Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.25.1 (proteasome)
28,817 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The inducible extracellular alkaline protease of Neurospora crassa was demonstrated to be a glycoprotein containing D-galactose residues by use of the enzyme-lectin conjugate horseradish peroxidase-Ricinus communis-agglutinin-120. The carbohydrate moiety of the protease appears to be a poor antigen since an antiserum made to the native enzyme recognizes epitopes determined only by the polypeptide portion of the enzyme. Immunochemical techniques were used to quantitatively precipitate protease labeled in vivo for electrophoretic analysis. Protease synthesis could not be detected in control, uninduced cultures, whereas ca. 0.4% of total cellular protein synthesis is devoted to protease formation under inducing conditions.
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PMID:Major extracellular protease of Neurospora crassa. 623 9

Proteins entering the secretory pathway may be glycosylated upon transfer of an oligosaccharide (Glc3Man9GlcNAc2) from a dolichol-P-P derivative to nascent polypeptide chains in the lumen of the endoplasmic reticulum (ER). Oligosaccharides are then deglucosylated by glucosidases I and II (GII). Also in the ER, glycoproteins acquire their final tertiary structures, and species that fail to fold properly are retained and eventually degraded in the proteasome. It has been proposed that in mammalian cells the monoglucosylated oligosaccharides generated either by partial deglucosylation of the transferred compound or by reglucosylation of glucose-free oligosaccharides by the UDP-Glc:glycoprotein glucosyltransferase (GT) are recognized by ER resident lectins (calnexin and/or calreticulin). GT is a sensor of glycoprotein conformation as it only glucosylates misfolded species. The lectin-monoglucosylated oligosaccharide interaction would retain glycoproteins in the ER until correctly folded, and also facilitate their acquisition of proper tertiary structures by preventing aggregation. GII would liberate glycoproteins from the calnexin/calreticulin anchor, but species not properly folded would be reglucosylated by GT, and so continue to be retained by the lectins. Only when the protein becomes properly folded would it cease to be retained by the lectins. This review presents evidence suggesting that a similar quality control mechanism of glycoprotein folding is operative in Schizosaccharomyces pombe and that the mechanism in Saccharomyces cerevisiae probably differs substantially from that occurring in mammalian and Sch. pombe cells.
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PMID:Reglucosylation of glycoproteins and quality control of glycoprotein folding in the endoplasmic reticulum of yeast cells. 987 90

We are studying endoplasmic reticulum-associated degradation (ERAD) with the use of a truncated variant of the type I ER transmembrane glycoprotein ribophorin I (RI). The mutant protein, RI(332), containing only the N-terminal 332 amino acids of the luminal domain of RI, has been shown to interact with calnexin and to be a substrate for the ubiquitin-proteasome pathway. When RI(332) was expressed in HeLa cells, it was degraded with biphasic kinetics; an initial, slow phase of approximately 45 min was followed by a second phase of threefold accelerated degradation. On the other hand, the kinetics of degradation of a form of RI(332) in which the single used N-glycosylation consensus site had been removed (RI(332)-Thr) was monophasic and rapid, implying a role of the N-linked glycan in the first proteolytic phase. RI(332) degradation was enhanced when the binding of glycoproteins to calnexin was prevented. Moreover, the truncated glycoprotein interacted with calnexin preferentially during the first proteolytic phase, which strongly suggests that binding of RI(332) to the lectin-like protein may result in the slow, initial phase of degradation. Additionally, mannose trimming appears to be required for efficient proteolysis of RI(332). After treatment of cells with the inhibitor of N-glycosylation, tunicamycin, destruction of the truncated RI variants was severely inhibited; likewise, in cells preincubated with the calcium ionophore A23187, both RI(332) and RI(332)-Thr were stabilized, despite the presence or absence of the N-linked glycan. On the other hand, both drugs are known to trigger the unfolded protein response (UPR), resulting in the induction of BiP and other ER-resident proteins. Indeed, only in drug-treated cells could an interaction between BiP and RI(332) and RI(332)-Thr be detected. Induction of BiP was also evident after overexpression of murine Ire1, an ER transmembrane kinase known to play a central role in the UPR pathway; at the same time, stabilization of RI(332) was observed. Together, these results suggest that binding of the substrate proteins to UPR-induced chaperones affects their half lives.
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PMID:Degradation of a short-lived glycoprotein from the lumen of the endoplasmic reticulum: the role of N-linked glycans and the unfolded protein response. 1058 43

The endoplasmic reticulum (ER) is the subcellular site where proteins following the secretory pathway acquire their proper tertiary and, in certain cases, quaternary structures. Species that are not yet properly folded are prevented from exit to the Golgi apparatus and, if permanently misfolded, are transported to the cytosol, where they are degraded in the proteasomes. This review deals with a mechanism, applicable to proteins that are N-glycosylated in the ER, by which the quality control of folding is performed. Protein-linked monoglucosylated glycans, formed by glucosidase I- and glucosidase II-dependent partial deglucosylation of the oligosaccharides transferred from dolichol diphosphate derivatives in N-glycosylation (Glc(3)Man(9)GlcNAc(2)), mediate glycoprotein recognition by two ER-resident lectins, membrane-bound calnexin (CNX) and its soluble homologue, calreticulin (CRT). A still not yet fully confirmed interaction between the lectins and the protein moieties of folding glycoproteins may occur after lectin recognition of monoglucosylated structures. Further deglucosylation of glycans by glucosidase II, and perhaps also by a change in CNX/CRT and/or in the substrate glycoprotein conformation, liberates the glycoproteins from their CNX/CRT anchors. Glycans may be then reglucosylated by the UDP-Glc:glycoprotein glucosyltransferase (GT), and thus be recognized again by CNX/CRT, but only when linked to not yet properly folded protein moieties, as this enzyme behaves as a sensor of glycoprotein conformation. Deglucosylation/reglucosylation cycles catalysed by the opposing activities of glucosidase II and GT only stop when proper folding is achieved. The interaction between CNX/CRT and a monoglucosylated glycan is one of the alternative mechanisms by which cells retain not yet properly folded glycoproteins in the ER; in addition, it enhances folding efficiency by preventing protein aggregation and thus allowing intervention of classical chaperones and other folding-assisting proteins. There is evidence suggesting that both glycoprotein glucosylation and mannose removal, respectively mediated by GT and ER mannosidase I, might be involved in cell recognition of permanently misfolded glycoproteins bound for proteasome degradation.
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PMID:Role of N-oligosaccharide endoplasmic reticulum processing reactions in glycoprotein folding and degradation. 1079 7

Misfolded proteins are recognized in the endoplasmic reticulum (ER), transported back to the cytoplasm and degraded by the proteasome. Processing intermediates of N-linked oligosaccharides on incompletely folded glycoproteins have an important role in their folding/refolding, and also in their targeting to proteolytic degradation. In Saccharomyces cerevisiae, we have identified a gene coding for a non-essential protein that is homologous to mannosidase I (HTM1) and that is required for degradation of glycoproteins. Deletion of the HTM1 gene does not affect oligosaccharide trimming. However, deletion of HTM1 does reduce the rate of degradation of the mutant glycoproteins such as carboxypeptidase Y, ABC-transporter Pdr5-26p and oligosaccharyltransferase subunit Stt3-7p, but not of mutant Sec61-2p, a non-glycoprotein. Our results indicate that although Htm1p is not involved in processing of N-linked oligosaccharides, it is required for their proteolytic degradation. We propose that this mannosidase homolog is a lectin that recognizes Man8GlcNAc2 oligosaccharides that serve as signals in the degradation pathway.
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PMID:Htm1p, a mannosidase-like protein, is involved in glycoprotein degradation in yeast. 1137 35

Protein folding and quality control in the early secretory pathway function as posttranslational checkpoints in eukaryote gene expression. Herein, an aberrant form of the hepatic secretory protein alpha1-antitrypsin was stably expressed in a human embryonic kidney cell line to elucidate the mechanisms by which glycoprotein endoplasmic reticulum-associated degradation (GERAD) is administered in cells from higher eukaryotes. After biosynthesis, genetic variant PI Z underwent alternative phases of secretion and degradation, the latter of which was mediated by the proteasome. Degradation required release from calnexin- and asparagine-linked oligosaccharide modification by endoplasmic reticulum mannosidase I, the latter of which occurred as PI Z was bound to the molecular chaperone grp78/BiP. That a distinct GERAD program operates in human embryonic kidney cells was supported by the extent of PI Z secretion, apparent lack of polymerization, inability of calnexin to participate in the degradation process, and sequestration of the glycoprotein folding sensor UDP-glucose:glycoprotein glucosyltransferase in the Golgi complex. Because UDP-glucose:glycoprotein glucosyltransferase sustains calnexin binding, its altered distribution is consistent with a GERAD program that hinders the reentry of substrates into the calnexin cycle, allowing grp78/BiP to partner with a lectin, other than calnexin, in the recognition of a two-component GERAD signal to facilitate substrate recruitment. How the processing of a mutant protein, rather than the mutation itself, can contribute to disease pathogenesis, is discussed.
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PMID:Organizational diversity among distinct glycoprotein endoplasmic reticulum-associated degradation programs. 1218 35

Adherence of the opportunistic fungus Aspergillus fumigatus to the extracellular matrix components is considered a crucial step in the establishment of the infection. Given the high carbohydrate content of these glycoproteins and the role of carbohydrate-protein interactions in numerous adherence processes, the presence of a lectin in A. fumigatus was investigated. Different fungal extracts obtained by sonication or grinding in liquid nitrogen from resting or swollen conidia, as well as from germ tubes and mycelium, were tested by hemagglutination assays using rabbit erythrocytes. A lectin activity was recovered in all the extracts tested. However, sonication of resting conidia resulted in the highest specific activity. Purification of the lectin was achieved by gel filtration followed by ion-exchange and hydrophobic-interaction chromatographies. Analysis of the purified lectin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed an apparent molecular mass of 32 kDa, which is similar to that of the alkaline protease already identified from different strains of A. fumigatus. However, as evidenced by the use of an alkaline protease-deficient mutant, the two activities were supported by distinct proteins. In addition, hemagglutination inhibition experiments using different saccharides and glycoproteins demonstrated the specificity of the lectin for sialic acid residues. Together these results suggest that this lectin may contribute to the attachment of conidia to the extracellular matrix components through the recognition of the numerous terminal sialic acid residues of their carbohydrate chains.
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PMID:Purification and partial characterization of a 32-kilodalton sialic acid-specific lectin from Aspergillus fumigatus. 1243 66

Previously we showed that two antithrombin mutants were degraded through an endoplasmic reticulum (ER)-associated degradation (ERAD) pathway [F. Tokunaga et al., FEBS Lett. 412 (1997) 65]. Here, we examined the combined effects of inhibitors of glycosidases, protein synthesis, proteasome, and tyrosine phosphatase on ERAD of a Glu313-deleted (DeltaGlu) mutant of antithrombin. We found that kifunensine, an ER mannosidase I inhibitor, suppressed ERAD, indicating that specific mannose trimming plays a critical role. Cycloheximide and puromycin, inhibitors of protein synthesis, also suppressed ERAD, the effects being cancelled by pretreatment with castanospermine. In contrast, kifunensine suppressed ERAD even in castanospermine-treated cells, suggesting that suppression of ERAD does not always require the binding of lectin-like ER chaperones-like calnexin and/or calreticulin. These results indicate that, besides proteasome inhibitors, inhibitors of ER mannosidase I and protein synthesis suppress ERAD of the antithrombin deltaGlu mutant at different stages, and processing of N-linked oligosaccharides highly correlated with the efficiency of ERAD.
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PMID:N-linked oligosaccharide processing, but not association with calnexin/calreticulin is highly correlated with endoplasmic reticulum-associated degradation of antithrombin Glu313-deleted mutant. 1262 72

Dendritic cells (DCs) facilitate HIV-1 spread in the host by capturing virions and transferring them to permissive lymphocytes in lymphoid organs. Lectins such as DC-specific ICAM-grabbing non-integrin (DC-SIGN) are involved in HIV-1 uptake by DCs, through high-affinity binding to viral envelope glycoproteins. We examined the role of DC-SIGN on the fate of incoming virions and on major histocompatibility complex class I (MHC-I)-restricted HIV-1 antigen presentation. We show that DC-SIGN expression in B-cell lines dramatically enhances viral internalization. In these cells, and also in primary DCs, most of the captured virions are rapidly degraded, likely in a lysosomal compartment. In addition, a fraction of incoming viral material is processed by the proteasome, leading to activation of anti-HIV-specific cytotoxic T lymphocytes (CTLs) by DC-SIGN-expressing cells. In DCs, DC-SIGN is not the only receptor involved, and redundant pathways of virus capture leading to antigen presentation likely coexist. Altogether, our results highlight new aspects of DC-SIGN interactions with HIV-1. The lectin does not significantly protect captured virions against degradation and promotes MHC-I exogenous presentation of HIV-1 antigens.
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PMID:DC-SIGN promotes exogenous MHC-I-restricted HIV-1 antigen presentation. 1457 49

The O-linked glycosylation of highly purified Drosophila 26S proteasome has been analyzed by immunological and lectin-binding methods. Five regulatory complex subunits and at least nine catalytic core subunits were recognized by two different monoclonal antibodies specific for O-linked N-acetylglucosamine-modified proteins, and by wheat germ agglutinin, which is specific for the N-acetylglucosamine sugar side-chain. The specificity of these reactions has been proved by competition studies with free N-acetylglucosamine. Three ATPase subunits of the regulatory complex, which are O-glycosylated, have previously been shown [FEBS Lett. 430 (1998) 269] to occur in phosphorylated form as well, indicating that several different post-translational modifications, with distinct regulatory potential, may be present on the same subunit.
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PMID:26S proteasome subunits are O-linked N-acetylglucosamine-modified in Drosophila melanogaster. 1465 13


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