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Structural insight into substrate recognition by the endoplasmic reticulum folding-sensor enzyme: crystal structure of third thioredoxin-like domain of UDP-glucose:glycoprotein glucosyltransferase.

ABSTRACT: The endoplasmic reticulum (ER) possesses a protein quality control system that supports the efficient folding of newly synthesized glycoproteins. In this system, a series of N-linked glycan intermediates displayed on proteins serve as quality tags. The ER folding-sensor enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) operates as the gatekeeper for ER quality control by specifically transferring monoglucose residues to incompletely folded glycoproteins, thereby allowing them to interact with lectin chaperone complexes to facilitate their folding. Despite its functional importance, no structural information is available for this key enzyme to date. To elucidate the folding-sensor mechanism in the ER, we performed a structural study of UGGT. Based on bioinformatics analyses, the folding-sensor region of UGGT was predicted to harbour three tandem thioredoxin (Trx)-like domains, which are often found in proteins involved in ER quality control. Furthermore, we determined the three-dimensional structure of the third Trx-like domain, which exhibits an extensive hydrophobic patch concealed by its flexible C-terminal helix. Our structural data suggest that this hydrophobic patch is involved in intermolecular interactions, thereby contributing to the folding-sensor mechanism of UGGT.

SUBMITTER: Zhu T

PROVIDER: S-EPMC4255179 | biostudies-literature | 2014 Dec

REPOSITORIES: biostudies-literature

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Structural insight into substrate recognition by the endoplasmic reticulum folding-sensor enzyme: crystal structure of third thioredoxin-like domain of UDP-glucose:glycoprotein glucosyltransferase.

Zhu Tong T Satoh Tadashi T Kato Koichi K

Scientific reports 20141204

The endoplasmic reticulum (ER) possesses a protein quality control system that supports the efficient folding of newly synthesized glycoproteins. In this system, a series of N-linked glycan intermediates displayed on proteins serve as quality tags. The ER folding-sensor enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) operates as the gatekeeper for ER quality control by specifically transferring monoglucose residues to incompletely folded glycoproteins, thereby allowing them to interac ...[more]

PMID: 25471383

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Project description: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.

Project description:The UDP-Glc:glycoprotein glucosyltransferase (UGGT) is the sensor of glycoprotein conformations in the glycoprotein folding quality control as it exclusively glucosylates glycoproteins not displaying their native conformations. Monoglucosylated glycoproteins thus formed may interact with the lectin-chaperones calnexin (CNX) and calreticulin (CRT). This interaction prevents premature exit of folding intermediates to the Golgi and enhances folding efficiency. Bioinformatic analysis showed that in C. elegans there are two open reading frames (F48E3.3 and F26H9.8 to be referred as uggt-1 and uggt-2, respectively) coding for UGGT homologues. Expression of both genes in Schizosaccharomyces pombe mutants devoid of UGGT activity showed that uggt-1 codes for an active UGGT protein (CeUGGT-1). On the other hand, uggt-2 coded for a protein (CeUGGT-2) apparently not displaying a canonical UGGT activity. This protein was essential for viability, although cnx/crt null worms were viable. We constructed transgenic worms carrying the uggt-1 promoter linked to the green fluorescent protein (GFP) coding sequence and found that CeUGGT-1 is expressed in cells of the nervous system. uggt-1 is upregulated under ER stress through the ire-1 arm of the unfolded protein response (UPR). Real-time PCR analysis showed that both uggt-1 and uggt-2 genes are expressed during the entire C. elegans life cycle. RNAi-mediated depletion of CeUGGT-1 but not of CeUGGT-2 resulted in a reduced lifespan and that of CeUGGT-1 and CeUGGT-2 in a developmental delay. We found that both CeUGGT1 and CeUGGT2 play a protective role under ER stress conditions, since 10 µg/ml tunicamycin arrested development at the L2/L3 stage of both uggt-1(RNAi) and uggt-2(RNAi) but not of control worms. Furthermore, we found that the role of CeUGGT-2 but not CeUGGT-1 is significant in relieving low ER stress levels in the absence of the ire-1 unfolding protein response signaling pathway. Our results indicate that both C. elegans UGGT homologues have distinct biological functions.

Dataset Information

Structural insight into substrate recognition by the endoplasmic reticulum folding-sensor enzyme: crystal structure of third thioredoxin-like domain of UDP-glucose:glycoprotein glucosyltransferase.

Publications

Structural insight into substrate recognition by the endoplasmic reticulum folding-sensor enzyme: crystal structure of third thioredoxin-like domain of UDP-glucose:glycoprotein glucosyltransferase.

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