Protein arginine methyltransferases (PRMTs) catalyze the posttranslational methylation of arginine using S-adenosylmethionine (SAM) as a methyl-donor. The PRMT family is widely expressed and has been implicated in biological functions such as RNA splicing, transcriptional control, signal transduction, and DNA repair. Therefore, specific inhibitors of individual PRMTs have potentially significant research and therapeutic value. In particular, PRMT1 is responsible for >85% of arginine methyltransferase activity, but currently available inhibitors of PRMT1 lack specificity, efficacy, and bioavailability. To address this limitation, we developed a high-throughput screening assay for PRMT1 that utilizes a hyper-reactive cysteine within the active site, which is lacking in almost all other PRMTs. This assay, which monitors the kinetics of the fluorescence polarization signal increase upon PRMT1 labeling by a rhodamine-containing cysteine-reactive probe, successfully identified two novel inhibitors selective for PRMT1 over other SAM-dependent methyltransferases.

Nuclear factor of activated T cell (NFAT) transcription factors are key regulators of gene transcription within immune cells. The NFAT-interacting protein, (NIP45), augments NFAT-driven IL-4 expression by a mechanism that relies on arginine methylation. To establish the function of NIP45 in vivo, we generated mice with a targeted deletion of the gene encoding this cofactor. NIP45-deficient T helper cells displayed profound defects in the expression of NFAT-regulated cytokine genes, including IL-4. Whereas NIP45 deficiency does not interfere with T helper cell NFAT activation or lineage-specific transcription-factor expression, NIP45 acts as an enhancer for the assembly of protein arginine methyltransferase 1 and the protein arginine methyltransferase 1-linked histone 4 arginine 3 methylation with the IL-4 promoter. Our study reveals an essential role for NIP45 in promoting robust cytokine expression in vivo, which is required for the efficient handling of parasites. We propose that NIP45 acts as a molecular rheostat serving to amplify the type-2 immune response.