Phosphorylation of IkappaBalpha in the C-terminal PEST domain by casein kinase II affects intrinsic protein stability.
The NF-kappaB/Rel transcription factors participate in the activation of immune system regulatory genes and viral early genes including the human immunodeficiency virus type 1 long terminal repeat. NF-kappaB/Rel proteins are coupled to inhibitory molecules, collectively termed IkappaB, which are responsible for cytoplasmic retention of NF-kappaB. Cell activation leads to the phosphorylation and degradation of IkappaBalpha, permitting NG-kappaB/Rel translocation to the nucleus and target gene activation. To further characterize the signaling events that contribute to IkappaBalpha phosphorylation, a kinase activity was isolated from Jurkat T cells that specifically interacted with IkappaBalpha in an affinity chromatography step and phosphorylated IkappaBalpha with high specificity in vitro. By using an in-gel kinase assay with recombinant IkappaBalpha as substrate, two forms of the kinase (43 and 38 kDa) were identified. Biochemical criteria and immunological cross-reactivity identified the kinase activity as the alpha catalytic subunit of casein kinase II (CKII). Deletion mutants of IkappaBalpha delta1 to delta4) localized phosphorylation to the C-terminal PEST domain of IkappaBalpha. Point mutation of residues T-291, S-283, and T-299 dramatically reduced phosphorylation of IkappaBalpha by the kinase in vitro. NIH-3T3 cells that stably expressed wild-type IkappaBalpha (wtIkappaB), double-point-mutated IkappaBalpha (T291A, S283A), or triple-point-mutated IkappaBalpha (T291A, S283A, T299A) under the control of the tetracycline-responsive promoter were generated. Constitutive phosphorylation of the triple point mutant was eliminated in vivo, although tumor necrosis factor-inducible IkappaBalpha degradation was unaffected. In cell lines and in transiently transfected cells, mutation of the CKII sites in IkappaBalpha resulted in a protein with increased intrinsic stability. Together with results demonstrating a role for N-terminal sites in inducer-mediated phosphorylation and degradation of IkappaBalpha, these studies indicate that CKII sites in the C-terminal PEST domain are important for constitutive phosphorylation and intrinsic stability of IkappaBalpha.
Lin
R
R
Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, Montreal, Quebec, Canada.
Beauparlant
P
P
Makris
C
C
Meloche
S
S
Hiscott
J
J
D001483:Base Sequence; D001665:Binding Sites; D047390:Casein Kinase II; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004795:Enzyme Stability; D006801:Humans; D020672:I-kappa B Proteins; D015151:Immunoblotting; D008969:Molecular Sequence Data; D000072000:NF-KappaB Inhibitor alpha; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D017385:Sequence Homology; D051841:Transcription Factor RelB; D014157:Transcription Factors
D004268:DNA-Binding Proteins; D020672:I-kappa B Proteins; D016328:NF-kappa B; C000606204:NFKBIA protein, human; D011518:Proto-Oncogene Proteins; C497593:RELB protein, human; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D000072000:NF-KappaB Inhibitor alpha; D051841:Transcription Factor RelB; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases