NF-kappaB activation through IKK-i-dependent I-TRAF/TANK phosphorylation.
BACKGROUND
NF-kappaB is an ubiquitously expressed transcription factor that plays an important role in the immune, anti-apoptotic and inflammatory responses. NF-kappaB is normally sequestered in the cytoplasm by interacting with inhibitory IkappaB molecules. Upon stimulation, IkappaB is phosphorylated and subsequently degraded by the proteasome, allowing NF-kappaB to translocate into the nucleus where they regulate target gene expression. Two kinases, IKK-alpha and IKK-beta, which are responsible for IkappaB phosphorylation were recently identified. We have recently identified a cytokine inducible IKK-i, a kinase related to IKK-alpha and -beta. IKK-i significantly induced NF-kappaB activation upon over-expression, as did IKK-alpha and IKK-beta. Unlike IKK-alpha and IKK-beta, IKK-i phosphorylated Ser36 but not Ser32 in vitro, suggesting that IKK-i activates NF-kappaB by distinct mechanisms from the conventional IKKs.


RESULTS
I-TRAF/TANK was isolated as a molecule that interacts specifically with inducible IkappaB kinase (IKK-i) by the yeast two-hybrid screening procedure. The association of IKK-i and I-TRAF is mediated via the interaction between the N-terminal domain of I-TRAF and the C-terminal portion of IKK-i. In vitro kinase assays demonstrate that IKK-i phosphorylates I-TRAF in the middle portion that associates with TRAF2. Interestingly, TRAF2 is freed from the I-TRAF/TRAF2 complex after I-TRAF phosphorylation. NF-kappaB activation by IKK-i is significantly blocked by coexpression of the N-terminal domain of I-TRAF, dominant negative TRAF2, and dominant negative NIK and IKK-beta. IKK-i over-expression also induced c-Jun N-terminal kinase. These results show that I-TRAF is a substrate of IKK-i. NF-kappaB activation by IKK-i may be mediated through phosphorylation of I-TRAF by IKK-i and subsequent liberation of TRAF2.


CONCLUSIONS
These results indicate that NF-kappaB activation by IKK-i is mediated through phosphorylation of I-TRAF/TANK by IKK-i and subsequent liberation of TRAF2.
Nomura
F
F
Department of Host Defense, Core Research for Evolutional Science and Technology of Japan Science and Technology Corporation, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
Kawai
T
T
Nakanishi
K
K
Akira
S
S
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D004789:Enzyme Activation; D005786:Gene Expression Regulation; D006801:Humans; D051550:I-kappa B Kinase; D016328:NF-kappa B / Q000235:genetics*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics* / Q000378:metabolism; D011506:Proteins / Q000235:genetics* / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D016328:NF-kappa B; D011506:Proteins; C484406:TANK protein, human; D017346:Protein Serine-Threonine Kinases; D051550:I-kappa B Kinase