Required for the activation of the NF-kappa-B factor Relish (Rel) by acting as an essential signaling component in transmitting the lipopolysaccharide (LPS) signal leading to cact degradation, which is required for direct activation of Rel (PubMed:10636911, PubMed:11018014, PubMed:11156609, PubMed:30119996). Phosphorylates inhibitors of NF-kappa-B (cact) thus leading to the dissociation of the inhibitor/NF-kappa-B complex and ultimately the degradation of the NF-kappa-B inhibitor (PubMed:10636911, PubMed:11018014, PubMed:11156609). Essential for antibacterial immune response (PubMed:11018014). Also required for antiviral immune response (PubMed:30119996). ATP + L-seryl-[I-kappa-B protein] = ADP + H(+) + O-phospho-L-seryl-[I-kappa-B protein] Interacts with key to form the I-kappa-B kinase complex. In vitro, interacts with cact. Expressed both maternally and zygotically throughout development. Highest expression is in early embryos and third larval instar. Autophosphorylated; upon LPS stimulation it is transiently activated, and can be autophosphorylated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. I-kappa-B kinase subfamily.