This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 3, 2005. Proteins conjugated by Lys-48-linked polyubiquitin chains are preferred substrates of the eukaryotic proteasome. Polyubiquitination requires an activating enzyme (E1), a conjugating enzyme (E2), and a ligase (E3). Occasionally, these enzymes only assemble short ubiquitin oligomers, and their extension to full length involves a ubiquitin elongating factor termed E4. Ufd2p, as the first E4 identified to date, is involved in the degradation of misfolded proteins of the endoplasmic reticulum and of a ubiquitin-beta-GAL fusion substrate in Saccharomyces cerevisiae. The mechanism of action of Ufd2p is unknown. Here we describe the crystal structure of the full-length yeast Ufd2p protein. Ufd2p has an elongated shape consisting of several irregular Armadillo-like repeats with two helical hairpins protruding from it and a U-box domain flexibly attached to its C terminus. The U-box of Ufd2p has a fold similar to that of the RING (Really Interesting New Gene) domain that is present in certain ubiquitin ligases. Accordingly, Ufd2p has all of the hallmarks of a RING finger-containing ubiquitin ligase: it associates with its cognate E2 Ubc4p via its U-box domain and catalyzes the transfer of ubiquitin from the E2 active site to Ufd2p itself or to an acceptor ubiquitin molecule to form unanchored diubiquitin oligomers. Thus, Ufd2p can function as a bona fide E3 ubiquitin ligase to promote ubiquitin chain elongation on a substrate.
|Evidence ID||Analyze ID||Interactor||Interactor Systematic Name||Interactor||Interactor Systematic Name||Type||Assay||Annotation||Action||Modification||Phenotype||Source||Reference||Note|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Gene Ontology Term||Gene Ontology Term ID||Qualifier||Aspect||Method||Evidence||Source||Assigned On||Reference||Annotation Extension|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Phenotype||Experiment Type||Experiment Type Category||Mutant Information||Strain Background||Chemical||Details||Reference|
|Evidence ID||Analyze ID||Regulator||Regulator Systematic Name||Target||Target Systematic Name||Experiment||Conditions||Strain||Source||Reference|