PKR stimulates NF-kappa B irrespective of its kinase function by interacting with the I kappa B kinase complex

The interferon (IFN)-induced double-stranded RNA-activated protein kinase P KR mediates inhibition of protein synthesis through phosphorylation of the or subunit of eukaryotic initiation factor 2 (eIF2 alpha) and is also invol ved in the induction of the IFN gene through the activation of the transcri ption factor NF-kappa B. NF-kappa B is retained in the cytoplasm through bi nding to its inhibitor I kappa B alpha. The critical step in NF-kappa B act ivation is the phosphorylation of I kappa B alpha by the I kappa B kinase ( IKK) complex. This activity releases NF-kappa B from I kappa B alpha and al lows its translocation to the nucleus. Here, we have studied the ability of PKR to activate NF-kappa B in a reporter assay and have shown for the firs t time that two catalytically inactive PKR mutants, PKR/KR296 and a deletio n mutant (PKR/De142) which lacks the potential eIF2 alpha-binding domain, c an also activate NF-kappa B. This result indicated that NF-kappa B activati on by PKR does not require its kinase activity and that it is independent o f the PKR-eIF2 alpha relationship. Transfection of either wild-type PKR or catalytically inactive PKR in PKR0/0 mouse embryo fibroblasts resulted in t he activation of the IKK complex. By using a glutathione S-transferase pull -dorm assay, we showed that PKR interacts with the IKK beta subunit of the IKK complex. This interaction apparently does not require the integrity of the IKK complex, as it was found to occur with extracts from cells deficien t in the NF-kappa B essential modulator, one of the components of the IKK c omplex. Therefore, our results reveal a novel pathway by which PKR can modu late the NF-kappa KB signaling pathway without using its kinase activity.