Systematic characterization by mass spectrometric analysis of phosphorylation sites in IRF-3 regulatory domain activated by IKK-i.

Interferon regulatory factor 3 (IRF-3) is a critical transcription factor that regulates innate immune responses against viral infection. Upon infection, IRF-3 is activated through phosphorylation of Ser/Thr residues in its C-terminal domain by the kinases, IKK-i and/or TBK-1. This phosphorylation triggers IRF-3 to interact with the co-activators to form a complex that activates target genes in the nucleus. However, the phosphorylation sites that determine the active/inactive status of IRF-3, estimated using biochemical methods such as mutagenesis and kinase assays, remain controversial. In the present study, phosphorylated IRF-3 189-427 (IRF-3 189C) was prepared by co-expression with IKK-i and was specifically fractionated into 3 major phosphorylation forms using anion-exchange chromatography. Identification of the phosphorylation sites was performed using systematic mass spectrometry approaches as follows: intact molecular mass analysis by nanoESI-MS, MS survey of phosphopeptides, and targeted MS/MS analysis of LC-MS/MS-based proteomics using a high-resolution Orbitrap mass spectrometer. Phosphorylated IRF-3 189C was clearly identified to exist as a mono-phosphoprotein (at Ser-402), and in two di-phosphoprotein forms (at Ser-386, -402 and Ser-396, -402). Thus, we demonstrated that Ser-386, -396 and -402 are directly phosphorylated by IKK-i in the co-expression system. These results will help provide new insights into the IRF-3 activation mechanism.