Persistent Activation of NF-κB by the Tax Transforming Protein Involves Chronic Phosphorylation of IκB Kinase Subunits IKKβ and IKKγ

The Tax transforming protein encoded by human T-cell leukemia virus type 1 (HTLV1) persistently activates transcription factor NF-kappaB and deregulates the expression of downstream genes that mediate cell cycle entry. We recently found that Tax binds to and chronically stimulates the catalytic function of I kappaB kinase (IKK), a cellular enzyme complex that phosphorylates and inactivates the I kappaB inhibitory subunit of NF-kappaB, We now demonstrate that the IKK beta catalytic subunit and IKK gamma regulatory subunit of IKK are chronically phosphorylated in HTLV1-infected and Tax-transfected 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 inhibitory 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 substitutions at Ser-177 and Ser-181 have the capacity to phosphorylate a recombinant lKK gamma substrate in vitro, We conclude that Tax-induced phosphorylation of IKK beta is required for IKK beta activation, phosphoryl group transfer to IKK gamma, and acquisition of the deregulated IKK phenotype.