A molecular mechanism for Toll-IL-1 receptor domain-containing adaptor molecule-1-mediated IRF-3 activation.
The Toll-IL-1 receptor (TIR) domain-containing adaptor molecule-1 (TICAM-1, also called TRIF) is a signaling adaptor for TLR3 and TLR4 that activates the transcription factors IRF-3, NF-kappaB, and AP-1, leading to induction of type I interferon and cytokines. The N-terminal region of TICAM-1 participates in IRF-3 activation, although the C-terminal region is involved in NF-kappaB activation. However, the mechanism by which TICAM-1 is activated and transmits signals is largely unknown. In this study, we identified Leu(194) as a critical amino acid for TICAM-1-mediated IRF-3 activation. When Leu(194) was substituted with Ala, the mutant TICAM-1 failed to recruit the IRF-3 kinase TBK1, resulting in lack of IRF-3 phosphorylation, although TRAF3 and NAP1 appeared to be recruited. The N-terminal 176 amino acids of TICAM-1 (N-terminal domain (NTD)) form a protease-resistant structural domain. A TICAM-1 mutant lacking the N-terminal 180 amino acids showed greater interferon-beta promoter activation than wild-type TICAM-1. Furthermore, immunoprecipitation and protein-protein interaction analysis revealed that the NTD interacted with the N terminus of TICAM-1-TIR. These results suggest that the NTD folds into the TIR domain structure to maintain the naive conformation of TICAM-1. Upon stimulation of TLR3/4, TICAM-1 oligomerizes through the TIR domain and the C-terminal region, which may break the intramolecular association and induce a conformational change that allows TBK1 access to TICAM-1.
Tatematsu
Megumi
M
Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Kita 15, Nishi 7, Kita-ku, Sapporo 060-8638, Japan.
Ishii
Akihiro
A
Oshiumi
Hiroyuki
H
Horiuchi
Masataka
M
Inagaki
Fuyuhiko
F
Seya
Tsukasa
T
Matsumoto
Misako
M
D033942:Adaptor Proteins, Vesicular Transport / Q000235:genetics / Q000378:metabolism*; D000409:Alanine / Q000235:genetics / Q000378:metabolism; D019943:Amino Acid Substitution; D001665:Binding Sites / Q000235:genetics; D002460:Cell Line; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D050838:Interferon Regulatory Factor-3 / Q000235:genetics / Q000378:metabolism*; D016899:Interferon-beta / Q000235:genetics; D007930:Leucine / Q000235:genetics / Q000378:metabolism; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D018613:Microscopy, Confocal; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D018808:Transcription Factor AP-1 / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D033942:Adaptor Proteins, Vesicular Transport; C494232:IRF3 protein, human; D050838:Interferon Regulatory Factor-3; D016328:NF-kappa B; C470409:TICAM1 protein, human; D018808:Transcription Factor AP-1; D016899:Interferon-beta; D008156:Luciferases; D007930:Leucine; D000409:Alanine