title: Nucleolar targeting of the fbw7 ubiquitin ligase by a pseudosubstrate and glycogen synthase kinase 3.
abstract: E3 ubiquitin ligases catalyze protein degradation by the ubiquitin-proteasome system, and their activity is tightly controlled. One level of regulation involves subcellular localization, and the Fbw7 tumor suppressor exemplifies this type of control. Fbw7 is the substrate-binding component of an SCF ubiquitin ligase that degrades critical oncoproteins. Alternative splicing produces three Fbw7 protein isoforms that occupy distinct compartments: Fbw7α is nucleoplasmic, Fbw7β is cytoplasmic, and Fbw7γ is nucleolar. We found that cancer-associated Fbw7 mutations that disrupt substrate binding prevent Fbw7γ nucleolar localization, implicating a substrate-like interaction in nucleolar targeting. We identified EBNA1-binding protein 2 (Ebp2) as the critical nucleolar factor that directly mediates Fbw7 nucleolar targeting. Ebp2 binds to Fbw7 like a substrate, and this is mediated by an Ebp2 degron that is phosphorylated by glycogen synthase kinase 3. However, despite these canonical substrate-like interactions, Fbw7 binding is largely uncoupled from Ebp2 turnover in vivo. Ebp2 thus acts like a pseudosubstrate that directly recruits Fbw7 to nucleoli.
authors.affiliation: Fred Hutchinson Cancer Research Center, Mailstop D2-100, 1100 Fairview Ave. N, Seattle, WA 98109, USA.
authors.forename: Markus
authors.initials: M
authors.lastname: Welcker
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Larimore
authors.forename: Lori
authors.initials: L
authors.lastname: Frappier
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Clurman
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D002466:Cell Nucleolus / Q000378:metabolism*; D002478:Cells, Cultured; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D006801:Humans; D009154:Mutation; D009369:Neoplasms / Q000235:genetics; D010766:Phosphorylation; D011485:Protein Binding; D020033:Protein Isoforms / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D016601:RNA-Binding Proteins; D044843:SKP Cullin F-Box Protein Ligases / Q000032:analysis* / Q000235:genetics / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; C432687:EBNA1BP2 protein, human; D020033:Protein Isoforms; D016601:RNA-Binding Proteins; D044843:SKP Cullin F-Box Protein Ligases; D038362:Glycogen Synthase Kinase 3