Identification of a novel 300‐kDa factor termed IκBαE3‐F1 that is required for ubiquitinylation of IκBα

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Abstract
Destruction of IκB by ubiquitinylation is required for signal‐dependent activation of NF‐κB. The IκBα ubiquitin‐ligase activity associated with phosphorylated IκBα (pIκBα) in HeLa cells was almost completely lost by washing under stringent conditions including 1 M NaCl; nevertheless, an SCFβTrCP complex containing Skp1, Cullin‐1, and F‐box/WD40 protein βTrCP was still bound to pIκBα, suggesting the existence of a putative factor that is loosely associated with pIκBα and may collaborate with SCFβTrCP. The factor was named IκBαE3‐F1 and was partially purified from HeLa cells. Gel filtration analysis revealed that IκBαE3‐F1 has an apparent molecular mass of approximately 300 kDa.