The alpha- and beta-subunits are required for expression of catalytic activity in the hetero-dimeric glucosidase II complex from human liver

GANAB and N-Glycans Substrates Are Relevant in Human Physiology, Polycystic Pathology and Multiple Sclerosis: A Review

Glycans are one of the four fundamental macromolecular components of living matter, and they are highly regulated in the cell. Their functions are metabolic, structural and modulatory. In particular, ER resident N-glycans participate with the Glc3Man9GlcNAc2 highly conserved sequence, in protein folding process, where the physiological balance between glycosylation/deglycosylation on the innermost glucose residue takes place, according GANAB/UGGT concentration ratio. However, under abnormal conditions, the cell adapts to the glucose availability by adopting an aerobic or anaerobic regimen of glycolysis, or to external stimuli through internal or external recognition patterns, so it responds to pathogenic noxa with unfolded protein response (UPR). UPR can affect Multiple Sclerosis (MS) and several neurological and metabolic diseases via the BiP stress sensor, resulting in ATF6, PERK and IRE1 activation. Furthermore, the abnormal GANAB expression has been observed in MS, systemic lupus erythematous, male germinal epithelium and predisposed highly replicating cells of the kidney tubules and bile ducts. The latter is the case of Polycystic Liver Disease (PCLD) and Polycystic Kidney Disease (PCKD), where genetically induced GANAB loss affects polycystin-1 (PC1) and polycystin-2 (PC2), resulting in altered protein quality control and cyst formation phenomenon. Our topics resume the role of glycans in cell physiology, highlighting the N-glycans one, as a substrate of GANAB, which is an emerging key molecule in MS and other human pathologies.

Engineering the endoplasmic reticulum secretory pathway in Trichoderma reesei for improved cellulase production

The filamentous fungus Trichoderma reesei is an extraordinarily efficient cell factory of industrial cellulase for production of biofuels and other bio-based products because of its excellent potential to secrete cellulolytic enzymes. Engineering the protein secretory pathway may be a powerful means for efficient protein production. However, it is uncertain whether this engineering approach could improve cellulase production in T. reesei. Herein, the endoplasmic reticulum (ER) secretory pathway was engineered for the production of cellulolytic enzymes by multiple strategies, including: (I) overexpression of the key components of protein folding (Pdi1, Ero1 and BiP); (II) overexpression of the glycosylation-related elements (Gpt1 and Gls2); (III) knockout of the ER mannosidase I (Mns1) encoding gene mns1. By utilizing these ER engineering strategies, the secretion of β-glucosidase was remarkably elevated in the engineered strains, ranging from 29.2 % to 112.5 %. Furthermore, it was found that engineering these components also regulated the ER stress resistance. More importantly, the total cellulase production was increased with varying degrees, which reached a maximum of 149.4 %, using the filter paper assay (FPA) as a characterization method. These results demonstrated that engineering the ER secretory pathway can enhance protein secretion, particularly for cellulase production, which shed light for the development of high-efficient cellulolytic enzymes for economically feasible bioethanol production from lignocellulosic biomass.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

N-Glycosylation

N-glycosylation is a highly conserved glycan modification, and more than 7000 proteins are N-glycosylated in humans. N-glycosylation has many biological functions such as protein folding, trafficking, and signal transduction. Thus, glycan modification to proteins is profoundly involved in numerous physiological and pathological processes. The N-glycan precursor is biosynthesized in the endoplasmic reticulum (ER) from dolichol phosphate by sequential enzymatic reactions to generate the dolichol-linked oligosaccharide composed of 14 sugar residues, Glc₃Man₉GlcNAc₂. The oligosaccharide is then en bloc transferred to the consensus sequence N-X-S/T (X represents any amino acid except proline) of nascent proteins. Subsequently, the N-glycosylated nascent proteins enter the folding step, in which N-glycans contribute largely to attaining the correct protein fold by recruiting the lectin-like chaperones, calnexin, and calreticulin. Despite the N-glycan-dependent folding process, some glycoproteins do not fold correctly, and these misfolded glycoproteins are destined to degradation by proteasomes in the cytosol. Properly folded proteins are transported to the Golgi, and N-glycans undergo maturation by the sequential reactions of glycosidases and glycosyltransferases, generating complex-type N-glycans. N-Acetylglucosaminyltransferases (GnT-III, GnT-IV, and GnT-V) produce branched N-glycan structures, affording a higher complexity to N-glycans. In this chapter, we provide an overview of the biosynthetic pathway of N-glycans in the ER and Golgi.

Novel GANAB variants associated with polycystic liver disease

BACKGROUND

Polycystic liver disease (PLD) is an inherited disorder characterized by numerous cysts in the liver. Autosomal dominant polycystic kidney and liver disease (ADPKD and ADPLD, respectively) have been linked to pathogenic GANAB variants. GANAB encodes the α-subunit of glucosidase II (GIIα). Here, we report the identification of novel GANAB variants in an international cohort of patients with the primary phenotype of PLD using molecular inversion probe analysis.

RESULTS

Five novel GANAB variants were identified in a cohort of 625 patients with ADPKD or ADPLD. In silico analysis revealed that these variants are likely to affect functionally important domains of glucosidase II α-subunit. Missense variant c.1835G>C p.(Arg612Pro) was predicted to disrupt the structure of the active site of the protein, likely reducing its activity. Frameshift variant c.687delT p.(Asp229Glufs*60) introduces a premature termination codon predicted to have no activity. Two nonsense variants (c.2509C>T; p.(Arg837*), and c.2656C>T; p.(Arg886*)) and splice variant c.2002+1G>C, which causes aberrant pre-mRNA splicing and affecting RNA processing, result in truncated proteins and are predicted to cause abnormal binding of α- and β-subunits of glucosidase II, thus affecting its enzymatic activity. Analysis of glucosidase II subunits in cell lines shows expression of a truncated GIIα protein in cells with c.687delT, c.2509C>T, c.2656C>T, and c.2002+1G>C variants. Incomplete colocalization of the subunits was present in cells with c.687delT or c.2002+1G>C variants. Other variants showed normal distribution of GIIα protein.

CONCLUSIONS

We identified five novel GANAB variants associated with PLD in both ADPKD and ADPLD patients supporting a common pathway in cystogenesis. These variants may lead to decreased or complete loss of enzymatic activity of glucosidase II which makes GANAB a candidate gene to be screened in patients with an unknown genetic background.

Genetics and mechanisms of hepatic cystogenesis

Polycystic liver disease (PLD) is a heterogeneous genetic condition. PKD1 and PKD2 germline mutations are found in patients with autosomal dominant polycystic kidney disease (ADPKD). Autosomal dominant polycystic liver disease (ADPLD) is associated with germline mutations in PRKCSH, SEC63, LRP5, and recently ALG8 and SEC61. GANAB mutations are found in both patient groups. Loss of heterozygosity of PLD-genes in cyst epithelium contributes to the development of hepatic cysts. A genetic interaction network is implied in hepatic cystogenesis that connects the endoplasmic glycoprotein control mechanisms and polycystin expression and localization. Wnt signalling could be the major downstream signalling pathway that results in hepatic cyst growth. PLD in ADPLD and ADPKD probably results from changes in one common final pathway that initiates cyst growth. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Substrate recognition of the catalytic α-subunit of glucosidase II from Schizosaccharomyces pombe

The recombinant catalytic α-subunit of N-glycan processing glucosidase II from Schizosaccharomyces pombe (SpGIIα) was produced in Escherichia coli. The recombinant SpGIIα exhibited quite low stability, with a reduction in activity to <40% after 2-days preservation at 4 °C, but the presence of 10% (v/v) glycerol prevented this loss of activity. SpGIIα, a member of the glycoside hydrolase family 31 (GH31), displayed the typical substrate specificity of GH31 α-glucosidases. The enzyme hydrolyzed not only α-(1→3)- but also α-(1→2)-, α-(1→4)-, and α-(1→6)-glucosidic linkages, and p-nitrophenyl α-glucoside. SpGIIα displayed most catalytic properties of glucosidase II. Hydrolytic activity of the terminal α-glucosidic residue of Glc2Man3-Dansyl was faster than that of Glc1Man3-Dansyl. This catalytic α-subunit also removed terminal glucose residues from native N-glycans (Glc2Man9GlcNAc2 and Glc1Man9GlcNAc2) although the activity was low.

α-Glucosidases and α-1,4-glucan lyases: structures, functions, and physiological actions

α-Glucosidases (AGases) and α-1,4-glucan lyases (GLases) catalyze the degradation of α-glucosidic linkages at the non-reducing ends of substrates to release α-glucose and anhydrofructose, respectively. The AGases belong to glycoside hydrolase (GH) families 13 and 31, and the GLases belong to GH31 and share the same structural fold with GH31 AGases. GH13 and GH31 AGases show diverse functions upon the hydrolysis of substrates, having linkage specificities and size preferences, as well as upon transglucosylation, forming specific α-glucosidic linkages. The crystal structures of both enzymes were determined using free and ligand-bound forms, which enabled us to understand the important structural elements responsible for the diverse functions. A series of mutational approaches revealed features of the structural elements. In particular, amino-acid residues in plus subsites are of significance, because they regulate transglucosylation, which is used in the production of industrially valuable oligosaccharides. The recently solved three-dimensional structure of GLase from red seaweed revealed the amino-acid residues essential for lyase activity and the strict recognition of the α-(1 → 4)-glucosidic substrate linkage. The former was introduced to the GH31 AGase, and the resultant mutant displayed GLase activity. GH13 and GH31 AGases hydrate anhydrofructose to produce glucose, suggesting that AGases are involved in the catabolic pathway used to salvage unutilized anhydrofructose.

Mutations in GANAB, Encoding the Glucosidase IIα Subunit, Cause Autosomal-Dominant Polycystic Kidney and Liver Disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a common, progressive, adult-onset disease that is an important cause of end-stage renal disease (ESRD), which requires transplantation or dialysis. Mutations in PKD1 or PKD2 (∼85% and ∼15% of resolved cases, respectively) are the known causes of ADPKD. Extrarenal manifestations include an increased level of intracranial aneurysms and polycystic liver disease (PLD), which can be severe and associated with significant morbidity. Autosomal-dominant PLD (ADPLD) with no or very few renal cysts is a separate disorder caused by PRKCSH, SEC63, or LRP5 mutations. After screening, 7%-10% of ADPKD-affected and ∼50% of ADPLD-affected families were genetically unresolved (GUR), suggesting further genetic heterogeneity of both disorders. Whole-exome sequencing of six GUR ADPKD-affected families identified one with a missense mutation in GANAB, encoding glucosidase II subunit α (GIIα). Because PRKCSH encodes GIIβ, GANAB is a strong ADPKD and ADPLD candidate gene. Sanger screening of 321 additional GUR families identified eight further likely mutations (six truncating), and a total of 20 affected individuals were identified in seven ADPKD- and two ADPLD-affected families. The phenotype was mild PKD and variable, including severe, PLD. Analysis of GANAB-null cells showed an absolute requirement of GIIα for maturation and surface and ciliary localization of the ADPKD proteins (PC1 and PC2), and reduced mature PC1 was seen in GANAB(+/-) cells. PC1 surface localization in GANAB(-/-) cells was rescued by wild-type, but not mutant, GIIα. Overall, we show that GANAB mutations cause ADPKD and ADPLD and that the cystogenesis is most likely driven by defects in PC1 maturation.

Insight into glucosidase II from the red marine microalga Porphyridium sp. (Rhodophyta)

N-glycosylation of proteins is one of the most important post-translational modifications that occur in various organisms, and is of utmost importance for protein function, stability, secretion, and loca-lization. Although the N-linked glycosylation pathway of proteins has been extensively characterized in mammals and plants, not much information is available regarding the N-glycosylation pathway in algae. We studied the α 1,3-glucosidase glucosidase II (GANAB) glycoenzyme in a red marine microalga Porphyridium sp. (Rhodophyta) using bioinformatic and biochemical approaches. The GANAB-gene was found to be highly conserved evolutionarily (compo-sed of all the common features of α and β subunits) and to exhibit similar motifs consistent with that of homolog eukaryotes GANAB genes. Phylogenetic analysis revealed its wide distribution across an evolutionarily vast range of organisms; while the α subunit is highly conserved and its phylogenic tree is similar to the taxon evolutionary tree, the β subunit is less conserved and its pattern somewhat differs from the taxon tree. In addition, the activity of the red microalgal GANAB enzyme was studied, including functional and biochemical characterization using a bioassay, indicating that the enzyme is similar to other eukaryotes ortholog GANAB enzymes. A correlation between polysaccharide production and GANAB activity, indicating its involvement in polysaccharide biosynthesis, is also demonstrated. This study represents a valuable contribution toward understanding the N-glycosylation and polysaccharide biosynthesis pathways in red microalgae.

Glucosidase II and MRH-domain containing proteins in the secretory pathway

N-glycosylation in the endoplasmic reticulum (ER) consists of the transfer of a preassembled glycan conserved among species (Glc3Man9GlcNAc2) from a lipid donor to a consensus sequence within a nascent protein that is entering the ER. The protein-linked glycans are then processed by glycosidases and glycosyltransferases in the ER producing specific structures that serve as signalling molecules for the fate of the folding glycoprotein: to stay in the ER during the folding process, to be retrotranslocated to the cytosol for proteasomal degradation if irreversibly misfolded, or to pursue transit through the secretory pathway as a mature glycoprotein. In the ER, each glycan signalling structure is recognized by a specific lectin. A domain similar to that of the mannose 6-phosphate receptors (MPRs) has been identified in several proteins of the secretory pathway. These include the beta subunit of glucosidase II (GII), a key enzyme in the early processing of the transferred glycan that removes middle and innermost glucoses and is involved in quality control of glycoprotein folding in the ER (QC), the lectins OS-9 and XTP3-B, proteins involved in the delivery of ER misfolded proteins to degradation (ERAD), the gamma subunit of the Golgi GlcNAc-1-phosphotransferase, an enzyme involved in generating the mannose 6-phosphate (M6P) signal for sorting acidic hydrolases to lysosomes, and finally the MPRs that deliver those hydrolytic enzymes to the lysosome. Each of the MRH-containing proteins recognizes a different signalling N-glycan structure. Three-dimensional structures of some of the MRH domains have been solved, providing the basis to understand recognition mechanisms.

Structural and biochemical characterization of novel bacterial α-galactosidases belonging to glycoside hydrolase family 31

We identified two bacterial enzymes as the first members that displayed α-galactosidase activity and the crystal structures provided insights into their novel substrate specificity. This is the first report of α-galactosidases which belong to the GH31 family.

N-linked sugar-regulated protein folding and quality control in the ER

Asparagine-linked glycans (N-glycans) are displayed on the majority of proteins synthesized in the endoplasmic reticulum (ER). Removal of the outermost glucose residue recruits the lectin chaperone malectin possibly involved in a first triage of defective polypeptides. Removal of a second glucose promotes engagement of folding and quality control machineries built around the ER lectin chaperones calnexin (CNX) and calreticulin (CRT) and including oxidoreductases and peptidyl-prolyl isomerases. Deprivation of the last glucose residue dictates the release of N-glycosylated polypeptides from the lectin chaperones. Correctly folded proteins are authorized to leave the ER. Non-native polypeptides are recognized by the ER quality control key player UDP-glucose glycoprotein glucosyltransferase 1 (UGT1), re-glucosylated and re-addressed to the CNX/CRT chaperone binding cycle to provide additional opportunity for the protein to fold in the ER. Failure to attain the native structure determines the selection of the misfolded polypeptides for proteasome-mediated degradation.

Expression of α-subunit of α-glucosidase II in adult mouse brain regions and selected organs

α-Glucosidase II (GII), a resident of endoplasmic reticulum (ER) and an important enzyme in the folding of nascent glycoproteins, is heterodimeric, consisting of α (GIIα) and β (GIIβ) subunits. The catalytic GIIα subunit, with the help of mannose 6-phosphate receptor homology domain of GIIβ, sequentially hydrolyzes two α1-3-linked glucose residues in the second step of N-linked oligosaccharide-mediated protein folding. The soluble GIIα subunit is retained in the ER through its interaction with the HDEL-containing GIIβ subunit. N-glycosylation and correct protein folding are crucial for protein stability and trafficking and cell surface expression of several proteins in the brain. Alterations in N-glycosylation lead to abnormalities in neuronal migration and mental retardation, various neurodegenerative diseases, and invasion of malignant gliomas. Inhibitors of GII are used to inhibit cell proliferation and migration in a variety of different pathologies, such as viral infection, cancer, and diabetes. Despite the widespread use of GIIα inhibitory drugs and the role of GIIα in brain function, little is known about its expression in brain and other tissues. Here, we report generation of a highly specific chicken antibody to the GIIα subunit and its characterization by Western blotting and immunoprecipitation using cerebral cortical extracts. By using this antibody, we showed that the GIIα protein is highly expressed in testis, kidney, and lung, with the lowest amount in heart. GIIα polypeptide levels in whole brain were comparable to those in spleen. However, a higher expression of GIIα protein was detected in the cerebral cortex, reflecting its continuous requirement in correct folding of cell surface proteins.

Engineering Yarrowia lipolytica to produce glycoproteins homogeneously modified with the universal Man3GlcNAc2 N-glycan core

Yarrowia lipolytica is a dimorphic yeast that efficiently secretes various heterologous proteins and is classified as “generally recognized as safe.” Therefore, it is an attractive protein production host. However, yeasts modify glycoproteins with non-human high mannose-type N-glycans. These structures reduce the protein half-life in vivo and can be immunogenic in man. Here, we describe how we genetically engineered N-glycan biosynthesis in Yarrowia lipolytica so that it produces Man₃GlcNAc₂ structures on its glycoproteins. We obtained unprecedented levels of homogeneity of this glycanstructure. This is the ideal starting point for building human-like sugars. Disruption of the ALG3 gene resulted in modification of proteins mainly with Man₅GlcNAc₂ and GlcMan₅GlcNAc₂ glycans, and to a lesser extent with Glc₂Man₅GlcNAc₂ glycans. To avoid underoccupancy of glycosylation sites, we concomitantly overexpressed ALG6. We also explored several approaches to remove the terminal glucose residues, which hamper further humanization of N-glycosylation; overexpression of the heterodimeric Apergillus niger glucosidase II proved to be the most effective approach. Finally, we overexpressed an α-1,2-mannosidase to obtain Man₃GlcNAc₂ structures, which are substrates for the synthesis of complex-type glycans. The final Yarrowia lipolytica strain produces proteins glycosylated with the trimannosyl core N-glycan (Man₃GlcNAc₂), which is the common core of all complex-type N-glycans. All these glycans can be constructed on the obtained trimannosyl N-glycan using either in vivo or in vitro modification with the appropriate glycosyltransferases. The results demonstrate the high potential of Yarrowia lipolytica to be developed as an efficient expression system for the production of glycoproteins with humanized glycans.

Impact of clostridial glucosylating toxins on the proteome of colonic cells determined by isotope-coded protein labeling and LC-MALDI

Background

The anaerobe Clostridium difficile produces two major virulence factors toxin A and B that inactivate Rho proteins by glucosylation of a pivotal threonine residue. Purified toxins induce reorganization of the cytoskeleton and cell death in colonic cells. Whether all toxin effects on target cells depend on catalytic glucosyltransferase activity is unclear at present. Thus, we conducted a proteome approach to compare the protein profile of target cells treated either with wild type toxin A (rTcdA wt) or with a catalytically inactive mutant toxin A (mutant rTcdA). Relative protein quantification was feasible using isotope-coded protein labeling techniques (ICPL) and mass spectrometry (LC-MALDI).

Results

Altogether we found a significant differential expression of thirty proteins after treatment with rTcdA wt or mutant rTcdA. Mutant rTcdA caused up-regulation of seven proteins and sixteen proteins were responsive to rTcdA wt after 5 h. Long-term effect of rTcdA wt on protein expression was the down-regulation of eleven proteins. Up- or down-regulation of several proteins was verified by western blot analysis confirming the MS results.

Conclusion

Our results indicate incubation time-dependent effects of the clostridial glucosylating toxin A on colonic cells. The rTcdA wt impact more cellular functions than actin cytoskeleton reorganization and apoptosis. Furthermore, these data give insight into glucosyltransferase independent effects of clostridial glucosylating toxins on target cells after short incubation time. Additionally, our data reveal pro-inflammatory and proliferative effects of mutant rTcdA after short-term incubation.

Promiscuous activity of ER glucosidase II discovered through donor specificity analysis of UGGT

In glycoprotein quality control system in the endoplasmic reticulum (ER), UGGT (UDP-glucose:glycoprotein glucosyltransferase) and glucosidase II (G-II) play key roles. UGGT serves as a glycoprotein folding sensor by virtue of its unique specificity to glucosylate glycoproteins at incompletely folded stage. By using various UDP-Glc analogues, we first analyzed donor specificity of UGGT, which was proven to be rather narrow. However, marginal activity was observed with UDP-galactose and UDP-glucuronic acid as well as with 3-, 4- and 6-deoxy glucose analogues to give corresponding transfer products. Intriguingly, G-II smoothly converted all of them back to Man(9)GlcNAc(2), providing an indication that G-II has a promiscuous activity as a broad specificity hexosidase.

Requirement of a homolog of glucosidase II beta-subunit for EFR-mediated defense signaling in Arabidopsis thaliana

EFR is a plasma-membrane resident receptor responsible for recognition of microbial elongation factor Tu (EF-Tu) and thus triggering plant innate immunity to fend off phytopathogens. Functional EFR must be subject to the endoplasmic reticulum quality control (ERQC) machinery for the correct folding and proper assembly in order to reach its final destination. Genetic studies have demonstrated that ERD2b, a counterpart of the yeast or mammalian HDEL receptor ERD2 for retaining proteins in the endoplasmic reticulum (ER) lumen, is required for EFR function in plants (Li et al., 2009). In this study, we characterized the Arabidopsis glucosidase II beta-subunit via the HDEL motif against the non-redundant protein database. Data mining also revealed that the glucosidase II beta-subunit gene has a highly similar expression pattern to ERD2b and the other known ERQC components involved in EFR biogenesis. Importantly, the T-DNA insertion lines of the glucosidase II beta-subunit gene showed that EFR-controlled responses were substantially reduced or completely blocked in these mutants. The responses include seedling growth inhibition, induction of marker genes, MAP kinase activation, and callose deposition, triggered by peptide elf18, a full mimic of EF-Tu. Taken together, our data indicate a requirement of the glucosidase II beta-subunit for EFR function.

Congenital disorders of glycosylation in hepatology: the example of polycystic liver disease

Autosomal dominant polycystic liver disease (PCLD) is a rare progressive disorder characterized by an increased liver volume due to many (>20) fluid-filled cysts of biliary origin. Disease causing mutations in PRKCSH or SEC63 are found in approximately 25% of the PCLD patients. Both gene products function in the endoplasmic reticulum, however, the molecular mechanism behind cyst formation remains to be elucidated. As part of the translocon complex, SEC63 plays a role in protein import into the ER and is implicated in the export of unfolded proteins to the cytoplasm during ER-associated degradation (ERAD). PRKCSH codes for the beta-subunit of glucosidase II (hepatocystin), which cleaves two glucose residues of Glc(3)Man(9)GlcNAc(2) N-glycans on proteins. Hepatocystin is thereby directly involved in the protein folding process by regulating protein binding to calnexin/calreticulin in the ER. A separate group of genetic diseases affecting protein N-glycosylation in the ER is formed by the congenital disorders of glycosylation (CDG). In distinct subtypes of this autosomal recessive multisystem disease specific liver symptoms have been reported that overlap with PCLD. Recent research revealed novel insights in PCLD disease pathology such as the absence of hepatocystin from cyst epithelia indicating a two-hit model for PCLD cystogenesis. This opens the way to speculate about a recessive mechanism for PCLD pathophysiology and shared molecular pathways between CDG and PCLD. In this review we will discuss the clinical-genetic features of PCLD and CDG as well as their biochemical pathways with the aim to identify novel directions of research into cystogenesis.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

UDP-GlC:glycoprotein glucosyltransferase-glucosidase II, the ying-yang of the ER quality control

The N-glycan-dependent quality control of glycoprotein folding prevents endoplasmic to Golgi exit of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones that recognize monoglucosylated polymannose glycans, a lectin-associated oxidoreductase acting on monoglucosylated glycoproteins, a glucosyltransferase that creates monoglucosytlated epitopes in protein-linked glycans and a glucosidase that removes the glucose units added by the glucosyltransferase. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded species or in not completely assembled complexes. The glucosidase is a dimeric heterodimer composed of a catalytic subunit and an additional one that is partially responsible for the ER localization of the enzyme and for the enhancement of the deglucosylation rate as its mannose 6-phosphate receptor homologous domain presents the substrate to the catalytic site. This review deals with our present knowledge on the glucosyltransferase and the glucosidase.

Sugar-binding activity of the MRH domain in the ER alpha-glucosidase II beta subunit is important for efficient glucose trimming

Glucosidase II (GII) is a glycan-processing enzyme that trims two alpha1,3-linked glucose residues from N-glycan on newly synthesized glycoproteins. Trimming of the first alpha1,3-linked glucose from Glc(2)Man(9)GlcNAc(2) (G2M9) is important for a glycoprotein to interact with calnexin/calreticulin (CNX/CRT), and cleavage of the innermost glucose from Glc(1)Man(9)GlcNAc(2) (G1M9) sets glycoproteins free from the CNX/CRT cycle and allows them to proceed to the Golgi apparatus. GII is a heterodimeric complex consisting of a catalytic alpha subunit (GIIalpha) and a tightly associated beta subunit (GIIbeta) that contains a mannose 6-phosphate receptor homology (MRH) domain. A recent study has suggested a possible involvement of the MRH domain of GIIbeta (GIIbeta-MRH) in the glucose trimming process via its putative sugar-binding activity. However, it remains unknown whether GIIbeta-MRH possesses sugar-binding activity and, if so, what role this activity plays in the function of GII. Here, we demonstrate that human GIIbeta-MRH binds to high-mannose-type glycans. Frontal affinity chromatography revealed that GIIbeta-MRH binds most strongly to the glycans with the alpha1,2-linked mannobiose structure. GII with the mutant GIIbeta that lost the sugar-binding activity of GIIbeta-MRH hydrolyzes p-nitrophenyl-alpha-glucopyranoside, but the capacity to remove glucose residues from G1M9 and G2M9 is significantly decreased. Our results clearly demonstrate the capacity of the GIIbeta-MRH to bind high-mannose-type glycans and its importance in efficient glucose trimming of N-glycans.

Glucosidase II beta subunit modulates N-glycan trimming in fission yeasts and mammals

Glucosidase II (GII) plays a key role in glycoprotein biogenesis in the endoplasmic reticulum (ER). It is responsible for the sequential removal of the two innermost glucose residues from the glycan (Glc(3)Man(9)GlcNAc(2)) transferred to Asn residues in proteins. GII participates in the calnexin/calreticulin cycle; it removes the single glucose unit added to folding intermediates and misfolded glycoproteins by the UDP-Glc:glycoprotein glucosyltransferase. GII is a heterodimer whose alpha subunit (GIIalpha) bears the glycosyl hydrolase active site, whereas its beta subunit (GIIbeta) role is controversial and has been reported to be involved in GIIalpha ER retention and folding. Here, we report that in the absence of GIIbeta, the catalytic subunit GIIalpha of the fission yeast Schizosaccharomyces pombe (an organism displaying a glycoprotein folding quality control mechanism similar to that occurring in mammalian cells) folds to an active conformation able to hydrolyze p-nitrophenyl alpha-d-glucopyranoside. However, the heterodimer is required to efficiently deglucosylate the physiological substrates Glc(2)Man(9)GlcNAc(2) (G2M9) and Glc(1)Man(9)GlcNAc(2) (G1M9). The interaction of the mannose 6-phosphate receptor homologous domain present in GIIbeta and mannoses in the B and/or C arms of the glycans mediates glycan hydrolysis enhancement. We present evidence that also in mammalian cells GIIbeta modulates G2M9 and G1M9 trimming.

Genetic analysis of glucosidase II beta-subunit in trimming of high-mannose-type glycans

Glucosidase II (G-II) is a glycoprotein-processing enzyme that successively cleaves two alpha1,3-linked glucose residues from N-linked oligosaccharides in the endoplasmic reticulum. G-II is a heterodimer whose alpha-subunit contains a glycosidase active site, but the function(s) of the beta-subunit remain poorly defined. We report here an in vivo enzymatic analysis using gene disruptants lacking either the G-II alpha- or beta-subunit in the filamentous fungus Aspergillus oryzae. Using synthetic oligosaccharides as probes, G-II activity of the membranous fraction of the gene disruptants was investigated. The fraction lacking the beta-subunit retained hydrolytic activity toward p-nitrophenyl alpha-D-glucopyranoside but was inactive toward both Glc(2)Man(9)GlcNAc(2) and Glc(1)Man(9)GlcNAc(2). When the fraction containing the beta-subunit was added to the one including the alpha-subunit, the glucosidase activity was restored. These results suggested that the beta-subunit confers the substrate specificity toward di- and monoglucosylated glycans on the glucose-trimming activity of the alpha-subunit.

N-glycan trimming by glucosidase II is essential for Arabidopsis development

Glucosidase II, one of the early N-glycan processing enzymes and a major player in the glycoprotein folding quality control, has been described as a soluble heterodimer composed of alpha and beta subunits. Here we present the first characterization of a plant glucosidase II alpha subunit at the molecular level. Expression of the Arabidopsis alpha subunit restored N-glycan maturation capacity in Schizosaccharomyces pombe alpha- or alphabeta-deficient mutants, but with a lower efficiency in the last case. Inactivation of the alpha subunit in a temperature sensitive Arabidopsis mutant blocked N-glycan processing after a first trimming by glucosidase I and strongly affected seedling development.

Endoplasmic reticulum alpha-glycosidases of Candida albicans are required for N glycosylation, cell wall integrity, and normal host-fungus interaction

The cell surface of Candida albicans is enriched in highly glycosylated mannoproteins that are involved in the interaction with the host tissues. N glycosylation is a posttranslational modification that is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) N-glycan is processed by alpha-glucosidases I and II and alpha1,2-mannosidase to generate Man(8)GlcNAc(2). This N-oligosaccharide is then elaborated in the Golgi to form N-glycans with highly branched outer chains rich in mannose. In Saccharomyces cerevisiae, CWH41, ROT2, and MNS1 encode for alpha-glucosidase I, alpha-glucosidase II catalytic subunit, and alpha1,2-mannosidase, respectively. We disrupted the C. albicans CWH41, ROT2, and MNS1 homologs to determine the importance of N-oligosaccharide processing on the N-glycan outer-chain elongation and the host-fungus interaction. Yeast cells of Cacwh41Delta, Carot2Delta, and Camns1Delta null mutants tended to aggregate, displayed reduced growth rates, had a lower content of cell wall phosphomannan and other changes in cell wall composition, underglycosylated beta-N-acetylhexosaminidase, and had a constitutively activated PKC-Mkc1 cell wall integrity pathway. They were also attenuated in virulence in a murine model of systemic infection and stimulated an altered pro- and anti-inflammatory cytokine profile from human monocytes. Therefore, N-oligosaccharide processing by ER glycosidases is required for cell wall integrity and for host-fungus interactions.

Structure of the bovine VASAP-60/PRKCSH gene, functional analysis of the promoter, and gene expression analysis

Vacuolar system-associated protein-60 (VASAP-60) constitutes the bovine ortholog of the human "protein kinase C substrate 80K-H" (PRKCSH or 80K-H). We characterized the bovine VASAP-60/PRKCSH gene structure and promoter, identified cis-acting elements controlling VASAP-60 expression, searched for mRNA splice variants, and analyzed mRNA expression in ovarian follicles. Expression of VASAP-60 mRNA showed a 2.4-fold increase (P<0.0001) in granulosa cells of dominant follicles compared to small follicles (2-4 mm) or ovulatory follicles, and no mRNA splice variant was identified. The bovine VASAP-60 gene encompasses 12.5 kb and is composed of 18 exons and 17 introns. Primer extension analysis revealed a single transcription initiation site, and the promoter lacks a TATA box. Promoter activity assays were performed with a series of deletion constructs in different bovine cell lines (endometrial epithelial glandular, kidney epithelial and aortic endothelial) to identify cis-acting elements. The -53/+16 bp fragment (+1 = transcription start site) conferred minimal promoter activity whereas activator and repressor elements were located in the -200/-53 bp and -653/-200 bp fragments, respectively. Analysis of cis-acting elements in the -200/-53 bp activation domain revealed by gel shift assays and chromatin immunoprecipitation assay that transcription factor YY1 binds to VASAP-60 promoter. This study is the first to report that VASAP-60 is up-regulated in granulosa cells of dominant follicles, to document the primary structure of the bovine VASAP-60 gene and promoter, and to demonstrate that YY1 binds to the VASAP-60 proximal promoter and may act as a positive transcriptional regulator.

The MRH protein Erlectin is a member of the endoplasmic reticulum synexpression group and functions in N-glycan recognition

Kremen1 and 2 (Krm1/2) are coreceptors for Dickkopf1 (Dkk1), an antagonist of Wnt/beta-catenin signaling, and play a role in head induction during early Xenopus development. In a proteomic approach we identified Erlectin, a novel protein that interacts with Krm2. Erlectin (XTP3-B) is member of a protein family containing mannose 6-phosphate receptor homology (MRH-, or PRKCSH-) domains implicated in N-glycan binding. Like other members of the MRH family, Erlectin is a luminal resident protein of the endoplasmic reticulum. It contains two MRH domains, of which one is essential for Krm2 binding, and this interaction is abolished by Krm2 deglycosylation. The overexpression of Erlectin inhibits transport of Krm2 to the cell surface. Analysis of its embryonic expression pattern in Xenopus reveals that Erlectin is member of the endoplasmic reticulum synexpression group. Erlectin morpholino antisense injection leads to head and axial defects during organogenesis stages in Xenopus embryos. The results indicate that Erlectin functions in N-glycan recognition in the endoplasmic reticulum, suggesting that it may regulate glycoprotein traffic.

Yeast GTB1 Encodes a Subunit of Glucosidase II Required for Glycoprotein Processing in the Endoplasmic Reticulum

Glucosidase II is essential for sequential removal of two glucose residues from N-linked glycans during glycoprotein biogenesis in the endoplasmic reticulum. The enzyme is a heterodimer whose alpha-subunit contains the glycosyl hydrolase active site. The function of the beta-subunit has yet to be defined, but mutations in the human gene have been linked to an autosomal dominant form of polycystic liver disease. Here we report the identification and characterization of a Saccharomyces cerevisiae gene, GTB1, encoding a polypeptide with 21% sequence similarity to the beta-subunit of human glucosidase II. The Gtb1 protein was shown to be a soluble glycoprotein (96-102 kDa) localized to the endoplasmic reticulum lumen where it was present in a complex together with the yeast alpha-subunit homologue Gls2p. Surprisingly, we found that Deltagtb1 mutant cells were specifically defective in the processing of monoglucosylated glycans. Thus, although Gls2p is sufficient for cleavage of the penultimate glucose residue, Gtb1p is essential for cleavage of the final glucose. Our data demonstrate that Gtb1p is required for normal glycoprotein biogenesis and reveal that the final two glucose-trimming steps in N-glycan processing are mechanistically distinct.

Glycoprotein reglucosylation

Proteins following the secretory pathway acquire their proper tertiary and in certain cases also quaternary structures in the endoplasmic reticulum (ER). Incompletely folded species are retained in the ER and eventually degraded. One of the molecular mechanisms by which cells achieve this conformational sorting is based on monoglucosylated N-glycans (Glc1Man5-9GlcNAc2) present on nascent glycoproteins in the ER. This chapter discusses two of the steps that regulate the abundance of such N-glycan structures, including glycoprotein deglucosylation (by glucosidase II) and reglucosylation (by the UDP-Glc:glycoprotein glucosyltransferase), as well as an overview of methods to evaluate the N-glycans prevalent during glycoprotein biogenesis in the ER.

Cloning and characterization of the glucosidase II alpha subunit gene of Trichoderma reesei: a frameshift mutation results in the aberrant glycosylation profile of the hypercellulolytic strain Rut-C30

We describe isolation and characterization of the gene encoding the glucosidase II alpha subunit (GIIalpha) of the industrially important fungus Trichoderma reesei. This subunit is the catalytic part of the glucosidase II heterodimeric enzyme involved in the structural modification within the endoplasmic reticulum (ER) of N-linked oligosaccharides present on glycoproteins. The gene encoding GIIalpha (gls2alpha) in the hypercellulolytic strain Rut-C30 contains a frameshift mutation resulting in a truncated gene product. Based on the peculiar monoglucosylated N-glycan pattern on proteins produced by the strain, we concluded that the truncated protein can still hydrolyze the first alpha-1,3-linked glucose residue but not the innermost alpha-1,3-linked glucose residue from the Glc2Man9GlcNAc2 N-glycan ER structure. Transformation of the Rut-C30 strain with a repaired T. reesei gls2alpha gene changed the glycosylation profile significantly, decreasing the amount of monoglucosylated structures and increasing the amount of high-mannose N-glycans. Full conversion to high-mannose carbohydrates was not obtained, and this was probably due to competition between the endogenous mutant subunit and the introduced wild-type GIIalpha protein. Since glucosidase II is also involved in the ER quality control of nascent polypeptide chains, its transcriptional regulation was studied in a strain producing recombinant tissue plasminogen activator (tPA) and in cultures treated with the stress agents dithiothreitol (DTT) and brefeldin A (BFA), which are known to block protein transport and to induce the unfolded protein response. While the mRNA levels were clearly upregulated upon tPA production or BFA treatment, no such enhancement was observed after DTT addition.

Human endo-alpha1,2-mannosidase is a Golgi-resident type II membrane protein

The cDNA for human endo-alpha1,2-mannosidase was reconstructed using two independent EST-clones and its properties characterized. The 2837 bp cDNA construct contained a 1389 bp open reading frame (ORF) encoding for 462 amino acids and an approximately 53.6 kDa protein, respectively. Hydrophobicity analysis of this amino acid sequence, as well as proteolytic degradation studies, indicate that the enzyme is a type II protein, anchored in the membrane via a 19 amino-acid long apolar sequence close to the N-terminus. Human endo-alpha1,2-mannosidase displays a high degree of sequence identity with the catalytic domain of the homologous rat liver endo-enzyme, but differs substantially in the N-terminal peptide region, which includes the transmembrane domain. No sequence similarity exists with other processing alpha-glycosidases. Based on sequence information provided by the 2837 bp construct, the cDNA consisting of the complete 1389 bp ORF was amplified by RT-PCR using human fibroblast RNA. Incubation of E. coli lysates with this cDNA, previously modified for boost translation by codon optimization, resulted in the synthesis of an approximately 52 kDa protein which degraded [(14)C]Glc(3)-Man(9)-GlcNAc(2) efficiently, indicating that the catalytic domain of the enzyme folds correctly under cell-free conditions. Transfection of the endo-alpha1,2-mannosidase wild-type cDNA into COS 1 cells resulted in a moderate (approximately 1.5-fold) but reproducible increase of activity compared with control cells, whereas >18-fold increase in activity was measured after expression of a chimera containing green-fluorescent-protein (GFP) attached to the N-terminus of the endo-alpha1,2-mannosidase polypeptide. This, together with the observation that GFP-endo-alpha1,2-mannosidase is expressed as a Golgi-resident type II protein, points to enzyme-specific parameters directing folding and membrane anchoring, as well as Golgi-targeting, not being affected by fusion of GFP to the endo-alpha1,2-mannosidase N-terminus.

Roles of N-linked glycans in the endoplasmic reticulum

From a process involved in cell wall synthesis in archaea and some bacteria, N-linked glycosylation has evolved into the most common covalent protein modification in eukaryotic cells. The sugars are added to nascent proteins as a core oligosaccharide unit, which is then extensively modified by removal and addition of sugar residues in the endoplasmic reticulum (ER) and the Golgi complex. It has become evident that the modifications that take place in the ER reflect a spectrum of functions related to glycoprotein folding, quality control, sorting, degradation, and secretion. The glycans not only promote folding directly by stabilizing polypeptide structures but also indirectly by serving as recognition "tags" that allow glycoproteins to interact with a variety of lectins, glycosidases, and glycosyltranferases. Some of these (such as glucosidases I and II, calnexin, and calreticulin) have a central role in folding and retention, while others (such as alpha-mannosidases and EDEM) target unsalvageable glycoproteins for ER-associated degradation. Each residue in the core oligosaccharide and each step in the modification program have significance for the fate of newly synthesized glycoproteins.

Immunolocalization of vacuolar system-associated protein-60 (VASAP-60)

We have characterized the localization of the protein termed VASAP-60 in different bovine tissues and cell lines, and have investigated if VASAP-60 interacts with other proteins. Monospecific polyclonal antibodies were raised against distinct fragments of VASAP-60: NH(2) (V(22) to Q(234)), central (A(246) to S(418)), and COOH (L(416) to L(533)). These three antibodies recognized an 88-kDa protein in immunoblotting analysis. The calculated Mr of VASAP-60 derived from its cDNA (60.1 kDa) was significantly lower than its Mr estimated by SDS-PAGE, and this was mainly attributed to the glutamic acid- and aspartic acid-rich composition of its central region (A(246) to S(418)). A 58-kDa proteolytically processed form of VASAP-60 was also identified. Immunocytochemistry demonstrated that VASAP-60 is found predominantly in the perinuclear region, colocalized with calnexin in the endoplasmic reticulum (ER), and partially colocalized with the endocytic marker DAMP. Immunohistochemical localization of VASAP-60 also demonstrated its presence within specialized vesicular structures not related to the ER. Immunoprecipitation using extracts prepared from S(35)Met/Cys metabolically labeled cells demonstrates that VASAP-60 interacts with 116-, 48.5-, and 26.5-kDa proteins. Therefore, VASAP-60 was found to be more widely distributed in the vacuolar system than anticipated, suggesting that VASAP-60 may function in intracellular transport events, rather than being an exclusive component of the quality control mechanism of newly synthesized proteins as thought previously.

The role of glucosidase II and endomannosidase in glucose trimming of asparagine-linked oligosaccharides

This review covers various aspects of glucose trimming reactions occurring on asparagine-linked oligosaccharides. Structural and functional features of two enzymes, glucosidase II and endo-alpha-mannosidase, prominently involved in this process are summarized and their striking differences in terms of substrate specificities are highlighted. Recent results of analyses by immunoelectron microscopy of their distribution pattern are presented which demonstrate that glucose trimming is not restricted to the endoplasmic reticulum (ER) but additionally is a function accommodated by the Golgi apparatus. The mutually exclusive subcellular distribution of glucosidase II and endomannosidase are discussed in terms of their significance for quality control of protein folding and N-glycosylation.

Mutations in PRKCSH cause isolated autosomal dominant polycystic liver disease

Autosomal dominant polycystic liver disease (ADPLD) is a distinct clinical and genetic entity that can occur independently from autosomal dominant polycystic kidney disease (ADPKD). We previously studied two large kindreds and reported localization of a gene for ADPLD to an approximately 8-Mb region, flanked by markers D19S586/D19S583 and D19S593/D19S579, on chromosome 19p13.2-13.1. Expansion of these kindreds and identification of an additional family allowed us to define flanking markers CA267 and CA048 in an approximately 3-Mb region containing >70 candidate genes. We used a combination of denaturing high-performance liquid chromatography (DHPLC) heteroduplex analysis and direct sequencing to screen a panel of 15 unrelated affected individuals for mutations in genes from this interval. We found sequence variations in a known gene, PRKCSH, that were not observed in control individuals, that segregated with the disease haplotype, and that were predicted to be chain-terminating mutations. In contrast to PKD1, PKD2, and PKHD1, PRKCSH encodes a previously described human protein termed "protein kinase C substrate 80K-H" or "noncatalytic beta-subunit of glucosidase II." This protein is highly conserved, is expressed in all tissues tested, and contains a leader sequence, an LDLa domain, two EF-hand domains, and a conserved C-terminal HDEL sequence. Its function may be dependent on calcium binding, and its putative actions include the regulation of N-glycosylation of proteins and signal transduction via fibroblast growth-factor receptor. In light of the focal nature of liver cysts in ADPLD, the apparent loss-of-function mutations in PRKCSH, and the two-hit mechanism operational in dominant polycystic kidney disease, ADPLD may also occur by a two-hit mechanism.

The cellulose-deficient Arabidopsis mutant rsw3 is defective in a gene encoding a putative glucosidase II, an enzyme processing N-glycans during ER quality control

rsw3 is a temperature-sensitive mutant of Arabidopsis thaliana showing radially swollen roots and a deficiency in cellulose. The rsw3 gene was identified by a map-based strategy, and shows high similarity to the catalytic alpha-subunits of glucosidase II from mouse, yeast and potato. These enzymes process N-linked glycans in the ER, so that they bind and then release chaperones as part of the quality control pathway, ensuring correct protein folding. Putative beta-subunits for the glucosidase II holoenzyme identified in the Arabidopsis and rice genomes share characteristic motifs (including an HDEL ER-retention signal) with beta-subunits in mammals and yeast. The genes encoding the putative alpha- and beta-subunits are single copy and, like the rsw3 phenotype, widely expressed. rsw3 reduces cell number more strongly than cell size in stamen filaments and probably stems. Most features of the rsw3 phenotype are shared with other cellulose-deficient mutants, but some--notably, production of multiple rosettes and a lack of secreted seed mucilage--are not and may reflect glucosidase II affecting processes other than cellulose synthesis. The rsw3 root phenotype develops more slowly than the rsw1 and rsw2 phenotypes when seedlings are transferred to the restrictive temperature. This is consistent with rsw3 reducing glycoprotein delivery from the ER to the plasma membrane whereas rsw1 and rsw2 act more rapidly by affecting the properties of already delivered enzymes.

The oligosaccharyltransferase complex from pig liver: cDNA cloning, expression and functional characterisation

Oligosaccharyltransferase (OST) is an oligomeric protein complex which catalyses the transfer en bloc of Glc(3)-Man(9)-GlcNAc(2) from Dol-PP to specific asparagine residues in the nascent polypeptide chain. In order to study the function of the pig enzyme subunits, we have cloned OST48, ribophorin I and ribophorin II and characterized these proteins after in vitro translation as well as after expression in COS-1 cells. The individual full-length cDNAs contained open reading frames (ORFs) encoding polypeptides with calculated molecular masses of approximately 48.9 kDa (OST48), approximately 68.7 kDa (ribophorin I) and approximately 69.3kDa (ribophorin II), respectively. A Kyte and Doolittle hydrophobicity analysis revealed that OST48, ribophorin I and ribophorin II possess a type I membrane topology with the bulk of their polypeptide chains directed towards the ER-lumen. In contrast to OST48, ribophorin I and II contain, respectively, three or two potential N-glycosylation sites of the Asn-Xaa-Thr/Ser type; only one is found to function as the acceptor site in each protein. Transfection of COS-1 cells with vector constructs encoding either OST48, ribophorin I, or a ribophorin I variant tagged with a myc-peptide sequence, resulted in the over-expression of polypeptides whose molecular masses were similar to those calculated from the respective cDNA ORFs. None of these three polypeptides, or ribophorin II, were found to display OST activity when over-expressed alone. By contrast, a modest but reproducible approximately 25% increase of activity was observed when OST48 together with ribophorin I, or OST48 and myc-tagged ribophorin I, were co-expressed, indicating that these two subunits are probably responsible for the catalytic activity in the hetero-oligomeric OST complex. The only modest over-expression of transferase activity suggests that either the dimeric enzyme complex is catalytically unstable, or that the OST48 and ribophorin I polypeptides are unable to fold properly when other subunit components of the hetero-oligomeric OST complex are lacking. OST48 as well as ribophorin I are expressed in COS-1 cells as ER-resident proteins. Whereas OST48 carries a double-lysine motif in the -3/-5 position of its cytosolic C-terminal domain, ribophorin I does not contain recognizable ER-retention information. Replacing the lysine residue in the -3 position by leucine resulted in plasma membrane expression of the OST48-Leu polypeptide, indicating that this sequence motif may be able to influence OST48 localisation. No cell surface staining was observed when OST48-Leu was co-expressed with ribophorin I. This suggests that localisation of OST48 in the ER is mediated by interaction with ribophorin I rather than by the double-lysine motif.