THE 90-KDA RIBOSOMAL S6 KINASE (PP90(RSK)) PHOSPHORYLATES THE N-TERMINAL REGULATORY DOMAIN OF I-KAPPA-B-ALPHA AND STIMULATES ITS DEGRADATION IN-VITRO

Nuclear factor kappa B (NF-kappa B) is a eukaryotic member of the Rel family of transcription factors whose biological activity is post-tran slationally regulated by its assembly with various ankyrin-rich cytopl asmic inhibitors, including I kappa B alpha. Expression of NF-kappa B in the nucleus occurs after signal-induced phosphorylation, ubiquitina tion, and proteasome-mediated degradation of I kappa B alpha. The indu ced proteolysis of I kappa B alpha unmasks the nuclear localization si gnal within NF-kappa B, allowing its rapid migration into the nucleus, where it activates the transcription of many target genes, At present , the identity of the I kappa B alpha kinase(s) that triggers the firs t step in I kappa B alpha degradation remains unknown. We have investi gated the potential function of the 90-kDa ribosomal S6 kinase, or pp9 0(rsk), as a signal-inducible I kappa B alpha kinase. pp90(rsk) lies d ownstream of mitogen-activated protein (MAP) kinase in the well charac terized Ras-Raf-MEK-MAP kinase pathway that is induced by various grow th factors and phorbol ester. We now show that pp90(rsk), but not pp70 (S6K) Or MAP kinase, phosphorylates the regulatory N terminus of I kap pa B alpha principally on serine 32 and triggers effective I kappa B a lpha degradation in vitro. When co-expressed in vivo in COS cells, I k appa B alpha and pp90(rsk) readily assemble into a complex that is imm unoprecipitated with antibodies specific for either partner. While pho rbol 12-myristate 13-acetate produced rapid activation of pp90(rsk), i n vivo, other potent NF-kappa B inducers, including tumor necrosis fac tor alpha and the Tax transactivator of human T-cell lymphotrophic vir us, type I, failed to activate pp90(rsk). These data suggest that more than a single I kappa B alpha kinase exists within the cell and that these I kappa B alpha kinases are differentially activated by differen t NF-kappa B inducers.