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 Thr ⁶⁶, 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.