Alternative splicing variants of IκBβ establish differential NF-κB signal responsiveness in human cells

To release transcription factor NF-kappa B into the nucleus, the mammalian I kappa B molecules I kappa B alpha and I kappa B beta 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-kappa B/Rel family, the different I kappa B molecules could vary in their specific NF-kappa B/Rel factor binding activities and could respond differently to activation signals. We have demonstrated that both mechanisms apply to differential regulation of NF-kappa B function by I kappa B beta relative to I kappa B alpha. Via alternative RNA processing, human I kappa B beta gives rise to different protein isoforms. I kappa B beta 1 and I kappa B beta 2, the major forms in human cells, differ in their carboxy-terminal PEST sequences. I kappa B beta 2 is the most abundant species in a number of human cell lines tested, whereas I kappa B beta 1 is the only form detected in murine cells. These isoforms are indistinguishable in their binding preferences to cellular NF-kappa B/Rel homo-and heterodimers, which are distinct from those of I kappa B alpha, and both are constitutively phosphorylated. In unstimulated B cells, however, I kappa B beta 1, but not I kappa B beta 2, 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 I kappa B beta 1 is nearly as responsive as I kappa B alpha, indicative of a shared activation mechanism, I kappa B beta 2 is only weakly degraded and often not responsive at all. Alternative splicing of the I kappa B beta pre-mRNA may thus provide a means to selectively control the amount of I kappa B beta-bound NF-kappa B heteromers to be released under NF-kappa B stimulating conditions.