R utilizes IRAK-M to mediate signalling in the Lypressin biological activity absence of IRAK-1 and IRAK-2. Since MyD88- 
or IRAK-4-deficient BMDMs had completely abolished TLR/IL-1R-mediated NFkB activation, indicating that TLR/IL-1R-induced IRAK-M-mediated NFkB activation in the IRAK-1/2-DKO-BMDMs must be MyD88IRAK-4 dependent. In the light of the crystal structure of MyD88IRAK-4IRAK-2 DD Myddosome, we hypothesized that IRAK-M might also be capable of forming a complex with MyD88IRAK-4 to mediate signalling. We indeed detected IL-1-induced interaction of IRAK-M with MyD88, IRAK-4, TRAF6 and MEKK3, but not with TAK1 when we restored IRAK-M-deficient MEFs with tagged-IRAK-M. Furthermore, TLR-induced interaction of IRAK-M with MyD88, IRAK-4, TRAF6 and MEKK3 can take place in the absence of IRAK-1 and IRAK-2 when we introduced tagged IRAK-M into the IRAK-1/2/M-TKO-BMDMs. These data indicate the formation of IRAK-M Myddosome, which is probably responsible for TLR-induced IRAK-M-mediated NFkB activation in the absence of IRAK-1 and IRAK-2 through the MEKK3-dependent, but not the TAK1-dependent pathway. The N-terminal DD of IRAK-M shares a considerable sequence identity to those of other members of the IRAK family. The similarity of topologies within this region of IRAK-M to the region of IRAK-2 suggested that this region of IRAK-M may adopt a similar protein fold. To test this prediction, we built a three-dimensional model of the region of IRAK-M DD. The model has secondary structure elements similar to those of other IRAK DDs with a hexahelical bundle. The model PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19828152 places a cluster of hydrophilic residues on the surface that is reminiscent to that of IRAK-2 DD when bound to IRAK-4 DD. IRAK-4-binding residues with IRAK-2 are highly conserved in IRAK-M DD, suggesting that IRAK-M is assembled with IRAK-4 DD in a similar manner to IRAK-2. In particular, the conserved W74 in helix H4 of the IRAK-M DD is likely a key residue for interacting with IRAK-4 DD. The model also a conserved salt bridge between E71 in helix H4 of the IRAK-M and R54 in IRAK-4, suggesting a critical role of the helix H4 segment in IRAK-M for recognizing IRAK-4. Interestingly, Q78 in IRAK-M is a polar residue and distinct from the equivalent residue in IRAK-2 but the hydrophobic side chain portion of Q78 seems to make the hydrophobic interaction with IRAK-4, whereas the hydrophilic part of Q78 in IRAK-M may interact with the backbone oxygen atoms of F25, E92 and F93 in IRAK-4. To test the proposed binding interface, we made individual and combined point mutants of IRAK-M in retroviral vector. We have previously shown that IRAK-M was able to restore IL-1-induced NFkB activation in 293-derived IRAK-1-deficient cells, in which IRAK-M and IRAK-2 expression was not detectable . Therefore, this cell line was used to test the ability of IRAK-M IRAK-M mediates TLR/IL-1R-induced NFjB activation and cytokine production H Zhou et al to form Myddosome in response to IL-1 stimulation. to IRAK-4. Furthermore, the W74A IRAK-M mutant completely lost the interaction with IRAK-4. We next used this cell line to examine the ability of IRAK-M mutants in mediating IL-1-dependent NFkB activation. & 2013 European Molecular Biology Organization IRAK-M mediates TLR/IL-1R-induced NFjB activation and cytokine production H Zhou et al IRAK-M wild-type and mutants were co-transfected with NFkB-dependent luciferase reporter construct into 293-I1A cells, followed by IL-1 treatment and luciferase assay. The W74A IRAK-M mutant completely lost