Association between HTLV-1 Tax and IκBα Is Dependent on the IκBα Phosphorylation State

Biological, molecular, and epidemiological data have demonstrated that human T cell leukemia virus type 1 (HTLV-1) encoded Tax protein plays a central role in the initiation of T cell malignancy. The 40-kDa Tax oncoprotein serves as a potent transcriptional activator that induces viral gene expression driven by the HTLV-1 long terminal repeats and also stimulates multiple cellular genes involved in T cell activation, cell cycle regulation, and gene activation. Since Tax has been shown to interact directly and indirectly with the NF-κB/IκB regulatory proteins, we examined the significance of anin vivoassociation between Tax and the IκBα inhibitor. Using GST affinity chromatography, Tax was shown to interact with the IκBα ankyrin repeats which are essential for interaction with the NF-κB/Rel proteins.In vivo,using IκBα mutants and co-immunoprecipitation, a preferential interaction between HTLV-1 Tax and N-terminally hypophosphorylated IκBα was detected. Tax also enhanced binding of IκBα to the proteasome subunit HsN3, resulting in a Tax-enhanced, constitutive degradation of wild-type and mutated forms of IκBα in the absence of phosphorylation and ubiquitination. Binding of IκBα to proteasome subunit HC9 was also observed, but this interaction occurred independently of Tax. Taken together, these results suggest a role for Tax as a viral chaperone resulting in the enhanced constitutive turnover of IκBα. The association of Tax with hypophosphorylated IκBα may prevent IκBα from binding to NF-κB and also target IκBα to the proteasome for degradation via a phosphorylation-independent pathway.