Vesicular-integral membrane protein, VIP36, recognizes high-mannose type glycans containing alpha1-->2 mannosyl residues in MDCK cells

Three novel L-type lectins from obscure puffer Takifugu obscurus promote antimicrobial immune response

L-type lectins (LTLs) have leguminous lectin domains that bind to high-mannose-type oligosaccharides. LTLs are involved in glycoprotein secretory pathways and associated with many immune responses. In the present research, three LTL homologs from obscure puffer Takifugu obscurus, designated as ToVIP36-1, ToVIP36-2, and ToVIP36-3, were first cloned and identified. The open reading frames of ToVIP36-1, ToVIP36-2, and ToVIP36-3 were 1068, 1002, and 1086 bp in length, respectively, and encode polypeptides with 355, 333, and 361 amino acids, respectively. Key conserved residues and functional domains, including lectin_leg-like domain (LTLD), transmembrane region, and C-terminal trafficking signal KRFY, were identified in all ToVIP36s. Quantitative real-time PCR analysis showed that the three ToVIP36s were widely expressed in six examined tissues and had relatively high expression levels in the liver and intestine. The expression levels of ToVIP36s were remarkably altered in the liver and kidney after induction by Vibrio harveyi and Staphylococcus aureus. Subsequently, the recombinant LTLDs of ToVIP36s (rToVIP36-LTLDs) were prepared by prokaryotic expression. Three rToVIP36-LTLD proteins agglutinated with S. aureus, V. harveyi, Vibrio parahaemolyticus, and Aeromonas hydrophila in a calcium-dependent manner. In the absence of calcium, rToVIP36-LTLD proteins bound to the bacteria by binding to lipopolysaccharides, peptidoglycans, d-mannose, and d-galactose and inhibited the growth of S. aureus and V. harveyi. Our results indicated that ToVIP36s function as pattern-recognition receptors in T. obscurus immunity, providing insights into the role of LTLs in the antibacterial immunity of fishes.

Identification of Protein Changes in the Urine of Hypothyroid Patients Treated with Thyroxine Using Proteomics Approach

The thyroid gland and thyroid hormones control a multitude of homeostatic functions including maintenance of fluid and electrolyte balance and normal functioning of the kidneys. Thyroid dysfunction alters the sytemic hemodynamic and metabolic balance, thereby affecting the kidney. In this study, we aimed to identify and characterize the urinary proteome of the patients with hypothyroidism. An untargeted proteomic approach with network analysis was used to identify changes in total urinary proteome in patients with newly diagnosed overt hypothyroidism. Urine samples were collected from nine age-matched patients' before and after l-thyroxine treatment. Differences in the abundance of urinary proteins between hypothyroid and euthyroid states were determined using a two-dimensional difference in gel electrophoresis (2D-DIGE) coupled to matrix-assisted laser desorption and ionization time-of-flight (MALDI TOF) mass spectrometry. Alterations in the abundance of urinary proteins, analyzed by Progenesis software, revealed statistically significant differential abundance in a total of 49 spots corresponding to 42 proteins, 28 up and 14 down (≥1.5-fold change, analysis of variance (ANOVA), p ≤ 0.05). The proteins identified in the study are known to regulate processes related to transport, acute phase response, oxidative stress, generation of reactive oxygen species, cellular proliferation, and endocytosis. Bioinformatic analysis using Ingenuity Pathway Analysis (IPA) identified dysregulation of pathways related to amino acid metabolism, molecular transport, and small-molecule biochemistry and involved the MAPK kinase, vascular endothelial growth factor (VEGF), PI3 kinase/Akt, protein kinase C (PKC), signaling pathways. The identified proteins were involved in the regulation of thyroglobulin (Tg) and thyrotropin (TSH) metabolism. Alterations in their levels indicate the presence of a compensatory mechanism aimed at increasing the regulation of Tg in the hypothyroid state.

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.

One-Step Enrichment of Intact Glycopeptides From Glycoengineered Chinese Hamster Ovary Cells

Recently, the glycoproteomic analysis of intact glycopeptides has emerged as an effective approach to decipher the glycan modifications of glycoproteins at the site-specific level. A rapid method to enrich intact glycopeptides is essential for the analysis of glycoproteins, especially for biopharmaceutical proteins. In this study, we established a one-step method for the rapid capture of intact glycopeptides for analysis by mass spectrometry. Compared to the conventional sequential enrichment method, the one-step intact glycopeptide enrichment method reduced the sample preparation time and improved the detection of intact glycopeptides with long sequences or non-polar amino acids. Moreover, an increased number of glycosite-containing peptides was identified by the one-step method compared with the sequential method. When we applied this method to the glycoproteomic analysis of glycoengineered Chinese hamster ovary (CHO)-K1 cells with α1,6-fucosyltransferase (FUT8) knockout, the results showed that the knockout of FUT8 altered the overall glycosylation profile of CHO-K1 cells with the elimination of core fucosylation and together with increases in high-mannose and sialylated N-glycans. Interestingly, the knockout of the FUT8 also appeared to regulate the expression of glycoproteins involved in several functions and pathways in CHO-K1 cells, such as the down-regulation of an intracellular lectin LMAN2 showing cellular adaptation to the alterations in FUT8 knockout cells. These findings indicate that the site-specific characterization of glycoproteins from glycoengineered CHO-K1 cells can be achieved rapidly using the one-step intact glycopeptide enrichment method, which could provide insights for bio-analysts and biotechnologists to better tailor therapeutic drugs.

Recombinant Expression and Purification of Animal Intracellular L-Type Lectins

Animal leguminous-type (L-type) lectins, including ERGIC-53 and VIP36 are responsible for intracellular transport and quality control of N-linked glycoproteins in the early secretory pathway. These lectins possess the carbohydrate recognition domain (CRD), which recognizes high-mannose-type glycans in a Ca²⁺-dependent manner. Here we describe the procedures involved in bacterial overproduction and purification of the CRDs of the animal L-type lectins.

Crystal structure of the legume lectin-like domain of an ERGIC-53-like protein from Entamoeba histolytica

ERGIC-53-like proteins are type I membrane proteins that belong to the class of intracellular cargo receptors and are known to be indispensable for the intracellular transport of glycoproteins. They are implicated in transporting glycoproteins between the endoplasmic reticulum and the Golgi body. The crystal structure of the legume lectin-like domain of an ERGIC-53-like protein from Entamoeba histolytica has been determined at 2.4 Å resolution. Although the overall structure of the domain resembles those of its mammalian and yeast orthologs (ERGIC-53 and Emp46, respectively), there are significant changes in the carbohydrate-binding site. A sequence-based search revealed the presence of several homologs of ERGIC-53 in different species of Entamoeba. This is the first report of the structural characterization of a member of this class of proteins from a protozoan and serves to further knowledge and understanding regarding the species-specific differences.

Hydroxymethylation of protein-encoding genes in the testes involved in precocious puberty of Eriocheir sinensis

To investigate the possible effects of epigenetic modification of testis protein-encoding genes on precocious puberty of Eriocheir sinensis, we used MeDIP-seq and hMeDIP-seq techniques to compare the methylation and hydroxymethylation of 263 E. sinensis protein-encoding genes known in the NCBI database in precocious testes with those in normally developing testes. The results showed that total methylation level of those genes was lower than their total hydroxymethylation level. Moreover, their total hydroxymethylation level in precocious testes was significantly lower than that in normal testes. In addition, no methylated genes had significant difference, but there were 37 different hydroxymethylated genes (DhMGs) in the precocious testes compared to the normal ones. Among the DhMGs, 21 were hypo-hydroxymethylated and 16 were hyper-hydroxymethylated. The hypo-hydroxymethylated DhMGs were associated with development, cell structural and cytoskeletal proteins, and response to stress. However, the hyper-hydroxymethylated DhMGs included immune-related genes, free radicals removement-related genes, protein folding-related genes, and so on. In addition, some DhMGs were hyper-hydroxymethylated while their homologous DhMGs were hypo-hydroxymethylated. The results of a qRT-PCR assay showed that the expression levels of 5 DhMGs randomly chosen presented a positive correlation with their hydroxymethylation levels. It can be seen that hydroxymethylation might regulate the expression of genes and be involved in precocious puberty to cause high mortality of crabs. Therefore, the hydroxymethylation level of DhMGs may be used as an evaluation index with economically meaningful growth and breeding traits.

YesU from Bacillus subtilis preferentially binds fucosylated glycans

The interaction of carbohydrate-binding proteins (CBPs) with their corresponding glycan ligands is challenging to study both experimentally and computationally. This is in part due to their low binding affinity, high flexibility, and the lack of a linear sequence in carbohydrates, as exists in nucleic acids and proteins. We recently described a function-prediction technique called SPOT-Struc that identifies CBPs by global structural alignment and binding-affinity prediction. Here we experimentally determined the carbohydrate specificity and binding affinity of YesU (RCSB PDB ID: 1oq1), an uncharacterized protein from Bacillus subtilis that SPOT-Struc predicted would bind high mannose-type glycans. Glycan array analyses however revealed glycan binding patterns similar to those exhibited by fucose (Fuc)-binding lectins, with SPR analysis revealing high affinity binding to Lewis^x and lacto-N-fucopentaose III. Structure based alignment of YesU revealed high similarity to the legume lectins UEA-I and GS-IV, and docking of Lewis^x into YesU revealed a complex structure model with predicted binding affinity of −4.3 kcal/mol. Moreover the adherence of B. subtilis to intestinal cells was significantly inhibited by Le^x and Le^y but by not non-fucosylated glycans, suggesting the interaction of YesU to fucosylated glycans may be involved in the adhesion of B. subtilis to the gastrointestinal tract of mammals.

Characterisation and functional analysis of an L-type lectin from the swimming crab Portunus trituberculatus

L-type lectins are involved in glycoprotein secretion and are associated with immune responses. Herein, an L-type lectin was identified in swimming crab (Portunus trituberculatus). The 1347 bp PtLTL cDNA includes a 26 bp 5'-untranslated region (UTR), a 547 bp 3'-UTR with a poly(A) tail, and a 774 bp open reading frame encoding a 257 amino acid protein with a putative 21 residue signalling peptide. The protein includes an L-type lectin carbohydrate recognition domain containing four conserved cysteines. The 714 bp cDNA fragment encoding the mature peptide of PtLTL1 was recombined into pET-21a (+) with a C-terminally hexa-histidine tag fused in-frame and expressed in Escherichia coli Origami (DE3). Recombinant PtLTL1 caused agglutination of all three Gram-positive and Gram-negative bacterial strains tested. In addition, erythrocyte agglutination and LPS-binding activity were observed. PtLTL1 mRNA transcripts were most abundant in P. trituberculatus hepatopancreas and hemocytes, and expression was up-regulated in hemocytes challenged with Vibrio alginolyticus, suggesting PtLTL functions in the immune response against bacterial pathogens.

Cloning and characterization of two different L-type lectin genes from the Chinese mitten crab Eriocheir sinensis

L-type lectins contain a leguminous lectin domain and bind to high-mannose type oligosaccharides. In the secretory pathway, L-type lectins play crucial functions in the trafficking, sorting, and targeting of maturing glycoproteins. This study identified two novel L-type lectins, designated as EsERGIC-53 and EsVIP36, from the Chinese mitten crab Eriocheir sinensis. The complete nucleotide sequence of ERGIC-53 cDNA was 1955 bp, containing a 1506 bp open reading frame (ORF) encoding a putative protein of 501 deduced amino acids. The full-length cDNA of VIP36 was 3474 bp with a 984 bp ORF encoding a 327-amino acid peptide. The deduced ERGIC-53 and VIP36 proteins contained a putative signal peptide and an L-type lectin-like domain. Phylogenetic analysis showed that ERGIC-53 and VIP36 belonged to different clades of L-type lectin family. Reverse transcription PCR showed that ERGIC-53 and VIP36 were expressed in all tested tissues. Quantitative real-time RT-PCR analysis revealed that ERGIC-53 and VIP36 transcripts in hepatopancreas were significantly induced at various time points after infection with lipopolysaccharide (LPS), peptidoglycan (PGN), Staphylococcus aureus, Vibrio parahaemolyticus, and Aeromonas hydrophila. A bacterium-binding experiment showed that both ERGIC-53 and VIP36 could bind to different microbes. Sugar binding assay revealed that these lectins could also bind to the glycoconjugates of bacteria surface, such as LPS, PGN, d-Mannose, and N-Acetyl-d-mannosamine. Moreover, these two L-type lectins agglutinated bacteria in a calcium-dependent manner, and both exerted the ability of facilitating the clearance of injected bacteria V. parahaemolyticus in the crab. Our results suggested that ERGIC-53 and VIP36 functioned as pattern recognition receptors in the immune system of E. sinensis.

Parallel quantification of lectin-glycan interaction using ultrafiltration

Using ultrafiltration membrane, a simple method for screening protein-ligand interaction was developed. The procedure comprises three steps: mixing ligand with protein, ultrafiltration of the solution, and quantification of unbound ligands by HPLC. By conducting analysis with variable protein concentrations, affinity constants were easily obtained. Multiple ligands can be analyzed simultaneously as a mixture, when concentration of ligands was controlled. Feasibility of this method for lectin-glycan interaction analysis was examined using fluorescently labeled high-mannose-type glycans and recombinant intracellular lectins or endo-α-mannosidase mutants. Estimated Ka values of malectin and VIP36 were in good agreement indeed with those evaluated by conventional methods such as isothermal titration calorimetry (ITC) or frontal affinity chromatography (FAC). Finally, several mutants of endo-α-mannosidase were produced and their affinities to monoglucosylated glycans were evaluated.

Molecular characterization of three L-type lectin genes from channel catfish, Ictalurus punctatus and their responses to Edwardsiella ictaluri challenge

L-type lectins have a leguminous lectin domain and can bind to high-mannose type oligosaccharides. In the secretory pathway, L-type lectins play crucial roles in selective protein trafficking, sorting and targeting. Three L-type lectins were cloned in the channel catfish, Ictalurus punctatus, the 53 kDa endoplasmic reticulum ER-Golgi intermediate compartment protein (ERGIC-53), the vesicular integral protein of 36 kDa (VIP36) and VIP36-like. Phylogenetic analysis indicated that the catfish genes are orthologous to their counterparts in other species. Southern blot analysis demonstrated that all three L-type lectin genes are likely single-copy genes in the catfish genome. Analysis of expression in healthy tissues using quantitative real time RT-PCR indicated that all three genes are expressed widely in all tested tissues, but with strong tissue preference of expression: ERGIC-53 was found to be abundantly expressed in the liver, VIP36 was found to be abundantly expressed in the head-kidney, whereas VIP36-like was found to be abundantly expressed in the brain. Upon infection with Edwardsiella ictaluri, expressions of the three genes all had significant up-regulation in the head-kidney, but had distinct expression patterns: ERGIC-53 was gradually induced with the highest expression 7 days after challenge in the head-kidney, but was down-regulated in the liver, spleen, and brain. VIP36 was highly induced in the head-kidney, and 3 days after challenge in the brain, but was not up-regulated in any other tissues or timepoints after challenge. Expression levels of the catfish VIP36-like gene appeared to also respond to infection, albeit with differing patterns among the tested tissues. Taken together, our results indicate that all three L-type lectin genes may be involved in the immune responses of catfish after infection with E. ictaluri.

VIP36 protein is a target of ectodomain shedding and regulates phagocytosis in macrophage Raw 264.7 cells

Ectodomain shedding is a posttranslational modification mechanism, which liberates extracellular domains of membrane proteins through juxtamembrane processing executed mainly by the ADAM (a disintegrin and metalloprotease) family of metalloproteases. Shedding is a unique and effective mechanism for inducing multifaceted effects through the soluble extracellular domains released and/or the remaining membrane-bound portions; however, the physiological functions of shedding are not yet fully understood. In this study, we performed unbiased proteomic screening for shedding targets in a lipopolysaccharide (LPS)-stimulated macrophage cell line to elucidate a new immunological function of shedding. We identified VIP36 (36-kDa vesicular integral membrane protein), a lectin domain-containing transmembrane protein postulated as a cargo receptor for Golgi-to-endoplasmic reticulum transport, as a new target for shedding and found that the shedding of VIP36 occurs mainly on the cell surface. In addition, we demonstrate that the amount of VIP36 precisely regulates phagocytosis in macrophages and that the shedding of VIP36 is required for this regulation. These results substantially expand our knowledge of the immunological and cell biological functions of both the shedding process and VIP36 itself.

Structural basis of carbohydrate recognition by calreticulin

The calnexin cycle is a process by which glycosylated proteins are subjected to folding cycles in the endoplasmic reticulum lumen via binding to the membrane protein calnexin (CNX) or to its soluble homolog calreticulin (CRT). CNX and CRT specifically recognize monoglucosylated Glc(1)Man(9)GlcNAc(2) glycans, but the structural determinants underlying this specificity are unknown. Here, we report a 1.95-Å crystal structure of the CRT lectin domain in complex with the tetrasaccharide α-Glc-(1→3)-α-Man-(1→2)-α-Man-(1→2)-Man. The tetrasaccharide binds to a long channel on CRT formed by a concave β-sheet. All four sugar moieties are engaged in the protein binding via an extensive network of hydrogen bonds and hydrophobic contacts. The structure explains the requirement for glucose at the nonreducing end of the carbohydrate; the oxygen O(2) of glucose perfectly fits to a pocket formed by CRT side chains while forming direct hydrogen bonds with the carbonyl of Gly(124) and the side chain of Lys(111). The structure also explains a requirement for the Cys(105)-Cys(137) disulfide bond in CRT/CNX for efficient carbohydrate binding. The Cys(105)-Cys(137) disulfide bond is involved in intimate contacts with the third and fourth sugar moieties of the Glc(1)Man(3) tetrasaccharide. Finally, the structure rationalizes previous mutagenesis of CRT and lays a structural groundwork for future studies of the role of CNX/CRT in diverse biological pathways.

Role of the lectin VIP36 in post-ER quality control of human alpha1-antitrypsin

The leguminous-type (L-type) lectin VIP36 localizes to the Golgi apparatus and cycles early in the secretory pathway. In vitro, VIP36 binds high-mannose glycans with a pH optimum of 6.5, a value similar to the luminal pH of the Golgi apparatus. Although the sugar-binding properties of VIP36 in vitro have been characterized in detail, the function of VIP36 in the intact cell remains unclear as no convincing glycoprotein cargo has been identified. Here, we used yellow fluorescent protein (YFP) fragment complementation to identify luminal interaction partners of VIP36. By screening a human liver cDNA library, we identified the glycoprotein alpha1-antitrypsin (alpha1-AT) as a cargo of VIP36. The VIP36/alpha1-AT complex localized to Golgi and endoplasmic reticulum (ER). In the living cell, VIP36 bound exclusively to the high-mannose form of alpha1-AT. The binding was increased when complex glycosylation was prevented by kifunensine and abolished when the glycosylation sites of alpha1-AT were inactivated by mutagenesis. Silencing VIP36 accelerated alpha1-AT transport, arguing against a role of VIP36 in anterograde traffic. The complex formed by VIP36 and alpha1-AT in the Golgi recycled back to the ER. The combined data are most consistent with a function of VIP36 in post-ER quality control of alpha1-AT.

Stimulation of N-Linked Glycosylation and Lipid-Linked Oligosaccharide Synthesis by Stress Responses in Metazoan Cells

Endoplasmic reticulum (ER) stress responses comprising the unfolded protein response (UPR) are activated by conditions that disrupt folding and assembly of proteins inside the ER lumenal compartment. Conditions known to be proximal triggers of the UPR include saturation of chaperones with misfolded protein, redox imbalance, disruption of Ca2+ levels, interference with N-linked glycosylation, and failure to dispose of terminally misfolded proteins. Potentially, ER stress responses can reprogram cells to correct all of these problems and thereby restore ER function to normal. This article will review literature on stimulation of N-linked glycosylation by ER stress responses, focusing on metazoan systems. The mechanisms involved will be contrasted with those mediating stimulation of N-linked glycosylation by cytoplasmic stress responses. This information will interest readers who study the biological roles of stress responses, the functions of N-linked glycans, and potential strategies for treatment of genetic disorders of N-linked glycosylation.

Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: different roles of the N-glycans of their beta subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic alpha-subunit and a glycosylated beta-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the beta-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective alpha subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled beta-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the beta-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase beta1-subunit is the major beta-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase beta1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the beta1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the alpha-subunit to the cytoskeleton. This intercellular beta1-beta1 interaction is also important for formation of cell-cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase beta2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the beta2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase beta-subunit determine apical sorting of this subunit.

Structural basis for recognition of high mannose type glycoproteins by mammalian transport lectin VIP36

VIP36 functions as a transport lectin for trafficking certain high mannose type glycoproteins in the secretory pathway. Here we report the crystal structure of VIP36 exoplasmic/luminal domain comprising a carbohydrate recognition domain and a stalk domain. The structures of VIP36 in complex with Ca(2+) and mannosyl ligands are also described. The carbohydrate recognition domain is composed of a 17-stranded antiparallel beta-sandwich and binds one Ca(2+) adjoining the carbohydrate-binding site. The structure reveals that a coordinated Ca(2+) ion orients the side chains of Asp(131), Asn(166), and His(190) for carbohydrate binding. This result explains the Ca(2+)-dependent carbohydrate binding of this protein. The Man-alpha-1,2-Man-alpha-1,2-Man, which corresponds to the D1 arm of high mannose type glycan, is recognized by eight residues through extensive hydrogen bonds. The complex structures reveal the structural basis for high mannose type glycoprotein recognition by VIP36 in a Ca(2+)-dependent and D1 arm-specific manner.

VIPL has sugar-binding activity specific for high-mannose-type N-glycans, and glucosylation of the α1,2 mannotriosyl branch blocks its binding

VIP36-like protein (VIPL) was identified as an endoplasmic reticulum (ER) resident protein with homology to VIP36, a cargo receptor involved in the transport of glycoproteins within cells. Although VIPL is structurally similar to VIP36, VIPL is thought not to be a lectin, because its sugar-binding activity has not been detected in several experiments. Here, recombinant soluble VIPL proteins (sVIPL) were expressed in Escherichia coli, biotinylated with biotin ligase and oligomerized with R-phycoerythrin (PE)-labeled streptavidin (SA). As measured with flow cytometry, PE-labeled sVIPL-SA bound to deoxymannojirimycin (DMJ)- or kifunensine (KIF)- but not to swainsonine (SW)-treated HeLaS3 cells in the presence of calcium. A surface plasmon resonance analysis showed that the avidity of sVIPL was enhanced after it formed a complex with SA. The binding of PE-labeled sVIPL-SA was abrogated by endo beta-N-acetylglucosaminidase H treatment of the DMJ- or KIF-treated cells. Competition with several high-mannose-type N-glycans inhibited VIPL binding, and indicated that VIPL recognizes the Manalpha1-2Manalpha1-2Man sequence. Glucosylation of the outer mannose residue of this portion decreased the binding. Although the biochemical characteristics of VIPL are similar to those of VIP36, the sugar-binding activity of VIPL was stronger at neutral pH, corresponding to the pH in the lumen of the ER, than under acidic conditions.

Search for additional influenza virus to cell interactions

Sialyl oligosaccharides have long been considered to be the sole receptors for influenza virus. However, according to [1] some viruses are able to grow in sialic-free MDCK cells. Here we attempted to reveal a possible second, non-sialic receptor, hypothesizing the involvement of additional carbohydrate lectin recognition in influenza virus reception process, first of all in situations when a lectin of the host cell could recognize the viral carbohydrate ligand. We tested the presence of galactose- and sialic acid-binding lectins, as well as mannoside- and sulfo-N-acetyllactosamine-recognizing properties of MDCK and Vero cells using polyacrylamide neoglycoconjugates and antibodies. MDCK cells bind galactoside probes stronger than Vero cells, whereas Vero cells bind preferentially sialoside, mannoside and various sulfo-oligosaccharide probes. The probing of viruses with the neoglycoconjugates revealed specific 6'-HSO (3) LacNAc (but not other sulfated oligosaccharides) binding property of A and B human strains. Affinity of 6'-HSO (3) LacNAc probe was comparable with affinity of 6'-SiaLac probe but the binding was not inhibited by the sialooligosaccharide.

Structures of the carbohydrate recognition domain of Ca2+-independent cargo receptors Emp46p and Emp47p

Emp46p and Emp47p are type I membrane proteins, which cycle between the endoplasmic reticulum (ER) and the Golgi apparatus by vesicles coated with coat protein complexes I and II (COPI and COPII). They are considered to function as cargo receptors for exporting N-linked glycoproteins from the ER. We have determined crystal structures of the carbohydrate recognition domains (CRDs) of Emp46p and Emp47p of Saccharomyces cerevisiae, in the absence and presence of metal ions. Both proteins fold as a beta-sandwich, and resemble that of the mammalian ortholog, p58/ERGIC-53. However, the nature of metal binding is distinct from that of Ca(2+)-dependent p58/ERGIC-53. Interestingly, the CRD of Emp46p does not bind Ca(2+) ion but instead binds K(+) ion at the edge of a concave beta-sheet whose position is distinct from the corresponding site of the Ca(2+) ion in p58/ERGIC-53. Binding of K(+) ion to Emp46p appears essential for transport of a subset of glycoproteins because the Y131F mutant of Emp46p, which cannot bind K(+) ion fails to rescue the transport in disruptants of EMP46 and EMP47 genes. In contrast the CRD of Emp47p binds no metal ions at all. Furthermore, the CRD of Emp46p binds to glycoproteins carrying high mannosetype glycans and the is promoted by binding not the addition of Ca(2+) or K(+) ion in These results suggest that Emp46p can be regarded as a Ca(2+)-independent intracellular lectin at the ER exit sites.

Sugar-binding Properties of VIP36, an Intracellular Animal Lectin Operating as a Cargo Receptor

The vesicular integral protein of 36 kDa (VIP36) is an intracellular animal lectin that acts as a putative cargo receptor, which recycles between the Golgi and the endoplasmic reticulum. Although it is known that VIP36 interacts with glycoproteins carrying high mannose-type oligosaccharides, detailed analyses of the sugar-binding specificity that discriminates isomeric oligosaccharide structures have not yet been performed. In the present study, we have analyzed, using the frontal affinity chromatography (FAC) method, the sugar-binding properties of a recombinant carbohydrate recognition domain of VIP36 (VIP36-CRD). For this purpose, a pyridylaminated sugar library, consisting of 21 kinds of oligosaccharides, including isomeric structures, was prepared and subjected to FAC analyses. The FAC data have shown that glucosylation and trimming of the D1 mannosyl branch interfere with the binding of VIP36-CRD. VIP36-CRD exhibits a bell-shaped pH dependence of sugar binding with an optimal pH value of approximately 6.5. By inspection of the specificity and optimal pH value of the sugar binding of VIP36 and its subcellular localization, together with the organellar pH, we suggest that VIP36 binds glycoproteins that retain the intact D1 mannosyl branch in the cis-Golgi network and recycles to the endoplasmic reticulum where, due to higher pH, it releases its cargos, thereby contributing to the quality control of glycoproteins.

Recombinant addition of N-glycosylation sites to the basolateral Na,K-ATPase beta1 subunit results in its clustering in caveolae and apical sorting in HGT-1 cells

In most polarized cells, the Na,K-ATPase is localized on the basolateral plasma membrane. However, an unusual location of the Na,K-ATPase was detected in polarized HGT-1 cells (a human gastric adenocarcinoma cell line). The Na,K-ATPase alpha1 subunit was detected along with the beta2 subunit predominantly on the apical membrane, whereas the Na,K-ATPase beta1 subunit was not found in HGT-1 cells. However, when expressed in the same cell line, a yellow fluorescent protein-linked Na,K-ATPase beta1 subunit was localized exclusively to the basolateral surface and resulted in partial redistribution of the endogenous alpha1 subunit to the basolateral membrane. The human beta2 subunit has eight N-glycosylation sites, whereas the beta1 isoform has only three. Accordingly, up to five additional N-glycosylation sites homologous to the ones present in the beta2 subunit were successively introduced in the beta1 subunit by site-directed mutagenesis. The mutated beta1 subunits were detected on both apical and basolateral membranes. The fraction of a mutant beta1 subunit present on the apical membrane increased in proportion to the number of glycosylation sites inserted and reached 80% of the total surface amount for the beta1 mutant with five additional sites. Clustered distribution and co-localization with caveolin-1 was detected by confocal microscopy for the endogenous beta2 subunit and the beta1 mutant with additional glycosylation sites but not for the wild type beta1 subunit. Hence, the N-glycans linked to the beta2 subunit of the Na,K-ATPase contain apical sorting information, and the high abundance of the beta2 subunit isoform, which is rich in N-glycans, along with the absence of the beta1 subunit, is responsible for the unusual apical location of the Na,K-ATPase in HGT-1 cells.

Cloning and Characterization of a Novel Animal Lectin Expressed in the Rat Sublingual Gland

We cloned a rat gene that is expressed primarily in the sublingual gland and named the predicted 503 amino-acid protein SLAMP (sublingual acinar membrane protein). SLAMP has 63% homology with human ERGIC-53-like protein, a member of the family of animal L-type lectins. Using a cDNA probe for SLAMP mRNA and rabbit antisera against SLAMP, we examined the expression and localization of SLAMP in major rat organs and tissues. With both Northern and Western blot analyses, abundant expression of SLAMP was demonstrated predominantly in the sublingual gland, with single sizes of the mRNA and protein 1.8 kb and 50 kDa, respectively, but not in other organs or tissues, including the parotid and submandibular glands. With immunohistochemistry, SLAMP was localized to the mucous acinar cells, but not to the serous demilunes or the duct system. With immunoelectron microscopy, SLAMP was localized predominantly to regions corresponding to the ER-Golgi intermediate compartment. Besides the sublingual gland, SLAMP immunore-activity was also demonstrated in mucous cells of the minor salivary glands in oral cavity and of Brunner's glands in the duodenum. These results suggested that rat SLAMP plays a specific role in the early secretory pathway of glycoproteins in specific types of mucous cells.

Characterization of O-mannosyltransferase family in Schizosaccharomyces pombe

Protein O-glycosylation is an essential protein modification in eukaryotic cells. In Saccharomyces cerevisiae, O-mannosylation is initiated in the lumen of the endoplasmic reticulum by O-mannosyltransferase gene products (Pmt1p-7p). A search of the Schizosaccharomyces pombe genome database revealed a total of three O-glycoside mannosyltransferase homologs (ogm1+, ogm2+, and ogm4+), closely related to Saccharomyces cerevisiae PMT1, PMT2, and PMT4. Although individual ogm genes were not found to be essential, ogm1Delta and ogm4Delta mutants exhibited aberrant morphology and failed to agglutinate during mating. The phenotypes of the ogm4Delta mutant were not complemented by overexpression of ogm1+ or ogm2+, suggesting that each of the Ogm proteins does not have overlapping functions. Heterologous expression of a chitinase from S. cerevisiae in the ogm mutants revealed that O-glycosylation of chitinase had decreased in ogm1Delta cells. A GFP-tagged Fus1p from S. cerevisiae was specifically not glycosylated and accumulated in the Golgi in ogm4Delta cells. These results indicate that O-glycosylation initiated by Ogm proteins plays crucial physiological roles and can serve as a sorting determinant for protein transport of membrane glycoproteins in S. pombe.

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.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

Localization of VIP36 in the post-Golgi secretory pathway also of rat parotid acinar cells

VIP36 (36-kD vesicular integral membrane protein), originally purified from Madin-Darby canine kidney (MDCK) epithelial cells, belongs to a family of animal lectins and may act as a cargo receptor. To understand its role in secretory processes, we performed morphological analysis of the rat parotid gland. Immunoelectron microscopy provided evidence that endogenous VIP36 is localized in the trans-Golgi network, on immature granules, and on mature secretory granules in acinar cells. Double-staining immunofluorescence experiments confirmed that VIP36 and amylase co-localized in the apical regions of the acinar cells. This is the first study to demonstrate that endogenous VIP36 is involved in the post-Golgi secretory pathway, suggesting that VIP36 plays a role in trafficking and sorting of secretory and/or membrane proteins during granule formation.

Profile-based data base scanning for animal L-type lectins and characterization of VIPL, a novel VIP36-like endoplasmic reticulum protein

Consensus profiles were established to screen data bases for novel animal L-type lectins. The profiles were generated from linear sequence motifs of the human L-type lectin-like membrane proteins ERGIC-53, ERGL, and VIP36 and by optimal alignment of the entire carbohydrate recognition domain of these proteins. The search revealed numerous orthologous and homologous L-type lectin-like proteins in animals, protozoans, and yeast, as well as the sequence of a novel family member related to VIP36, named VIPL for VIP36-like. Sequence analysis suggests that VIPL is a ubiquitously expressed protein and appeared earlier in evolution than VIP36. The cDNA of VIPL was cloned and expressed in cell culture. VIPL is a high-mannose type I membrane glycoprotein with similar domain organization as VIP36. Unlike VIP36 and ERGIC-53 that are predominantly associated with postendoplasmic reticulum (ER) membranes and cycle in the early secretory pathway, VIPL is a non-cycling resident protein of the ER. Mutagenesis experiments indicate that ER retention of VIPL involves a RKR di-arginine signal. Overexpression of VIPL redistributed ERGIC-53 to the ER without affecting the cycling of the KDEL-receptor and the overall morphology of the early secretory pathway. The results suggest that VIPL may function as a regulator of ERGIC-53.

Lectin control of protein folding and sorting in the secretory pathway

Glycan moieties are essential for folding, sorting and targeting of glycoproteins through the secretory pathway to various cellular compartments. The molecular mechanisms that underlie these processes, however, are only now coming to light. Recent crystallographic and NMR studies of proteins located in the endoplasmic reticulum (ER), Golgi complex and ER-Golgi intermediate compartment have illuminated their roles in glycoprotein folding and secretion. Calnexin and calreticulin, both ER-resident proteins, have lectin domains that are crucial for their function as chaperones. The crystal structure of the carbohydrate-recognition domain of ER-Golgi intermediate compartment (ERGIC)-53 complements the biochemical and functional characterization of the protein, confirming that a lectin domain is essential for the role of this protein in sorting and transfer of glycoproteins from the ER to the Golgi complex. The lectin domains of calnexin and ERGIC-53 are structurally similar, although there is little primary sequence similarity. By contrast, sequence similarity between ERGIC-53 and vesicular integral membrane protein (VIP36), a Golgi-resident protein, leaves little doubt that a similar lectin domain is central to the transport and/or sorting functions of VIP36. The theme emerging from these studies is that carbohydrate recognition and modification are central to mediation of glycoprotein folding and secretion.

Crystal structure of the carbohydrate recognition domain of p58/ERGIC-53, a protein involved in glycoprotein export from the endoplasmic reticulum

p58/ERGIC-53 is an animal calcium-dependent lectin that cycles between the endoplasmic reticulum (ER) and the Golgi complex and appears to act as a cargo receptor for a subset of soluble glycoproteins exported from the ER. We have determined the crystal structure of the carbohydrate recognition domain (CRD) of p58, the rat homologue of human ERGIC-53, to 1.46 A resolution. The fold and ligand binding site are most similar to those of leguminous lectins. The structure also resembles that of the CRD of the ER folding chaperone calnexin and the neurexins, a family of non-lectin proteins expressed on neurons. The CRD comprises one concave and one convex beta-sheet packed into a beta-sandwich. The ligand binding site resides in a negatively charged cleft formed by conserved residues. A large surface patch of conserved residues with a putative role in protein-protein interactions and oligomerization lies on the opposite side of the ligand binding site. Together with previous functional data, the structure defines a new and expanding class of calcium-dependent animal lectins and provides a starting point for the understanding of glycoprotein sorting between the ER and the Golgi.

Involvement of VIP36 in intracellular transport and secretion of glycoproteins in polarized Madin-Darby canine kidney (MDCK) cells

VIP36, an intracellular lectin that recognizes high mannose-type glycans (Hara-Kuge, S., Ohkura, T., Seko, A., and Yamashita, K. (1999) Glycobiology 9, 833-839), was shown to localize not only to the early secretory pathway but also to the plasma membrane of Madin-Darby canine kidney (MDCK) cells. In the plasma membrane, VIP36 exhibited an apical-predominant distribution, the apical/basolateral ratio being approximately 2. Like VIP36, plasma membrane glycoproteins recognized by VIP36 were found in the apical and basolateral membranes in the ratio of approximately 2 to 1. In addition, secretory glycoproteins recognized by VIP36 were secreted approximately 2-fold more efficiently from the apical membrane than from the basolateral membrane. Thus, the apical/basolateral ratio of the transport of VIP36-recognized glycoproteins was correlated with that of VIP36 in MDCK cells. Upon overproduction of VIP36 in MDCK cells, the apical/basolateral ratios of both VIP36 and VIP36-recognized glycoproteins were changed from approximately 2 to approximately 4, and the secretion of VIP36-recognized glycoproteins was greatly stimulated. In contrast to the overproduction of VIP36, that of a mutant version of VIP36, which has no lectin activity, was of no effect on the distribution of glycoproteins to apical and basolateral membranes and inhibited the secretion of VIP36-recognized glycoproteins. Furthermore, the overproduction of VIP36 greatly stimulated the secretion of a major apical secretory glycoprotein of MDCK cells, clusterin, which was found to carry at least one high mannose-type glycan and to be recognized by VIP36. In contrast to the secretion of clusterin, that of a non-glycosylated apical-secretion protein, galectin-3, was not stimulated through the overproduction of VIP36. These results indicated that VIP36 was involved in the transport and sorting of glycoproteins carrying high mannose-type glycan(s).

Characteristics and structural requirements of apical sorting of the rat growth hormone through the O-glycosylated stalk region of intestinal sucrase-isomaltase

The apical sorting of the small intestinal membrane glycoprotein sucrase-isomaltase (SI) depends on the presence of O-linked glycans and the transmembrane domain. Here, we investigate the role of O-glycans carried by the Ser/Thr-rich stalk region of SI as an apical sorting signal and evaluate the spatial requirements for an efficient recognition of this signal. Several hybrid proteins are generated comprising the unsorted and unglycosylated protein, the rat growth hormone (rGH), fused to either the transmembrane domain of SI (GH-SI(TM)), or the transmembrane and the stalk domains (GH-SI(SR/TM)). Both constructs are randomly distributed over the apical and basolateral membranes of MDCK cells indicating that neither the transmembrane domain nor the O-glycans are sufficient per se for an apical delivery. Only when a polyglycine spacer is inserted between the stalk region of SI and the luminal part of rGH in the GH-SI(Gly/SR/TM) fusion protein does efficient apical sorting of an O-glycosylated protein as well as a time-dependent association with detergent-insoluble lipid microdomains occur. Obviously, the polyglycine spacer facilitates the accessibility of the O-glycans in GH-SI(Gly/SR/TM) to a putative sorting receptor, whereas these glycans are inadequately recognized in GH-SI(SR/TM). We conclude that the O-glycans in the stalk region of SI act as an apical sorting signal within a sorting machinery that comprises at least a carbohydrate-binding protein and fulfills specific spatial requirements provided, for example by a polyglycine spacer in the context of rGH or the P-domain within the SI enzyme complex.

Quantitative ER <--> Golgi transport kinetics and protein separation upon Golgi exit revealed by vesicular integral membrane protein 36 dynamics in live cells

To quantitatively investigate the trafficking of the transmembrane lectin VIP36 and its relation to cargo-containing transport carriers (TCs), we analyzed a C-terminal fluorescent-protein (FP) fusion, VIP36-SP-FP. When expressed at moderate levels, VIP36-SP-FP localized to the endoplasmic reticulum, Golgi apparatus, and intermediate transport structures, and colocalized with epitope-tagged VIP36. Temperature shift and pharmacological experiments indicated VIP36-SP-FP recycled in the early secretory pathway, exhibiting trafficking representative of a class of transmembrane cargo receptors, including the closely related lectin ERGIC53. VIP36-SP-FP trafficking structures comprised tubules and globular elements, which translocated in a saltatory manner. Simultaneous visualization of anterograde secretory cargo and VIP36-SP-FP indicated that the globular structures were pre-Golgi carriers, and that VIP36-SP-FP segregated from cargo within the Golgi and was not included in post-Golgi TCs. Organelle-specific bleach experiments directly measured the exchange of VIP36-SP-FP between the Golgi and endoplasmic reticulum (ER). Fitting a two-compartment model to the recovery data predicted first order rate constants of 1.22 +/- 0.44%/min for ER --> Golgi, and 7.68 +/- 1.94%/min for Golgi --> ER transport, revealing a half-time of 113 +/- 70 min for leaving the ER and 1.67 +/- 0.45 min for leaving the Golgi, and accounting for the measured steady-state distribution of VIP36-SP-FP (13% Golgi/87% ER). Perturbing transport with AlF(4)(-) treatment altered VIP36-SP-GFP distribution and changed the rate constants. The parameters of the model suggest that relatively small differences in the first order rate constants, perhaps manifested in subtle differences in the tendency to enter distinct TCs, result in large differences in the steady-state localization of secretory components.

Carbohydrate recognition site of interleukin-2 in relation to cell proliferation

Interleukin-2 (IL-2) is a cytokine with important roles in the immune system. IL-2 initially binds a high mannose-type glycan and a specific peptide sequence of the IL-2 receptor alpha-subunit and sequentially forms a high affinity complex of IL-2.IL-2 receptor alpha-, beta-, and gamma-subunits. This formation induces cellular signaling and cell proliferation (Fukushima, K., and Yamashita, K. (2001) J. Biol. Chem. 276, 7351-7356). To determine the carbohydrate-binding site of IL-2, we prepared wild-type and point-mutated (35)S-IL-2 by an in vitro transcription and translation method. We found that wild-type (35)S-IL-2 tends to form a dimer spontaneously, and the dimeric form has both carbohydrate recognition activity and cell proliferation activity. Moreover, substitution of Asn-26 in IL-2 with Gln or Asp conserved the dimeric form and affected the carbohydrate recognition activities in correspondence with the cell proliferation activities, suggesting that Asn-26 in IL-2 is involved in the carbohydrate recognition site. These results suggest that the carbohydrate recognition of IL-2 dimer triggers formation of high affinity complex (IL-2.IL-2Ralpha, -beta, -gamma)(2), and the hetero-octamer stimulates IL-2-dependent T-cell proliferation by intensifying cellular signaling.

Galectins structure and function--a synopsis

The 20 or so galectins expected to be found in man, and their many possible functional effects promise a rich and fruitful research field in the future. At present, the biomedically most promising areas for use of galectins or their ligands are in inflammation, immunity, and cancer. Many good stories can be formulated, but the field lacks the cohesion of knowing basic galectin function. The only basic common denominators among galectins are beta-galactoside binding, and the unusual combination of intra- and extracellular expression with non-classical secretion in between. Maybe that is all there is, and nature has used these properties for multiple, otherwise unrelated functions. Then again, maybe there is some deeper common function that has so far been overlooked. If it exists, this probably lies somewhere in the detailed integration of galectin activity in the complexities of cell physiology.

Intracellular functions of N-linked glycans

N-linked oligosaccharides arise when blocks of 14 sugars are added cotranslationally to newly synthesized polypeptides in the endoplasmic reticulum (ER). These glycans are then subjected to extensive modification as the glycoproteins mature and move through the ER via the Golgi complex to their final destinations inside and outside the cell. In the ER and in the early secretory pathway, where the repertoire of oligosaccharide structures is still rather small, the glycans play a pivotal role in protein folding, oligomerization, quality control, sorting, and transport. They are used as universal "tags" that allow specific lectins and modifying enzymes to establish order among the diversity of maturing glycoproteins. In the Golgi complex, the glycans acquire more complex structures and a new set of functions. The division of synthesis and processing between the ER and the Golgi complex represents an evolutionary adaptation that allows efficient exploitation of the potential of oligosaccharides.

Interleukin-2 carbohydrate recognition modulates CTLL-2 cell proliferation

Interleukin-2 (IL-2) specifically recognizes high-mannose type glycans with five or six mannosyl residues. To determine whether the carbohydrate recognition activity of IL-2 contributes to its physiological activity, the inhibitory effects of high-mannose type glycans on IL-2-dependent CTLL-2 cell proliferation were investigated. Man(5)GlcNAc(2)Asn added to CTLL-2 cell cultures inhibited not only phosphorylation of tyrosine kinases but also IL-2-dependent cell proliferation. We found that a complex of IL-2, IL-2 receptor alpha, beta, gamma subunits, and tyrosine kinases was formed in rhIL-2-stimulated CTLL-2 cells. Among the components of this complex, only the IL-2 receptor alpha subunit was stained with Galanthus nivalis agglutinin which specifically recognizes high-mannose type glycans. This staining was diminished after digestion of the glycans with endo-beta-N-acetylglucosaminidase H or D, suggesting that at least a N-glycan containing Man(5)GlcNAc(2) is linked to the extracellular portion of the IL-2 receptor alpha subunit. Our findings indicate that IL-2 binds the IL-2 receptor alpha subunit through Man(5)GlcNAc(2) and a specific peptide sequence on the surface of CTLL-2 cells. When IL-2 binds to the IL-2Ralpha subunit, this may trigger formation of the high affinity complex of IL-2-IL-2Ralpha, -beta, and -gamma subunits, leading to cellular signaling.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Lectins and traffic in the secretory pathway

Evidence is accumulating that intracellular animal lectins play important roles in quality control and glycoprotein sorting along the secretory pathway. Calnexin and calreticulin in conjunction with associated chaperones promote correct folding and oligomerization of many glycoproteins in the endoplasmic reticulum (ER). The mannose lectin ERGIC-53 operates as a cargo receptor in transport of glycoproteins from ER to Golgi and the homologous lectin VIP36 may operate in quality control of glycosylation in the Golgi. Exit from the Golgi of lysosomal hydrolases to endosomes requires mannose 6-phosphate receptors and exit to the apical plasma membrane may also involve traffic lectins. Here we discuss the features of these lectins and their role in glycoprotein traffic in the secretory pathway.

ERGIC-53 and traffic in the secretory pathway

The ER-Golgi intermediate compartment (ERGIC) marker ERGIC-53 is a mannose-specific membrane lectin operating as a cargo receptor for the transport of glycoproteins from the ER to the ERGIC. Lack of functional ERGIC-53 leads to a selective defect in secretion of glycoproteins in cultured cells and to hemophilia in humans. Beyond its interest as a transport receptor, ERGIC-53 is an attractive probe for studying numerous aspects of protein trafficking in the secretory pathway, including traffic routes, mechanisms of anterograde and retrograde traffic, retention of proteins in the ER, and the function of the ERGIC. Understanding these fundamental processes of cell biology will be crucial for the elucidation and treatment of many inherited and acquired diseases, such as cystic fibrosis, Alzheimer's disease and viral infections.

Mannose-dependent endoplasmic reticulum (ER)-Golgi intermediate compartment-53-mediated ER to Golgi trafficking of coagulation factors V and VIII

The endoplasmic reticulum-Golgi intermediate compartment (ERGIC) is the site of segregation of secretory proteins for anterograde transport, via packaging into COPII-coated transport vesicles. ERGIC-53 is a homo-hexameric transmembrane lectin localized to the ERGIC that exhibits mannose-selective properties in vitro. Null mutations in ERGIC-53 were recently shown to be responsible for the autosomal recessive bleeding disorder, combined deficiency of coagulation factors V and VIII. We have studied the effect of defective ER to Golgi cycling by ERGIC-53 on the secretion of factors V and VIII. The secretion efficiency of factor V and factor VIII was studied in a tetracycline-inducible HeLa cell line overexpressing a wild-type ERGIC-53 or a cytosolic tail mutant of ERGIC-53 (KKAA) that is unable to exit the ER due to mutation of two COOH-terminal phenylalanine residues to alanines. The results show that efficient trafficking of factors V and VIII requires a functional ERGIC-53 cycling pathway and that this trafficking is dependent on post-translational modification of a specific cluster of asparagine (N)-linked oligosaccharides to a fully glucose-trimmed, mannose9 structure.

The lectin ERGIC-53 is a cargo transport receptor for glycoproteins

Soluble secretory proteins are transported from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC) in vesicles coated with COP-II coat proteins. The sorting of secretory cargo into these vesicles is thought to involve transmembrane cargo-receptor proteins. Here we show that a cathepsin-Z-related glycoprotein binds to the recycling, mannose-specific membrane lectin ERGIC-53. Binding occurs in the ER, is carbohydrate- and calcium-ion-dependent and is affected by untrimmed glucose residues. Binding does not, however, require oligomerization of ERGIC-53, although oligomerization is required for exit of ERGIC-53 from the ER. Dissociation of ERGIC-53 occurs in the ERGIC and is delayed if ERGIC-53 is mislocalized to the ER. These results strongly indicate that ERGIC-53 may function as a receptor facilitating ER-to-ERGIC transport of soluble glycoprotein cargo.