Ubiquitylation of BAG-1 suggests a novel regulatory mechanism during the sorting of chaperone substrates to the proteasome.
BAG-1 is a ubiquitin domain protein that links the molecular chaperones Hsc70 and Hsp70 to the proteasome. During proteasomal sorting BAG-1 can cooperate with another co-chaperone, the carboxyl terminus of Hsc70-interacting protein CHIP. CHIP was recently identified as a Hsp70- and Hsp90-associated ubiquitin ligase that labels chaperone-presented proteins with the degradation marker ubiquitin. Here we show that BAG-1 itself is a substrate of the CHIP ubiquitin ligase in vitro and in vivo. CHIP mediates attachment of ubiquitin moieties to BAG-1 in conjunction with ubiquitin-conjugating enzymes of the Ubc4/5 family. Ubiquitylation of BAG-1 is strongly stimulated when a ternary Hsp70.BAG-1.CHIP complex is formed. Complex formation results in the attachment of an atypical polyubiquitin chain to BAG-1, in which the individual ubiquitin moieties are linked through lysine 11. The noncanonical polyubiquitin chain does not induce the degradation of BAG-1, but it stimulates a degradation-independent association of the co-chaperone with the proteasome. Remarkably, this stimulating activity depends on the simultaneous presentation of the integrated ubiquitin-like domain of BAG-1. Our data thus reveal a cooperative recognition of sorting signals at the proteolytic complex. Attachment of polyubiquitin chains to delivery factors may represent a novel mechanism to regulate protein sorting to the proteasome.
Alberti
Simon
S
Institut für Zellbiologie und Bonner Forum Biomedizin, Rheinische Friedrich-Wilhelms-Universität Bonn, Ulrich-Haberland-Strasse 61a, Bonn D-53121, Germany.
Demand
Jens
J
Esser
Claudia
C
Emmerich
Niels
N
Schild
Hansjorg
H
Hohfeld
Jorg
J
D002352:Carrier Proteins / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004268:DNA-Binding Proteins; D006801:Humans; D018832:Molecular Chaperones / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D014157:Transcription Factors; D025801:Ubiquitin / Q000378:metabolism*
C091732:BCL2-associated athanogene 1 protein; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D014157:Transcription Factors; D025801:Ubiquitin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex