| dc.description.abstract | Humans and animals are constantly facing a variety of potential pathogens including bacteria and viruses daily. In order to survive and fight against their “enemies”, they have developed both innate immune system and adaptive immune system to eliminate infection. The latter one remembers the previous specific pathogens and gets rid of them when they invade again whereas the innate immune system serves as the first line to defend against a new pathogen in an efficient and timely manner.
The innate immune system is able to detect the pathogen-associated molecular patterns (PAMPs) such as nucleic acids and lipopolysaccharide (LPS) through pattern recognition receptors (PRRs), which leads to the induction of type-I interferons (IFN-I) by activating a transcription factor known as IRF-3. Although extensive studies on the functions of IRF-3 have been reported over years, the mechanism of IRF-3 activation still remains not fully understood.
IRF-3 belongs to the interferon regulatory factor (IRF) family and contains a DNA binding domain (DBD), IRF-3 association domain (IAD), and auto-inhibitory elements flanking on both sides of IAD. Within the C-terminal auto-inhibitory segment, there is a serine rich repeat containing seven serine/threonine residues that has been shown to be the potential sites of phosphorylation. In this study, we solved the crystal structures of both phosphorylated human and mouse IRF-3 bound to CBP (cAMP response element binding protein (CREB)-binding protein), which demonstrate that phosphorylated IRF-3 forms a dimer through phosphorylated Ser386 (pSer379 in mouse IRF-3) and a downstream pLxIS motif. Besides, size-exclusion chromatography and cell-based studies show that mutations of key residues interacting with pSer386 severely impair IRF-3 activation and IFN-β induction. By contrast, phosphorylation of Ser396 within the pLxIS motif of human IRF-3 only plays a moderate role in IRF-3 activation.
The mouse IRF-3/CBP complex structure reveals that Arg373 and Arg205 of mIRF-3, corresponding to Arg211 and Arg380 of hIRF-3, are critical for the dimerization of mIRF-3. However, the replacement of Glu388 and Asn389 of human IRF-3 by mouse Lys381 makes this region restructured and distinct from human IRF-3. Biochemical studies also confirmed this discrepancy. Therefore, the mechanism of mouse IRF-3 activation is similar but distinct from human IRF-3.
Taken together, these structural and functional studies reveal the detailed mechanism of IRF-3 activation upon phosphorylation. | en |
