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-κB is an inducible transcription factor mediating innate immune responses whose activity is controlled by the multiprotein Ikappa;B kinase (IKK) "signalsome". The core IKK consists of two catalytic serine kinases, IKKα and IKKβ and a noncatalytic subunit, IKKγ. IKKγ 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 IKKγ mRNA isoform, encoding an in-frame deletion of exon 5, termed IKKγ-Δ. Using a specific reverse transcription-PCR assay, we find that IKKγ-Δ is widely expressed in cultured human cells and normal human tissues. Because IKKγ-Δ 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-κB activity. Coimmunoprecipitation and confocal colocalization assays indicate IKKγ-Δ has strong homo- and heterotypic association with wild-type (WT) IKKγ and, like IKKγ WT, associates with the IKKβ kinase. Similarly, IKKγ-Δ mediates IKK kinase activity and downstream NFκB-dependent transcription in response to tumor necrosis factor (TNF) and the NF-κB-inducing kinase-IKKα signaling pathway. Surprisingly, however, in contrast IKKγ WT, IKKγ-Δ is not able to mediate HTLV-1 Tax-induced NFκB-dependent transcription, even though IKKγ-Δ binds and colocalizes with Tax. These observations suggest that IKKγ-Δ 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 IKKγ-Δ expression, therefore, may affect signal transduction cascades coupling to IKK.