title: The Hsc70 co-chaperone CHIP targets immature CFTR for proteasomal degradation.
abstract: The folding of both wild-type and mutant forms of the cystic-fibrosis transmembrane-conductance regulator (CFTR), a plasma-membrane chloride-ion channel, is inefficient. Most nascent CFTR is retained in the endoplasmic reticulum and degraded by the ubiquitin proteasome pathway. Aberrant folding and defective trafficking of CFTRDeltaF508 is the principal cause of cystic fibrosis, but how the endoplasmic-reticulum quality-control system targets CFTR for degradation remains unknown. CHIP is a cytosolic U-box protein that interacts with Hsc70 through a set of tetratricorepeat motifs. The U-box represents a modified form of the ring-finger motif that is found in ubiquitin ligases and that defines the E4 family of polyubiquitination factors. Here we show that CHIP functions with Hsc70 to sense the folded state of CFTR and targets aberrant forms for proteasomal degradation by promoting their ubiquitination. The U-box appeared essential for this process because overexpresion of CHIPDeltaU-box inhibited the action of endogenous CHIP and blocked CFTR ubiquitination and degradation. CHIP is a co-chaperone that converts Hsc70 from a protein-folding machine into a degradation factor that functions in endoplasmic-reticulum quality control.
authors.affiliation: Department of Cell Biology and Cystic Fibrosis Center, University of Alabama Medical Center, 1918 University Boulevard, Birmingham, Alabama 35294-0005, USA.
authors.forename: G C
authors.initials: GC
authors.lastname: Meacham
authors.forename: C
authors.initials: C
authors.lastname: Patterson
authors.forename: W
authors.initials: W
authors.lastname: Zhang
authors.forename: J M
authors.initials: JM
authors.lastname: Younger
authors.forename: D M
authors.initials: DM
authors.lastname: Cyr
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism / Q000648:ultrastructure; D017353:Gene Deletion; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D008025:Ligases; D018832:Molecular Chaperones / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D017434:Protein Structure, Tertiary / Q000502:physiology; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D018840:HSP70 Heat-Shock Proteins; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D014452:Ubiquitins; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; C467728:STUB1 protein, human; D044767:Ubiquitin-Protein Ligases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases