title: Ubiquitin chains are remodeled at the proteasome by opposing ubiquitin ligase and deubiquitinating activities.
abstract: The ubiquitin ligase Hul5 was recently identified as a component of the proteasome, a multisubunit protease that degrades ubiquitin-protein conjugates. We report here a proteasome-dependent conjugating activity of Hul5 that endows proteasomes with the capacity to extend ubiquitin chains. hul5 mutants show reduced degradation of multiple proteasome substrates in vivo, suggesting that the polyubiquitin signal that targets substrates to the proteasome can be productively amplified at the proteasome. However, the products of Hul5 conjugation are subject to disassembly by a proteasome-bound deubiquitinating enzyme, Ubp6. A hul5 null mutation suppresses a ubp6 null mutation, suggesting that a balance of chain-extending and chain-trimming activities is required for proper proteasome function. As the association of Hul5 with proteasomes was found to be strongly stabilized by Ubp6, these enzymes may be situated in proximity to one another. We propose that through dynamic remodeling of ubiquitin chains, proteasomes actively regulate substrate commitment to degradation.
authors.affiliation: Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA; Institut de Biologia Molecular de Barcelona, CSIC, Jordi Girona 18-26, Barcelona 08034, Spain.
authors.forename: Bernat
authors.initials: B
authors.lastname: Crosas
authors.forename: John
authors.initials: J
authors.lastname: Hanna
authors.forename: Donald S
authors.initials: DS
authors.lastname: Kirkpatrick
authors.forename: Dan Phoebe
authors.initials: DP
authors.lastname: Zhang
authors.forename: Yoshiko
authors.initials: Y
authors.lastname: Tone
authors.forename: Nathaniel A
authors.initials: NA
authors.lastname: Hathaway
authors.forename: Christa
authors.initials: C
authors.lastname: Buecker
authors.forename: David S
authors.initials: DS
authors.lastname: Leggett
authors.forename: Marion
authors.initials: M
authors.lastname: Schmidt
authors.forename: Randall W
authors.initials: RW
authors.lastname: King
authors.forename: Steven P
authors.initials: SP
authors.lastname: Gygi
authors.forename: Daniel
authors.initials: D
authors.lastname: Finley
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism; D010450:Endopeptidases / Q000378:metabolism / Q000502:physiology; D008025:Ligases / Q000378:metabolism* / Q000502:physiology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D025801:Ubiquitin / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism* / Q000502:physiology
chemical_list: D002352:Carrier Proteins; C122019:RPN10 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; C468547:HUL5 protein, S cerevisiae; D044767:Ubiquitin-Protein Ligases; D010450:Endopeptidases; D046988:Proteasome Endopeptidase Complex; C109028:UBP6 protein, S cerevisiae; D008025:Ligases