Nedd8 Modification of Cul-1 Activates SCF(β(TrCP))-Dependent Ubiquitination of IκBα

Regulation of NF-κB occurs through phosphorylation-dependent ubiquitination of IκBα, which is degraded by the 26S proteasome. Recent studies have shown that ubiquitination of IκBα is carried out by a ubiquitin-ligase enzyme complex called SCF(β(TrCP)). Here we show that Nedd8 modification of the Cul-1 component of SCF(β(TrCP)) is important for function of SCF(β(TrCP)) in ubiquitination of IκBα. In cells, Nedd8-conjugated Cul-1 was complexed with two substrates of SCF(β(TrCP)), phosphorylated IκBα and β-catenin, indicating that Nedd8–Cul-1 conjugates are part of SCF(β(TrCP)) in vivo. Although only a minute fraction of total cellular Cul-1 is modified by Nedd8, the Cul-1 associated with ectopically expressed βTrCP was highly enriched for the Nedd8-conjugated form. Moreover, optimal ubiquitination of IκBα required Nedd8 and the Nedd8-conjugating enzyme, Ubc12. The site of Nedd8 ligation to Cul-1 is essential, as SCF(β(TrCP)) containing a K720R mutant of Cul-1 only weakly supported IκBα ubiquitination compared to SCF(β(TrCP)) containing WT Cul-1, suggesting that the Nedd8 ligation of Cul-1 affects the ubiquitination activity of SCF(β(TrCP)). These observations provide a functional link between the highly related ubiquitin and Nedd8 pathways of protein modification and show how they operate together to selectively target the signal-dependent degradation of IκBα.