ecessary for its transcriptional function. Specific lysine residues of IRF5 were previously reported to be required for K63-ubiquitination . To test the requirement of these lysines for transcriptional activity, we 6 IRF5 Activation . To determine if the specific phosphorylation of IRF5 contributes to its pro-apoptotic effects, we evaluated the IRF5 aspartic acid gain-of-function mutations. Cells expressing wt IRF5 or the IRF5 mutants tagged with GFP were analyzed with flow cytometry for cell death by permeability to propidium order AZ-505 iodide or for apoptosis by staining with annexin V . Expression of wt IRF5 had a modest effect on cell death, however the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189963 transcriptionally active SS451,462DD mutation had a dramatic pro-apoptotic effect. The promotion of apoptosis correlated with the transcriptional activity of SS451,462DD. Unexpectedly the S158D mutation that did not transcriptionally induce the IL12p40 gene, also promoted apoptosis, albeit to a lesser extent. The S158D mutation is within the NES of IRF5 and localize the protein to the nucleus presumably due to an inhibition of nuclear export. The results indicate both transcriptional activation and nuclear presence contribute to the proapoptotic effects of IRF5. The increases in apoptosis seen with these aspartic acid mutants were not due to differences in protein expression. Discussion Engagement of pattern recognition receptors stimulates innate immune responses that promote clearance of microbial infections, but in addition these responses can contribute to inflammation and autoimmunity. IRF5 has been shown to respond to TLR and NOD signaling, and polymorphisms of IRF5 are associated with increased risk of autoimmune disease. Since IRF5 may be a potential target of autoimmune intervention, knowledge of the mechanisms that activate IRF5 is needed. Post-translation phosphorylation has been shown to regulate the molecular switch in IRF5 from latency to activation, however the precise phosphorylation sites in IRF5 remained to be determined. In this study we identified amino acids in IRF5 that are phosphorylated in response to signaling molecules downstream of PRRs. Our subsequent mutagenesis studies clearly establish the contribution of IRF5 modified amino 
acids to transcriptional induction, nuclear trafficking, and apoptosis. We investigated the effects of several potential activators of IRF5; TBK-1, TRAF6, and RIP2. TBK-1 was previously identified as a kinase that phosphorylates the carboxyl terminus of the related transcription factor, IRF3, in response to viral infection. TRAF6 is an E3-K63 ubiquitin ligase that can activate effector enzymes downstream of TLR and NOD signaling. RIP2 kinase plays an essential role in the biological responses to NOD signaling. We tested the effect of these activators on induction of the IL12 p40 subunit gene by IRF5. TBK-1 had a modest effect on the ability of IRF5 to induce the IL12p40 reporter, however TRAF6 and especially RIP2 were far more effective activators of IRF5. Although the gene induction by IRF5 in the presence of TBK-1 was modest, phosphorylation by TBK-1 produced a significant shift in the mobility of IRF5 in SDS-PAGE. For this reason we identified the residues that are phosphorylated on IRF5 by TBK-1 with mass spectrometry. Ser-158 and Ser-309 were found to be phosphorylated, and mutations of these serines eliminated specific shifts in IRF5 mobility by SDS-PAGE. These residues did not appear to be modified by TRAF6 or RIP2. In addition, these r
