An alternative splice product of IκB kinase (IKKγ), IKKγ-Δ, differentially mediates cytokine and human T-cell leukemia virus type 1 tax-induced NF-κB activation

NF-kappa B is an inducible transcription factor mediating innate immune responses whose activity is controlled by the multiprotein I kappa B kinase (IKK) "signalsome". The core IKK consists of two catalytic serine kinases, IKK alpha and IKK beta, and a noncatalytic subunit, IKK gamma. IKK gamma is required for IKK activity by mediating kinase oligomerization and serving to couple the core catalytic subunits to upstream mitogen-activated protein 3-kinase cascades. We have discovered an alternatively spliced IKK gamma mRNA isoform, encoding an in-frame deletion of exon 5, termed IKK-gamma-Delta. Using a specific reverse transcription-PCR assay, we find that IKK-gamma-Delta is widely expressed in cultured human cells and normal human tissues. Because IKK-gamma-Delta protein is lacking a critical coiled-coil domain important in protein-protein interactions, we sought to determine its signaling properties by examining its ability to self associate, couple to activators of the canonical pathway, and mediate human T-cell leukemia virus type 1 (HTLV-1) Tax-induced NF-kappa B activity. Coimmunoprecipitation and confocal colocalization assays indicate IKK-gamma-Delta has strong homo- and heterotypic association with wild-type (WT) IKK-gamma and, like IKK-gamma WT, associates with the IKK beta kinase. Similarly, IKK gamma-Delta mediates IKK kinase activity and downstream NF-kappa B-dependent transcription in response to tumor necrosis factor (TNF) and the NF-kappa B-inducing kinase-IKK alpha signaling pathway. Surprisingly, however, in contrast to IKK gamma WT, IKK gamma-Delta is not able to mediate HTLV-1 Tax-induced NF-kappa B-dependent transcription, even though IKK-gamma-Delta binds and colocalizes with Tax. These observations suggest that IKK gamma-Delta is a functionally distinct alternatively spliced mRNA product differentially mediating TNF-induced, but not Tax-induced, signals converging on the IKK signalsome. Differing levels of IKK gamma-Delta expression, therefore, may affect signal transduction cascades coupling to IKK.