Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway.

Abstract

The transcription factor NF-KB is sequestered in the cytoplasm by the inhibitor protein IKBc~. Extracellular inducers of NF-KB activate signal transduction pathways that result in the phosphorylation and subsequent degradation of IKBa. At present, the link between phosphorylation of IKB~x and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of IKBcx targets the protein to the ubiquitin-proteasome pathway. IKBa is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of IKBa in vivo prevent ubiquitination in vitro. Ubiquitinated IgBa remains associated with NF-KB, and the bound IKBa is degraded by the 26S proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of IKBr

NF-κB mediates proteolysis-inducing factor induced protein degradation and expression of the ubiquitin–proteasome system in skeletal muscle

“…In I-kBa DN, the phosphorylation sites required for degradation (Ser 32 and Ser 36 ) are absent (truncation of amino acids 1 -36), while in I-kBa S32/A36 there are point mutations of Ser 32 and Ser 36 to alanine. This prevented ubiquitin conjugation and proteolysis of either protein (Brockman et al, 1993;Chen et al, 1995). The control cells were transfected with the empty pCMV4 vector.…”

Analysis of changes in the proteome of HL-60 promyeloid leukemia cells induced by the proteasome inhibitor PSI

“…Prominent events in proteasome inhibitor induced apoptosis include the production of reactive oxygen species (ROS) [20,21], activation of the stress kinases JNK [22][23][24] and p38 [8,10,25,26] as well as abrogation of cytoprotective p42/p44 MAPK signaling [27,28]. Furthermore, transcriptional activation of NFκB dependent survival promoting genes, such as cIAP-1 and cIAP-2 [29], XIAP [30], A1 and A20 [31,32] and Bcl-xL [33] is inhibited by blocking the degradation of IκB [34,35]. Mutations in the canonical or alternative pathways of NFκB-activation, which lead to chronic activation of this transcription factor, have been shown to occur quite frequently in tumor cells [36].…”

Defective IκBα in Hodgkin cell lines with constitutively active NF-κB

“…Signal induced activation of NF-kB is preceded by phosphorylation and rapid degradation of IkBa (Beg and Baldwin, 1993;Brown et al, 1993;Henkel et al, 1993;Mellits et al, 1993) mediated by the ubiquitin proteasome pathway (Palombella et al, 1994). The target residues for signal induced modi®cation are located in the exposed N-terminus of the IkBa protein (Ja ray et al, 1995) with S32 and S36 being the sites of phosphoryla-tion Brown et al, 1995;DiDonato et al, 1996;Ro et al, 1996;Traenckner et al, 1995) and K21 and K22 being the principal sites of ubiquitination (Baldi et al, 1996;Chen et al, 1995;Rodriguez et al, 1996;Scherer et al, 1995). Signal induced modi®cation of the IkBa N-terminus, although necessary, is not su cient to promote degradation of the protein.…”