Alternative Splicing Variants of IκBβ Establish Differential NF-κB Signal Responsiveness in Human Cells

ABSTRACT To release transcription factor NF-κB into the nucleus, the mammalian IκB molecules IκBα and IκBβ are inactivated by phosphorylation and proteolytic degradation. Both proteins contain conserved signal-responsive phosphorylation sites and have conserved ankyrin repeats. To confer specific physiological functions to members of the NF-κB/Rel family, the different IκB molecules could vary in their specific NF-κB/Rel factor binding activities and could respond differently to activation signals. We have demonstrated that both mechanisms apply to differential regulation of NF-κB function by IκBβ relative to IκBα. Via alternative RNA processing, human IκBβ gives rise to different protein isoforms. IκBβ1 and IκBβ2, the major forms in human cells, differ in their carboxy-terminal PEST sequences. IκBβ2 is the most abundant species in a number of human cell lines tested, whereas IκBβ1 is the only form detected in murine cells. These isoforms are indistinguishable in their binding preferences to cellular NF-κB/Rel homo- and heterodimers, which are distinct from those of IκBα, and both are constitutively phosphorylated. In unstimulated B cells, however, IκBβ1, but not IκBβ2, is found in the nucleus. Furthermore, the two forms differ markedly in their efficiency of proteolytic degradation after stimulation with several inducing agents tested. While IκBβ1 is nearly as responsive as IκBα, indicative of a shared activation mechanism, IκBβ2 is only weakly degraded and often not responsive at all. Alternative splicing of the IκBβ pre-mRNA may thus provide a means to selectively control the amount of IκBβ-bound NF-κB heteromers to be released under NF-κB stimulating conditions.