IRAK1 recruits IKK complex upon TLR7/8 or 9 stimulation

IRAK1 recruits IKK complex upon TLR7/8 or 9 stimulation The role of IRAK1 kinase activity in the activation of NF-kappa-B by IL-1/TLR is still uncertain. It has been shown that a kinase-dead IRAK1 mutants can still activate NF-kappa-B. Furthermore, stimulation of IRAK1-deficient I1A 293 cells with LMP1 (latent membrane protein 1- a known viral activator of NF-kappa-B) leads to TRAF6 polyubiquitination and IKKbeta activation [Song et al 2006]. On the other hand, IRAK1 enhances p65 Ser536 phosphorylation [Song et al 2006] and p65 binding to the promoter of NF-kappa-B dependent target genes [Liu G et al 2008]. IRAK1 has also been shown to be itself Lys63-polyubiquitinated (probably by Pellino proteins, which have E3 ligase activity). Mutation of the ubiquitination sites on IRAK1 prevented interaction with the NEMO subunit of IKK complex and subsequent IL-1/TLR-induced NF-kappa-B activation [Conze et al 2008]. These data suggest that kinase activity of IRAK1 is not essential for its ability to activate NF-kappa-B, while its Lys63-polyubuquitination allows IRAK1 to bind NEMO thus facilitating association of TRAF6 and TAK1 complex with IKK complex followed by induction of NF-kappa-B. Upon IL-1/TLR stimulation IRAK1 protein can undergo covalent modifications including phosphorylation [Kollewe et al 2004], ubiquitination [Conze DB et al 2008] and sumoylation [Huang et al 2004]. Depending upon the nature of its modification, IRAK1 may perform distinct functions including activation of IRF5/7 [Uematsu et al 2005, Schoenemeyer et al 2005], NF-kappa-B [Song et al 2006], and Stat1/3 [Huang et al 2004, Nguyen et al 2003].