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      Cooh-Terminal Truncations Promote Proteasome-Dependent Degradation of Mature Cystic Fibrosis Transmembrane Conductance Regulator from Post-Golgi Compartments

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          Abstract

          Impaired biosynthetic processing of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel, constitutes the most common cause of CF. Recently, we have identified a distinct category of mutation, caused by premature stop codons and frameshift mutations, which manifests in diminished expression of COOH-terminally truncated CFTR at the cell surface. Although the biosynthetic processing and plasma membrane targeting of truncated CFTRs are preserved, the turnover of the complex-glycosylated mutant is sixfold faster than its wild-type (wt) counterpart. Destabilization of the truncated CFTR coincides with its enhanced susceptibility to proteasome-dependent degradation from post-Golgi compartments globally, and the plasma membrane specifically, determined by pulse–chase analysis in conjunction with cell surface biotinylation. Proteolytic cleavage of the full-length complex-glycosylated wt and degradation intermediates derived from both T70 and wt CFTR requires endolysosomal proteases. The enhanced protease sensitivity in vitro and the decreased thermostability of the complex-glycosylated T70 CFTR in vivo suggest that structural destabilization may account for the increased proteasome susceptibility and the short residence time at the cell surface. These in turn are responsible, at least in part, for the phenotypic manifestation of CF. We propose that the proteasome-ubiquitin pathway may be involved in the peripheral quality control of other, partially unfolded membrane proteins as well.

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          Most cited references86

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          Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA.

          Overlapping complementary DNA clones were isolated from epithelial cell libraries with a genomic DNA segment containing a portion of the putative cystic fibrosis (CF) locus, which is on chromosome 7. Transcripts, approximately 6500 nucleotides in size, were detectable in the tissues affected in patients with CF. The predicted protein consists of two similar motifs, each with (i) a domain having properties consistent with membrane association and (ii) a domain believed to be involved in ATP (adenosine triphosphate) binding. A deletion of three base pairs that results in the omission of a phenylalanine residue at the center of the first predicted nucleotide-binding domain was detected in CF patients.
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            The function of heat-shock proteins in stress tolerance: degradation and reactivation of damaged proteins.

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              Ubiquitin-dependent protein degradation.

              A growing number of cellular regulatory mechanisms are being linked to protein modification by the polypeptide ubiquitin. These include key transitions in the cell cycle, class I antigen processing, signal transduction pathways, and receptor-mediated endocytosis. In most, but not all, of these examples, ubiquitination of a protein leads to its degradation by the 26S proteasome. Following attachment of ubiquitin to a substrate and binding of the ubiquitinated protein to the proteasome, the bound substrate must be unfolded (and eventually deubiquitinated) and translocated through a narrow set of channels that leads to the proteasome interior, where the polypeptide is cleaved into short peptides. Protein ubiquitination and deubiquitination are both mediated by large enzyme families, and the proteasome itself comprises a family of related but functionally distinct particles. This diversity underlies both the high substrate specificity of the ubiquitin system and the variety of regulatory mechanisms that it serves.
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                Author and article information

                Contributors
                Journal
                J Cell Biol
                The Journal of Cell Biology
                The Rockefeller University Press
                0021-9525
                1540-8140
                28 May 2001
                : 153
                : 5
                : 957-970
                Affiliations
                [a ]Program in Lung and Cell Biology, The Hospital for Sick Children, Toronto M5G 1X8, Ontario, Canada
                [b ]Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto M5G 1X8, Ontario, Canada
                [c ]Department of Pharmacology, University of Toronto, Toronto M5G 1X8, Ontario, Canada
                Article
                0011019
                2174331
                11381082
                9ffdcebc-d906-4aeb-bf6d-ba17127fe637
                © 2001 The Rockefeller University Press
                History
                : 3 November 2000
                : 30 March 2001
                : 30 March 2001
                Categories
                Original Article

                Cell biology
                functional complementation,ubiquitination,cystic fibrosis,structural destabilization,lysosomal proteolysis

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