Removing Ubiquitin to Regulate NF-κB Activity

Science's STKE  27 Mar 2007:
Vol. 2007, Issue 379, pp. tw102
DOI: 10.1126/stke.3792007tw102

As part of a negative feedback mechanism, the transcription factor nuclear factor κΒ (NF-κB) induces IκBα transcription, restoring the amount of its negative regulator, IκBα, in the cell. However, the rapidity with which functional IκBα recovers implies the existence of other mechanisms to stabilize IκBα, even in the face of phosphorylation by IκB kinase β (IKKβ). This event leads to ubiquitinylation of IκBα, targeting it for proteasomal degradation and resulting in the release of active NF-κB. Schweitzer et al. have uncovered the crucial role that the COP9 signalosome (CSN) plays in this process. CSN is an eight-subunit (CSN1-8) protein complex that controls the activities of multisubunit cullin-RING ubiquitin ligases (CRLs), one member of which, SCFβTrCP, ubiquitinylates IκBα. The authors performed coimmunoprecipitation studies that established that CSN2 associated with IκBα after treatment of transfected HeLa cells with tumor necrosis factor-α (TNF-α). Through coimmunoprecipitation and electrophoretic mobility shift assays, and knockdown of CSN2 by small interfering RNA (siRNA), the authors showed that the recovery in IκBα after treatment with TNF-α was much reduced in CSN2-deficient cells compared with control cells. Earlier accumulation and enhanced amounts of ubiquitinylated IκBα were also seen in cells deficient in CSN2. One component of the CSN complex is USP15, a deubiquitinylase (DUB). HeLa cells transfected with a USP15-specific siRNA showed reduced recovery of IκBα after TNF-α-stimulated degradation compared with control cells. In vitro DUB assays, using ubiquitinylated IκBα as a substrate, demonstrated that both purified CSN and USP15 deubiquitinylated IκBα. Together, these data demonstrate a previously unknown role for CSN in regulating NF-κB activity through the stabilization of IκBα.

K. Schweitzer, P. M. Bozko, W. Dubiel, M. Naumann, CSN controls NF-κB by deubiquitinylation of IκBα. EMBO J. 26, 1532-1541 (2007). [PubMed]