ALTERNATIVE SPLICING VARIANTS OF I-KAPPA-B-BETA ESTABLISH DIFFERENTIAL NF-KAPPA-B SIGNAL RESPONSIVENESS IN HUMAN-CELLS

To release transcription factor NF-kappa B into the nucleus, the mamma lian I kappa B molecules I kappa B alpha and I kappa B beta are inacti vated by phosphorylation and proteolytic degradation. Both proteins co ntain conserved signal-responsive phosphorylation sites and have conse rved ankyrin repeats. To confer specific physiological functions to me mbers 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 demonstra ted that both mechanisms apply to differential regulation of NF-kappa B function by I kappa B beta relative to I kappa B alpha. Via alternat ive RNA processing, human I kappa B beta gives rise to different prote in 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 preferenc es to cellular NF-kappa B/Rel homo-and heterodimers, which are distinc t from those of I kappa B alpha, and both are constitutively phosphory lated. In unstimulated B cells, however, I kappa B beta 1, but not I k appa B beta 2, is found in the nucleus, Furthermore, the two forms dif fer markedly in their efficiency of proteolytic degradation after stim ulation with several inducing agents tested. While I kappa B beta 1 is nearly as responsive as I kappa B alpha, indicative of a shared activ ation mechanism, I kappa B beta 2 is only weakly degraded and often no t responsive at all. Alternative splicing of the I kappa B beta pre-mR NA may thus provide a means to selectively control the amount of I kap pa B beta-bound NF-kappa B heteromers to be released under NF-kappa B stimulating conditions.