title: The Cullin3 ubiquitin ligase functions as a Nedd8-bound heterodimer.
abstract: Cullins are members of a family of scaffold proteins that assemble multisubunit ubiquitin ligase complexes to confer substrate specificity for the ubiquitination pathway. Cullin3 (Cul3) forms a catalytically inactive BTB-Cul3-Rbx1 (BCR) ubiquitin ligase, which becomes functional upon covalent attachment of the ubiquitin homologue neural-precursor-cell-expressed and developmentally down regulated 8 (Nedd8) near the C terminus of Cul3. Current models suggest that Nedd8 activates cullin complexes by providing a recognition site for a ubiquitin-conjugating enzyme. Based on the following evidence, we propose that Nedd8 activates the BCR ubiquitin ligase by mediating the dimerization of Cul3. First, Cul3 is found as a neddylated heterodimer bound to a BTB domain-containing protein in vivo. Second, the formation of a Cul3 heterodimer is mediated by a Nedd8 molecule, which covalently attaches itself to one Cul3 molecule and binds to the winged-helix B domain at the C terminus of the second Cul3 molecule. Third, complementation experiments revealed that coexpression of two distinct nonfunctional Cul3 mutants can rescue the ubiquitin ligase function of the BCR complex. Likewise, a substrate of the BCR complex binds heterodimeric Cul3, suggesting that the Cul3 complex is active as a dimer. These findings not only provide insight into the architecture of the active BCR complex but also suggest assembly as a regulatory mechanism for activation of all cullin-based ubiquitin ligases.
authors.affiliation: Department of Molecular Biology, Cell Biology and Biochemistry, Center for Genomics and Proteomics, Brown University, Providence, RI 02903, USA.
authors.forename: Wananit
authors.initials: W
authors.lastname: Wimuttisuk
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Singer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D044842:Cullin Proteins / Q000737:chemistry / Q000378:metabolism*; D019927:Cyclin E / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008954:Models, Biological; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D014452:Ubiquitins / Q000378:metabolism*
chemical_list: D044842:Cullin Proteins; D019927:Cyclin E; D050505:Mutant Proteins; D014452:Ubiquitins