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Peric D, Durrant-Arico C, Delenda C, Dupré T, De Lonlay P, de Baulny HO, Pelatan C, Bader-Meunier B, Danos O, Chantret I, Moore SEH. The compartmentalisation of phosphorylated free oligosaccharides in cells from a CDG Ig patient reveals a novel ER-to-cytosol translocation process. PLoS One 2010; 5:e11675. [PMID: 20652024 PMCID: PMC2907391 DOI: 10.1371/journal.pone.0011675] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2010] [Accepted: 06/14/2010] [Indexed: 11/18/2022] Open
Abstract
BACKGROUND Biosynthesis of the dolichol linked oligosaccharide (DLO) required for protein N-glycosylation starts on the cytoplasmic face of the ER to give Man(5)GlcNAc(2)-PP-dolichol, which then flips into the ER for further glycosylation yielding mature DLO (Glc(3)Man(9)GlcNAc(2)-PP-dolichol). After transfer of Glc(3)Man(9)GlcNAc(2) onto protein, dolichol-PP is recycled to dolichol-P and reused for DLO biosynthesis. Because de novo dolichol synthesis is slow, dolichol recycling is rate limiting for protein glycosylation. Immature DLO intermediates may also be recycled by pyrophosphatase-mediated cleavage to yield dolichol-P and phosphorylated oligosaccharides (fOSGN2-P). Here, we examine fOSGN2-P generation in cells from patients with type I Congenital Disorders of Glycosylation (CDG I) in which defects in the dolichol cycle cause accumulation of immature DLO intermediates and protein hypoglycosylation. METHODS AND PRINCIPAL FINDINGS In EBV-transformed lymphoblastoid cells from CDG I patients and normal subjects a correlation exists between the quantities of metabolically radiolabeled fOSGN2-P and truncated DLO intermediates only when these two classes of compounds possess 7 or less hexose residues. Larger fOSGN2-P were difficult to detect despite an abundance of more fully mannosylated and glucosylated DLO. When CDG Ig cells, which accumulate Man(7)GlcNAc(2)-PP-dolichol, are permeabilised so that vesicular transport and protein synthesis are abolished, the DLO pool required for Man(7)GlcNAc(2)-P generation could be depleted by adding exogenous glycosylation acceptor peptide. Under conditions where a glycotripeptide and neutral free oligosaccharides remain predominantly in the lumen of the ER, Man(7)GlcNAc(2)-P appears in the cytosol without detectable generation of ER luminal Man(7)GlcNAc(2)-P. CONCLUSIONS AND SIGNIFICANCE The DLO pools required for N-glycosylation and fOSGN2-P generation are functionally linked and this substantiates the hypothesis that pyrophosphatase-mediated cleavage of DLO intermediates yields recyclable dolichol-P. The kinetics of cytosolic fOSGN2-P generation from a luminally-generated DLO intermediate demonstrate the presence of a previously undetected ER-to-cytosol translocation process for either fOSGN2-P or DLO.
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Affiliation(s)
- Delphine Peric
- INSERM U773 CRB3, Paris, France
- Université Denis Diderot, Paris 7, Paris, France
| | | | | | - Thierry Dupré
- INSERM U773 CRB3, Paris, France
- Université Denis Diderot, Paris 7, Paris, France
- AP-HP, Hôpital Bichat-Claude Bernard, Biochimie Métabolique et Cellulaire, Paris, France
| | - Pascale De Lonlay
- Département de Pédiatrie, Hôpital Necker-Enfants Malades, Paris, France
| | | | - Cécile Pelatan
- Centre Hospitalier, Service de Pédiatrie, Le Mans, France
| | | | - Olivier Danos
- Généthon: Evry, France
- INSERM U781, Hôpital Necker-Enfants Malades, Paris, France
| | - Isabelle Chantret
- INSERM U773 CRB3, Paris, France
- Université Denis Diderot, Paris 7, Paris, France
| | - Stuart E. H. Moore
- INSERM U773 CRB3, Paris, France
- Université Denis Diderot, Paris 7, Paris, France
- * E-mail:
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2
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Haga Y, Totani K, Ito Y, Suzuki T. Establishment of a real-time analytical method for free oligosaccharide transport from the ER to the cytosol. Glycobiology 2009; 19:987-94. [PMID: 19494346 DOI: 10.1093/glycob/cwp075] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
During N-glycosylation of proteins, significant amounts of free unconjugated glycans are also generated in the lumen of the endoplasmic reticulum (ER). These ER-derived free glycans are translocated into the cytosol by a putative transporter on the ER membrane for further processing. However, the molecular nature of the transporter remains to be determined. Here, we report the establishment of a novel assay method for free oligosaccharide transport from the ER lumen using chemically synthesized fluorescence-labeled N-glycan derivatives. In this method, fluorescence-labeled glycan substrates were encapsulated inside mouse liver microsomes, followed by incubation with the cytosol and a fluorescence-quenching agent (anti-fluorophore antibody). The rate of substrate efflux was then monitored in real time by the decrease in the fluorescence intensity. The present data clearly demonstrated that the oligosaccharide transport activity under the current assay conditions was both ATP and cytosol dependent. The transporter activity was also found to be glycan structure specific because free glucosylated glycans were unable to be transported out of the microsomes. This new assay method will be a useful tool for identifying the transporter protein on the ER membrane.
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Affiliation(s)
- Yoshimi Haga
- Glycometabolome Team, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan
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3
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Suzuki T, Funakoshi Y. Free N-linked oligosaccharide chains: formation and degradation. Glycoconj J 2007; 23:291-302. [PMID: 16897173 DOI: 10.1007/s10719-006-6975-x] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2005] [Revised: 12/21/2005] [Accepted: 12/27/2005] [Indexed: 01/09/2023]
Abstract
There is growing evidence that N-linked glycans play pivotal roles in protein folding and intra- and/or intercellular trafficking of N-glycosylated proteins. It has been shown that during the N-glycosylation of proteins, significant amounts of free oligosaccharides (free OSs) are generated in the lumen of the endoplasmic reticulum (ER) by a mechanism which remains to be clarified. Free OSs are also formed in the cytosol by enzymatic deglycosylation of misfolded glycoproteins, which are subjected to destruction by a cellular system called "ER-associated degradation (ERAD)." While the precise functions of free OSs remain obscure, biochemical studies have revealed that a novel cellular process enables them to be catabolized in a specialized manner, that involves pumping free OSs in the lumen of the ER into the cytosol where further processing occurs. This process is followed by entry into the lysosomes. In this review we summarize current knowledge about the formation, processing and degradation of free OSs in eukaryotes and also discuss the potential biological significance of this pathway. Other evidence for the occurrence of free OSs in various cellular processes is also presented.
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Affiliation(s)
- Tadashi Suzuki
- 21st COE (Center of Excellence) Program and Department of Biochemistry, Osaka University Graduate School of Medicine, Suita, Osaka 565-0871, Japan.
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4
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Ali BRS, Edwards LC, Field MC. Reconstitution of glycopeptide export in mixed detergent-solubilised and resealed microsomes depleted of lumenal components. JOURNAL OF BIOCHEMICAL AND BIOPHYSICAL METHODS 2005; 62:1-12. [PMID: 15656939 DOI: 10.1016/j.jbbm.2004.01.013] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/01/2003] [Revised: 12/22/2003] [Accepted: 01/01/2004] [Indexed: 02/05/2023]
Abstract
Export of macromolecules from the endoplasmic reticulum (ER) lumen into the cytosol is a major aspect of the quality control systems operating within the early secretory system. Glycopeptides are exported from the ER by an ATP- and GTP-dependent pathway, which shares many similarities to the protein export system. Significantly, for glycopeptides, there is no requirement for cytosolic factors, biochemically distinguishing the glycopeptide and protein paths and probably reflecting the lower conformational complexity of the former substrate. Genetic studies in yeast, and biochemical data from higher eukaryotes, indicate that glycopeptides utilise the Sec61 translocon. Here, we report a new system allowing access to lumenal ER components, facilitating assessment of their importance in glycopeptide retrotranslocation and potentially other processes. Saponin, in combination with CHAPS, but not saponin alone, facilitated removal of >95% of lumenal protein disulphide isomerase (PDI) and BiP. Upon resealing, these microsomes retained glycopeptide export competence. These data suggest that the majority of lumenal components of the ER are most likely nonessential for glycopeptide export. In addition, export competence was highly sensitive to the addition of external protease, indicating a role for protein factors with cytoplasmically exposed determinants.
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Affiliation(s)
- Bassam R S Ali
- Department of Biological Sciences, Imperial College, London, SW7 2AY, UK
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5
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Mellor H, Neville D, Harvey D, Platt F, Dwek R, Butters T. Cellular effects of deoxynojirimycin analogues: inhibition of N-linked oligosaccharide processing and generation of free glucosylated oligosaccharides. Biochem J 2004; 381:867-75. [PMID: 15128289 PMCID: PMC1133898 DOI: 10.1042/bj20031824] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2003] [Revised: 04/21/2004] [Accepted: 05/06/2004] [Indexed: 11/17/2022]
Abstract
In the accompanying paper [Mellor, Neville, Harvey, Platt, Dwek and Butters (2004) Biochem. J. 381, 861-866] we treated HL60 cells with N-alk(en)yl-deoxynojirimycin (DNJ) compounds to inhibit glucosphingolipid (GSL) biosynthesis and identified a number of non-GSL-derived, small, free oligosaccharides (FOS) most likely produced due to inhibition of the oligosaccharide-processing enzymes a-glucosidases I and II. When HL60 cells were treated with concentrations of N-alk(en)ylated DNJ analogues that inhibited GSL biosynthesis completely, N-butyl- and N-nonyl-DNJ inhibited endoplasmic reticulum (ER) glucosidases I and II, but octadecyl-DNJ did not, probably due to the lack of ER lumen access for this novel, long-chain derivative. Glucosidase inhibition resulted in the appearance of free Glc1-3Man structures, which is evidence of Golgi glycoprotein endomannosidase processing of oligosaccharides with retained glucose residues. Additional large FOS was also detected in cells following a 16 h treatment with N-butyl- and N-nonyl-DNJ. When these FOS structures (>30, including >20 species not present in control cells) were characterized by enzyme digests and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS, all were found to be polymannose-type oligosaccharides, of which the majority were glucosylated and had only one reducing terminal GlcNAc (N-acetylglucosamine) residue (FOS-GlcNAc1), demonstrating a cytosolic location. These results support the proposal that the increase in glucosylated FOS results from enzyme-mediated cytosolic cleavage of oligosaccharides from glycoproteins exported from the ER because of misfolding or excessive retention. Importantly, the present study characterizes the cellular properties of DNJs further and demonstrates that side-chain modifications allow selective inhibition of protein and lipid glycosylation pathways. This represents the most detailed characterization of the FOS structures arising from ER a-glucosidase inhibition to date.
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Affiliation(s)
- Howard R. Mellor
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
| | - David C. A. Neville
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
| | - David J. Harvey
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
| | - Frances M. Platt
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
| | - Raymond A. Dwek
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
| | - Terry D. Butters
- Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K
- To whom correspondence should be addressed (e-mail )
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6
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Neuendorf E, Weber A, Saalmueller A, Schatzl H, Reifenberg K, Pfaff E, Groschup MH. Glycosylation Deficiency at Either One of the Two Glycan Attachment Sites of Cellular Prion Protein Preserves Susceptibility to Bovine Spongiform Encephalopathy and Scrapie Infections. J Biol Chem 2004; 279:53306-16. [PMID: 15448157 DOI: 10.1074/jbc.m410796200] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The conversion into abnormally folded prion protein (PrP) plays a key role in prion diseases. PrP(C) carries two N-linked glycan chains at amino acid residues 180 and 196 (mouse). Previous in vitro data indicated that the conversion process may not require glycosylation of PrP. However, it is conceivable that these glycans function as intermolecular binding sites during the de novo infection of cells on susceptible organisms and/or play a role for the interaction of both PrP isoforms. Such receptor-like properties could contribute to the formation of specific prion strains. However, in earlier studies, mutations at the glycosylation sites of PrP led to intracellular trafficking abnormalities, which made it impossible to generate PrP glycosylation-deficient mice that were susceptible to bovine spongiform encephalopathy (BSE) or scrapie. We have now tested more than 25 different mutations at both consensus sites and found one nonglycosylated (T182N/T198A) and two monoglycosylated (T182N and T198A) mutants that rather retained authentic cellular trafficking properties. In vitro all three mutants were converted into PrP(res). PrP mutant T182N/T198A also provoked a strong dominant-negative inhibition on the endogenous wild type PrP conversion reaction. By using the two monoglycosylated mutants, we generated transgenic mice overexpressing PrP(C) in their brains at levels of 2-4 times that of nontransgenic mice. Most interestingly, such mice proved readily susceptible to a challenge with either scrapie (Chandler and Me7) or with BSE. Incubation times were comparable or in some instances even significantly shorter than those of nontransgenic mice. These data indicate that diglycosylation of PrP(C) is not mandatory for prion infection in vivo.
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Affiliation(s)
- Erdmute Neuendorf
- Institute for Novel and Emerging Infectious Diseases, Federal Research Centre for Virus Diseases of Animals, 17943 Greifswald, Isle of Riems, Germany
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7
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Suzuki T, Lennarz WJ. Glycopeptide export from the endoplasmic reticulum into cytosol is mediated by a mechanism distinct from that for export of misfolded glycoprotein. Glycobiology 2002; 12:803-11. [PMID: 12499402 DOI: 10.1093/glycob/cwf095] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
When glycoproteins formed in the endoplasmic reticulum (ER) are misfolded, they are generally translocated into the cytosol for ubiquitination and are subsequently degraded by the proteasome. This system, the so-called ER-associated glycoprotein degradation, is important for eukaryotes to maintain the quality of glycoproteins generated in the ER. It has been established in yeast that several distinct proteins are involved in this translocation and degradation processes. Small glycopeptides formed in the ER are exported to the cytosol in a similar manner. This glycopeptide export system is conserved from yeast to mammalian cells, suggesting its basic biological significance for eukaryotic cells. These two export systems (for misfolded glycoproteins and glycopeptides) share some properties, such as a requirement for ATP and involvement of Sec61p, a central membrane protein presumably forming a dislocon channel for export of proteins. However, the machinery of glycopeptide export is poorly understood. In this study, various mutants known to have an effect on export/degradation of misfolded glycoproteins were examined for glycopeptide export activity with a newly established assay method. Surprisingly, most of the mutants were found not to exhibit a defect in glycopeptide export. The only gene that was found to be required on efficient export of both types of substrates was PMR1, the gene encoding the medial-Golgi Ca(2+)/Mn(2+)-ion pump. These results provide evidence that although the systems involved in export of misfolded glycoproteins and glycopeptides share some properties, they have exhibited distinct differences.
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Affiliation(s)
- Tadashi Suzuki
- Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo and PRESTO, Japan Science and Technology Corporation (JST), Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
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8
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Durrant C, Moore SEH. Perturbation of free oligosaccharide trafficking in endoplasmic reticulum glucosidase I-deficient and castanospermine-treated cells. Biochem J 2002; 365:239-47. [PMID: 11942856 PMCID: PMC1222649 DOI: 10.1042/bj20011786] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2001] [Revised: 03/27/2002] [Accepted: 04/10/2002] [Indexed: 11/17/2022]
Abstract
Free oligosaccharides (FOS) are generated both in the endoplasmic reticulum (ER) and in the cytosol during glycoprotein biosynthesis. ER lumenal FOS possessing the di-N-acetylchitobiose moiety at their reducing termini (FOSGN2) are exported into the cytosol where they, along with their cytosolically generated counterparts possessing a single N-acetylglucosamine residue at their reducing termini (FOSGN1), are trimmed in order to be imported into lysosomes for final degradation. Both the ER and lysosomal FOS transport processes are unable to translocate triglucosylated FOS across membranes. In the present study, we have examined FOS trafficking in HepG2 cells treated with the glucosidase inhibitor castanospermine. We have shown that triglucosylated FOSGN2 generated in the ER are transported to the Golgi apparatus where they are deglucosylated by endomannosidase and acquire complex, sialic acid-containing structures before being secreted into the extracellular space by a Brefeldin A-sensitive pathway. FOSGN2 are also secreted from glucosidase I-deficient Lec23 cells and from the castanospermine-treated parental Chinese-hamster ovary cell line. Despite the secretion of FOSGN2 from Lec23 cells, we noted a transient intracellular accumulation (60 nmol/g cells) of triglucosylated FOSGN1 in these cells. Finally, in glucosidase I-compromised cells, FOS trafficking was severely perturbed leading to both the secretion of FOSGN2 into the extracellular space and a growth-dependent pile up of triglucosylated FOSGN1 in the cytosol. The possibility that these abnormalities contributed to the severe and rapidly progressive pathology in a patient with congenital disorders of glycosylation type IIb (glucosidase I deficiency) is discussed.
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Affiliation(s)
- Christelle Durrant
- Unité de Glycobiologie et Signalisation Cellulaire, U504, Bâtiment INSERM, 16 avenue Paul Vaillant-Couturier, 94807 Villejuif Cedex, France
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9
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Spiro MJ, Spiro RG. Release of polymannose oligosaccharides from vesicular stomatitis virus G protein during endoplasmic reticulum-associated degradation. Glycobiology 2001; 11:803-11. [PMID: 11588156 DOI: 10.1093/glycob/11.10.803] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
To further explore the localization of the N-deglycosylation involved in the endoplasmic reticulum (ER)-associated quality control system we studied HepG2 cells infected with vesicular stomatitis virus (VSV) and its ts045 mutant, as in this system oligosaccharide release can be attributed solely to the VSV glycoprotein (G protein). We utilized the restricted intracellular migration of the mutant protein as well as dithiothreitol (DTT), low temperature, and a castanospermine (CST)-imposed glucosidase blockade to determine in which intracellular compartment deglycosylation takes place. Degradation of the VSV ts045 G protein was considerably greater at the nonpermissive than at the permissive temperature; this was reflected by a substantial increase in polymannose oligosaccharide release. Under both conditions these oligosaccharides were predominantly in the characteristic cytosolic form, which terminates in a single N-acetylglucosamine (OS-GlcNAc(1)); this was also the case in the presence of DTT, which retains the G protein completely in the ER. However when cells infected with the VSV mutant were examined at 15 degrees C or exposed to CST, both of which represent conditions that impair ER-to-cytosol transport, the released oligosaccharides were almost exclusively (> 95%) in the vesicular OS-GlcNAc(2) form; glucosidase blockade had a similar effect on the wild-type virus. Addition of puromycin to glucosidase-inhibited cells resulted in a pronounced reduction (> 90%) in oligosaccharide release, which reflected a comparable impairment in glycoprotein biosynthesis and indicated that the OS-GlcNAc(2) components originated from protein degradation rather than hydrolysis of oligosaccharide lipids. Our findings are consistent with N-deglycosylation of the VSV G protein in the ER and the subsequent transport of the released oligosaccharides to the cytosol where OS-GlcNAc(2) to OS-GlcNAc(1) conversion by an endo-beta-N-acetylglucosaminidase takes place. Studies with the ts045 G protein at the nonpermissive temperature permitted us to determine that it can be processed by Golgi endomannosidase although remaining endo H sensitive, supporting the concept that it recycles between the ER and cis-Golgi compartments.
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Affiliation(s)
- M J Spiro
- Department of Medicine, Harvard Medical School, Joslin Diabetes Center, 1 Joslin Place, Boston, Massachusetts 02215, USA
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10
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Ali BR, Tjernberg A, Chait BT, Field MC. A microsomal GTPase is required for glycopeptide export from the mammalian endoplasmic reticulum. J Biol Chem 2000; 275:33222-30. [PMID: 10913137 DOI: 10.1074/jbc.m003845200] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Bidirectional transport of proteins via the Sec61p translocon across the endoplasmic reticulum (ER) membrane is a recognized component of the ER quality control machinery. Following translocation and engagement by the luminal quality control system, misfolded and unassembled proteins are exported from the ER lumen back to the cytosol for degradation by the proteasome. Additionally, other ER contents, including oligosaccharides, oligopeptides, and glycopeptides, are efficiently exported from mammalian and yeast systems, indicating that bidirectional transport across ER membranes is a general eukaryotic phenomenon. Glycopeptide and protein export from the ER in in vitro systems is both ATP- and cytosol-dependent. Using a well established system to study glycopeptide export and conventional liquid chromatography, we isolated a single polypeptide species of 23 kDa from rat liver cytosol that was capable of fully supporting glycopeptide export from rat microsomes in the presence of an ATP-regenerating system. The protein was identified by mass spectrometric sequence analysis as guanylate kinase (GK), a housekeeping enzyme critical in the regulation of cellular GTP levels. We confirmed the ability of GK to substitute for complete cytosol by reconstitution of glycopeptide export from rat liver microsomes using highly purified recombinant GK from Saccharomyces cerevisiae. Most significantly, we found that the GK (and hence the cytosolic component) requirement was fully bypassed by low micromolar concentrations of GDP or GTP. Similarly, export was inhibited by non-hydrolyzable analogues of GDP and GTP, indicating a requirement for GTP hydrolysis. Membrane integrity was fully maintained under assay conditions, as no ER luminal proteins were released. Competence for glycopeptide export was abolished by very mild protease treatment of microsomes, indicating the presence of an essential protein on the cytosolic face of the ER membrane. These data demonstrate that export of glycopeptide export is controlled by a microsomal GTPase and is independent of cytosolic protein factors.
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Affiliation(s)
- B R Ali
- Wellcome Trust Laboratories for Molecular Parasitology, Department of Biochemistry, Imperial College of Science, Technology, and Medicine, Exhibition Road, London SW7 2AY, United Kingdom
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11
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Ali BR, Claxton S, Field MC. Export of a misprocessed GPI-anchored protein from the endoplasmic reticulum in vitro in an ATP- and cytosol-dependent manner. FEBS Lett 2000; 483:32-6. [PMID: 11033351 DOI: 10.1016/s0014-5793(00)02073-1] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Strict quality control mechanisms within the mammalian endoplasmic reticulum act to prevent misfolded and unprocessed proteins from entering post-endoplasmic reticulum (ER) compartments. Following translocation into the ER lumen via the Sec61p translocon, nascent polypeptide chains fold and are modified in an environment that contains numerous chaperones and other folding mediators. Recently it has emerged that polypeptides failing to acquire the native state are re-exported from the ER to the cytosol for ultimate degradation by the proteasome ubiquitin system, apparently mediated again via Sec61p. Substrates for this degradation pathway include proteins destined to become glycosyl phosphatidylinositol (GPI)-anchored, but which fail to be processed and retain the C-terminal GPI signal peptide. In order to characterise this process we have used a model GPI-anchored mutant protein, prepro mini human placental alkaline phosphatase (PLAP) W179, which cannot be processed efficiently on account of being a poor substrate for the transamidase which cleaves the GPI signal peptide and adds the GPI anchor in a coupled reaction. In vitro transcription, translation and translocation into canine pancreatic microsomes resulted in ER-targeting signal sequence cleavage and formation of prominiPLAP in the ER lumen. We were able to show that prominiPLAPW179 could be exported from the microsomes in a time-dependent manner and that release requires both ATP and cytosol. Export was not supported by GTP, indicating a biochemical distinction from glycopeptide export which we showed recently requires GTP hydrolysis. The process was not affected by redox, unlike several other GPI-anchored model proteins. These data demonstrate that misprocessed proteins can be exported in vitro from mammalian microsomes, facilitating identification of factors involved in this process.
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Affiliation(s)
- B R Ali
- Centre for Molecular Microbiology and Infection, Wellcome Trust Laboratories for Molecular Parasitology, Department of Biochemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, SW7 2AY, London, UK
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12
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Cacan R, Verbert A. Transport of free and N-linked oligomannoside species across the rough endoplasmic reticulum membranes. Glycobiology 2000; 10:645-8. [PMID: 10910969 DOI: 10.1093/glycob/10.7.645] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The N-glycosylation process occurs in the rough endoplasmic reticulum. It requires the transport of glycosyl donors into the lumen and the exit of the glycosylated products toward the secretory pathway. Besides this main flow, the formation of free oligomannosides, glycopeptides, and misfolded glycoproteins which do not enter the secretory pathway and are cleared out of the endoplasmic reticulum by specific transports has been demonstrated. This review focuses on the export mechanisms of these three side products of the N-glycosylation process and discusses their physiological significance.
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Affiliation(s)
- R Cacan
- Laboratoire de Chimie Biologique, CNRS-UMR 8576, Université des Sciences et Technologies de Lille, Villeneuve d'Ascq Cedex, France
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