Regulation of Interleukin Receptor-associated Kinase (IRAK) Phosphorylation and Signaling by Iota Protein Kinase C

We have previously shown that the activity of the interleukin-1 (IL-1) receptor-associated kinase (IRAK) is required for nerve growth factor (NGF)-induced activation of NF-κB and cell survival ((2002) J. Biol. Chem. 277, 28010–28018). Herein we demonstrate that NGF induces co-association of IRAK with atypical protein kinase C iota (PKC) and that the iota PKC·IRAK complex is recruited to the p75 neurotrophin receptor. Recruitment of IRAK to the receptor was dependent upon the activity of the iota PKC. Moreover, transfection of kinase-dead iota PKC blocked both NGF- and IL-1-induced IRAK activation and the activity of NF-κB. Hence, iota PKC lies upstream of IRAK in the κB pathway. Examining the primary structure of IRAK, we identified three putative PKC phosphorylation sites; iota PKC selectively phosphorylated peptide 1 (RTAS) within the death domain domain at Thr66, which is highly conserved among all IRAK family members. Mutation of Thr66 to Ala impaired the autokinase activity of IRAK and reduced its association with iota PKC but not TRAF6, resulting in impaired NGF- as well as IL-1-induced NF-κB activation. These findings provide insight into the underlying mechanism whereby IRAK regulates the κB pathway and reveal that IRAK is a substrate of iota PKC.