Requirement of the Lec35 gene for all known classes of monosaccharide-P-dolichol-dependent glycosyltransferase reactions in mammals

Evidence that the wzxE gene of Escherichia coli K-12 encodes a protein involved in the transbilayer movement of a trisaccharide-lipid intermediate in the assembly of enterobacterial common antigen

The assembly of many bacterial cell surface polysaccharides requires the transbilayer movement of polyisoprenoid-linked saccharide intermediates across the cytoplasmic membrane. It is generally believed that transverse diffusion of glycolipid intermediates is mediated by integral membrane proteins called translocases or "flippases." The bacterial genes proposed to encode these translocases have been collectively designated wzx genes. The wzxE gene of Escherichia coli K-12 has been implicated in the transbilayer movement of Fuc4NAc-ManNAcA-GlcNAc-P-P-undecaprenol (lipid III), the donor of the trisaccharide repeat unit in the biosynthesis of enterobacterial common antigen (ECA). Previous studies (Feldman, M. F., Marolda, C. L., Monteiro, M. A., Perry, M. B., Parodi, A. J., and Valvano, M. (1999) J. Biol. Chem. 274, 35129-35138) provided indirect evidence that the wzx(016) gene product of E. coli K-12 encoded a translocase capable of mediating the transbilayer movement of N-acetylglucosaminylpyrophosphorylundecaprenol (GlcNAc-P-P-Und), an early intermediate in the synthesis of ECA and many lipopolysaccharide O antigens. Therefore, genetic and biochemical studies were conducted to determine if the putative Wzx(O16) translocase was capable of mediating the transport of N-acetylglucosaminylpyrophosphorylnerol (GlcNAc-P-P-Ner), a water-soluble analogue of GlcNAc-P-P-Und. [(3)H]GlcNAc-P-P-Ner was transported into sealed, everted cytoplasmic membrane vesicles of E. coli K-12 as well as a deletion mutant lacking both the wzx(016) and wzxC genes. In contrast, [(3)H]GlcNAc-P-P-Ner was not transported into membrane vesicles prepared from a wzxE-null mutant, and metabolic radiolabeling experiments revealed the accumulation of lipid III in this mutant. The WzxE transport system exhibited substrate specificity by recognizing both a pyrophosphoryl-linked saccharide and an unsaturated alpha-isoprene unit in the carrier lipid. These results support the conclusion that the wzxE gene encodes a membrane protein involved in the transbilayer movement of lipid III in E. coli.

Human disorders in N-glycosylation and animal models

Genes that cause human disorders in N-linked oligosaccharide biosynthesis have appeared much faster than animal model systems to study them. In most models, a single gene is altered or deleted while other genes and the environment are held constant. Since humans have variable genetic backgrounds and environments, model systems may only partially mimic the actual disorders. Mutations in seven of the 30-40 genes needed for the synthesis and transfer of oligosaccharides from the lipid donor to the nascent protein acceptors in the endoplasmic reticulum cause Type I Congenital Disorders of Glycosylation (CDG). Since all of these gene products ultimately contribute to the same final step, one might suspect that all the diseases would be very similar. However, even patients with mutations in the same gene show considerable phenotypic variability. Modifier, or susceptibility genes in the background likely explain some variations of the "primary" gene chosen for study. Add to this the stress of infections, dietary insufficiencies, and the demands of growth itself. These issues are particularly important during development when the temporal and spatial specific interplay of cell adhesions and signals has only a single opportunity. Multiple hypomorphic alleles of genes in the same pathway may have synergistic effects. Investigators designing model systems to study human glycosylation disorders may want to construct strains with several heterozygous hypomorphic alleles in rate-limiting steps in the glycosylation pathway.

Rapid activation of glycogen phosphorylase by the endoplasmic reticulum unfolded protein response

Endoplasmic reticulum (ER) stress is associated with misfolding of ER proteins and triggers the unfolded protein response (UPR). The UPR, in turn, helps restore normal ER function. Since fastidious N-linked glycosylation is critical for folding of most ER proteins, this study examined whether metabolic interconversions of precursors used for glycan assembly were controlled by the UPR. Thus, eight enzymes and factors with key roles in hexose phosphate metabolism were assayed in cytoplasmic extracts from primary dermal fibroblasts treated with UPR inducers. Stimulation of only one activity by the UPR was detected, AMP-independent glycogen phosphorylase (GP). GP activation required only 20 min of ER stress, with concurrent decreases in cellular glycogen and elevations of its metabolites Glc-1-P and Glc-6-P. Addition of phosphatase inhibitors to enzyme extracts from unstressed cells mimicked the effect of ER stress on GP activity, suggesting that phosphorylation of GP or a regulatory factor was involved. These data show that the UPR can modulate hexose metabolism in a manner beneficial for protein glycosylation. Since activation of GP appears to occur by a rapid post-translational process, it may be part of a general strategy of ER damage control, preceding the well-known transcription-dependent processes of the UPR that are manifested hours after the occurrence of ER stress.

Coupling of the dolichol-P-P-oligosaccharide pathway to translation by perturbation-sensitive regulation of the initiating enzyme, GlcNAc-1-P transferase

In mammalian cells, inhibition of translation interferes with synthesis of the lipid-linked oligosaccharide (LLO) Glc3Man9GlcNAc2-P-P-dolichol as measured with radioactive sugar precursors. Conflicting hypotheses have been proposed, and the fundamental basis for this regulation has remained elusive. Here, fluorophore-assisted carbohydrate electrophoresis (FACE) was used to measure LLO concentrations directly in cells treated with translation blockers. Further, LLO biosynthetic enzymes were assayed in vitro with endogenous acceptor substrates using either cells gently permeabilized with streptolysin-O (SLO) or microsomes from homogenized cells. In Chinese hamster ovary (CHO)-K1 cells treated with translation blockers, FACE did not detect changes in concentrations of Glc3Man9GlcNAc2-P-P-dolichol or early LLO intermediates. These results do not support earlier proposals for feedback repression of LLO initiation by accumulated Glc3Man9GlcNAc2-P-P-dolichol, or inhibition of a GDP-mannose dependent transferase. With microsomes from cells treated with translation blockers, there was no interference with LLO initiation by GlcNAc-1-P transferase (GPT), mannose-P-dolichol synthase, glucose-P-dolichol synthase, or LLO synthesis in vitro, as reported previously. Surprisingly, inhibition of all of these was detected with the SLO in vitro system. Additional experiments with the SLO system showed that the three transferases shared a limited pool of dolichol-P that was trapped as Glc3Man9GlcNAc2-P-P-dolichol by translation arrest. Overexpression of GPT was unable to reverse the effects of translation arrest on LLO initiation, and experiments with FACE and the SLO system showed that overexpressed GPT was not functional in vivo, although it was highly active in microsomal assays. Thus, the combined use of the SLO in vitro system and FACE showed that LLO biosynthesis depends upon a limited primary pool of dolichol-P. Physical perturbation associated with microsome preparation appears to make available a secondary pool of dolichol-P, masking inhibition by translation arrest, as well as activating a nonfunctional fraction of GPT. The implications of these results for the organization of the LLO pathway are discussed.

Common origin and evolution of glycosyltransferases using Dol-P-monosaccharides as donor substrate

On the basis of the analysis of 64 glycosyltransferases from 14 species we propose that several successive duplications of a common ancestral gene, followed by divergent evolution, have generated the mannosyltransferases and the glucosyltransferases involved in asparagine-linked glycosylation (ALG) and phosphatidyl-inositol glycan anchor (PIG or GPI), which use lipid-related donor and acceptor substrates. Long and short conserved peptide motifs were found in all enzymes. Conserved and identical amino acid positions were found for the alpha 2/6- and the alpha 3/4-mannosyltransferases and for the alpha 2/3-glucosyltransferases, suggesting unique ancestors for these three superfamilies. The three members of the alpha 2-mannosyltransferase family (ALG9, PIG-B, and SMP3) and the two members of the alpha 3-glucosyltransferase family (ALG6 and ALG8) shared 11 and 30 identical amino acid positions, respectively, suggesting that these enzymes have also originated by duplication and divergent evolution. This model predicts a common genetic origin for ALG and PIG enzymes using dolichyl-phospho-monosaccharide (Dol-P-monosaccharide) donors, which might be related to similar spatial orientation of the hydroxyl acceptors. On the basis of the multiple sequence analysis and the prediction of transmembrane topology we propose that the endoplasmic reticulum glycosyltransferases using Dol-P-monosaccharides as donor substrate have a multispan transmembrane topology with a first large luminal conserved loop containing the long motif and a small cytosolic conserved loop containing the short motif, different from the classical type II glycosyltransferases, which are anchored in the Golgi by a single transmembrane domain.

Loss of albumin and megalin binding to renal cubilin in rats results in albuminuria after total body irradiation

The role of the renal apical brush-border membrane (BBM) endocytic receptors cubilin and megalin in the onset of albuminuria in rats exposed to a single dose of total body irradiation (TBI) has been investigated. Albuminuria was evident as immunoblot (IB) analysis of the urine samples from TBI rats revealed excretion of large amounts of albumin. IB analysis of the BBM proteins did not reveal any significant changes in cubilin or megalin levels, but (125)I-albumin binding to BBM from TBI rats declined by 80% with a fivefold decrease (from 0.5 to 2.5 microM) in the affinity for albumin. IB analysis of cubilin from the BBM demonstrated a 75% loss when purified using albumin, but not intrinsic factor (IF)-cobalamin (Cbl) ligand affinity chromatography. Immunoprecipitation (IP) of Triton X-100 extract of the BBM with antiserum to cubilin followed by IB of the immune complex with an antiserum to megalin revealed a 75% loss of association between megalin and cubilin. IP studies with antiserum to cubilin or megalin and IB with antiserum to the cation-independent mannose 6-phosphate/insulin-like growth factor II-receptor (CIMPR) revealed that CIMPR interacted with both cubilin and megalin. In addition, TBI did not disrupt the association of CIMPR with either cubilin or megalin in BBM. These results suggest that albuminuria noted in TBI rats is due to selective loss of albumin and megalin, but not CIMPR or IF-Cbl binding by cubilin. Furthermore, these results also suggest that albumin and IF-Cbl binding to cubilin occur at distinct sites and that in the rat renal BBM, CIMPR interacts with both cubilin and megalin.

Transmembrane movement of dolichol linked carbohydrates during N-glycoprotein biosynthesis in the endoplasmic reticulum

The process of N-linked glycosylation of secretory proteins is characterized by enzymatic reactions occurring on both sides of the endoplasmic reticulum (ER) membrane. On either side multiple glycosyltransferases participate in the stepwise addition of monosaccharides to core oligosaccharide unit that is attached to the lipid carrier dolichyl pyrophosphate. Cytoplasm-oriented glycosyltransferases use nucleotide-activated sugars as substrates, whereas lumen-oriented transferases that act later in the pathway make use of dolichyl phosphate-linked monosaccharides. The completely assembled core oligosaccharide is transferred to proteins on the lumenal side of the ER. The topological organization of this biosynthetic pathway requires the translocation of lipid-linked mono- and oligo-saccharides across the ER membrane. The transfer of the substrates and intermediates depend on specific translocators, i.e. so called flippases.

Analyses of dolichol pyrophosphate-linked oligosaccharides in cell cultures and tissues by fluorophore-assisted carbohydrate electrophoresis

Lipid-linked oligosaccharides (LLOs) are the precursors of asparagine (N)-linked glycans, which are essential information carriers in many biological systems, and defects in LLO synthesis cause Type I congenital disorders of glycosylation. Due to the low abundance of LLOs and the limitations of the chemical and physical methods previously used to detect them, simple and sensitive nonradioactive methods for LLO analysis are lacking. Thus, almost all studies of LLO synthesis have relied on metabolic labeling of the oligosaccharides with radioactive sugar precursors. We report that LLOs in cell cultures and tissues can be easily detected and quantified with a sensitivity of 1-2 pmol by fluorophore-assisted carbohydrate electrophoresis (FACE). These analyses required efficient removal of contaminants, most likely trace quantities of glycogen breakdown products, that interfered with FACE. Studies with CHO-K1 cells showed that LLOs detected by FACE and by metabolic labeling had similar turnover rates. Glc(3)Man(9)GlcNAc(2)-P-P-dolichol was the most prominent LLO detected by FACE in normal cultured cells and mouse tissues. However, the relative amounts of Glc(0-2)Man(5-9)GlcNAc(2)-P-P-dolichol intermediates in tissues, such as liver and kidney, were unexpectedly greater than for cultured cells. IV injection of D-mannose, raising the circulatory concentration by three- to fourfold, did not affect LLO composition. Thus, the relative accumulation of LLO intermediates in mouse liver and kidney is not likely due to inadequate D-mannose in the circulation. In summary, FACE is a facile, accurate, and sensitive method for LLO analysis, permitting investigations not feasible by metabolic labeling.

Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds

Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.

Mutant mammalian cells as tools to delineate the sterol regulatory element-binding protein pathway for feedback regulation of lipid synthesis

The tools of somatic cell genetics have been instrumental in unraveling the pathway by which sterol regulatory element-binding proteins (SREBPs) control lipid metabolism in animal cells. SREBPs are membrane-bound transcription factors that enhance the synthesis and uptake of cholesterol and fatty acids. The activities of the SREBPs are controlled by the cholesterol content of cells through feedback inhibition of proteolytic processing. When cells are replete with sterols, SREBPs remain bound to membranes of the endoplasmic reticulum (ER) and are therefore inactive. When cells are depleted of sterols, the SREBPs move to the Golgi complex where two proteases release the active portions of the SREBPs, which then enter the nucleus and activate transcription of target genes. This processing requires three membrane proteins-a sterol-sensing escort protein (SCAP) that transports SREBPs from the ER to the Golgi and two Golgi-located proteases (S1P and S2P) that release SREBPs from membranes. The existence of all three proteins was revealed through analysis of mutant mammalian cells in tissue culture. Their cDNAs and genes were isolated by genetic complementation or by expression cloning. The somatic cell genetic approach described in this article should prove useful for unraveling other complex biochemical pathways in animal cells.

MPDU1 mutations underlie a novel human congenital disorder of glycosylation, designated type If

Deficiencies in the pathway of N-glycan biosynthesis lead to severe multisystem diseases, known as congenital disorders of glycosylation (CDG). The clinical appearance of CDG is variable, and different types can be distinguished according to the gene that is altered. In this report, we describe the molecular basis of a novel type of the disease in three unrelated patients diagnosed with CDG-I. Serum transferrin was hypoglycosylated and patients' fibroblasts accumulated incomplete lipid-linked oligosaccharide precursors for N-linked protein glycosylation. Transfer of incomplete oligosaccharides to protein was detected. Sequence analysis of the Lec35/MPDU1 gene, known to be involved in the use of dolichylphosphomannose and dolichylphosphoglucose, revealed mutations in all three patients. Retroviral-based expression of the normal Lec35 cDNA in primary fibroblasts of patients restored normal lipid-linked oligosaccharide biosynthesis. We concluded that mutations in the Lec35/MPDU1 gene cause CDG. This novel type was termed CDG-If.

A mutation in the human MPDU1 gene causes congenital disorder of glycosylation type If (CDG-If)

We describe a new congenital disorder of glycosylation, CDG-If. The patient has severe psychomotor retardation, seizures, failure to thrive, dry skin and scaling with erythroderma, and impaired vision. CDG-If is caused by a defect in the gene MPDU1, the human homologue of hamster Lec35, and is the first disorder to affect the use, rather than the biosynthesis, of donor substrates for lipid-linked oligosaccharides. This leads to the synthesis of incomplete and poorly transferred precursor oligosaccharides lacking both mannose and glucose residues. The patient has a homozygous point mutation (221T-->C, L74S) in a semiconserved amino acid of MPDU1. Chinese hamster ovary Lec35 cells lack a functional Lec35 gene and synthesize truncated lipid-linked oligosaccharides similar to the patient's. They lack glucose and mannose residues donated by Glc-P-Dol and Man-P-Dol. Transfection with the normal human MPDU1 allele nearly completely restores normal glycosylation, whereas transfection with the patient's MPDU1 allele only weakly restores normal glycosylation. This work provides a new clinical picture for another CDG that may involve synthesis of multiple types of glycoconjugates.

An inner membrane enzyme in Salmonella and Escherichia coli that transfers 4-amino-4-deoxy-L-arabinose to lipid A: induction on polymyxin-resistant mutants and role of a novel lipid-linked donor

Attachment of the cationic sugar 4-amino-4-deoxy-l-arabinose (l-Ara4N) to lipid A is required for the maintenance of polymyxin resistance in Escherichia coli and Salmonella typhimurium. The enzymes that synthesize l-Ara4N and transfer it to lipid A have not been identified. We now report an inner membrane enzyme, expressed in polymyxin-resistant mutants, that adds one or two l-Ara4N moieties to lipid A or its immediate precursors. No soluble factors are required. A gene located near minute 51 on the S. typhimurium and E. coli chromosomes (previously termed orf5, pmrK, or yfbI) encodes the l-Ara4N transferase. The enzyme, renamed ArnT, consists of 548 amino acid residues in S. typhimurium with 12 possible membrane-spanning regions. ArnT displays distant similarity to yeast protein mannosyltransferases. ArnT adds two l-Ara4N units to lipid A precursors containing a Kdo disaccharide. However, as shown by mass spectrometry and NMR spectroscopy, it transfers only a single l-Ara4N residue to the 1-phosphate moiety of lipid IV(A), a precursor lacking Kdo. Proteins with full-length sequence similarity to ArnT are present in genomes of other bacteria thought to synthesize l-Ara4N-modified lipid A, including Pseudomonas aeruginosa and Yersinia pestis. As shown in the following article (Trent, M. S., Ribeiro, A. A., Doerrler, W. T., Lin, S., Cotter, R. J., and Raetz, C. R. H. (2001) J. Biol. Chem. 276, 43132-43144), ArnT utilizes the novel lipid undecaprenyl phosphate-alpha-l-Ara4N as its sugar donor, suggesting that l-Ara4N transfer to lipid A occurs on the periplasmic side of the inner membrane.

Cystinosin, the protein defective in cystinosis, is a H(+)-driven lysosomal cystine transporter

Cystinosis is an inherited lysosomal storage disease characterized by defective transport of cystine out of lysosomes. However, the causative gene, CTNS, encodes a seven transmembrane domain lysosomal protein, cystinosin, unrelated to known transporters. To investigate the molecular function of cystinosin, the protein was redirected from lysosomes to the plasma membrane by deletion of its C-terminal GYDQL sorting motif (cystinosin-DeltaGYDQL), thereby exposing the intralysosomal side of cystinosin to the extracellular medium. COS cells expressing cystinosin-DeltaGYDQL selectively take up L-cystine from the extracellular medium at acidic pH. Disruption of the transmembrane pH gradient or incubation of the cells at neutral pH strongly inhibits the uptake. Cystinosin-DeltaGYDQL is directly involved in the observed cystine transport, since this activity is highly reduced when the GYDQL motif is restored and is abolished upon introduction of a point mutation inducing early-onset cystinosis. We conclude that cystinosin represents a novel H(+)-driven transporter that is responsible for cystine export from lysosomes, and propose that cystinosin homologues, such as mammalian SL15/Lec35 and Saccharomyces cerevisiae ERS1, may perform similar transport processes at other cellular membranes.

Balancing N-linked glycosylation to avoid disease

Complete loss of N-glycosylation is lethal in both yeast and mammals. Substantial deficiencies in some rate-limiting biosynthetic steps cause human congenital disorders of glycosylation (CDG). Patients have a range of clinical problems including variable degrees of mental retardation, liver dysfunction, and intestinal disorders. Over 60 mutations in phosphomannomutase (encoded by PMM2) diminish activity and cause CDG-Ia. The severe mutation R141H in PMM2 is lethal when homozygous, but heterozygous in about 1/70 Northern Europeans. Another disorder, CDG-Ic, is caused by mutations in ALG6, an alpha 1,3glucosyl transferase used for lipid-linked precursor synthesis, yet some function-compromising mutations occur at a high frequency in this gene also. Maintenance of seemingly deleterious mutations implies a selective advantage or positive heterosis. One possible explanation for this is that production of infective viruses such as hepatitis virus B and C, or others that rely heavily on host N-glycosylation, is substantially inhibited when only a tiny fraction of their coat proteins is misglycosylated. In contrast, this reduced glycosylation does not affect the host. Prevalent functional mutations in rate-limiting glycosylation steps could provide some resistance to viral infections, but the cost of this insurance is CDG. A balanced glycosylation level attempts to accommodate these competing agendas. By assessing the occurrence of a series of N-glycosylation-compromising alleles in multi-genic diseases, it may be possible to determine whether impaired glycosylation is a risk factor or a major determinant underlying their pathology.