Crosstalk between the NF-κB activating IKK-complex and the CSN signalosome

A great variety of signalling pathways regulating inflammation, cell development and cell survival require NF-κB transcription factors, which are normally inactive due to binding to inhibitors, such as IκBα. The canonical activation pathway of NF-κB is initiated by phosphorylation of the inhibitor by an IκB kinase (IKK) complex triggering ubiquitination of IκB molecules by SCF-type E3-ligase complexes and rapid degradation by 26S-proteasomes. The ubiquitination machinery is regulated by the COP9 signalosome (CSN). We show that IκB kinases interact with the CSN-complex, as well as the SCF-ubiquitination machinery, providing an explanation for the rapid signalling-induced ubiquitination and degradation of IκBα. Furthermore, we reveal that IKK’s phosphorylate not only IκBα, but also the CSN-subunit Csn5/JAB1 (c-Jun activation domain binding protein-1) and that IKK2 influences ubiquitination of Csn5/JAB1. Our observations imply that the CSN complex acts as an inhibitor of constitutive NF-κB activity in non-activated cells. Knock-down of Csn5/JAB1 clearly enhanced basal NF-κB activity and improved cell survival under stress. The inhibitory effect of Csn5/JAB1 requires a functional MPN(+) metalloprotease domain, which is responsible for cleaving ubiquitin-like Nedd8-modifications. Upon activation of cells with tumour necrosis factor-α, the CSN complex dissociates from IKK’s allowing full and rapid activation of the NF-κB pathway by the concerted action of interacting protein complexes.