Cloning, sequence, and expression of a cDNA encoding hamster UDP-GlcNAc:dolichol phosphate N-acetylglucosamine-1-phosphate transferase

NFAT5 promotes oral squamous cell carcinoma progression in a hyperosmotic environment

Epidermal growth factor receptor (EGFR) is highly expressed in several types of cancer cells including oral squamous cell carcinoma (OSCC). EGF/EGFR signaling is recognized as an important molecular target in cancer therapy. However, cancer cells often become tolerant to EGF/EGFR signaling-targeted therapies. In the tumor microenvironment, the tumor incites inflammation and the inflammation-derived cytokines make a considerable impact on cancer development. In addition, hyperosmolarity is also induced, but the role of osmotic stress in cancer development has not been fully understood. This study demonstrates molecular insights into hyperosmolarity effect on OSCC development and shows that NFAT5 transcription factor plays an important functional role in enhancing the oral cancer cell proliferation by inducing the EGFR translocation from the endoplasmic reticulum to the plasma membrane through increase the expression of DPAGT1, an essential enzyme for catalyzing the first committed step of N-linked protein glycosylation. These results suggest that hyperosmolarity-induced intra-nuclear translocation of NFAT5 essential for DPAGT1 activation and EGFR subcellular translocation responsible for OSCC tumor progression.

Computational Chemogenomics Drug Repositioning Strategy Enables the Discovery of Epirubicin as a New Repurposed Hit for Plasmodium falciparum and P. vivax

Widespread resistance against antimalarial drugs thwarts current efforts for controlling the disease and urges the discovery of new effective treatments. Drug repositioning is increasingly becoming an attractive strategy since it can reduce costs, risks, and time-to-market. Herein, we have used this strategy to identify novel antimalarial hits. We used a comparative in silico chemogenomics approach to select Plasmodium falciparum and Plasmodium vivax proteins as potential drug targets and analyzed them using a computer-assisted drug repositioning pipeline to identify approved drugs with potential antimalarial activity.

Widespread resistance against antimalarial drugs thwarts current efforts for controlling the disease and urges the discovery of new effective treatments. Drug repositioning is increasingly becoming an attractive strategy since it can reduce costs, risks, and time-to-market. Herein, we have used this strategy to identify novel antimalarial hits. We used a comparative in silico chemogenomics approach to select Plasmodium falciparum and Plasmodium vivax proteins as potential drug targets and analyzed them using a computer-assisted drug repositioning pipeline to identify approved drugs with potential antimalarial activity. Among the seven drugs identified as promising antimalarial candidates, the anthracycline epirubicin was selected for further experimental validation. Epirubicin was shown to be potent in vitro against sensitive and multidrug-resistant P. falciparum strains and P. vivax field isolates in the nanomolar range, as well as being effective against an in vivo murine model of Plasmodium yoelii. Transmission-blocking activity was observed for epirubicin in vitro and in vivo. Finally, using yeast-based haploinsufficiency chemical genomic profiling, we aimed to get insights into the mechanism of action of epirubicin. Beyond the target predicted in silico (a DNA gyrase in the apicoplast), functional assays suggested a GlcNac-1-P-transferase (GPT) enzyme as a potential target. Docking calculations predicted the binding mode of epirubicin with DNA gyrase and GPT proteins. Epirubicin is originally an antitumoral agent and presents associated toxicity. However, its antiplasmodial activity against not only P. falciparum but also P. vivax in different stages of the parasite life cycle supports the use of this drug as a scaffold for hit-to-lead optimization in malaria drug discovery.

Clinical features of congenital myasthenic syndrome due to mutations in DPAGT1

BACKGROUND

A newly defined congenital myasthenic syndrome (CMS) caused by DPAGT1 mutations has recently been reported. While many other CMS-associated proteins have discrete roles localised to the neuromuscular junction, DPAGT1 is ubiquitously expressed, modifying many proteins, and as such is an unexpected cause of isolated neuromuscular involvement.

METHODS

We present detailed clinical characteristics of five patients with CMS caused by DPAGT1 mutations.

RESULTS

Patients have prominent limb girdle weakness and minimal craniobulbar symptoms. Tubular aggregates on muscle biopsy are characteristic but may not be apparent on early biopsies. Typical myasthenic features such as pyridostigmine and 3, 4- diaminopyridine responsiveness, and decrement on repetitive nerve stimulation are present.

CONCLUSIONS

These patients mimic myopathic disorders and are likely to be under-diagnosed. The descriptions here should facilitate recognition of this disorder. In particular minimal craniobulbar involvement and tubular aggregates on muscle biopsy help to distinguish DPAGT1 CMS from the majority of other forms of CMS. Patients with DPAGT1 CMS share similar clinical features with patients who have CMS caused by mutations in GFPT1, another recently identified CMS subtype.

Hyaluronan synthase 1 (HAS1) requires higher cellular UDP-GlcNAc concentration than HAS2 and HAS3

Mammals have three homologous genes encoding proteins with hyaluronan synthase activity (Has1-3), all producing an identical polymer from UDP-N-acetylglucosamine and UDP-glucuronic acid. To compare the properties of these isoenzymes, COS-1 cells, with minor endogenous hyaluronan synthesis, were transfected with human Has1-3 isoenzymes. HAS1 was almost unable to secrete hyaluronan or form a hyaluronan coat, in contrast to HAS2 and HAS3. This failure of HAS1 to synthesize hyaluronan was compensated by increasing the cellular content of UDP-N-acetyl glucosamine by ∼10-fold with 1 mm glucosamine in the growth medium. Hyaluronan synthesis driven by HAS2 was less affected by glucosamine addition, and HAS3 was not affected at all. Glucose-free medium, leading to depletion of the UDP-sugars, markedly reduced hyaluronan synthesis by all HAS isoenzymes while raising its concentration from 5 to 25 mm had a moderate stimulatory effect. The results indicate that HAS1 is almost inactive in cells with low UDP-sugar supply, HAS2 activity increases with UDP-sugars, and HAS3 produces hyaluronan at high speed even with minimum substrate content. Transfected Has2 and particularly Has3 consumed enough UDP-sugars to reduce their content in COS-1 cells. Comparison of different human cell types revealed ∼50-fold differences in the content of UDP-N-acetylhexosamines and UDP-glucuronic acid, correlating with the expression level of Has1, suggesting cellular coordination between Has1 expression and the content of UDP-sugars.

Identification of DPAGT1 as a new gene in which mutations cause a congenital myasthenic syndrome

Congenital myasthenic syndromes (CMS) are a group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. This is a heterogenous group of disorders with 15 different genes implicated in the development of the disease. Using whole-exome sequencing we identified DPAGT1 as a new gene associated with CMS. DPAGT1 catalyses the first step of N-linked protein glycosylation. DPAGT1 patients are characterized by weakness of limb muscles, response to treatment with cholinesterase inhibitors, and the presence of tubular aggregates on muscle biopsy. We showed that DPAGT1 is required for glycosylation of acetylcholine receptor (AChR) subunits and efficient export of AChR to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1-associated CMS is reduced levels of AChRs at the endplate region. This finding demonstrates that impairment of the N-linked glycosylation pathway can lead to the development of CMS.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

At the membrane frontier: a prospectus on the remarkable evolutionary conservation of polyprenols and polyprenyl-phosphates

Long-chain polyprenols and polyprenyl-phosphates are ubiquitous and essential components of cellular membranes throughout all domains of life. Polyprenyl-phosphates, which include undecaprenyl-phosphate in bacteria and the dolichyl-phosphates in archaea and eukaryotes, serve as specific membrane-bound carriers in glycan biosynthetic pathways responsible for the production of cellular structures such as N-linked protein glycans and bacterial peptidoglycan. Polyprenyl-phosphates are the only form of polyprenols with a biochemically-defined role; however, unmodified or esterified polyprenols often comprise significant percentages of the cellular polyprenol pool. The strong evolutionary conservation of unmodified polyprenols as membrane constituents and polyprenyl-phosphates as preferred glycan carriers in biosynthetic pathways is poorly understood. This review surveys the available research to explore why unmodified polyprenols have been conserved in evolution and why polyprenyl-phosphates are universally and specifically utilized for membrane-bound glycan assembly.

Hetero-oligomeric interactions between early glycosyltransferases of the dolichol cycle

N-Linked glycosylation begins with the formation of a dolichol-linked oligosaccharide in the endoplasmic reticulum (ER). The first two steps of this pathway lead to the formation of GlcNAc(2)-PP-dolichol, whose synthesis is sequentially catalyzed by the Alg7p GlcNAc phosphotransferase and by the dimeric Alg13p/Alg14p UDP-GlcNAc transferase on the cytosolic face of the endoplasmic reticulum. Here, we show that the Alg7p, Alg13p, and Alg14p glycosyltransferases form a functional multienzyme complex. Coimmunoprecipitation and gel filtration assays demonstrate that the Alg7p/Alg13p/Alg14p complex is a hexamer with a native molecular weight of approximately 200 kDa and an Alg7p:Alg13:Alg14p stoichiometry of 1:1:1. These results highlight and extend the striking parallels that exist between these eukaryotic UDP-GlcNAc transferases and their bacterial MraY and MurG homologs that catalyze the first two steps of the lipid-linked peptidoglycan precursor. In addition to their preferred substrate and lipid acceptors, these enzymes are similar in their structure, chemistry, temporal, and spatial organization. These similarities point to an evolutionary link between the early steps of N-linked glycosylation and those of peptidoglycan synthesis.

Transmembrane molecules for phylogenetic analyses of pathogenic protists: Leishmania-specific informative sites in hydrophilic loops of trans- endoplasmic reticulum N-acetylglucosamine-1-phosphate transferase

A sequence database was created for the Leishmania N-acetylglucosamine-1-phosphate transferase (nagt) gene from 193 independent isolates. PCR products of this single-copy gene were analyzed for restriction fragment length polymorphism based on seven nagt sequences initially available. We subsequently sequenced 77 samples and found 19 new variants (genotypes). Alignment of all 26 nagt sequences is gap free, except for a single codon addition or deletion. Phylogenetic analyses of the sequences allow grouping the isolates into three subgenera, each consisting of recognized species complexes, i.e., subgenus Leishmania (L. amazonensis-L. mexicana, L. donovani-L. infantum, L. tropica, L. major, and L. turanica-L. gerbilli), subgenus Viannia (L. braziliensis, L. panamensis), and one unclassified (L. enriettii) species. This hierarchy of grouping is also supported by sequence analyses of selected samples for additional single-copy genes present on different chromosomes. Intraspecies divergence of nagt varies considerably with different species complexes. Interestingly, species complexes with less subspecies divergence are more widely distributed than those that are more divergent. The relevance of this to Leishmania evolutionary adaptation is discussed. Heterozygosity of subspecies variants contributes to intraspecies diversity, which is prominent in L. tropica but not in L. donovani-L. infantum. This disparity is thought to result from the genetic recombination of the respective species at different times as a rare event during their predominantly clonal evolution. Phylogenetically useful sites of nagt are restricted largely to several extended hydrophilic loops predicted from hypothetical models of Leishmania NAGT as an endoplasmic reticulum transmembrane protein. In silico analyses of nagt from fungi and other protozoa further illustrate the potential value of this and, perhaps, other similar transmembrane molecules for phylogenetic analyses of single-cell eukaryotes.

A hypomorphic allele of the first N-glycosylation gene, ALG7, causes mitochondrial defects in yeast

The modification of proteins at asparagine residues with oligosaccharides (N-glycans) plays critical roles in diverse cell functions. N-glycans originate from a common lipid-linked oligosaccharide (LLO) precursor whose synthesis is initiated by the Dol-P-dependent GlcNAc-1-P transferase (GPT) encoded by an essential ALG7 gene. To identify cellular processes affected by ALG7 and N-glycosylation, we replaced the genomic copy of ALG7 with its hypomorphic allele in two genetically distinct haploid yeast cells. We show that ALG7 knockdown gave rise to an unexpected phenotype of mitochondrial dysfunction. The alg7 mutants did not grow on glycerol and DNA arrays revealed the absence of mitochondrial genes' expression. Accordingly, the alg7 mutants displayed no detectable mtDNA and respiratory activity. Both mutants exhibited diminished abundance of LLO and under-glycosylation of carboxypeptidase Y (CPY). Moreover, another N-glycosylation mutant with a LLO defect, alg6, was respiratory deficient. Collectively, our studies provide evidence that the dysregulation of N-glycosylation in haploid yeast cells leads to mitochondrial dysfunction.

Deficiency of UDP-GlcNAc:Dolichol Phosphate N-Acetylglucosamine-1 Phosphate Transferase (DPAGT1) causes a novel congenital disorder of Glycosylation Type Ij

Defects in the assembly of dolichol-linked oligosaccharide or its transfer to proteins result in severe, multi-system human diseases called Type I congenital disorders of glycosylation. We have identified a novel CDG type, CDG-Ij, resulting from deficiency in UDP-GlcNAc: dolichol phosphate N-acetyl-glucosamine-1 phosphate transferase (GPT) activity encoded by DPAGT1. The patient presents with severe hypotonia, medically intractable seizures, mental retardation, microcephaly, and exotropia. Metabolic labeling of cultured dermal fibroblasts from the patient with [2-(3)H]-mannose revealed lowered incorporation of radiolabel into full-length dolichol-linked oligosaccharides and glycoproteins. In vitro enzymatic analysis of microsomal fractions from the cultured cells indicated that oligosaccharyltransferase activity is normal, but the GPT activity is reduced to approximately 10% of normal levels while parents have heterozygous levels. The patient's paternal DPAGT1 allele contains a point mutation (660A>G) that replaces a highly conserved tyrosine with a cysteine (Y170C). The paternal allele cDNA produces a full-length protein with almost no activity when over-expressed in CHO cells. The maternal allele makes only about 12% normal mature mRNA, while the remainder shows a complex exon skipping pattern that shifts the reading frame encoding a truncated non-functional GPT protein. Thus, we conclude that the DPAGT1 gene defects are responsible for the CDG symptoms in this patient. Hum Mutat 22:144-150, 2003.

Characterisation of the gptA gene, encoding UDP N-acetylglucosamine: dolichol phosphate N-acetylglucosaminylphosphoryl transferase, from the filamentous fungus, Aspergillus niger

The production of asparagine (N)-linked oligosaccharides is of vital importance in the formation of glycosylated proteins in eukaryotes and is mediated by the dolichol pathway. As part of studies to allow manipulation of this pathway, the gene coding for the production of the enzyme UDP N-acetylglucosamine: dolichol phosphate N-acetylglucosaminylphosphoryl transferase (GPT), catalysing the first step in the assembly of dolichol-linked oligosaccharides, was cloned from the filamentous fungus Aspergillus niger. Degenerate-PCR was used to amplify a 470-bp fragment of the gene, which was labelled as a probe to obtain a full-length clone from a genomic library of A. niger. This contained a 1557-bp open reading frame encoding a highly hydrophobic protein of 468 amino acids with a predicted molecular weight of 51.4 kDa. The gene contained two intron sequences and putative dolichol recognition sites (PDRSs) were present in the deduced amino acid sequence. Comparison with other eukaryotic GPTs revealed the A. niger GPT to share 45-47% identity with yeasts (Saccharomyces cerevisiae and Schizosaccharomyces pombe) and 41-42% identity with mammals (mouse, hamster, human). Nested-PCR of a cDNA library was used to confirm the position of an intron. A complete cDNA clone of A. niger gpt was obtained by employing a recombinant PCR approach. This was used to rescue a conditional lethal mutant of S. cerevisiae carrying a dysfunctional gpt gene by heterologous expression, confirming that the gpt genes from A. niger and S. cerevisiae are functionally equivalent.

Conserved aspartic acids are essential for the enzymic activity of the WecA protein initiating the biosynthesis of O-specific lipopolysaccharide and enterobacterial common antigen in Escherichia coli

The integral membrane protein WecA mediates the transfer of N-acetylglucosamine (GlcNAc) 1-phosphate to undecaprenyl phosphate (Und-P) with the formation of a phosphodiester bond. Bacteria employ this reaction during the biosynthesis of enterobacterial common antigen as well as of many O-specific lipopolysaccharides (LPSs). Alignment of a number of prokaryotic and eukaryotic WecA-homologous sequences identified a number of conserved aspartic acid (D) residues in putative cytoplasmic loops II and III of the inner-membrane protein. Site-directed mutagenesis was used to study the role of the conserved residues D90, D91 (loop II), D156 and D159 (loop III). As controls, D35, D94 and D276 were also mutagenized. The resulting WecA derivatives were assessed for function by complementation analysis of O-antigen biosynthesis, by the ability to incorporate radiolabelled precursor to a biosynthetic intermediate, by detection of the terminal GlcNAc residue in LPS and by a tunicamycin competition assay. It was concluded from these analyses that the conserved aspartic acid residues are functionally important, but also that they participate differently in the transfer reaction. Based on these results it is proposed that D90 and D91 are important in forwarding the reaction product to the next biosynthetic step, while D156 and D159 are a part of the catalytic site of the enzyme.

Conserved cytoplasmic motifs that distinguish sub-groups of the polyprenol phosphate:N-acetylhexosamine-1-phosphate transferase family

WecA, MraY and WbcO are conserved members of the polyprenol phosphate:N-acetylhexosamine-1-phosphate transferase family involved in the assembly of bacterial cell walls, and catalyze reactions involving a membrane-associated polyprenol phosphate acceptor substrate and a cytoplasmically located UDP-D-amino sugar donor. MraY, WbcO and WecA purportedly utilize different UDP-sugars, although the molecular basis of this specificity is largely unknown. However, domain variations involved in specificity are predicted to occur on the cytoplasmic side of the membrane, adjacent to conserved domains involved in the mechanistic activity, and with access to the cytoplasmically located sugar nucleotides. Conserved C-terminal domains have been identified that satisfy these criteria. Topological analyses indicate that they form the highly basic, fifth cytoplasmic loop between transmembrane regions IX and X. Four diverse loops are apparent, for MraY, WecA, WbcO and RgpG, that uniquely characterize these sub-groups of the transferase family, and a correlation is evident with the known or implied UDP-sugar specificity.

Cloning of the human cDNA which can complement the defect of the yeast mannosyltransferase I-deficient mutant alg 1

The assembly of the lipid-linked oligosaccharide, Glc(3)Man(9)GlcNAc(2)-P-P-Dol, occurs on the rough ER membrane in an ordered stepwise manner. The process is highly conserved among eukaryotes. In order to isolate the human mannosyltransferase I (MT-I) gene involved in the process, we used the Saccharomyces cerevisiae MT-I gene ( ALG1 ), which has already been cloned. On searching the EST database with the amino acid sequence of the ALG1 gene product, we detected seven related human EST clones. A human fetal brain cDNA library was screened by PCR using gene-specific primers based on the EST nucleotide sequences and a 430 bp cDNA fragment was amplified. The cDNA library was rescreened with this 430 bp cDNA, and two cDNA clones (HR1-3 and HR1-4) were isolated and sequenced. On a homology search of the EST database with the nucleotide sequence of HR1-3, we detected a novel human EST clone, AA675921 (GenBank accession number). Based on the nucleotide sequences of AA675921 and HR1-4, we designed gene-specific PCR primers, which allowed to amplify a 1.8 kb cDNA from human fetal brain cDNA. This cDNA was cloned and shown to contain an ORF encoding a protein of 464 amino acids. We designated this ORF as Hmat-1. The amino acid sequence deduced from the Hmat-1 gene showed several highly conserved regions shared with the yeast and nematode MT-I sequences. Furthermore, this 1.8 kb cDNA successfully complemented the S. cerevisiae alg1-1 mutation, indicating that the Hmat-1 gene encodes the human MT-I and that the function of this enzyme was conserved between yeast and human.

The N-terminal region of the Escherichia coli WecA (Rfe) protein, containing three predicted transmembrane helices, is required for function but not for membrane insertion

The correct site for translation initiation for Escherichia coli WecA (Rfe), presumably involved in catalyzing the transfer of N-acetylglucosamine 1-phosphate to undecaprenylphosphate, was determined by using its FLAG-tagged derivatives. The N-terminal region containing three predicted transmembrane helices was found to be necessary for function but not for membrane localization of this protein.

A recessive deletion in the GlcNAc-1-phosphotransferase gene results in peri-implantation embryonic lethality

Formation of the dolichol oligosaccharide precursor is essential for the production of asparagine- (N-) linked oligosaccharides (N-glycans) in eukaryotic cells. The first step in precursor biosynthesis requires the enzyme UDP-GlcNAc: dolichol phosphate N-acetylglucosamine-1-phosphate transferase (GPT). Without GPT activity, subsequent steps necessary in constructing the oligosaccharide precursor cannot occur. Inhibition of this biosynthetic step using tunicamycin, a GlcNAc analog, produces a deficiency in N-glycosylation in cell lines and embryonic lethality during preimplantation development in vitro, suggesting that N-glycan formation is essential in early embryogenesis. In exploring structure-function relationships among N-glycans, and since tunicamycin has various reported biochemical activities; we have generated a germline deletion in the mouse GPT gene. GPT mutant embryos were analyzed and the phenotypes obtained were compared with previous studies using tunicamycin. We find that embryos homozygous for a deletion in the GPT gene complete preimplantation development and also implant in the uterine epithelium, but die shortly thereafter between days 4-5 postfertilization with cell degeneration apparent among both embryonic and extraembryonic cell types. Of cells derived from these early embryos, neither trophoblast nor embryonic endodermal lineages are able to survive in culture in vitro. These results indicate that GPT function is essential in early embryogenesis and suggest that N-glycosylation is needed for the viability of cells comprising the peri-implantation stage embryo.

Overexpression of a gene that encodes the first enzyme in the biosynthesis of asparagine-linked glycans makes plants resistant to tunicamycin and obviates the tunicamycin-induced unfolded protein response

The cytotoxic drug tunicamycin kills cells because it is a specific inhibitor of UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-P transferase (GPT), an enzyme that catalyzes the initial step of the biosynthesis of dolichol-linked oligosaccharides. In the presence of tunicamycin, asparagine-linked glycoproteins made in the endoplasmic reticulum are not glycosylated with N-linked glycans, and therefore may not fold correctly. Such proteins may be targeted for breakdown. Cells that are treated with tunicamycin normally experience an unfolded protein response and induce genes that encode endoplasmic reticulum chaperones such as the binding protein (BiP). We isolated a cDNA clone for Arabidopsis GPT and overexpressed it in Arabidopsis. The transgenic plants have a 10-fold higher level of GPT activity and are resistant to 1 microg/mL tunicamycin, a concentration that kills control plants. Transgenic plants grown in the presence of tunicamycin have N-glycosylated proteins and the drug does not induce BiP mRNA levels as it does in control plants. BiP mRNA levels are highly induced in both control and GPT-expressing plants by azetidine-2-carboxylate. These observations suggest that excess GPT activity obviates the normal unfolded protein response that cells experience when exposed to tunicamycin.

Transmembrane topology of pmt1p, a member of an evolutionarily conserved family of protein O-mannosyltransferases

The identification of the evolutionarily conserved family of dolichyl-phosphate-D-mannose:protein O-mannosyltransferases (Pmts) revealed that protein O-mannosylation plays an essential role in a number of physiologically important processes. Strikingly, all members of the Pmt protein family share almost identical hydropathy profiles; a central hydrophilic domain is flanked by amino- and carboxyl-terminal sequences containing several putative transmembrane helices. This pattern is of particular interest because it diverges from structural models of all glycosyltransferases characterized so far. Here, we examine the transmembrane topology of Pmt1p, an integral membrane protein of the endoplasmic reticulum, from Saccharomyces cerevisiae. Structural predictions were directly tested by site-directed mutagenesis of endogenous N-glycosylation sites, by fusing a topology-sensitive monitor protein domain to carboxyl-terminal truncated versions of the Pmt1 protein and, in addition, by N-glycosylation scanning. Based on our results we propose a seven-transmembrane helical model for the yeast Pmt1p mannosyltransferase. The Pmt1p amino terminus faces the cytoplasm, whereas the carboxyl terminus faces the lumen of the endoplasmic reticulum. A large hydrophilic segment that is oriented toward the lumen of the endoplasmic reticulum is flanked by five amino-terminal and two carboxyl-terminal membrane spanning domains. We could demonstrate that this central loop is essential for the function of Pmt1p.

Conserved sequences in enzymes of the UDP-GlcNAc/MurNAc family are essential in hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase

The UDP-GlcNAc/MurNAc family of eukaryotic and prokaryotic enzymes use UDP-GlcNAc or UDP-MurNAc-pentapeptide as donors, dolichol-P or polyprenol-P as acceptors, and generate sugar-P-P-polyisoprenols. A series of six conserved sequences, designated A through F and ranging from 5 to 13 amino acid residues, has been identified in this family. To determine whether these conserved sequences are required for enzyme function, various mutations were examined in hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT). Scramble mutations of sequences B-F, generated by scrambling the residues within each sequence, demonstrated that each is important in GPT. While E and F scrambles appeared to prevent stable expression of GPT, scrambling of B-D resulted in GPT mutants that could be stably expressed and bound tunicamycin, but lacked enzymatic activity. Further, the C and D scramble mutants had an unexpected sorting defect. Replacement of sequences B-F with prokaryotic counterparts from either the B.subtilis mraY or E.coli rfe genes also affected GPT by preventing expression of the mutant protein (B, F) or inhibiting its enzymatic activity (C-E). For the C-E replacements, no acquisition of acceptor activity for polyprenol-P, the fully unsaturated natural bacterial acceptor, was detected. These studies show that the conserved sequences of the UDP-GlcNAc/MurNAc family are important, and that the eukaryotic and prokaryotic counterparts are not freely interchangeable. Since several mutants were efficiently expressed and bound tunicamycin, yet lacked enzymatic activity, the data are consistent with these sequences having a direct role in product formation.

Cloning and functional expression of the human GlcNAc-1-P transferase, the enzyme for the committed step of the dolichol cycle, by heterologous complementation in Saccharomyces cerevisiae

The gene for the human dolichol cycle GlcNAc-1-P transferase (ALG7/GPT) was cloned by screening a human lung fibroblast cDNA library. The library was constructed in a Saccharomyces cerevisiae expression vector, and the positive clone was identified by complementation of the conditional lethal S.cerevisiae strain YPH-A7-GAL. This strain was constructed by replacing the endogenous promoter of the GPT-gene by the stringently regulated GAL1-promoter. This construct allows to specifically suppress the endogenous enzyme activity. The insert of the positive clone displayed an open reading frame of 1200 nucleotides, coding for a putative protein of 400 amino acids with a calculated molecular weight of 44.7 kDa. The deduced protein sequence shows a homology of over 90% when compared with other mammalian GPT sequences, thus resembling the close phylogenetic relationship between mammalian species. This homology however decreases to 40-50% when compared to more distantly related organisms such as S.cerevisiae , Schizosaccharomyces pombe , or Leishmania amazonensis . Biochemical characterization of the recombinant protein showed that it is functionally expressed in the S.cerevisiae strain YPH-A7-GAL. GlcNAc- and GlcNAc2-PP-Dolichol biosynthesis could be shown with isolated S.cerevisiae membranes from cells harboring the recombinant plasmid and grown on glucose thus suppressing transcription of the endogenous gene. Synthesis could be stimulated by dolicholphosphate and was inhibited by tunicamycin. These results show that we have cloned the human GlcNAc-1-P transferase by heterologous complementation in S. cerevisiae, a strategy that may be useful for the cloning and characterization of glycosyltransferases from a variety of organisms.

Oligomerization of hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase, an enzyme with multiple transmembrane spans

Hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT), which initiates N-linked glycosylation by catalyzing the synthesis of GlcNAc-P-P-dolichol, has multiple transmembrane spans and a catalytic site that probably exists on the cytosolic face of the endoplasmic reticulum membrane (Dan, N., Middleton, R. M., and Lehrman, M. A. (1996) J. Biol. Chem. 271, 30717-30725). In this report, we demonstrate that GPT forms functional oligomers, probably dimers. Oligomers were detected by chemical cross-linking of GPT and by a dominant-negative effect caused by co-expression of enzymatically inactive (but properly folded) GPT mutants. The GPT mutants had no effect on two other dolichol-P-dependent endoplasmic reticulum enzymes. Mixing experiments indicated that mature GPT was competent for oligomerization. Oligomerization appeared to be favored in detergent extracts compared with intact microsomes. Detergent treatments were found to prevent, rather than promote, nonspecific aggregation of GPT. These results demonstrate that GPT subunits can physically interact and influence each other. The implications of oligomerization for enzyme function are discussed. From these results, we conclude that GPT is one of a very small number of multitransmembrane span enzymes that can form multimers.

Characterization of recombinant yeast dolichyl mannosyl phosphate synthase and site-directed mutagenesis of its cysteine residues

Dolichyl mannosyl phosphate synthase is associated with membranes of the rough endoplasmic reticulum and catalyzes mannosyl transfer from GDP-mannose to the hydrophobic long-chain acceptor dolichyl-phosphate. The gene for the yeast enzyme encodes a protein with a molecular mass of 30.36 kDa containing three cysteine residues, at positions 93, 172 and 259 [Orlean, P., Albright, C. & Robbins, P. W. (1988) J. Biol. Chem. 263, 17499-17507]. Inhibition of the synthase by thiol-specific reagents, including N-ethylmaleimide, p-hydroxymercuribenzoate, 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2), and lucifer yellow iodoacetamide (LYI), suggests that sulfhydryl groups might play a role in the catalytic mechanism of the enzyme. Titration of the synthase with Nbs2 or LYI indicated that 1 mol sulfhydryl/mol protein was accessible to these reagents, and that saturation of this site completely inhibited enzyme activity. To ascertain the reactive group and its possible function in enzyme catalysis, each of the cysteine residues was replaced individually by site-directed mutagenesis. The mutant enzymes had specific activities comparable to that of the wild-type enzyme, demonstrating that none of the cysteine residues were essential for catalytic activity. All of the mutant proteins except those containing a substitution at Cys93 were inhibited by thiol-blocking reagents, indicating that Cys93 might be physically located near the catalytic site of the enzyme. GDP-mannose, dolichyl phosphate and substrate analogs were found to protect against Nbs2 inactivation, further suggesting that Cys93 was physically near, or within, the substrate-binding site of the enzyme.

Hamster UDP-N-acetylglucosamine:dolichol-P N-acetylglucosamine-1-P transferase has multiple transmembrane spans and a critical cytosolic loop

UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT) is an endoplasmic reticulum (ER) enzyme responsible for synthesis of GlcNAc-P-P-dolichol, the committed step of dolichol-P-P-oligosaccharide synthesis. The sequence of hamster GPT predicted multiple transmembrane segments (Zhu, X., and Lehrman, M. A. (1990) J. Biol. Chem. 265, 14250-14255). GPT has also been predicted to act on the cytosolic face of the ER membrane, based on topological studies of its substrates and products. In this report we test these predictions by: (i) immunofluorescence microscopy with antibodies specific for native GPT sequences or epitope tags inserted into GPT, after selective permeabilization of the plasma membrane with digitonin; (ii) insertion of Factor Xa cleavage sites; (iii) in vitro translation of GPT; and (iv) site-directed mutagenesis. The loops between the 1st and 2nd and between the 9th and 10th predicted transmembrane spans of GPT were found to be cytosolic. In contrast, the loop between the 6th and 7th transmembrane spans, as well as the carboxyl terminus, were lumenal. Thus, hamster GPT must cross the ER membrane at least three times, consistent with previous computer-assisted predictions. There was no apparent N-glycosylation or signal sequence cleavage detected by in vitro translation. The cytosolic loop between the 9th and 10th transmembrane spans is the largest hydrophilic segment in GPT and, as judged by site-directed mutagenesis, has a number of conserved residues essential for activity. Hence, these results directly support the hypothesis that dolichol-P-P-oligosaccharide assembly is initiated in the cytosol and that a downstream intermediate must translocate to the lumenal face of the ER membrane.

Expression cloning of a novel suppressor of the Lec15 and Lec35 glycosylation mutations of Chinese hamster ovary cells

Lec15 and Lec35 are recessive Chinese hamster ovary (CHO) cell glycosylation mutations characterized by inefficient synthesis and utilization, respectively, of mannose-P-dolichol (MPD). Consequently, Lec15 and Lec35 cells accumulate Man5GlcNAc2-P-P-dolichol and glucosaminyl-acylphosphatidylinositol. This report describes the cloning of a suppressor (termed SL15) of the Lec15 and Lec35 mutations from a CHO cDNA library by functional expression in Lec15 cells, employing phytohemagglutinin/swainsonine selection. The SL15 protein has a predicted molecular weight of 26,693 with two potential membrane spanning regions and a likely C-terminal endoplasmic reticulum retention signal (Lys-Lys-Glu-Gln). Lec15 cells transfected with SL15 have normal levels of MPD synthase activity in vitro and convert Man5GlcNAc2-P-P-dolichol to Glc0-3Man9GlcNAc2-P-P-dolichol in vivo. Surprisingly, SL15 also corrects the defective mannosylation in Lec35 cells. The SL15 protein bears no apparent similarity to Saccharomyces cerevisiae MPD synthase (the DPM1 protein), but is highly similar to the hypothetical F38E1.9 protein encoded on Caenorhabditis elegans chromosome 5. These results indicate a novel function for the SL15 protein and suggest that MPD synthesis is more complex than previously suspected.

Negative regulatory element involved in the hormonal regulation of GlcNAc-1-P transferase gene in mouse mammary gland

The gene encoding UDP-GlcNAc:dolichol phosphate N-acetylglucosamine-1-phosphate transferase (GPT), the enzyme that initiates the pathway for the biosynthesis of asparagine-linked glycoproteins, is ubiquitously expressed in eukaryotic cells. However, its expression in the mammary gland is developmentally and hormonally regulated; transcription of the mouse mammary GPT gene is stimulated by the lactogenic hormones, insulin, glucocorticoid, and prolactin. The involvement of cisacting elements in regulating the expression of the mouse GPT (mGPT) gene was investigated by transient transfections of various GPT promoter/luciferase (Luc) constructs into primary mouse mammary epithelial cells. A series of 5'-deletions of the GPT promoter identified a distal negative regulatory region (base pairs -1057 to -968) and deletion of this region results in enhanced hormonal induction (approximately 7-fold) with no effect on basal promoter activity. Electrophoretic mobility shift assays (EMSA) performed with nuclear extracts from different developmental stages of mouse mammary gland demonstrated that the binding activity of the nuclear proteins to the distal negative regulatory region was predominant in virgin stage as compared with pregnant and lactating stages. EMSA performed with nuclear extracts from virgin explants showed that the binding activity was markedly decreased after cultivation with the combination of the three lactogenic hormones. DNase I footprinting analysis identified two pentamer direct repeat motifs, AGGAA and GAAAC, within the negative regulatory region. EMSA competition experiments showed that mutations within the direct repeats failed to compete for binding of the nuclear proteins to labeled wild type oligonucleotide. Transcription from the promoter containing the mutated direct repeats was increased greatly, consistent with the conclusion that these motifs functions in vivo to repress GPT gene expression. These data suggest the importance of the negative regulatory region in hormonal control of mGPT gene expression in mammary gland.

UDP-N-acetylglucosamine:dolichyl-phosphate N-acetylglucosamine-1-phosphate transferase is amplified in tunicamycin-resistant soybean cells

A tunicamycin-resistant soybean cell line was developed by gradually increasing the concentration of tunicamycin in the growth medium. At the final stage, the resistant cells could survive in media containing 60 micrograms/ml of tunicamycin, whereas normal cells show a greatly retarded growth rate at 0.5 microgram/ml of antibiotic. The tunicamycin-resistant cells had a greater than 40-fold increase in the activity of the enzyme UDP-GlcNAc:dolichyl-P GlcNAc1P transferase, a 2-3-fold increase in the activity of dolichyl-P-mannose synthase, but no increase in the activities of other enzymes of the lipid-linked saccharide pathway such as dolichyl-P-glucose synthase or mannosyl transferases. There was also no change in the activities of the glycoprotein-processing enzymes, glucosidase I or glucosidase II, as compared to wild-type cells. The increase in GlcNAc1P transferase was due to an increased production of enzyme, as seen by a dramatic increase in the amount of a 39-kDa protein, which is presumed to be this enzyme protein. The GlcNAc1P transferase from tunicamycin-resistant cells was equally sensitive to tunicamycin as was the wild-type enzyme, but was considerably more labile to temperatures above 30 degrees C. The activity in tunicamycin-resistant cells was greatly stimulated by exogenous dolichyl-P. The spectrum of oligosaccharides from labeled lipid-linked oligosaccharides was similar in wild-type and tunicamycin-resistant soybean cells, but the resistant cells had significantly greater amounts of the shorter and much lower amounts of the larger-sized oligosaccharides.

Diminished activity of the first N-glycosylation enzyme, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), gives rise to mutant phenotypes in yeast

The enzyme which initiates the dolichol pathway of protein N-glycosylation, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), is encoded by the ALG7 gene. Essential for viability, ALG7 has been evolutionarily conserved and shown to be involved in a variety of functions. ALG7 is an early growth-response gene in yeast, and downregulation of ALG7 expression results in diminished N-glycosylation and secretion of Xenopus oocyte proteins. We have now investigated the consequences of diminished GPT activity in yeast using mutant ALG7 genes with deletions in the 3' untranslated region (3' UTR). We show that a 2.5- to 4-fold reduction in GPT activity gave rise to distinct phenotypes, whose severity was inversely related to the level of GPT activity. These phenotypes included hypersensitivity to tunicamycin, enlarged cell size, extensive aggregation, lack of a typical stationary (G0) arrest, and defective spore germination. We conclude that yeast cells are sensitive to GPT dosage, and that attenuation of GPT activity interferes with various functions in the yeast life cycle.

From peptidoglycan to glycoproteins: common features of lipid-linked oligosaccharide biosynthesis

The peptidoglycan layer of bacterial cell walls is biosynthesised using a lipid carrier undecaprenyl phosphate to assemble and transport the MurNAc(GlcNAc)-pentapeptide precursor. Similar lipid-linked cycles are involved in the biosynthesis of other bacterial exopolysaccharides and eukaryotic asparagine-linked glycoproteins, the latter involving the structurally related dolichyl phosphate as a lipid carrier. Recent protein sequence data and common inhibitors of the bacterial and eukaryotic systems have revealed functional similarities between the two systems. Biological and physical studies on the lipid carriers themselves have provided clues to their role in oligosaccharide translocation, but have not revealed significant differences in function between undecaprenyl phosphate and dolichyl phosphate. The presence of dolichyl phosphate and a family of saturated isoprenoid lipids in Archaebacteria suggests a possible evolutionary link between the two systems.

Identification of cpsD, a gene essential for type III capsule expression in group B streptococci

We showed previously that a mutant strain of group B Streptococcus (GBS) defective in capsule production was avirulent. This study describes the derivation of an unencapsulated mutant from a highly encapsulated wild-type strain of type III GBS, COH1, by transposon mutagenesis with Tn916 delta E. The mutant, COH1-13, was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat sepsis model compared with the wild-type strain. No capsular polysaccharide was evident in the cytoplasm or on the cell surface of the mutant strain. The Tn916 delta E insertion site in COH1-13 was mapped to the same chromosomal location as the Tn916 insertion site in the unencapsulated type III mutant COH31-15 reported previously. Nucleotide sequencing of DNA flanking the insertion site in COH1-13 revealed an open reading frame, designated cpsD, with significant homology to the rfbP gene of Salmonella typhimurium. RfbP encodes a galactosyl transferase enzyme that catalyses the transfer of galactose to undecaprenol phosphate, the initial step in O-polysaccharide synthesis. A particulate fraction of a lysate of wild-type strain GBS COH1 mediated the transfer of galactose from UDP-galactose to an endogenous acceptor. The galactose-acceptor complex partitioned into organic solvents, suggesting it is lipid in nature or membrane-associated. Galactosyl transferase activity was significantly reduced in the unencapsulated mutant strain COH1-13. These results, together with the similarity in deduced amino acid sequence between cpsD and rfbP suggest that cpsD encodes a galactosyl transferase essential for assembly of the GBS type III capsular polysaccharide.

Clinical aspects of glycoprotein biosynthesis

Glycoproteins are widely distributed among species in soluble and membrane-bound forms, associated with many different functions. The heterogenous sugar moieties of glycoproteins are assembled in the endoplasmic reticulum and in the Golgi and are implicated in many roles that require further elucidation. Glycoprotein-bound oligosaccharides show significant changes in their structures and relative occurrences during growth, development, and differentiation. Diverse alterations of these carbohydrate chains occur in diseases such as cancer, metastasis, leukemia, inflammatory, and other diseases. Structural alterations may correlate with activities of glycosyltransferases that assemble glycans, but often the biochemical origin of these changes remains unclear. This suggests a multitude of biosynthetic control mechanisms that are functional in vivo but have not yet been unraveled by in vitro studies. The multitude of carbohydrate alterations observed in disease states may not be the primary cause but may reflect the growth and biochemical activity of the affected cell. However, knowledge of the control mechanisms in the biosynthesis of glycoprotein glycans may be helpful in understanding, diagnosing, and treating disease.

Oligosaccharyltransferase activity is associated with a protein complex composed of ribophorins I and II and a 48 kd protein

Oligosaccharyltransferase catalyzes the N-linked glycosylation of asparagine residues on nascent polypeptides in the lumen of the rough endoplasmic reticulum (RER). A protein complex composed of 66, 63, and 48 kd subunits copurified with oligosaccharyltransferase from canine pancreas. The 66 and 63 kd subunits were shown by protein immunoblotting to be identical to ribophorin I and II, two previously identified RER glycoproteins that colocalize with membrane-bound ribosomes. The transmembrane segment of ribophorin I was found to be homologous to a recently proposed dolichol recognition consensus sequence. Based on a revision of the consensus sequence, we propose a model for the interaction of dolichol with the glycosyltransferases that catalyze the assembly and transfer of lipid-linked oligosaccharides.

Topography of glycosylation reactions in the endoplasmic reticulum

A variety of distinct protein glycosylation reactions occur in the endoplasmic reticulum (ER) of eukaryotic cells. In some instances, both the proteins to be glycosylated and the precursor sugar donors must be translocated across the membrane from the cytoplasm to the lumen of the ER. Elucidation of the individual steps in each of the glycosylation pathways has revealed the topographic complexity of these reactions.

Specific lipids enhance the activity of UDP-GlcNAc: dolichol phosphate GlcNAc-1-phosphate transferase in rat liver endoplasmic reticulum membrane vesicles

The rate of the reaction catalyzed by UDP-N-acetylglucosamine (GlcNAc):dolichol phosphate GlcNAc-1-phosphate transferase in rat liver endoplasmic reticulum vesicles was shown to be influenced by particular lipids. Utilizing in vitro assay conditions where the membrane vesicles retained latency of glucose-6-phosphatase activity, the addition of phosphatidylethanolamine, cardiolipin, or monogalactosyldiglyceride resulted in severalfold increases in the rate of dolichol pyrophosphate N-acetylglucosamine synthesis. Other phospholipids were not stimulatory. These rates were dependent on the concentrations of the exogenous lipids and of the substrate dolichol phosphate. In the presence of cardiolipin, the membrane-bound enzyme became more susceptible to inactivation by protease K and to inhibition by tunicamycin. Titration of cardiolipin-containing endoplasmic reticulum vesicles with adriamycin indicated that the majority of the cardiolipin was exposed on the outer surface. These results suggest that the particular lipids altered membrane structure in a way that allowed further access of the enzyme to substrate, inhibitor, and other molecules. Lipids observed in these studies to be stimulatory are known to exist in the macromolecular hexagonal phase and may therefore be affecting the GlcNAc-1-phosphate transferase by locally disrupting the bilayer structure of the membrane. As other dolichol-utilizing enzymes have been previously observed by other investigators to be similarly influenced by such lipids, the effects may be common to enzymes of the dolichol cycle.