a striking degree, indicating that high levels of this enzyme may promote tumorigenesis, at least in part by facilitating NF-κB-induced gene expression.histone | mass spectrometry T he NF-κB family is comprised of two protein subfamilies: c-Rel, RelB, and RelA (p65), which include transactivation domains in their C termini, and p100 (p52) and p105 (p50), which include a number of ankyrin repeats in their C termini and have transrepressive functions. All family members include an N-terminal DNA-binding region, the Rel homology domain (1). In the absence of an activating stimulus, NF-κB is located in the cytoplasm in a complex with the inhibitory IκB protein. In the classic pathway of NF-κB activation, extracellular signals activate IκB kinase, which phosphorylates IκBα, leading to its ubiquitination and degradation by proteasomes. NF-κB, liberated from IκB, translocates to the nucleus to regulate its target genes (2). The NF-κB pathway is regulated by means of several different posttranslational modifications, including ubiquitination, phosphorylation, acetylation, sumoylation, and nitrosylation (3). The recent work of several laboratories has revealed the reversible methylation of nonhistone proteins by enzymes that were discovered on the basis of their activities toward histones. For NF-κB, we found that in response to an activating signal, such as treatment with IL-1β, the p65 subunit is reversibly methylated on two specific lysine residues by chromatin remodeling enzymes in ways that profoundly aff...