title: Autoregulation of an E2 enzyme by ubiquitin-chain assembly on its catalytic residue.
abstract: Cells have quality-control mechanisms to recognize non-native protein structures and either help the proteins fold or promote their degradation. Ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s) work together to assemble polyubiquitin chains on misfolded or misassembled proteins, which are then degraded by the proteasome. Here, we find that Ubc7, a yeast E2, can itself undergo degradation when its levels exceed that of its binding partner Cue1, a transmembrane protein that tethers Ubc7 to the endoplasmic reticulum. Unassembled, and thus mislocalized, Ubc7 is targeted to the proteasome by Ufd4, a homologous to E6-AP C-terminus (HECT)-class E3. Ubc7 is autoubiquitinated by a novel mechanism wherein the catalytic cysteine, instead of a lysine residue, provides the polyubiquitin chain acceptor site, and this cysteine-linked chain functions as a degradation signal. The polyubiquitin chain can also be transferred to a lysine side chain, suggesting a mechanism for polyubiquitin chain assembly that precedes substrate modification.
authors.affiliation: Department of Molecular Biophysics & Biochemistry, Yale University, New Haven, CT 06520, USA.
authors.forename: Tommer
authors.initials: T
authors.lastname: Ravid
authors.forename: Mark
authors.initials: M
authors.lastname: Hochstrasser
mesh_terms: D000596:Amino Acids / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D003545:Cysteine / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D015966:Gene Expression Regulation, Fungal; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D010957:Plasmids / Q000235:genetics; D025821:Polyubiquitin / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D025801:Ubiquitin / Q000378:metabolism*; D044763:Ubiquitin-Conjugating Enzymes / Q000235:genetics / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism; D015003:Yeasts / Q000235:genetics / Q000378:metabolism
chemical_list: D000596:Amino Acids; D005656:Fungal Proteins; D025801:Ubiquitin; D025821:Polyubiquitin; D049452:Green Fluorescent Proteins; D044763:Ubiquitin-Conjugating Enzymes; D044767:Ubiquitin-Protein Ligases; D046988:Proteasome Endopeptidase Complex; D003545:Cysteine