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Chen EY, Clarke DM. The PEST sequence does not contribute to the stability of the cystic fibrosis transmembrane conductance regulator. BMC BIOCHEMISTRY 2002; 3:29. [PMID: 12361483 PMCID: PMC130031 DOI: 10.1186/1471-2091-3-29] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/10/2002] [Accepted: 10/02/2002] [Indexed: 11/13/2022]
Abstract
BACKGROUND Endoplasmic reticulum retention of misfolded cystic fibrosis transmembrane conductance regulator (CFTR) mutants and their rapid degradation is the major cause of cystic fibrosis (CF). An important goal is to understand the mechanism of how the misfolded proteins are recognized, retained, and targeted for degradation. RESULTS Using a web-based algorithm, PESTFind, we found a PEST sequence in the regulatory (R) domain of CFTR. The PEST sequence is found in many short-lived eukaryotic proteins and plays a role in their degradation. To determine its role in the stability and degradation of misprocessed CFTR, we introduced a number of site-directed mutations into the PEST sequence in the cDNA of DeltaF508 CFTR, the most prevalent misprocessed mutation found in CF patients. Analysis of these mutants showed that the disruption of the PEST sequence plays a minor role in the degradation of the CFTR mutants. Multiple mutations to the PEST sequence within the R domain of CFTR inhibit maturation of CFTR and prevent the formation of a 100 kDa degradation product. The mutations, however, do not improve the stability of the mutant DeltaF508 CFTR. CONCLUSION These observations show that disruption of the structure of the R domain of CFTR can inhibit maturation of the protein and that the predicted PEST sequence plays no significant role in the degradation of CFTR.
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Affiliation(s)
- Eva Y Chen
- Canadian Institutes for Health Research Group in Membrane Biology, Departments of Medicine and Biochemistry, University of Toronto, Toronto, Ontario, M5S 1A8, Canada
| | - David M Clarke
- Canadian Institutes for Health Research Group in Membrane Biology, Departments of Medicine and Biochemistry, University of Toronto, Toronto, Ontario, M5S 1A8, Canada
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52
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Johnston JA, Illing ME, Kopito RR. Cytoplasmic dynein/dynactin mediates the assembly of aggresomes. CELL MOTILITY AND THE CYTOSKELETON 2002; 53:26-38. [PMID: 12211113 DOI: 10.1002/cm.10057] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Aggresomes are pericentrosomal cytoplasmic structures into which aggregated, ubiquitinated, misfolded proteins are sequestered. Misfolded proteins accumulate in aggresomes when the capacity of the intracellular protein degradation machinery is exceeded. Previously, we demonstrated that an intact microtubule cytoskeleton is required for the aggresome formation [Johnston et al., 1998: J. Cell Biol. 143:1883-1898]. In this study, we have investigated the involvement of microtubules (MT) and MT motors in this process. Induction of aggresomes containing misfolded DeltaF508 CFTR is accompanied by a redistribution of the retrograde motor cytoplasmic dynein that colocalizes with aggresomal markers. Coexpression of the p50 (dynamitin) subunit of the dynein/dynactin complex prevents the formation of aggresomes, even in the presence of proteasome inhibitors. Using in vitro microtubule binding assays in conjunction with immunogold electron microscopy, our data demonstrate that misfolded DeltaF508 CFTR associate with microtubules. We conclude that cytoplasmic dynein/dynactin is responsible for the directed transport of misfolded protein into aggresomes. The implications of these findings with respect to the pathogenesis of neurodegenerative disease are discussed.
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Affiliation(s)
- Jennifer A Johnston
- Department of Biological Sciences, Stanford University, Stanford, California, USA.
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53
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Trombetta ES, Parodi AJ. N-glycan processing and glycoprotein folding. ADVANCES IN PROTEIN CHEMISTRY 2002; 59:303-44. [PMID: 11868276 DOI: 10.1016/s0065-3233(01)59010-5] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Affiliation(s)
- E S Trombetta
- Instituto de Investigaciones Biotecnológicas, Universidad de San Martín, (1650) San Martin, Pcia. de Buenos Aires, Argentina
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54
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Haggie PM, Stanton BA, Verkman AS. Diffusional mobility of the cystic fibrosis transmembrane conductance regulator mutant, delta F508-CFTR, in the endoplasmic reticulum measured by photobleaching of GFP-CFTR chimeras. J Biol Chem 2002; 277:16419-25. [PMID: 11877404 DOI: 10.1074/jbc.m112361200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR) cause cystic fibrosis. The most common disease-causing mutation, DeltaF508, is retained in the endoplasmic reticulum (ER) and is unable to function as a plasma membrane chloride channel. To investigate whether the ER retention of DeltaF508-CFTR is caused by immobilization and/or aggregation, we have measured the diffusional mobility of green fluorescent protein (GFP) chimeras of wild type (wt)-CFTR and DeltaF508-CFTR by fluorescence recovery after photobleaching. GFP-labeled DeltaF508-CFTR was localized in the ER and wt-CFTR in the plasma membrane and intracellular membranes in transfected COS7 and Chinese hamster ovary K1 cells. Both chimeras localized to the ER after brefeldin A treatment. Spot photobleaching showed that CFTR diffusion (diffusion coefficient approximately 10(-9) cm(2)/s) was not significantly slowed by the DeltaF508 mutation and that nearly all wt-CFTR and DeltaF508-CFTR diffused throughout the ER without restriction. Stabilization of molecular chaperone interactions by ATP depletion produced remarkable DeltaF508-CFTR immobilization ( approximately 50%) and slowed diffusion (6.5 x 10(-10) cm(2)/s) but had little effect on wt-CFTR. Fluorescence depletion experiments revealed that the immobilized DeltaF508-CFTR in ATP-depleted cells remained in an ER pattern. The mobility of wt-CFTR and DeltaF508-CFTR was reduced by maneuvers that alter CFTR processing or interactions with molecular chaperones, including tunicamycin, geldanamycin, and lactacystin. Photobleaching of the fluorescent ER lipid diOC(4)(3) showed that neither ER restructuring nor fragmentation during these maneuvers was responsible for the slowing and immobilization of CFTR. These results suggest that (a) the ER retention of DeltaF508-CFTR is not due to restricted ER mobility, (b) the majority of DeltaF508-CFTR is not aggregated or bound to slowly moving membrane proteins, and (c) DeltaF508-CFTR may interact to a greater extent with molecular chaperones than does wt-CFTR.
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Affiliation(s)
- Peter M Haggie
- Department of Medicine, Cardiovascular Research Institute, University of California, San Francisco, California 94143-0521, USA
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55
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Suzuki T, Park H, Lennarz WJ. Cytoplasmic peptide:N-glycanase (PNGase) in eukaryotic cells: occurrence, primary structure, and potential functions. FASEB J 2002; 16:635-41. [PMID: 11978727 DOI: 10.1096/fj.01-0889rev] [Citation(s) in RCA: 113] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
A cytoplasmic peptide:N-glycanase has been implicated in the proteasomal degradation of newly synthesized misfolded glycoproteins exported from the endoplasmic reticulum. The gene encoding this enzyme (Png1p) has been identified in yeast. Based on sequence analysis, Png1p was classified as a member of the 'transglutaminase-like superfamily' that contains a putative catalytic triad of amino acids (cysteine, histidine, and aspartic acid). More recent studies in yeast indicate that Png1p can bind to the 26S proteasome through its interaction with the DNA repair protein Rad23p. A mouse homologue of Png1p (mPng1p) bound not only to the Rad23 protein, but also to various proteins related to ubiquitin and/or the proteasome through an extended amino-terminal domain. This NH2 terminus of mPng1p, which is not found in yeast, contains a PUB domain predicted to be involved in the ubiquitin-related pathway. This review will focus on the primary structure and potential functions of the cytoplasmic PNGases.
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Affiliation(s)
- Tadashi Suzuki
- Department of Biochemistry and Cell Biology and the Institute of Cell and Developmental Biology, State University of New York at Stony Brook, New York, USA
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56
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Tsai B, Ye Y, Rapoport TA. Retro-translocation of proteins from the endoplasmic reticulum into the cytosol. Nat Rev Mol Cell Biol 2002; 3:246-55. [PMID: 11994744 DOI: 10.1038/nrm780] [Citation(s) in RCA: 488] [Impact Index Per Article: 22.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Proteins that are misfolded in the endoplasmic reticulum are transported back into the cytosol for destruction by the proteasome. This retro-translocation pathway has been co-opted by certain viruses, and by plant and bacterial toxins. The mechanism of retro-translocation is still mysterious, but several aspects of this process are now being unravelled.
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Affiliation(s)
- Billy Tsai
- Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
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57
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Lazrak A, Thome U, Myles C, Ware J, Chen L, Venglarik CJ, Matalon S. cAMP regulation of Cl(-) and HCO(-)(3) secretion across rat fetal distal lung epithelial cells. Am J Physiol Lung Cell Mol Physiol 2002; 282:L650-8. [PMID: 11880289 DOI: 10.1152/ajplung.00370.2001] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We isolated and cultured fetal distal lung epithelial (FDLE) cells from 17- to 19-day rat fetuses and assayed for anion secretion in Ussing chambers. With symmetrical Ringer solutions, basal short-circuit currents (I(sc)) and transepithelial resistances were 7.9 +/- 0.5 microA/cm(2) and 1,018 +/- 73 Omega.cm(2), respectively (means +/- SE; n = 12). Apical amiloride (10 microM) inhibited basal I(sc) by approximately 50%. Subsequent addition of forskolin (10 microM) increased I(sc) from 3.9 +/- 0.63 microA/cm(2) to 7.51 +/- 0.2 microA/cm(2) (n = 12). Basolateral bumetanide (100 microM) decreased forskolin-stimulated I(sc) from 7.51 +/- 0.2 microA/cm(2) to 5.62 +/- 0.53, whereas basolateral 4,4'-dinitrostilbene-2,2'-disulfonate (5 mM), an inhibitor of HCO secretion, blocked the remaining I(sc). Forskolin addition evoked currents of similar fractional magnitudes in symmetrical Cl(-)- or HCO(-)(3)-free solutions; however, no response was seen using HCO(-)(3)- and Cl(-)-free solutions. The forskolin-stimulated I(sc) was inhibited by glibenclamide but not apical DIDS. Glibenclamide also blocked forskolin-induced I(sc) across monolayers having nystatin-permeablized basolateral membranes. Immunolocalization studies were consistent with the expression of cystic fibrosis transmembrane conductance regulator (CFTR) protein in FDLE cells. In aggregate, these findings indicate the presence of cAMP-activated Cl(-) and HCO(-)(3) secretion across rat FDLE cells mediated via CFTR.
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Affiliation(s)
- Ahmed Lazrak
- Department of Anesthesiology, University of Alabama at Birmingham, 619 19th Street S., Birmingham, AL 35233, USA
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58
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Fiebiger E, Story C, Ploegh HL, Tortorella D. Visualization of the ER-to-cytosol dislocation reaction of a type I membrane protein. EMBO J 2002; 21:1041-53. [PMID: 11867532 PMCID: PMC125905 DOI: 10.1093/emboj/21.5.1041] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The human cytomegalovirus gene products US2 and US11 induce proteasomal degradation of MHC class I heavy chains. We have generated an enhanced green fluorescent protein-class I heavy chain (EGFP-HC) chimeric molecule to study its dislocation and degradation in US2- and US11-expressing cells. The EGFP-HC fusion is stable in control cells, but is degraded rapidly in US2- or US11-expressing cells. Proteasome inhibitors induce in a time-dependent manner the accumulation of EGFP-HC molecules in US2- and US11-expressing cells, as assessed biochemically and by cytofluorimetry of intact cells. Pulse-chase analysis and subcellular fractionation show that EGFP-HC proteins are dislocated from the endoplasmic reticulum and can be recovered as deglycosylated fluorescent intermediates in the cytosol. These results raise the possibility that dislocation of glycoproteins from the ER may not require their full unfolding.
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Affiliation(s)
| | | | - Hidde L. Ploegh
- Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Armenise Building, Boston, MA 02115, USA
Corresponding author e-mail:
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59
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van Laar T, van der Eb AJ, Terleth C. A role for Rad23 proteins in 26S proteasome-dependent protein degradation? Mutat Res 2002; 499:53-61. [PMID: 11804604 DOI: 10.1016/s0027-5107(01)00291-3] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Treatment of cells with genotoxic agents affects protein degradation in both positive and negative ways. Exposure of S. cerevisiae to the alkylating agent MMS resulted in activation of genes that are involved in ubiquitin- and 26S proteasome-dependent protein degradation. This process partially overlaps with the activation of the ER-associated protein degradation pathway. The DNA repair protein Rad23p and its mammalian homologues have been shown to inhibit degradation of specific substrates in response to DNA damage. Particularly the recently identified inhibition of degradation by mouse Rad23 protein (mHR23) of the associated nucleotide excision repair protein XPC was shown to stimulate DNA repair.Recently, it was shown that Rad23p and the mouse homologue mHR23B also associate with Png1p, a deglycosylation enzyme. Png1p-mediated deglycosylation plays a role in ER-associated protein degradation after accumulation of malfolded proteins in the endoplasmic reticulum. Thus, if stabilization of proteins that are associated with the C-terminus of Rad23p is a general phenomenon, then Rad23 might be implicated in the stimulation of ER-associated protein degradation as well. Interestingly, the recently identified HHR23-like protein Mif1 is also thought to play a role in ER-associated protein degradation. The MIF1 gene is strongly activated in response to ER-stress. Mif1 contains a ubiquitin-like domain which is most probably involved in binding to S5a, a subunit of the 19S regulatory complex of the 26S proteasome. On the basis of its localization in the ER-membrane, it is hypothesized that Mif1 could play a role in the translocation of the 26S proteasome towards the ER-membrane, thereby enhancing ER-associated protein degradation.
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Affiliation(s)
- Theo van Laar
- MGC Department of Radiation Genetics and Chemical Mutagenesis, Leiden University Medical Centre, P.O. Box 9503, 2300 RA Leiden, The Netherlands.
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60
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Fewell SW, Travers KJ, Weissman JS, Brodsky JL. The action of molecular chaperones in the early secretory pathway. Annu Rev Genet 2002; 35:149-91. [PMID: 11700281 DOI: 10.1146/annurev.genet.35.102401.090313] [Citation(s) in RCA: 217] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The endoplasmic reticulum (ER) serves as a way-station during the biogenesis of nearly all secreted proteins, and associated with or housed within the ER are factors required to catalyze their import into the ER and facilitate their folding. To ensure that only properly folded proteins are secreted and to temper the effects of cellular stress, the ER can target aberrant proteins for degradation and/or adapt to the accumulation of misfolded proteins. Molecular chaperones play critical roles in each of these phenomena.
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Affiliation(s)
- S W Fewell
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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61
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Oberdorf J, Carlson EJ, Skach WR. Redundancy of mammalian proteasome beta subunit function during endoplasmic reticulum associated degradation. Biochemistry 2001; 40:13397-405. [PMID: 11683650 DOI: 10.1021/bi011322y] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Misfolded proteins in the endoplasmic reticulum (ER) are degraded by N-terminal threonine proteases within the 26S proteasome. Each protease is formed by an activated beta subunit, beta5/X, beta1/Y, or beta2/Z, that exhibits chymotrypsin-like, peptidylglutamyl-peptide hydrolyzing, or trypsin-like activity, respectively. Little is known about the relative contribution of specific beta subunits in the degradation of endogenous protein substrates. Using active site proteasome inhibitors and a reconstituted degradation system, we now show that all three active beta subunits can independently contribute to ER-associated degradation of the cystic fibrosis transmembrane conductance regulator (CFTR). Complete inactivation (>99.5%) of the beta5/X subunit decreased the rate of ATP-dependent conversion of CFTR to trichloroacetic acid soluble fragments by only 40%. Similarly, proteasomes containing only active beta1/Y or beta2/Z subunits degraded CFTR at approximately 50% of the rate observed for fully functional proteasomes. Simultaneous inhibition (>93%) of all three beta subunits blocked CFTR degradation by approximately 90%, and inhibition of both protease and ATPase activities was required to completely prevent generation of small peptide fragments. Our results demonstrate both a conserved hierarchy (ChT-L > PGPH > or = T-L) as well as a redundancy of beta subunit function and provide insight into the mechanism by which active site proteasome inhibitors influence degradation of endogenous protein substrates at the ER membrane.
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Affiliation(s)
- J Oberdorf
- Molecular Medicine Division, Department of Cell and Developmental Biology, Oregon Health Sciences University, Portland, Oregon 97201, USA
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62
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Park H, Suzuki T, Lennarz WJ. Identification of proteins that interact with mammalian peptide:N-glycanase and implicate this hydrolase in the proteasome-dependent pathway for protein degradation. Proc Natl Acad Sci U S A 2001; 98:11163-8. [PMID: 11562482 PMCID: PMC58701 DOI: 10.1073/pnas.201393498] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Peptide:N-glycanase (PNGase) cleaves oligosaccharide chains from glycopeptides and glycoproteins. Recently the deduced amino acid sequence of a cytoplasmic PNGase has been identified in various eukaryotes ranging from yeast to mammals, suggesting that deglycosylation may play a central role in some catabolic process. Several lines of evidence indicate that the cytoplasmic enzyme is involved in the quality control system for newly synthesized glycoproteins. Two-hybrid library screening by using mouse PNGase as the target yielded several PNGase-interacting proteins that previously had been implicated in proteasome-dependent protein degradation: mHR23B, ubiquitin, a regulatory subunit of the 19S proteasome, as well as a protein containing an ubiquitin regulatory motif (UBX) and an ubiquitin-associated motif (UBA). These findings by using the two-hybrid system were further confirmed either by in vitro binding assays or size fractionation assays. These results suggest that PNGase may be required for efficient proteasome-mediated degradation of misfolded glycoproteins in mammalian cells.
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Affiliation(s)
- H Park
- Department of Biochemistry, State University of New York, Stony Brook, NY 11794-5215, USA
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63
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Farmery MR, Bulleid NJ. Major histocompatibility class I folding, assembly, and degradation: a paradigm for two-stage quality control in the endoplasmic reticulum. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 2001; 67:235-68. [PMID: 11525384 DOI: 10.1016/s0079-6603(01)67030-2] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Protein folding in living cells is a complex process involving many interdependent factors. The primary site for folding of nascent proteins destined for secretion is the endoplasmic reticulum (ER). Several disease states, including cystic fibrosis, are brought about because of irregularities in protein folding. Under normal cellular conditions, "quality control" mechanisms ensure that only correctly folded proteins are exported from the ER, with incorrectly folded or incompletely assembled proteins being degraded. Quality control mechanisms can be divided into two broad processes: (1) Primary quality control involves general mechanisms that are not specific for individual proteins; these monitor the fidelity of nascent protein folding in the ER and mediate the destruction of incompletely folded proteins. (2) Partially folded or assembled proteins may be subject to secondary quality control mechanisms that are protein- or protein-family-specific. Here we use the folding and assembly of major histocompatibility complex (MHC) class I as an example to illustrate the processes of quality control in the ER. MHC class I, a trimeric complex assembled in the ER of virally infected or malignant cells, presents antigenic peptide to cytotoxic T lymphocytes; this mediates cell killing and thereby prevents the spread of infection or malignancy. The folding and assembly of MHC class I is subjected to both primary and secondary quality control mechanisms that lead either to correct folding, assembly, and secretion or to degradation via a proteasome-associated mechanism.
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Affiliation(s)
- M R Farmery
- University of Manchester, School of Biological Sciences, United Kingdom
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64
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Rhim AD, Stoykova L, Glick MC, Scanlin TF. Terminal glycosylation in cystic fibrosis (CF): a review emphasizing the airway epithelial cell. Glycoconj J 2001; 18:649-59. [PMID: 12386452 DOI: 10.1023/a:1020815205022] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Altered terminal glycosylation, with increased fucosylation and decreased sialylation is a hallmark of the cystic fibrosis (CF) glycosylation phenotype. Oligosaccharides purified from the surface membrane glycoconjugates of CF airway epithelial cells have the Lewis x, selectin ligand in terminal positions. This review is focused on the investigations of the glycoconjugates of the CF airway epithelial cell surface. Two of the major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins which recognize fucose in alpha-1,3 linkage and asialoglycoconjugates. Therefore, consideration has been given to the possibility that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to both the chronic infection and the robust, but ineffective, inflammatory response in the CF lung. Since the glycosylation phenotype of CF airway epithelial cells have been modulated by the expression of wtCFTR, the hypotheses which have been proposed to relate altered function of CFTR to the regulation of the glycosyltransferases are discussed. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as new approaches to the therapy of CF.
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Affiliation(s)
- A D Rhim
- The Cystic Fibrosis Center and Department of Pediatrics, University of Pennsylvania School of Medicine, The Children's Hospital of Philadelphia, 3516 Civic Center Boulevard, Philadelphia, PA 19104, USA
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65
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Glick MC, Kothari VA, Liu A, Stoykova LI, Scanlin TF. Activity of fucosyltransferases and altered glycosylation in cystic fibrosis airway epithelial cells. Biochimie 2001; 83:743-7. [PMID: 11530206 DOI: 10.1016/s0300-9084(01)01323-2] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Cystic fibrosis (CF) glycoconjugates have a glycosylation phenotype of increased fucosylation and/or decreased sialylation when compared with non-CF. A major increase in fucosyl residues linked alpha 1,3 to antennary GlcNAc was observed when surface membrane glycoproteins of CF airway epithelial cells were compared to those of non-CF airway cells. Importantly, the increase in the fucosyl residues was reversed with transfection of CF cells with wild type CFTR cDNA under conditions which brought about a functional correction of the Cl(-) channel defect in the CF cells. In contrast, examination of fucosyl residues in alpha 1,2 linkage by a specific alpha 1,2 fucosidase showed that cell surface glycoproteins of the non-CF cells had a higher percentage of fucose in alpha 1,2 linkage than the CF cells. Airway epithelial cells in primary culture had a similar reciprocal relationship of alpha 1,2- and alpha 1,3-fucosylation when CF and non-CF surface membrane glycoconjugates were compared. In striking contrast, the enzyme activity and the mRNA of alpha 1,2 fucosyltransferase did not reflect the difference in glycoconjugates observed between the CF and non-CF cells. We hypothesize that mutated CFTR may cause faulty compartmentalization in the Golgi so that the nascent glycoproteins encounter alpha 1,3FucT before either the sialyl- or alpha 1,2 fucosyltransferases. In subsequent compartments, little or no terminal glycosylation can take place since the sialyl- or alpha 1,2 fucosyltransferases are unable to utilize a substrate, which is fucosylated in alpha 1,3 position on antennary GlcNAc. This hypothesis, if proven correct, could account for the CF glycophenotype.
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Affiliation(s)
- M C Glick
- The Cystic Fibrosis Center, Department of Pediatrics, University of Pennsylvania School of Medicine, The Childrens Hospital of Philadelphia, 3516 Civic Center Blvd., ARC 402, Philadelphia, PA 19104-4318, USA.
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66
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Zaman K, McPherson M, Vaughan J, Hunt J, Mendes F, Gaston B, Palmer LA. S-nitrosoglutathione increases cystic fibrosis transmembrane regulator maturation. Biochem Biophys Res Commun 2001; 284:65-70. [PMID: 11374871 DOI: 10.1006/bbrc.2001.4935] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Endogenous S-nitrosoglutathione (GSNO) is known to increase the expression of certain proteins at concentrations present in the normal human airway. We hypothesized that GSNO would increase expression and maturation of the cystic fibrosis transmembrane conductance regulator (CFTR). Cells expressing DeltaF508 and wild type CFTR were exposed to GSNO and analyzed for expression and maturation by Western blot analysis. Physiologically relevant concentrations of GSNO resulted in dose- and time-dependent increases in expression. The GSNO-induced increases were eliminated by cycloheximide, suggesting a posttranscriptional effect. Unlike proteasome inhibitors, GSNO resulted in an increase CFTR maturation. The GSNO effect could be reversed by dithiothreitol and inhibited by acivicin, a gamma glutamyl transpeptidase inhibitor. These observations suggest that GSNO leads to maturation of mutated DeltaF508 CFTR, a process associated with restoration of CFTR function. Because endogenous levels of GSNO are low in the cystic fibrosis (CF) airway, these results raise the possibility that GSNO replacement therapy could be an effective treatment for CF.
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Affiliation(s)
- K Zaman
- Department of Pediatrics, University of Virginia, Charlottesville, Virgina 22908, USA
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67
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Mosse CA, Hsu W, Engelhard VH. Tyrosinase degradation via two pathways during reverse translocation to the cytosol. Biochem Biophys Res Commun 2001; 285:313-9. [PMID: 11444844 DOI: 10.1006/bbrc.2001.5153] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Previous studies established that after inhibition of proteasome activity, tyrosinase could be detected in the cytosol after initial translation in the endoplasmic reticulum (ER), with a molecular weight consistent with that of a full-length, deglycosylated polypeptide. Here we show that most of these molecules are glycosylated, but have been proteolyzed at the carboxyl terminus by a protease that is insensitive to proteasome inhibitors. We also demonstrate the inhibitor-dependent accumulation of a membrane species that appears structurally homologous to the glycosylated and partially proteolyzed cytosolic form. Under some circumstances, cytosolic tyrosinase that had been deglycosylated and not proteolyzed prior to proteasomal degradation could also be detected. The presence of cytosolic tyrosinase was dependent upon glycosylation of the molecule during synthesis in the ER. These results suggest the existence of at least two alternative pathways for degradation of tyrosinase in the cytosol.
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Affiliation(s)
- C A Mosse
- Department of Microbiology, University of Virginia, Charlottesville, VA 22908, USA
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68
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Kiser GL, Gentzsch M, Kloser AK, Balzi E, Wolf DH, Goffeau A, Riordan JR. Expression and degradation of the cystic fibrosis transmembrane conductance regulator in Saccharomyces cerevisiae. Arch Biochem Biophys 2001; 390:195-205. [PMID: 11396922 DOI: 10.1006/abbi.2001.2385] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Many cystic fibrosis disease-associated mutations cause a defect in the biosynthetic processing and trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Yeast mutants, defective at various steps of the secretory pathway, have been used to dissect the mechanisms of biosynthetic processing and intracellular transport of several proteins. To exploit these yeast mutants, we have employed an expression system in which the CFTR gene is driven by the promoter of a structurally related yeast ABC protein, Pdr5p. Pulse-chase experiments revealed a turnover rate similar to that of nascent CFTR in mammalian cells. Immunofluorescence microscopy showed that most CFTR colocalized with the endoplasmic reticulum (ER) marker protein Kar2p and not with a vacuolar marker. Degradation was not influenced by the vacuolar protease mutants Pep4p and Prb1p but was sensitive to the proteasome inhibitor lactacystin beta-lactone. Blocking ER-to-Golgi transit with the sec18-1 mutant had little influence on turnover indicating that it occurred primarily in the ER compartment. Degradation was slowed in cells deficient in the ER degradation protein Der3p as well as the ubiquitin-conjugating enzymes Ubc6p and Ubc7p. Finally a mutation (sec61-2) in the translocon protein Sec61p that prevents retrotranslocation across the ER membrane also blocked degradation. These results indicate that whereas approximately 75% of nascent wild-type CFTR is degraded at the ER of mammalian cells virtually all of the protein meets this fate on heterologous expression in Saccharomyces cerevisiae.
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Affiliation(s)
- G L Kiser
- S. C. Johnson Medical Research Center, Mayo Clinic Scottsdale, Scottsdale, Arizona 85259, USA
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69
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Kamhi-Nesher S, Shenkman M, Tolchinsky S, Fromm SV, Ehrlich R, Lederkremer GZ. A novel quality control compartment derived from the endoplasmic reticulum. Mol Biol Cell 2001; 12:1711-23. [PMID: 11408579 PMCID: PMC37335 DOI: 10.1091/mbc.12.6.1711] [Citation(s) in RCA: 155] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Degradation of proteins that, because of improper or suboptimal processing, are retained in the endoplasmic reticulum (ER) involves retrotranslocation to reach the cytosolic ubiquitin-proteasome machinery. We found that substrates of this pathway, the precursor of human asialoglycoprotein receptor H2a and free heavy chains of murine class I major histocompatibility complex (MHC), accumulate in a novel preGolgi compartment that is adjacent to but not overlapping with the centrosome, the Golgi complex, and the ER-to-Golgi intermediate compartment (ERGIC). On its way to degradation, H2a associated increasingly after synthesis with the ER translocon Sec61. Nevertheless, it remained in the secretory pathway upon proteasomal inhibition, suggesting that its retrotranslocation must be tightly coupled to the degradation process. In the presence of proteasomal inhibitors, the ER chaperones calreticulin and calnexin, but not BiP, PDI, or glycoprotein glucosyltransferase, concentrate in the subcellular region of the novel compartment. The "quality control" compartment is possibly a subcompartment of the ER. It depends on microtubules but is insensitive to brefeldin A. We discuss the possibility that it is also the site for concentration and retrotranslocation of proteins that, like the mutant cystic fibrosis transmembrane conductance regulator, are transported to the cytosol, where they form large aggregates, the "aggresomes."
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Affiliation(s)
- S Kamhi-Nesher
- Department of Cell Research and Immunology, George Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel, 69978
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70
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Zhang Y, Nijbroek G, Sullivan ML, McCracken AA, Watkins SC, Michaelis S, Brodsky JL. Hsp70 molecular chaperone facilitates endoplasmic reticulum-associated protein degradation of cystic fibrosis transmembrane conductance regulator in yeast. Mol Biol Cell 2001; 12:1303-14. [PMID: 11359923 PMCID: PMC34585 DOI: 10.1091/mbc.12.5.1303] [Citation(s) in RCA: 211] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2000] [Revised: 11/29/2000] [Accepted: 02/15/2001] [Indexed: 11/11/2022] Open
Abstract
Membrane and secretory proteins fold in the endoplasmic reticulum (ER), and misfolded proteins may be retained and targeted for ER-associated protein degradation (ERAD). To elucidate the mechanism by which an integral membrane protein in the ER is degraded, we studied the fate of the cystic fibrosis transmembrane conductance regulator (CFTR) in the yeast Saccharomyces cerevisiae. Our data indicate that CFTR resides in the ER and is stabilized in strains defective for proteasome activity or deleted for the ubiquitin-conjugating enzymes Ubc6p and Ubc7p, thus demonstrating that CFTR is a bona fide ERAD substrate in yeast. We also found that heat shock protein 70 (Hsp70), although not required for the degradation of soluble lumenal ERAD substrates, is required to facilitate CFTR turnover. Conversely, calnexin and binding protein (BiP), which are required for the proteolysis of ER lumenal proteins in both yeast and mammals, are dispensable for the degradation of CFTR, suggesting unique mechanisms for the disposal of at least some soluble and integral membrane ERAD substrates in yeast.
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Affiliation(s)
- Y Zhang
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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71
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Sommer T, Jarosch E, Lenk U. Compartment-specific functions of the ubiquitin-proteasome pathway. Rev Physiol Biochem Pharmacol 2001; 142:97-160. [PMID: 11190579 DOI: 10.1007/bfb0117492] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Affiliation(s)
- T Sommer
- Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Strasse 10, 13092 Berlin, Germany
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72
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Petaja-Repo UE, Hogue M, Laperriere A, Bhalla S, Walker P, Bouvier M. Newly synthesized human delta opioid receptors retained in the endoplasmic reticulum are retrotranslocated to the cytosol, deglycosylated, ubiquitinated, and degraded by the proteasome. J Biol Chem 2001; 276:4416-23. [PMID: 11054417 DOI: 10.1074/jbc.m007151200] [Citation(s) in RCA: 175] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have previously shown that only a fraction of the newly synthesized human delta opioid receptors is able to leave the endoplasmic reticulum (ER) and reach the cell surface (Petäjä-Repo, U. E, Hogue, M., Laperrière, A., Walker, P., and Bouvier, M. (2000) J. Biol. Chem. 275, 13727-13736). In the present study, we investigated the fate of those receptors that are retained intracellularly. Pulse-chase experiments revealed that the disappearance of the receptor precursor form (M(r) 45,000) and of two smaller species (M(r) 42,000 and 39,000) is inhibited by the proteasome blocker, lactacystin. The treatment also promoted accumulation of the mature receptor form (M(r) 55,000), indicating that the ER quality control actively routes a significant proportion of rescuable receptors for proteasome degradation. In addition, degradation intermediates that included full-length deglycosylated (M(r) 39,000) and ubiquitinated forms of the receptor were found to accumulate in the cytosol upon inhibition of proteasome function. Finally, coimmunoprecipitation experiments with the beta-subunit of the Sec61 translocon complex revealed that the receptor precursor and its deglycosylated degradation intermediates interact with the translocon. Taken together, these results support a model in which misfolded or incompletely folded receptors are transported to the cytoplasmic side of the ER membrane via the Sec61 translocon, deglycosylated and conjugated with ubiquitin prior to degradation by the cytoplasmic 26 S proteasomes.
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Affiliation(s)
- U E Petaja-Repo
- Département de Biochimie, Université de Montréal, Montréal, Quebec H3C 3J7, Canada.
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73
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Kunzelmann K, Nitschke R. Defects in processing and trafficking of cystic fibrosis transmembrane conductance regulator. EXPERIMENTAL NEPHROLOGY 2000; 8:332-42. [PMID: 11014930 DOI: 10.1159/000020687] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
In most epithelial tissues Cl(-) transport relies on the cystic fibrosis transmembrane conductance regulator (CFTR) which has dual function as a Cl(-) channel and as a regulator of other ion channels. More than 900 different mutations in the CFTR gene are the cause for defective transport of Cl(-) and Na(+) and impaired secretion or absorption of electrolytes in cystic fibrosis. However, the CFTR mutation delta F508 is the most common reason for the frequently inherited disease among the Caucasian population. Maturation and processing of delta F508-CFTR is defective which leads to expression of only very little but functional CFTR in the cell membrane. Understanding the processing and trafficking of CFTR may give a clue to the question as to how the expression and residual function of delta F508-CFTR can be enhanced, and may lead to the development of new pharmacological tools for the treatment of cystic fibrosis.
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Affiliation(s)
- K Kunzelmann
- Department of Physiology and Pharmacology, University of Queensland, St. Lucia, Brisbane, Australia.
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74
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Wilson CM, Farmery MR, Bulleid NJ. Pivotal role of calnexin and mannose trimming in regulating the endoplasmic reticulum-associated degradation of major histocompatibility complex class I heavy chain. J Biol Chem 2000; 275:21224-32. [PMID: 10801790 DOI: 10.1074/jbc.m000567200] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have established a mammalian semipermeabilized cell system that faithfully reconstitutes the proteasome-mediated degradation of major histocompatibility complex Class I heavy chain. We show that degradation required unfolding of the protein and was cytosol- and ATP-dependent and that dislocation and degradation required proteasome activity. When the interaction of heavy chain with calnexin was prevented, the rate of degradation was accelerated, suggesting that an interaction with calnexin stabilized heavy chain. Stabilization of heavy chain to degradation was also achieved either by preventing mannose trimming or by removal of the N-linked glycosylation site. This demonstrates that glycosylation and mannose trimming are required to ensure degradation of heavy chain. When degradation or mannose trimming was inhibited, heavy chain formed a prolonged interaction with immunoglobulin heavy chain binding protein, ERp57, and protein disulfide isomerase. Taken together, these results indicate that calnexin association and mannose trimming provide a mechanism to regulate the folding, assembly, and degradation of glycoproteins entering the secretory pathway.
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Affiliation(s)
- C M Wilson
- School of Biological Sciences, 2.205 Stopford Building, University of Manchester, Manchester M13 9PT, United Kingdom
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75
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Heymann JA, Subramaniam S. Integration of deletion mutants of bovine rhodopsin into the membrane of the endoplasmic reticulum. Mol Membr Biol 2000; 17:165-74. [PMID: 11128975 DOI: 10.1080/09687680050197392] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/16/2022]
Abstract
Newly synthesized eukaryotic membrane proteins must be integrated into the membrane of the endoplasmic reticulum with the correct topology to enable the subsequent acquisition of the correctly folded, functional conformation. Here, an analysis is presented of N-terminal glycosylation and steady-state membrane orientation of a series of truncation mutants of the seven-helix protein rhodopsin expressed in COS-1 cells. Mutants containing one, three, or five N-terminal transmembrane segments of rhodopsin, as well as mutants containing only the first transmembrane segment, but with hydrophilic extensions at the C-terminus were studied. The findings demonstrate that the C-terminal transmembrane segments play a crucial role in determining the final orientation of rhodopsin, and that the commitment to the correct orientation occurs only after the synthesis of at least three transmembrane segments. The experiments also suggest that the molecular machinery involved in the integration of a newly synthesized seven-helix membrane protein into the endoplasmic reticulum membrane is sensitive to the overall hydrophobicity of the sequence that follows the first transmembrane segment.
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Affiliation(s)
- J A Heymann
- National Cancer Institute, NIH, Laboratory of Biochemistry, Bethesda, MD 20892, USA.
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76
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Van Oene M, Lukacs GL, Rommens JM. Cystic fibrosis mutations lead to carboxyl-terminal fragments that highlight an early biogenesis step of the cystic fibrosis transmembrane conductance regulator. J Biol Chem 2000; 275:19577-84. [PMID: 10764788 DOI: 10.1074/jbc.m002186200] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Inefficient delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) to the surface of cells contributes to disease in the majority of cystic fibrosis patients. Analysis of cystic fibrosis-associated missense mutations in the first nucleotide binding domain (NBD1), including A455E, S549R, Y563N, and P574H, revealed reduced levels of mature CFTR with elevated levels of carboxyl-terminal polypeptide fragments of 105 and 90 kDa. These fragments appear early in biogenesis and degrade rapidly in four distinct cell types tested including the bronchial epithelial IB3-1 cell line. They were detected at highest levels with CFTRA455E where the 105-kDa fragment accounted for 40% of newly synthesized polypeptide but for only 20 and 7% of nascent wild type and mutant DeltaF508 proteins, respectively. The bands represent core- and unglycosylated forms of the same CFTR fragment supporting that precursor forms are correctly inserted into the membrane of the endoplasmic reticulum. Proteolytic cleavage would be predicted to occur on the cytosolic face of the endoplasmic reticulum within the NBD1-R domain segment, but pharmacological testing did not support involvement of the 26 S proteasome. The examined missense mutations in NBD1 manifest differently than the major mutant, DeltaF508, and highlight a critical conformational aspect of biogenesis of CFTR.
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Affiliation(s)
- M Van Oene
- Department of Molecular and Medical Genetics, Program in Genetics & Genomic Biology, University of Toronto, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada
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77
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Suzuki T, Park H, Hollingsworth NM, Sternglanz R, Lennarz WJ. PNG1, a yeast gene encoding a highly conserved peptide:N-glycanase. J Cell Biol 2000; 149:1039-52. [PMID: 10831608 PMCID: PMC2174826 DOI: 10.1083/jcb.149.5.1039] [Citation(s) in RCA: 172] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2000] [Accepted: 04/06/2000] [Indexed: 11/29/2022] Open
Abstract
It has been proposed that cytoplasmic peptide:N-glycanase (PNGase) may be involved in the proteasome-dependent quality control machinery used to degrade newly synthesized glycoproteins that do not correctly fold in the ER. However, a lack of information about the structure of the enzyme has limited our ability to obtain insight into its precise biological function. A PNGase-defective mutant (png1-1) was identified by screening a collection of mutagenized strains for the absence of PNGase activity in cell extracts. The PNG1 gene was mapped to the left arm of chromosome XVI by genetic approaches and its open reading frame was identified. PNG1 encodes a soluble protein that, when expressed in Escherichia coli, exhibited PNGase activity. PNG1 may be required for efficient proteasome-mediated degradation of a misfolded glycoprotein. Subcellular localization studies indicate that Png1p is present in the nucleus as well as the cytosol. Sequencing of expressed sequence tag clones revealed that Png1p is highly conserved in a wide variety of eukaryotes including mammals, suggesting that the enzyme has an important function.
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Affiliation(s)
- Tadashi Suzuki
- Department of Biochemistry and Cell Biology, Institute of Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
| | - Hangil Park
- Department of Biochemistry and Cell Biology, Institute of Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
| | - Nancy M. Hollingsworth
- Department of Biochemistry and Cell Biology, Institute of Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
| | - Rolf Sternglanz
- Department of Biochemistry and Cell Biology, Institute of Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
| | - William J. Lennarz
- Department of Biochemistry and Cell Biology, Institute of Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
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78
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Petaja-Repo UE, Hogue M, Laperriere A, Walker P, Bouvier M. Export from the endoplasmic reticulum represents the limiting step in the maturation and cell surface expression of the human delta opioid receptor. J Biol Chem 2000; 275:13727-36. [PMID: 10788493 DOI: 10.1074/jbc.275.18.13727] [Citation(s) in RCA: 244] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Synthesis and maturation of G protein-coupled receptors are complex events that require an intricate combination of processes that include protein folding, post-translational modifications, and transport through distinct cellular compartments. Relatively little is known about the nature and kinetics of specific steps involved in these processes. Here, the human delta opioid receptor expressed in human embryonic kidney 293S cells is used as a model to delineate these steps and to establish the kinetics of receptor synthesis, glycosylation, and transport. We found that the receptor is synthesized as a core-glycosylated M(r) 45,000 precursor that is converted to the fully mature M(r) 55,000 receptor with a half-time of about 120 min. In addition to trimming and processing of two N-linked oligosaccharides, maturation involves addition of O-glycans containing N-acetylgalactosamine, galactose, and sialic acid. In contrast to N-glycosylation, which is initiated co-translationally and is completed when the protein reaches the trans-Golgi network, O-glycosylation was found to occur only after the receptor exits from the endoplasmic reticulum (ER) and was terminated as early as the trans-Golgi cisternae. Once the carbohydrates are fully processed and the receptor reaches the trans-Golgi network, it is transported to the cell surface in about 10 min. The exit from the ER was found to be the limiting step in overall processing of the receptor. This indicates that early events in the folding of the receptor are probably rate-limiting and that receptor folding intermediates are retained in the ER until they can adopt the correct conformation. The overall low efficiency of receptor maturation, less than 50% of the precursor being processed to the fully glycosylated protein, further suggests that only a fraction of the synthesized receptors attain properly folded conformation that allows exit from the ER. This indicates that folding and ER export are key events in control of receptor cell surface expression. Whether or not the low efficiency of the ER export is a general feature among G protein-coupled receptors remains to be investigated.
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Affiliation(s)
- U E Petaja-Repo
- Département de Biochimie, Université de Montréal, Montréal, Quebec H3C 3J7, Canada
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79
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Abstract
After endocytosis cholera toxin is transported to the endoplasmic reticulum (ER), from where its A1 subunit (CTA1) is assumed to be transferred to the cytosol by an as-yet unknown mechanism. Here, export of CTA1 from the ER to the cytosol was investigated in a cell-free assay using either microsomes loaded with CTA1 by in vitro translation or reconstituted microsomes containing CTA1 purified from V. cholerae. Export of CTA1 from the microsomes was time- and adenosine triphosphate-dependent and required lumenal ER proteins. By coimmunoprecipitation CTA1 was shown to be associated during export with the Sec61p complex, which mediates import of proteins into the ER. Export of CTA1 was inhibited when the Sec61p complexes were blocked by nascent polypeptides arrested during import, demonstrating that the export of CTA1 depended on translocation-competent Sec61p complexes. Export of CTA1 from the reconstituted microsomes indicated the de novo insertion of the toxin into the Sec61p complex from the lumenal side. Our results suggest that Sec61p complex-mediated protein export from the ER is not restricted to ER-associated protein degradation but is also used by bacterial toxins, enabling their entry into the cytosol of the target cell.
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Affiliation(s)
- A Schmitz
- Institut für Zellbiologie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53121 Bonn, Germany.
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80
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Teckman JH, Gilmore R, Perlmutter DH. Role of ubiquitin in proteasomal degradation of mutant alpha(1)-antitrypsin Z in the endoplasmic reticulum. Am J Physiol Gastrointest Liver Physiol 2000; 278:G39-48. [PMID: 10644560 DOI: 10.1152/ajpgi.2000.278.1.g39] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
A delay in intracellular degradation of the mutant alpha(1)-antitrypsin (alpha(1)AT)Z molecule is associated with greater retention within the endoplasmic reticulum (ER) and susceptibility to liver disease in a subgroup of patients with alpha(1)AT deficiency. Recent studies have shown that alpha(1)ATZ is ordinarily degraded in the ER by a mechanism that involves the proteasome, as demonstrated in intact cells using human fibroblast cell lines engineered for expression of alpha(1)ATZ and in a cell-free microsomal translocation assay system programmed with purified alpha(1)ATZ mRNA. To determine whether the ubiquitin system is required for proteasomal degradation of alpha(1)ATZ and whether specific components of the ubiquitin system can be implicated, we have now used two approaches. First, we overexpressed a dominant-negative ubiquitin mutant (UbK48R-G76A) by transient transfection in the human fibroblast cell lines expressing alpha(1)ATZ. The results showed that there was marked, specific, and selective inhibition of alpha(1)ATZ degradation mediated by UbK48R-G76A, indicating that the ubiquitin system is at least in part involved in ER degradation of alpha(1)ATZ. Second, we subjected reticulocyte lysate to DE52 chromatography and tested the resulting well-characterized fractions in the cell-free system. The results showed that there were both ubiquitin-dependent and -independent proteasomal mechanisms for degradation of alpha(1)ATZ and that the ubiquitin-conjugating enzyme E2-F1 may play a role in the ubiquitin-dependent proteasomal mechanism.
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Affiliation(s)
- J H Teckman
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
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81
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Abstract
Cotranslational protein translocation across and integration into the membrane of the endoplasmic reticulum (ER) occur at sites termed translocons. Translocons are composed of several ER membrane proteins that associate to form an aqueous pore through which secretory proteins and lumenal domains of membrane proteins pass from the cytoplasm to the ER lumen. These sites are not passive holes in the bilayer, but instead are quite dynamic both structurally and functionally. Translocons cycle between ribosome-bound and ribosome-free states, and convert between translocation and integration modes of operation. These changes in functional state are accompanied by structural rearrangements that alter translocon conformation, composition, and interactions with ligands such as the ribosome and BiP. Recent studies have revealed that the translocon is a complex and sophisticated molecular machine that regulates the movement of polypeptides through the bilayer, apparently in both directions as well as laterally into the bilayer, all while maintaining the membrane permeability barrier.
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Affiliation(s)
- A E Johnson
- Department of Medical Biochemistry, Texas A&M University, College Station 77843, USA.
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82
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Yamaguchi A, Hori O, Stern DM, Hartmann E, Ogawa S, Tohyama M. Stress-associated endoplasmic reticulum protein 1 (SERP1)/Ribosome-associated membrane protein 4 (RAMP4) stabilizes membrane proteins during stress and facilitates subsequent glycosylation. J Cell Biol 1999; 147:1195-204. [PMID: 10601334 PMCID: PMC2168098 DOI: 10.1083/jcb.147.6.1195] [Citation(s) in RCA: 92] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Application of differential display to cultured rat astrocytes subjected to hypoxia allowed cloning of a novel cDNA, termed stress-associated endoplasmic reticulum protein 1 (SERP1). Expression of SERP1 was enhanced in vitro by hypoxia and/or reoxygenation or other forms of stress, causing accumulation of unfolded proteins in endoplasmic reticulum (ER) stress, and in vivo by middle cerebral artery occlusion in rats. The SERP1 cDNA encodes a 66-amino acid polypeptide which was found to be identical to ribosome-associated membrane protein 4 (RAMP4) and bearing 29% identity to yeast suppressor of SecY 6 protein (YSY6p), suggesting participation in pathways controlling membrane protein biogenesis at ER. In cultured 293 cells subjected to ER stress, overexpression of SERP1/RAMP4 suppressed aggregation and/or degradation of newly synthesized integral membrane proteins, and subsequently, facilitated their glycosylation when the stress was removed. SERP1/RAMP4 interacted with Sec61alpha and Sec61beta, which are subunits of translocon, and a molecular chaperon calnexin. Furthermore, Sec61alpha and Sec61beta, but not SERP1/RAMP4, were found to associate with newly synthesized integral membrane proteins under stress. These results suggest that stabilization of membrane proteins in response to stress involves the concerted action of a rescue unit in the ER membrane comprised of SERP1/RAMP4, other components of translocon, and molecular chaperons in ER.
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Affiliation(s)
- Atsushi Yamaguchi
- Department of Anatomy and Neuroscience, Graduate School of Medicine, Osaka University, Suita City, Osaka 565-0871, Japan
- Core Research for Evolutional Science and Technology, Japan Science and Technology, Tokyo 105, Japan
| | - Osamu Hori
- Department of Anatomy III, Kanazawa University, School of Medicine, Kanazawa City, Ishikawa 290-8640, Japan
- Core Research for Evolutional Science and Technology, Japan Science and Technology, Tokyo 105, Japan
| | - David M. Stern
- Department of Surgery, Department of Physiology and Cellular Biophysics, College of Physicians and Surgeons, Columbia University, New York, New York 10032
| | - Enno Hartmann
- Abteilung Biochemie II, Zentrum Biochemie und Moleculare Zellbiologie, Georg-August-Universität, 37073 Göttingen, Germany
| | - Satoshi Ogawa
- Department of Anatomy III, Kanazawa University, School of Medicine, Kanazawa City, Ishikawa 290-8640, Japan
- Core Research for Evolutional Science and Technology, Japan Science and Technology, Tokyo 105, Japan
| | - Masaya Tohyama
- Department of Anatomy and Neuroscience, Graduate School of Medicine, Osaka University, Suita City, Osaka 565-0871, Japan
- Core Research for Evolutional Science and Technology, Japan Science and Technology, Tokyo 105, Japan
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83
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Abstract
Cystic fibrosis (CF) is a common genetic disease for which the gene was identified within the last decade. Pulmonary disease predominates in this ultimately fatal disease and current therapy only slows the progression. CF transmembrane regulator (CFTR), the gene product, is an integral membrane glycoprotein that normally functions as a chloride channel in epithelial cells. The most common mutation, deltaF508, results in mislocalization and altered glycosylation of CFTR. Altered fucosylation and sialylation are hallmarks of both membrane and secreted glycoproteins in CF and the focus here is on these investigations. Oligosaccharides from CF membrane glycoproteins have the Lewis x, selectin ligand in terminal positions. In addition, two major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins, which recognize fucose in alpha1,3 linkage and asialoglycoconjugates. We speculate that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to the chronic infection and robust inflammatory response in the CF lung. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as therapy for CF.
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Affiliation(s)
- T F Scanlin
- Cystic Fibrosis Center, Children's Hospital of Philadelphia, PA 19104-4318, USA
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84
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Brodsky JL, McCracken AA. ER protein quality control and proteasome-mediated protein degradation. Semin Cell Dev Biol 1999; 10:507-13. [PMID: 10597633 DOI: 10.1006/scdb.1999.0321] [Citation(s) in RCA: 257] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
A variety of mutant polypeptides that are associated with human disease are targeted for degradation by an endoplasmic reticulum (ER) quality control system. In addition, physiological signals and viral gene products can target the degradation of several ER resident proteins and secreted proteins passing through the ER. Although the mechanism of protein quality control and the site of degradation were obscure, recent data indicate that degradation requires the cytosolic proteasome. Biochemical and genetic analyses have indicated that both lumenal and integral membrane proteins are selected for proteolysis and exported to the cytosol by a process that in several cases requires ER associated molecular chaperones.
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Affiliation(s)
- J L Brodsky
- Department of Biological Sciences, University of Pittsburgh, PA 15260, USA
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85
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Skowronek MH, Rotter M, Haas IG. Molecular characterization of a novel mammalian DnaJ-like Sec63p homolog. Biol Chem 1999; 380:1133-8. [PMID: 10543453 DOI: 10.1515/bc.1999.142] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
We identified a human cDNA sequence encoding a polypeptide of 760 amino acids that shares 53% homology and 25.6% identity with the yeast DnaJ-like endoplasmic reticulum (ER) translocon component Sec63p. Three epitope-specific antisera revealed a protein of an apparent molecular mass of 83 kDa, both in human cell extracts and in dog pancreatic microsomes. Biochemical analyses show that it is an integral membrane protein of the rough ER, which has the DnaJ domain located in the ER lumen. The novel Sec63 protein could thus represent a key component of the mammalian ER protein translocation machinery.
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86
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Story CM, Furman MH, Ploegh HL. The cytosolic tail of class I MHC heavy chain is required for its dislocation by the human cytomegalovirus US2 and US11 gene products. Proc Natl Acad Sci U S A 1999; 96:8516-21. [PMID: 10411907 PMCID: PMC17548 DOI: 10.1073/pnas.96.15.8516] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The US2 and US11 glycoproteins of human cytomegalovirus facilitate destruction of MHC class I heavy chains by proteasomal proteolysis through acceleration of endoplasmic reticulum-to-cytosol dislocation. Modification of the class I heavy chain was used to probe the structural requirements for this sequence of reactions. The cytosolic domain of the class I heavy chain is required for dislocation to the cytosol and for its subsequent destruction. However, interactions between US2 or US11 and the heavy chain are maintained in the absence of the class I cytosolic domain, as shown by chemical crosslinking in vivo and coprecipitation when translated in vitro. Thus, substrate recognition and accelerated destruction of the heavy chain, as facilitated by US2 or US11, are separable events.
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Affiliation(s)
- C M Story
- Department of Pathology, Harvard Medical School, Boston, MA 02115, USA
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87
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Lencer WI, Hirst TR, Holmes RK. Membrane traffic and the cellular uptake of cholera toxin. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1450:177-90. [PMID: 10395933 DOI: 10.1016/s0167-4889(99)00070-1] [Citation(s) in RCA: 193] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/16/2022]
Abstract
In nature, cholera toxin (CT) and the structurally related E. coli heat labile toxin type I (LTI) must breech the epithelial barrier of the intestine to cause the massive diarrhea seen in cholera. This requires endocytosis of toxin-receptor complexes into the apical endosome, retrograde transport into Golgi cisternae or endoplasmic reticulum (ER), and finally transport of toxin across the cell to its site of action on the basolateral membrane. Targeting into this pathway depends on toxin binding ganglioside GM1 and association with caveolae-like membrane domains. Thus to cause disease, both CT and LTI co-opt the molecular machinery used by the host cell to sort, move, and organize their cellular membranes and substituent components.
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Affiliation(s)
- W I Lencer
- Combined Program in Pediatric Gastroenterology, Children's Hospital, Harvard Medical School, Harvard Digestive Diseases Center, Boston, MA, USA.
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88
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Johnston JA, Ward CL, Kopito RR. Aggresomes: a cellular response to misfolded proteins. J Cell Biol 1998; 143:1883-98. [PMID: 9864362 PMCID: PMC2175217 DOI: 10.1083/jcb.143.7.1883] [Citation(s) in RCA: 1671] [Impact Index Per Article: 64.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/1998] [Revised: 11/09/1998] [Indexed: 11/22/2022] Open
Abstract
Intracellular deposition of misfolded protein aggregates into ubiquitin-rich cytoplasmic inclusions is linked to the pathogenesis of many diseases. Why these aggregates form despite the existence of cellular machinery to recognize and degrade misfolded protein and how they are delivered to cytoplasmic inclusions are not known. We have investigated the intracellular fate of cystic fibrosis transmembrane conductance regulator (CFTR), an inefficiently folded integral membrane protein which is degraded by the cytoplasmic ubiquitin-proteasome pathway. Overexpression or inhibition of proteasome activity in transfected human embryonic kidney or Chinese hamster ovary cells led to the accumulation of stable, high molecular weight, detergent-insoluble, multiubiquitinated forms of CFTR. Using immunofluorescence and transmission electron microscopy with immunogold labeling, we demonstrate that undegraded CFTR molecules accumulate at a distinct pericentriolar structure which we have termed the aggresome. Aggresome formation is accompanied by redistribution of the intermediate filament protein vimentin to form a cage surrounding a pericentriolar core of aggregated, ubiquitinated protein. Disruption of microtubules blocks the formation of aggresomes. Similarly, inhibition of proteasome function also prevented the degradation of unassembled presenilin-1 molecules leading to their aggregation and deposition in aggresomes. These data lead us to propose that aggresome formation is a general response of cells which occurs when the capacity of the proteasome is exceeded by the production of aggregation-prone misfolded proteins.
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Affiliation(s)
- J A Johnston
- Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA
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