Pseudosubstrate regulation of the SCF β-TrCP ubiquitin ligase by hnRNP-U

• Published in: Genes & development 2002-02, Vol.16 (4), p.439-451
• Main Authors: Davis, Matti, Hatzubai, Ada, Andersen, Jens S., Ben-Shushan, Etti, Fisher, Gregory Zvi, Yaron, Avraham, Bauskin, Asne, Mercurio, Frank, Mann, Matthias, Ben-Neriah, Yinon
• Format: Article
• Language: English
• Subjects: Research Paper
• ISSN: 0890-9369; 1549-5477
• DOI: 10.1101/gad.218702

β-TrCP/E3RS (E3RS) is the F-box protein that functions as the receptor subunit of the SCF β-TrCP ubiquitin ligase (E3). Surprisingly, although its two recognized substrates, IκBα and β-catenin, are present in the cytoplasm, we have found that E3RS is located predominantly in the nucleus. Here we report the isolation of the major E3RS-associated protein, hnRNP-U, an abundant nuclear phosphoprotein. This protein occupies E3RS in a specific and stoichiometric manner, stabilizes the E3 component, and is likely responsible for its nuclear localization. hnRNP-U binding was abolished by competition with a pIκBα peptide, or by a specific point mutation in the E3RS WD region, indicating an E3–substrate-type interaction. However, unlike pIκBα, which is targeted by SCF β-TrCP for degradation, the E3-bound hnRNP-U is stable and is, therefore, a pseudosubstrate. Consequently, hnRNP-U engages a highly neddylated active SCF β-TrCP , which dissociates in the presence of a high-affinity substrate, resulting in ubiquitination of the latter. Our study points to a novel regulatory mechanism, which secures the localization, stability, substrate binding threshold, and efficacy of a specific protein-ubiquitin ligase.