IKK gamma/NEMO facilitates the recruitment of the I kappa B proteins into the I kappa B kinase complex

IKK gamma /NEMO is an essential regulatory component of the I kappaB kinase complex that is required for NF-kappaB activation in response to various s timuli including tumor necrosis factor-alpha and interleukin-1 beta. To inv estigate the mechanism by which IKK gamma /NEMO regulates the IKK complex, we examined the ability of IKK gamma /NEMO to recruit the I kappaB proteins into this complex. IKK gamma /NEMO binding to wild-type, but not to a kina se-deficient IKK beta protein, facilitated the association of I kappaB alph a and I kappaB beta with the high molecular weight IKK complex. Following t umor necrosis factor-alpha treatment of HeLa cells, the majority of the pho sphorylated form of endogenous I kappaB alpha was associated with the high molecular weight IKK complex in HeLa cells and parental mouse embryo fibrob lasts but not in IKK gamma /NEMO-deficient cells. Finally, we demonstrate t hat IKK gamma /NEMO facilitates the association of the I kappaB proteins an d IKK beta and leads to increases in IKK beta kinase activity. These result s suggest that an important function of IKK gamma /NEMO is to facilitate th e association of both IKK beta and I kappaB in the high molecular weight IK K complex to increase I kappaB phosphorylation.