Persistent activation of NF-kappa B by the tax transforming protein involves chronic phosphorylation of I kappa B kinase subunits IKK beta and IKK gamma

The Tax transforming protein encoded by human T-cell leukemia virus type 1 (HTLV1) persistently activates transcription factor NF-kappaB and deregulat es the expression of downstream genes that mediate cell cycle entry. We rec ently found that Tax binds to and chronically stimulates the catalytic func tion of I kappaB kinase (IKK), a cellular enzyme complex that phosphorylate s and inactivates the I kappaB inhibitory subunit of NF-kappaB, We now demo nstrate that the IKK beta catalytic subunit and IKK gamma regulatory subuni t of IKK are chronically phosphorylated in HTLV1-infected and Tax-transfect ed cells. Alanine substitutions at Ser-177 and Ser-181 in the T loop of IKK beta protect both of these IKK subunits from Tax-directed phosphorylation and prevent the induction of I kappaB kinase activity. Each of these inhibi tory effects is recapitulated in Tax transfectants expressing the bacterial protein YopJ, a potent in vivo agonist of T loop phosphorylation, Moreover , ectopically expressed forms of IKK beta that contain glutamic acid substi tutions at Ser-177 and Ser-181 have the capacity to phosphorylate a recombi nant lKK gamma substrate in vitro, We conclude that Tax-induced phosphoryla tion of IKK beta is required for IKK beta activation, phosphoryl group tran sfer to IKK gamma, and acquisition of the deregulated IKK phenotype.