Protein Folding Stability Can Determine the Efficiency of Escape from Endoplasmic Reticulum Quality Control*
Abstract
A fraction of each secreted protein is retained and degraded by the endoplasmic reticulum (ER) quality control apparatus that restricts export to correctly folded proteins. The intrinsic biophysical attributes that determine efficiency of escape from this proofreading process have been examined by expressing mutants of bovine pancreatic trypsin inhibitor (BPTI) in yeast. Secretion efficiency is strongly correlated with thermodynamic stability for a series of six point mutations of BPTI. No correlation of secretion efficiency with either oxidative folding or refolding rates in vitro is found; both the rapidly folded Y35L BPTI mutant and the slowly unfolded G36D BPTI mutant exhibit low secretion efficiency. Elimination of cysteines 14 and 38 by mutagenesis does not increase secretion efficiency, indicating that intramolecular thiol/disulfide rearrangements are not primarily responsible for retention and degradation of destabilized BPTI variants. Mutant yeast strains with diminished ER-associated degradation do not secrete BPTI more efficiently, indicating that retention and degradation are separable processes. These data support a model for ER quality control, wherein protein folding is functionally reversible and the relative rates of folding, unfolding, vesicular export, and retention determine secretion efficiency.
Footnotes
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↵* This work was funded by the National Institutes of Health Grant GM50673 and the National Science Foundation Grant BES 95–31407.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: 600 S. Mathews, Urbana, IL 61801. Tel.: 217-333-2631; Fax: 217-333-5052; E-mail:wittrup{at}uiuc.edu.
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↵1 The abbreviations used are: ER, endoplasmic reticulum; BPTI, bovine pancreatic trypsin inhibitor;l-BAPA,N-α-benzoyl-arginine-p-nitroanilide; Tricine,N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine; WT, wild type.
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↵2 J. M. Kowalski, R. N. Parekh, J. Mao, K. D. Wittrup, manuscript in preparation.
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- Received March 6, 1998.
- Revision received May 18, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
