Cysteine-179 of IκB kinase β plays a critical role in enzyme activation by promoting phosphorylation of activation loop serines

I kappa B kinase beta (IKKO) subunit of IKK beta complex is essential for the activation of NF-kappa B in response to various proinflammatory signals. Cys-179 in the activation loop of IKK beta is known to be the target site for IKK inhibitors such as cyclopentenone prostaglandins, arsenite, and antirheumatic gold compounds. Here we show that a mutant IKK beta in which Cys-179 is substituted with alanine had decreased activity when it was expressed in HEK-293 cells, and TNF stimulation did not restore the activity. Phosphorylation of activation loop serines (Ser-177 and Ser-181) which is required for IKKO activation was reduced in the IKK beta(C179A) mutant. The activity of IKK beta (C179A) was partially recovered when its phosphorylation was enforced by coexpression with mitogen-activated protein kinase kinase kinases (MAPKKK) such as NF-kappa B inducing kinase (NIK) and MAPK/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) or when the serine residues were replaced with phospho-mimetic glutamate. The IKKO (C179A) mutant was normal in dimer formation, while its activity abnormally responded to the change in the concentration of substrate ATP in reaction mixture. Our results suggest that Cys-179 of IKKO plays a critical role in enzyme activation by promoting phosphorylation of activation-loop serines and interaction with ATP.