Regulation of IRAK-1 activation by its C-terminal domain.
Macrophages are important mediators of the immune response to infection by virtue of their ability to secrete cytokines that trigger inflammation. Toll-like receptors (TLRs) are largely responsible for meditating the activation of macrophages by pathogens. IRAK-1 is a proximal protein kinase in TLR signalling pathways and hence its activation must be tightly regulated. However, the mechanisms which control the activation of IRAK-1 are poorly understood. IRAK-1 contains a death domain at its N-terminus that mediates its interaction with other death domain containing proteins, a central Ser/Thr kinase domain, and a C-terminal domain that contains binding motifs for TRAF6. We show here that deletion of the death domain or the majority of the C-terminal domain markedly enhanced the capacity of IRAK-1 to activate NF-kappaB in a TLR-independent manner in RAW 264.7 macrophages. Furthermore, the C-terminal truncation mutant spontaneously oligomerised and formed complexes with the negative regulator IRAK-M in the absence of TLR activation. In contrast to the binding of IRAK-M to IRAK-1, the death domain of IRAK-1 was not required for the interaction of IRAK-4 with IRAK-1. On the basis of these results we propose a model in which IRAK-1 is held in a closed, inactive conformation via an intramolecular mechanism involving its C-terminal domain and possibly the death domain. Phosphorylation of IRAK-1 by IRAK-4 in response to TLR activation may then release IRAK-1 from the inhibitory constraint exerted by its C-terminal domain.
Nguyen
Thao
T
Department of Medicine and Cooperative Research Centre for Chronic Inflammatory Diseases, The University of Melbourne, Royal Melbourne Hospital, Parkville, Victoria 3050, Australia.
De Nardo
Dominic
D
Masendycz
Paul
P
Hamilton
John A
JA
Scholz
Glen M
GM
D000818:Animals; D002460:Cell Line; D005786:Gene Expression Regulation; D006801:Humans; D053592:Interleukin-1 Receptor-Associated Kinases / Q000737:chemistry* / Q000378:metabolism*; D008264:Macrophages / Q000276:immunology; D051379:Mice; D008954:Models, Biological; D016328:NF-kappa B / Q000378:metabolism; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D017384:Sequence Deletion; D015398:Signal Transduction; D048029:TNF Receptor-Associated Factor 6 / Q000378:metabolism; D051193:Toll-Like Receptors / Q000378:metabolism*
D016328:NF-kappa B; D048029:TNF Receptor-Associated Factor 6; D051193:Toll-Like Receptors; D053592:Interleukin-1 Receptor-Associated Kinases; C507772:Irak3 protein, mouse