Molecular cloning and expression of rat liver endo-alpha-mannosidase, an N-linked oligosaccharide processing enzyme

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.

Generation and degradation of free asparagine-linked glycans

Asparagine (N)-linked protein glycosylation, which takes place in the eukaryotic endoplasmic reticulum (ER), is important for protein folding, quality control and the intracellular trafficking of secretory and membrane proteins. It is known that, during N-glycosylation, considerable amounts of lipid-linked oligosaccharides (LLOs), the glycan donor substrates for N-glycosylation, are hydrolyzed to form free N-glycans (FNGs) by unidentified mechanisms. FNGs are also generated in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins during ER-associated degradation. FNGs derived from LLOs and misfolded glycoproteins are eventually merged into one pool in the cytosol and the various glycan structures are processed to a near homogenous glycoform. This article summarizes the current state of our knowledge concerning the formation and catabolism of FNGs.

Protein N-glycosylation, protein folding, and protein quality control

Quality control of protein folding represents a fundamental cellular activity. Early steps of protein N-glycosylation involving the removal of three glucose and some specific mannose residues in the endoplasmic reticulum have been recognized as being of importance for protein quality control. Specific oligosaccharide structures resulting from the oligosaccharide processing may represent a glycocode promoting productive protein folding, whereas others may represent glyco-codes for routing not correctly folded proteins for dislocation from the endoplasmic reticulum to the cytosol and subsequent degradation. Although quality control of protein folding is essential for the proper functioning of cells, it is also the basis for protein folding disorders since the recognition and elimination of non-native conformers can result either in loss-of-function or pathological-gain-of-function. The machinery for protein folding control represents a prime example of an intricate interactome present in a single organelle, the endoplasmic reticulum. Here, current views of mechanisms for the recognition and retention leading to productive protein folding or the eventual elimination of misfolded glycoproteins in yeast and mammalian cells are reviewed.

Endomannosidase undergoes phosphorylation in the Golgi apparatus

Glucose residues from N-linked oligosaccharides are removed by glucosidases I and II in the endoplasmic reticulum (ER) or by the alternate endomannosidase pathway in the Golgi apparatus. Our morphological analysis demonstrates that recombinant rat endomannosidase exhibited a cis- and medial-Golgi localization alike the endogenous enzyme and its ER to Golgi transport is COP II mediated. Recombinant endomannosidase undergoes a posttranslational modification, which is not related to N-or O-glycosylation. A shift in molecular mass of recombinant endomannosidase was observed upon phosphatase digestion but not for ER-retained CHO cell endomannosidase. Furthermore, immunoprecipitation of (35)S- and (33)P-labeled endomannosidase expressed in CHO-K1 cells suggests that recombinant endomannosidase undergoes phosphorylation. Substitution of the single cytoplasmic threonine residue of rat endomannosidase by either an alanine or valine residue resulted in the same posttranslational modification alike the wild-type enzyme. The subcellular localization and the in vivo activity of the mutant endomannosidase were not affected. Thus, endomannosidase phosphorylation is occurring in luminal sequences. Modification was prevented when endomannosidase was synthesized using reticulocyte lysates in the presence of canine microsomes. Treatment of cells with brefeldin A blocked the posttranslational modification of endomannosidase, suggesting that phosphorylation is occurring in the Golgi apparatus, the residence of endomannosidase.

Association of variants in MANEA with cocaine-related behaviors

CONTEXT

Cocaine dependence (CD) and related behaviors are highly heritable, but no genetic association has been consistently demonstrated. A recent genome-wide study of drug dependence identified an association between cocaine-induced paranoia (CIP) and a single-nucleotide polymorphism (SNP) in the alpha-endomannosidase (MANEA) locus in a family-based sample of European Americans and African Americans.

OBJECTIVE

To conduct a comprehensive genetic association study of the MANEA locus with CD and CIP.

DESIGN

Genome-wide association study.

SETTING

Four university hospitals.

PARTICIPANTS

A total of 3992 individuals from 2 family-based and 2 case-control samples.

INTERVENTION

Participants were classified as having CD or CIP or as a control using the Semi-Structured Assessment for Drug Dependence and Alcoholism. They were genotyped for 11 SNPs spanning MANEA and its surrounding region.

MAIN OUTCOME MEASURE

Association of CD and CIP with individual SNPs and haplotypes.

RESULTS

Cocaine-induced paranoia was associated with 6 SNPs in the European American families and 9 SNPs in the African American families. The strongest evidence in the total sample of families was observed in 3 markers located in the promoter and 3' untranslated regions (P < .001). The association of MANEA SNPs with CD in both family samples was much weaker. In the African American case-control sample, multiple markers were significantly associated with CIP and CD; CIP and CD were also significantly associated with a 2-SNP haplotype in the European American case-control sample. The A allele of the 3' untranslated region SNP rs9387522 was associated with increased risk of CIP in all 4 data sets.

CONCLUSIONS

Our findings suggest that CD and associated behaviors may involve biological pathways not typically thought to be associated with brain metabolism.

Free N-linked oligosaccharide chains: formation and degradation

There is growing evidence that N-linked glycans play pivotal roles in protein folding and intra- and/or intercellular trafficking of N-glycosylated proteins. It has been shown that during the N-glycosylation of proteins, significant amounts of free oligosaccharides (free OSs) are generated in the lumen of the endoplasmic reticulum (ER) by a mechanism which remains to be clarified. Free OSs are also formed in the cytosol by enzymatic deglycosylation of misfolded glycoproteins, which are subjected to destruction by a cellular system called "ER-associated degradation (ERAD)." While the precise functions of free OSs remain obscure, biochemical studies have revealed that a novel cellular process enables them to be catabolized in a specialized manner, that involves pumping free OSs in the lumen of the ER into the cytosol where further processing occurs. This process is followed by entry into the lysosomes. In this review we summarize current knowledge about the formation, processing and degradation of free OSs in eukaryotes and also discuss the potential biological significance of this pathway. Other evidence for the occurrence of free OSs in various cellular processes is also presented.

The evolution of N-glycan-dependent endoplasmic reticulum quality control factors for glycoprotein folding and degradation

Asn-linked glycans (N-glycans) play important roles in the quality control (QC) of glycoprotein folding in the endoplasmic reticulum (ER) lumen and in ER-associated degradation (ERAD) of proteins by cytosolic proteasomes. A UDP-Glc:glycoprotein glucosyltransferase glucosylates N-glycans of misfolded proteins, which are then bound and refolded by calreticulin and/or calnexin in association with a protein disulfide isomerase. Alternatively, an alpha-1,2-mannosidase (Mns1) and mannosidase-like proteins (ER degradation-enhancing alpha-mannosidase-like proteins 1, 2, and 3) are part of a process that results in the dislocation of misfolded glycoproteins into the cytosol, where proteins are degraded in the proteasome. Recently we found that numerous protists and fungi contain 0-11 sugars in their N-glycan precursors versus 14 sugars in those of animals, plants, fungi, and Dictyostelium. Our goal here was to determine what effect N-glycan precursor diversity has on N-glycan-dependent QC systems of glycoprotein folding and ERAD. N-glycan-dependent QC of folding (UDP-Glc:glycoprotein glucosyltransferase, calreticulin, and/or calnexin) was present and active in some but not all protists containing at least five mannose residues in their N-glycans and was absent in protists lacking Man. In contrast, N-glycan-dependent ERAD appeared to be absent from the majority of protists. However, Trypanosoma and Trichomonas genomes predicted ER degradation-enhancing alpha-mannosidase-like protein and Mns1 orthologs, respectively, each of which had alpha-mannosidase activity in vitro. Phylogenetic analyses suggested that the diversity of N-glycan-dependent QC of glycoprotein folding (and possibly that of ERAD) was best explained by secondary loss. We conclude that N-glycan precursor length has profound effects on N-glycan-dependent QC of glycoprotein folding and ERAD.

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.

Intact α-1,2-endomannosidase is a typical type II membrane protein

Rat endomannosidase is a glycosidic enzyme that catalyzes the cleavage of di-, tri-, or tetrasaccharides (Glc(1-3)Man), from N-glycosylation intermediates with terminal glucose residues. To date it is the only characterized member of this class of endomannosidic enzymes. Although this protein has been demonstrated to localize to the Golgi lumenal membrane, the mechanism by which this occurs has not yet been determined. Using the rat endomannosidase sequence, we identified three homologs, one each in the human, mouse, and rat genomes. Alignment of the four encoded protein sequences demonstrated that the newly identified sequences are highly conserved but differed significantly at the N-terminus from the previously reported protein. In this study we have cloned two novel endomannosidase sequences from rat and human cDNA libraries, but were unable to amplify the open reading frame of the previously reported rat sequence. Analysis of the rat genome confirmed that the 59- and 39-termini of the previously reported sequence were in fact located on different chromosomes. This, in combination with our inability to amplify the previously reported sequence, indicated that the N-terminus of the rat endomannosidase sequence previously published was likely in error (a cloning artifact), and that the sequences reported in the current study encode the intact proteins. Furthermore, unlike the previous sequence, the three ORFs identified in this study encode proteins containing a single N-terminal transmembrane domain. Here we demonstrate that this region is responsible for Golgi localization and in doing so confirm that endomannosidase is a type II membrane protein, like the majority of other secretory pathway glycosylation enzymes.

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.

Interferon-gamma increases the expression of glycosylated CD95 in B-leukemic cells: an inducible model to study the role of glycosylation in CD95-signalling and trafficking

B-lineage acute leukaemia cells are generally resistant to CD95-mediated apoptosis. In this report, the CD95-resistant B-leukaemia lines SEM, RS4;11, and REH were used to investigate the mechanisms of resistance to CD95-signalling. We found that interferon-gamma (IFN-gamma) treatment increased the presence of high molecular weight forms of CD95 in these cells as judged by Western analysis, and treatment of protein extracts with Peptide: N -glycosidase F indicated that the majority of high molecular weight forms were due to N-linked glycosylation. Treatment of whole cells with neuraminidase from Vibrio cholerae substantially reduced the relative molecular mass of CD95 observed after IFN-gamma treatment and partially sensitized the three leukaemia lines to CD95-mediated death. To further characterize the different steps of oligosaccharide processing that may regulate CD95 signalling, the leukaemic cells were treated with IFN-gamma and the glycosidase inhibitors castanospermine, 1-deoxymannojirimycin (DMM), and swainsonine. Treatment with DMM, a mannosidase inhibitor, efficiently reduced the appearance of high molecular weight forms of CD95 after IFN-gamma treatment, and sensitized SEM and REH cells to CD95-mediated death. However, the IFN-gamma-induced increases of CD95 on the cell surface were not altered by treatment with any of the glycosidase inhibitors, suggesting that the generation of complex oligosaccharide structures is not required for trafficking of CD95, but may instead be used as a mechanism of partially blocking CD95 signalling in these cells. In conclusion, IFN-gamma treatment of the B-lineage leukaemia lines provides a novel, inducible system for the further characterization of post-translational modifications involved in modulating sensitivity to CD95-signalling.

Golgi apparatus immunolocalization of endomannosidase suggests post-endoplasmic reticulum glucose trimming: implications for quality control

Trimming of N-linked oligosaccharides by endoplasmic reticulum (ER) glucosidase II is implicated in quality control of protein folding. An alternate glucosidase II-independent deglucosylation pathway exists, in which endo-alpha-mannosidase cleaves internally the glucose-substituted mannose residue of oligosaccharides. By immunogold labeling, we detected most endomannosidase in cis/medial Golgi cisternae (83.8% of immunogold labeling) and less in the intermediate compartment (15.1%), but none in the trans-Golgi apparatus and ER, including its transitional elements. This dual localization became more pronounced under 15 degrees C conditions indicative of two endomannosidase locations. Under experimental conditions when the intermediate compartment marker p58 was retained in peripheral sites, endomannosidase was redistributed to the Golgi apparatus. Double immunogold labeling established a mutually exclusive distribution of endomannosidase and glucosidase II, whereas calreticulin was observed in endomannosidase-reactive sites (17.3% in intermediate compartment, 5.7% in Golgi apparatus) in addition to the ER (77%). Our results demonstrate that glucose trimming of N-linked oligosaccharides is not limited to the ER and that protein deglucosylation by endomannosidase in the Golgi apparatus and intermediate compartment additionally ensures that processing to mature oligosaccharides can continue. Thus, endomannosidase localization suggests that a quality control of N-glycosylation exists in the Golgi apparatus.

Use of recombinant endomannosidase for evaluation of the processing of N-linked oligosaccharides of glycoproteins and their oligosaccharide-lipid precursors

Although glucose residues in a triglucosyl sequence are essential for the N-glycosylation of proteins and in their monoglucosyl form have been implicated in lectin-like interactions with chaperones, their removal is required for the formation of mature carbohydrate units and represents the initial steps in the glycoprotein processing sequence. In order to provide a probe for the glucosylation state of newly synthesized glycoproteins obtained from normal or altered cells, we have evaluated the usefulness of recombinant endo-alpha-mannosidase employing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to monitor the change in molecular mass brought about by the release of glucosylated mannose (Glc(1-3)Man). With this approach the presence of two triglucosylated-N-linked oligosaccharides in vesicular stomatis virus (VSV) G protein formed by castanospermine-treated CHO cells or the glucosidase I deficient Lec23 mutant could be clearly demonstrated and an even more pronounced change in migration was observed upon endomannosidase treatment of their more heavily N-glycosylated lysosomal membrane glycoproteins. Furthermore, the G protein of the temperature sensitive VSV ts045 mutant was found to be sensitive to endomannosidase, resulting in a change in electrophoretic mobility consistent with the presence of mono-glucosylated-N-linked oligosaccharides. The finding that endomannosidase also acts effectively on oligosaccharide lipids, as assessed by SDS-PAGE or thin layer chromatography, indicated that it would be a valuable tool in assessing the glucosylation state of these biosynthetic intermediates in normal cells as well as in mutants or altered metabolic states, even if the polymannose portion is truncated. Endomannosidase can also be used to determine the glucosylation state of the polymannose oligosaccharides released during glycoprotein quality control and when used together with endo-beta-N- acetylglucosaminidase H can distinguish between those terminating in a single N-acetylglucosamine or in a di-N-acetylchitobiose sequence.

N-Glycan processing by a lepidopteran insect alpha1,2-mannosidase

Protein glycosylation pathways are relatively poorly characterized in insect cells. As part of an overall effort to address this problem, we previously isolated a cDNA from Sf9 cells that encodes an insect alpha1,2-mannosidase (SfManI) which requires calcium and is inhibited by 1-deoxymannojirimycin. In the present study, we have characterized the substrate specificity of SfManI. A recombinant baculovirus was used to express a GST-tagged secreted form of SfManI which was purified from the medium using an immobilized glutathione column. The purified SfManI was then incubated with oligosaccharide substrates and the resulting products were analyzed by HPLC. These analyses showed that SfManI rapidly converts Man(9)GlcNAc(2)to Man(6)Glc-NAc(2)isomer C, then more slowly converts Man(6)GlcNAc(2)isomer C to Man(5)GlcNAc(2). The slow step in the processing of Man(9)GlcNAc(2)to Man(5)GlcNAc(2)by SfManI is removal of the alpha1,2-linked mannose on the middle arm of Man(9)GlcNAc(2). In this respect, SfManI is similar to mammalian alpha1,2-mannosidases IA and IB. However, additional HPLC and(1)H-NMR analyses demonstrated that SfManI converts Man(9)GlcNAc(2)to Man(5)GlcNAc(2)primarily through Man(7)GlcNAc(2)isomer C, the archetypal Man(9)GlcNAc(2)missing the lower arm alpha1,2-linked mannose residues. In this respect, SfManI differs from mammalian alpha1,2-mannosidases IA and IB, and is the first alpha1,2-mannosidase directly shown to produce Man(7)GlcNAc(2)isomer C as a major processing intermediate.

Processing of viral envelope glycoprotein by the endomannosidase pathway: evaluation of host cell specificity

Endo-alpha-D-mannosidase is an enzyme involved in N-linked oligosaccharide processing which through its capacity to cleave the internal linkage between the glucose-substituted mannose and the remainder of the polymannose carbohydrate unit can provide an alternate pathway for achieving deglucosylation and thereby make possible the continued formation of complex oligosaccharides during a glucosidase blockade. In view of the important role which has been attributed to glucose on nascent glycoproteins as a regulator of a number of biological events, we chose to further define the in vivo action of endomannosidase by focusing on the well characterized VSV envelope glycoprotein (G protein) which can be formed by the large array of cell lines susceptible to infection by this pathogen. Through an assessment of the extent to which the G protein was converted to an endo-beta-N-acetylglucosaminidase (endo H)-resistant form during a castanospermine imposed glucosidase blockade, we found that utilization of the endomannosidase-mediated deglucosylation route was clearly host cell specific, ranging from greater than 90% in HepG2 and PtK1 cells to complete absence in CHO, MDCK, and MDBK cells, with intermediate values in BHK, BW5147.3, LLC-PK1, BRL, and NRK cell lines. In some of the latter group the electrophoretic pattern after endo H treatment suggested that only one of the two N-linked oligosaccharides of the G protein was processed by endomannosidase. In the presence of the specific endomannosidase inhibitor, Glcalpha1-->3(1-deoxy)mannojirimycin, the conversion of the G protein into an endo H-resistant form was completely arrested. While the lack of G protein processing by CHO cells was consistent with the absence of in vitro measured endomannosidase activity in this cell line, the failure of MDBK and MDCK cells to convert the G protein into an endo H-resistant form was surprising since these cell lines have substantial levels of the enzyme. Similarly, we observed that influenza virus hemagglutinin was not processed in castanospermine-treated MDCK cells. Our findings suggest that studies which rely on glucosidase inhibition to explore the function of glucose in controlling such critical biological phenomena as intracellular movement or quality control should be carried out in cell lines in which the glycoprotein under study is not a substrate for endomannosidase action.