Recruitment of a ROC1-CUL1 ubiquitin ligase by Skp1 and HOS to catalyze the ubiquitination of I kappa B alpha

Activation of the transcription factor NF-kappa B in response to proinflamm atory stimuli requires the phosphorylation-triggered and ubiquitin-dependen t degradation of the NF-kappa B inhibitor, I kappa B alpha. Here, we show t he in vitro reconstitution of the phosphorylation-dependent ubiquitination of I kappa B alpha with purified components. ROC1, a novel SCF-associated p rotein, is recruited by cullin 1 to form a quaternary SCFHOS-ROC1 holoenzym e (with Skp1 and the beta-TRCP homolog HOS). SCFHOS-ROC1 binds IKK beta-pho sphorylated I kappa B alpha and catalyzes its ubiquitination in the presenc e of ubiquitin, E1, and Cdc34. ROC1 plays a unique role in the ubiquitinati on reaction by heterodimerizing with cullin 1 to catalyze ubiquitin polymer ization.