Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P11021 (BiP)
 2,049 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Glycosylation plays a crucial role in glycoprotein stability and its correct folding. Murine acid sphingomyelinase (ASM) is a lysosomal glycoprotein. We studied the functional role of its individual N-linked oligosaccharides needed to maintain enzymatic activity and protein stability. Mutagenized cDNA constructs were heterologously expressed. All six potential N-glycosylation sites were modified. Incomplete glycosylation of the most distant C-terminal site resulted in two isoforms. Oligosaccharides at N-84, N-173, and N-611 were found to be of minor importance for enzymatic activity. The glycosylation defect at N-333 or N-393 reduced the enzymatic activity to 40% and at N-518 to less than 20%. These mutations did not effect the Km value. Glycosylation at N-333 and N-393 mainly contributed to the enzyme stability and prevented degradation at lysosomal acidic pH, whereas the low residual enzymatic activity of mutant ASM deficient in glycosylation at N-518 was caused by protein misfolding. The mutant protein was also prone to proteolysis when trapped in the endoplasmic reticulum/cis-Golgi after brefeldin A application. Insufficiently glycosylated ASM formed a stable complex with BiP, an immunoglobulin heavy chain-binding protein, and thus remained in the endoplasmic reticulum. 32PO4 labeling revealed that the glycosylation mutants of ASM were phosphorylated predominantly at mannose residues of oligosaccharides linked to N-84, N-333, and N-393.
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PMID:Functional analysis of the glycosylation of murine acid sphingomyelinase. 894 61

Thyroglobulin (Tg), the major protein secreted by thyroid epithelial cells and precursor of thyroid hormones, is a large dimeric glycoprotein with multiple disulfide bonds. The folding and assembly of this complex molecule begins in the endoplasmic reticulum (ER) and is likely to involve a variety of reactions catalyzed by molecular chaperones (Kuznetsov, G., Chen, L. B., and Nigam, S. K. (1994) J. Biol. Chem. 269, 22990-22995). By coimmunoprecipitation in rat thyroid cells, we were able to demonstrate that BiP, grp94, ERp72, and grp170, four proteins believed to function as specific molecular chaperones, complex with Tg during its maturation. The same complex of the four putative chaperones with Tg was observed in cells treated with tunicamycin, indicating that these four ER chaperones stably associate with Tg when it is misfolded/misassembled due to inhibition of its glycosylation. BiP, grp94, and ERp72 were also found to associate with Tg in cells in which misfolding was induced by perturbing ER calcium stores. To determine if the assembly of a complex between the four chaperones and Tg under conditions of misglycosylation was unique to the maturation of this particular secretory protein or a more general phenomenon, adenovirus-transformed rat thyroid cells that do not synthesize Tg were analyzed. In these transformed cells, the only protein these same four chaperones were found to complex with was a protein of approximately 200 kDa. This protein was subsequently identified as thrombospondin, which, like Tg, is a large oligomeric secreted glycoprotein with multiple disulfide bonds. We therefore propose that these ER chaperones complex together with a variety of large oligomeric secretory glycoproteins as they fold and assemble in the ER.
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PMID:Multiple molecular chaperones complex with misfolded large oligomeric glycoproteins in the endoplasmic reticulum. 900 56

A temperature sensitive secretory line, MadIA214, was selected from mutagenized Chinese hamster ovary cells that express two heterologous export marker proteins: a secretory form of the human placental alkaline phosphatase (SeAP), and the Kd heavy chain of mouse MHC class I. SeAP secretion in MadIA214 was extremely reduced at elevated temperature (40 degrees C), while the export of functional H-2Kd molecules to the plasma membrane was only slightly affected. This mutant constitutively transferred onto newly synthesized proteins a truncated oligosaccharide core, Man5GlcNAc2, which was monoglucosylated in the protein-bound form. Nevertheless, the final oligosaccharide-structures associated to mature SeAP and H-2Kd were similar in mutant and wild-type glycoproteins. The inaccessibility in MadIA214 endoplasmic reticulum (ER) of one or more components required for oligosaccharide chain elongation is supported by the reconstitution of a correct core structure, obtained after disruption of cellular compartments, but not after cell permeabilisation or blocking ER-to-Golgi transport. The increased association of the ER-chaperone BiP with immature SeAP correlated with the thermodependent decrease in SeAP secretion. The retention of incompletely folded polypeptides in MadIA214 parallels both a marked ER-dilation and an important glycoprotein degradation documented by the formation of soluble oligomannosides with one GlcNAc residue. Our data provide the first in vivo evidence that the initial step in N-glycosylation differentially governs glycoprotein maturation, transport and degradation.
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PMID:Differential fate of glycoproteins carrying a monoglucosylated form of truncated N-glycan in a new CHO line, MadIA214214, selected for a thermosensitive secretory defect. 905 85

Halothane causes an idiosyncratic hepatitis that is thought to result, in part, from immune reactions against one or more lumenal endoplasmic reticulum (ER) proteins that have been covalently modified by the trifluoroacetyl chloride metabolite of halothane. In this study, we have identified a 170 kDa protein target of halothane in the liver of rats. The 170 kDa protein was first detected when proteins in lysates of hepatocytes from halothane-treated rats were immunoprecipitated with antisera against several resident ER proteins. This 170 kDa protein was found to be associated with other protein targets of halothane, including protein disulfide isomerase, a protein disulfide isomerase isoform, a 59 kDa carboxylesterase, and 78 kDa glucose-regulated protein. Immunoblotting with antiserum directed against the trifluoroacetylated hapten indicated that the 170 kDa protein was trifluoroacetylated. Based upon its subcellular localization, molecular mass, N-terminal amino acid sequence, and antigenicity, the trifluoroacetylated 170 kDa protein was identified as UDP-glucose:glycoprotein glucosyltransferase (UGGT), a lumenal ER protein that is thought to have a role in the folding of N-linked glycoproteins. Moreover, treatment of rats with halothane caused a 44% decrease in the activity of liver microsomal UGGT, and at least 36% of the change in the activity of the enzyme could be due to a decrease in the level of the protein. The results suggest that the function of UGGT in folding of N-linked glycoproteins may be affected by other resident ER proteins or xenobiotics such as halothane.
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PMID:UDP-glucose:glycoprotein glucosyltransferase associates with endoplasmic reticulum chaperones and its activity is decreased in vivo by the inhalation anesthetic halothane. 907 3

Four well-characterized monoclonal antibodies (MAbs) directed against rabies virus glycoprotein (G) were used to study G folding in vivo. Two of the MAbs were able to immunoprecipitate incompletely oxidized folding intermediates. The two others recognized G only after folding was completed. By using these MAbs, the ability of G to undergo low-pH-induced conformational changes during folding was also investigated. It appeared that some domains acquire this ability before folding is completed. In addition, interactions between unfolded G and some of the molecular chaperones were analyzed. Unfolded G was associated with BiP and calnexin. Association with BiP was maximal immediately after the pulse, whereas association with calnexin was maximal after 5 to 10 min of chase. The effects of tunicamycin and castanospermine on chaperone binding and folding were also studied. In the presence of both drugs, calnexin binding was reduced, consistent with the view that calnexin specifically recognizes monoglucosylated oligosaccharides, but some residual binding was still observed, indicating that calnexin also recognizes the polypeptide chain. In the presence of both drugs, association with BiP was increased and prolonged and folding was impaired. However, the global effects of the drugs were different, since folding was much more efficient in the presence of castanospermine than in the presence of tunicamycin. Taken together, these results provide the basis to draw a schematic view of rabies virus glycoprotein folding.
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PMID:Folding of rabies virus glycoprotein: epitope acquisition and interaction with endoplasmic reticulum chaperones. 909 49

Calsequestrin (CSQ), the major low-affinity Ca(2+)-binding glycoprotein of striated muscle fibers, is concentrated to yield aggregates that occupy the lumen of the terminal cisternae of the sarcoplasmic reticulum (SR). When infected or transfected into L6 myoblast, the protein is also concentrated, however, in dense vacuoles apparently separate from the endoplasmic reticulum (ER). CSQ-rich cells appear otherwise normal; in particular, neither other proteins involved in Ca2+ homeostasis nor ER chaperones are increased. The CSQ dense vacuoles are shown herein to be specialized ER subdomains as demonstrated by 1) the endoglycosidase H sensitivity of their CSQ and 2) two markers, calreticulin and calnexin (but not others, protein disulfide isomerase and BiP), intermixed with the vacuole content. Their formation is shown to start with the aggregation of CSQ at discrete sites of the ER lumen. When cells were transfected with both CSQ and calreticulin, only the first gave rise to vacuoles; the second remained diffusely distributed within the ER lumen. The possibility that CSQ aggregation is an artifact of overexpression appears unlikely because 1) within dense vacuoles CSQ molecules are not disulfide cross-linked, 2) their turnover is relatively slow (t = 12 h), and 3) segregated CSQ is bound to large amounts of Ca2+. Transfection of a tagged CSQ into cells already overexpressing the protein revealed the continuous import of the newly synthesized protein into preassembled vacuoles. The tendency to aggregation appears, therefore, as a property contributing to the segregation of CSQ within the ER lumen and to its accumulation within specialized subdomains. The study of L6 cells expressing CSQ-rich vacuoles might thus ultimately help to unravel mechanisms by which the complexity of the sarcoplasmic reticulum is established in muscle fibers.
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PMID:Overexpression of calsequestrin in L6 myoblasts: formation of endoplasmic reticulum subdomains and their evolution into discrete vacuoles where aggregates of the protein are specifically accumulated. 930 74

The Sindbis virus envelope protein spike is a hetero-oligomeric complex composed of a trimer of glycoprotein E1-E2 heterodimers. Spike assembly is a multistep process which occurs in the endoplasmic reticulum (ER) and is required for the export of E1 from the ER. PE2 (precursor to E2), however, can transit through the secretory pathway and be expressed at the cell surface in the absence of E1. Although oligomer formation does not appear to be required for the export of PE2, there is evidence that defects in E1 folding can affect PE2 transit from the ER. Temperature-sensitive mutant ts23 of Sindbis virus contains two amino acid substitutions in E1, while PE2 and capsid protein have the wild-type sequence; however, at the nonpermissive temperature, both E1 and PE2 are retained within the ER and can be isolated in protein aggregates with the molecular chaperone GRP78-BiP. We previously demonstrated that the temperature sensitivity for ts23 was lost as oligomer formation took place at the permissive temperature, suggesting that temperature sensitivity is initiated early in the process of viral spike assembly (M. Carleton and D. T. Brown, J. Virol. 70:952-959, 1996). Experiments described herein investigated the defects in envelope protein maturation that occur in ts23-infected cells and which result in retention of both envelope proteins in the ER. The data demonstrate that in ts23-infected cells incubated at the nonpermissive temperature, E1 folding is disrupted early after synthesis, resulting in the rapid incorporation of both E1 and PE2 into disulfide-stabilized aggregates. Furthermore, the aberrant E1 conformation which is responsible for induction of the ts phenotype requires the formation of intramolecular disulfide bridges formed prior to E1 association with PE2 and the completion of E1 folding.
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PMID:The formation of intramolecular disulfide bridges is required for induction of the Sindbis virus mutant ts23 phenotype. 931 53

Unlike properly folded and assembled proteins, most misfolded and incompletely assembled proteins are retained in the endoplasmic reticulum of mammalian cells and degraded without transport to the Golgi complex. To analyze the mechanisms underlying this unique sorting process and its fidelity, the fate of C-terminally truncated fragments of influenza hemagglutinin was determined. An assortment of different fragments was generated by adding puromycin at low concentrations to influenza virus-infected tissue culture cells. Of the fragments generated, < 2% was secreted, indicating that the system for detecting defects in newly synthesized proteins is quite stringent. The majority of secreted species corresponded to folding domains within the viral spike glycoprotein. The retained fragments acquired a partially folded structure with intra-chain disulfide bonds and conformation-dependent antigenic epitopes. They associated with two lectin-like endoplasmic reticulum chaperones (calnexin and calreticulin) but not BiP/GRP78. Inhibition of the association with calnexin and calreticulin by the addition of castanospermine significantly increased fragment secretion. However, it also caused association with BiP/GRP78. These results indicated that the association with calnexin and calreticulin was involved in retaining the fragments. They also suggested that BiP/GRP78 could serve as a backup for calnexin and calreticulin in retaining the fragments. In summary, the results showed that the quality control system in the secretory pathway was efficient and sensitive to folding defects, and that it involved multiple interactions with endoplasmic reticulum chaperones.
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PMID:Quality control in the secretory pathway: the role of calreticulin, calnexin and BiP in the retention of glycoproteins with C-terminal truncations. 934 35

To acquire information on the relationships between structural maturation of proteins in the endoplasmic reticulum (ER) and their transport along the secretory pathway, we have analyzed the destiny of an assembly-defective form of the trimeric vacuolar storage glycoprotein phaseolin. In leaves of transgenic tobacco, where assembly-competent phaseolin is correctly targeted to the vacuole, defective phaseolin remains located in the ER or a closely related compartment where it represents a major ligand of the chaperone BiP. Defective phaseolin maintained susceptibility to endoglycosidase H and was slowly degraded by a process that is not inhibited by heat shock or brefeldin A, indicating that degradation does not involve transport along the secretory pathway. These results provide evidence for the presence of a quality control mechanism in the ER of plant cells that avoids intracellular trafficking of severely defective proteins and eventually leads to their degradation.
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PMID:Protein quality control along the route to the plant vacuole. 1118 57

The stress-induced unfolded protein response (UPR) is the only signaling pathway known to regulate expression of genes encoding the resident endoplasmic reticulum (ER) molecular chaperones and folding enzymes, yet these genes are constitutively expressed in all cells. We have examined the expression of ER chaperones in several cell lines that are dependent on a variety of cytokines for growth and survival. When the various cell lines were deprived of essential growth factors, mRNA levels of the ER chaperones BiP and GRP94 decreased dramatically. Re-stimulation of ligand-deprived cells with the appropriate growth factor induced BiP and GRP94 as delayed-early response genes. Cytokine induction of BiP and GRP94 biosynthesis was not preceded by a burst of glycoprotein traffic through the ER nor accompanied by expression of the CHOP transcription factor. The glycosylation inhibitor tunicamycin potently induced expression of both ER chaperones and CHOP in ligand-deprived cells, demonstrating that the UPR pathway remains functionally intact in the absence of growth factor-mediated signaling. Therefore, basal expression of ER chaperones is dependent upon and regulated by a mitogenic pathway distinct from the stress-inducible UPR cascade and this probably controls expression of ER chaperones and folding enzymes needed to assist protein biogenesis in the ER of normal, non-stressed cells.
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PMID:A pathway distinct from the mammalian unfolded protein response regulates expression of endoplasmic reticulum chaperones in non-stressed cells. 938 97


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