Topography of initiation of N-glycosylation reactions

Proteomic analysis of diabetic retinas

Introduction

As a metabolic disease, diabetes often leads to health complications such as heart failure, nephropathy, neurological disorders, and vision loss. Diabetic retinopathy (DR) affects as many as 100 million people worldwide. The mechanism of DR is complex and known to impact both neural and vascular components in the retina. While recent advances in the field have identified major cellular signaling contributing to DR pathogenesis, little has been reported on the protein post-translational modifications (PTM) - known to define protein localization, function, and activity - in the diabetic retina overall. Protein glycosylation is the enzymatic addition of carbohydrates to proteins, which can influence many protein attributes including folding, stability, function, and subcellular localization. O-linked glycosylation is the addition of sugars to an oxygen atom in amino acids with a free oxygen atom in their side chain (i.e., threonine, serine). To date, more than 100 congenital disorders of glycosylation have been described. However, no studies have identified the retinal O-linked glycoproteome in health or disease. With a critical need to expedite the discovery of PTMomics in diabetic retinas, we identified both global changes in protein levels and the retinal O-glycoproteome of control and diabetic mice.

Methods

We used liquid chromatography/mass spectrometry-based proteomics and high throughput screening to identify proteins differentially expressed and proteins differentially O-glycosylated in the retinas of wildtype and diabetic mice.

Results

Changes in both global expression levels of proteins and proteins differentially glycosylated in the retinas of wild-type and diabetic mice have been identified. We provide evidence that diabetes shifts both global expression levels and O-glycosylation of metabolic and synaptic proteins in the retina.

Discussion

Here we report changes in the retinal proteome of diabetic mice. We highlight alterations in global proteins involved in metabolic processes, maintaining cellular structure, trafficking, and neuronal processes. We then showed changes in O-linked glycosylation of individual proteins in the diabetic retina.

Identification of novel potential interaction partners of UDP-galactose (SLC35A2), UDP-N-acetylglucosamine (SLC35A3) and an orphan (SLC35A4) nucleotide sugar transporters

Nucleotide sugar transporters (NSTs) are ER and Golgi-resident members of the solute carrier 35 (SLC35) family which supply substrates for glycosylation by exchanging lumenal nucleotide monophosphates for cytosolic nucleotide sugars. Defective NSTs have been associated with congenital disorders of glycosylation (CDG), however, molecular basis of many types of CDG remains poorly characterized. To better understand the biology of NSTs, we identified potential interaction partners of UDP-galactose transporter (SLC35A2), UDP-N-acetylglucosamine transporter (SLC35A3) and an orphan nucleotide sugar transporter SLC35A4 of to date unassigned specificity. For this purpose, each of the SLC35A2-A4 proteins was used as a bait in four independent pull-down experiments and the identity of the immunoprecipitated material was discovered using MS techniques. From the candidate list obtained, we selected a few for which the interaction was confirmed in vitro using the NanoBiT system, a split luciferase-based luminescent technique. NSTs have been shown to interact with two ATPases (ATP2A2, ATP2C1), Golgi pH regulator B (GPR89B) and calcium channel (TMCO1), which may reflect the regulation of glycosylation by ion homeostasis, and with basigin (BSG). Our findings provide a starting point for the NST interaction network discovery in order to better understand how glycosylation is regulated and linked to other cellular processes. SIGNIFICANCE: Despite the facts that nucleotide sugar transporters are a key component of the protein glycosylation machinery, and deficiencies in their activity underlie serious metabolic diseases, biology, function and regulation of these essential proteins remain enigmatic. In this study we have advanced the field by identifying sets of new potential interaction partners for UDP-galactose transporter (SLC35A2), UDP-N-acetylglucosamine transporter (SLC35A3) and an orphan transporter SLC35A4 of yet undefined role. Several of these new interactions were additionally confirmed in vitro using the NanoBiT system, a split luciferase complementation assay. This work is also significant in that it addresses the overall challenge of discovering membrane protein interaction partners by a detailed comparison of 4 different co-immunoprecipitation strategies and by custom sample preparation and data processing workflows.

Structure and mechanism of an active lipid-linked oligosaccharide flippase

The flipping of membrane-embedded lipids containing large, polar head groups is slow and energetically unfavourable, and is therefore catalysed by flippases, the mechanisms of which are unknown. A prominent example of a flipping reaction is the translocation of lipid-linked oligosaccharides that serve as donors in N-linked protein glycosylation. In Campylobacter jejuni, this process is catalysed by the ABC transporter PglK. Here we present a mechanism of PglK-catalysed lipid-linked oligosaccharide flipping based on crystal structures in distinct states, a newly devised in vitro flipping assay, and in vivo studies. PglK can adopt inward- and outward-facing conformations in vitro, but only outward-facing states are required for flipping. While the pyrophosphate-oligosaccharide head group of lipid-linked oligosaccharides enters the translocation cavity and interacts with positively charged side chains, the lipidic polyprenyl tail binds and activates the transporter but remains exposed to the lipid bilayer during the reaction. The proposed mechanism is distinct from the classical alternating-access model applied to other transporters.

Protein glycosylation in Candida

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

Specific transbilayer translocation of dolichol-linked oligosaccharides by an endoplasmic reticulum flippase

The oligosaccharide donor for protein N-glycosylation, Glc(3)Man(9)GlcNAc(2)-PP-dolichol, is synthesized via a multistep pathway that starts on the cytoplasmic face of the endoplasmic reticulum (ER) and ends in the lumen where the glycosylation reaction occurs. This necessitates transbilayer translocation or flipping of the lipid intermediate Man(5)GlcNAc(2)-PP-dolichol (M5-DLO) across the ER membrane. The mechanism by which M5-DLO-or any other lipid-is flipped across the ER is unknown, except that specific transport proteins or flippases are required. We recently demonstrated M5-DLO flipping activity in proteoliposomes reconstituted from detergent-solubilized ER membrane proteins and showed that it was ATP-independent and required a trypsin-sensitive protein that sedimented at approximately 4S. By using an activity-enriched fraction devoid of glycerophospholipid flippase activity, we now report that M5-DLO is rapidly flipped in the reconstituted system with a time constant tau <2 min, whereas its triantennary structural isomer is flipped slowly with tau >200 min. DLOs larger than M5-DLO are also poorly translocated, with tau ranging from approximately 10 min to >200 min. We conclude that (i) the number and arrangement of mannoses in the DLO glycan has a profound effect on the ability of the DLO to be translocated by the flippase, (ii) glycan size per se does not dictate whether a DLO will be flipped, and (iii) the flippase is highly specific for M5-DLO. Our results suggest a simple structural model for the interaction between the DLO head group and the flippase.

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.

Galactosyltransferase--still up and running

The following review on galactosyltransferase (gal-T1) intends to cover genetic, biochemical, structural, biotechnological, cell biological and medical aspects of this enzyme in a comprehensive manner from discovery to the present day which have brought to light a genetic defect of this enzyme. Early work has only been included if it appeared relevant to ongoing issues. Following the evolution of a research topic over 40 years is in itself a fascinating endeavor as it permits to observe the ins and outs of hypotheses, fashions and errors. Gal-T1 is a beautiful example as it has been involved in almost every aspect of life science. Importantly, there is a future to this enzyme as a research topic, since many questions still remain unanswered: to which extent is it a representative Golgi protein? What is the role of the gene family of gal-Ts? Does gal-T1 exert any functions other than a catalytic one? Why is it phosphorylated? Does it form homodimers in vivo? Surely, there is room for further work, which is likely to reveal further insights into cellular trafficking and signaling and, in the context of the gene family, shall contribute to understanding development and morphogenesis.

Congenital disorders of glycosylation type Ig is defined by a deficiency in dolichyl-P-mannose:Man7GlcNAc2-PP-dolichyl mannosyltransferase

Type I congenital disorders of glycosylation (CDG I) are diseases presenting multisystemic lesions including central and peripheral nervous system deficits. The disease is characterized by under-glycosylated serum glycoproteins and is caused by mutations in genes encoding proteins involved in the stepwise assembly of dolichol-oligosaccharide used for protein N-glycosylation. We report that fibroblasts from a type I CDG patient, born of consanguineous parents, are deficient in their capacity to add the eighth mannose residue onto the lipid-linked oligosaccharide precursor. We have characterized cDNA corresponding to the human ortholog of the yeast gene ALG12 that encodes the dolichyl-P-Man:Man(7)GlcNAc(2)-PP-dolichyl alpha6-mannosyltransferase that is thought to accomplish this reaction, and we show that the patient is homozygous for a point mutation (T571G) that causes an amino acid substitution (F142V) in a conserved region of the protein. As the pathological phenotype of the fibroblasts of the patient was largely normalized upon transduction with the wild type gene, we demonstrate that the F142V substitution is the underlying cause of this new CDG, which we suggest be called CDG Ig. Finally, we show that the fibroblasts of the patient are capable of the direct transfer of Man(7)GlcNAc(2) from dolichol onto protein and that this N-linked structure can be glucosylated by UDP-glucose:glycoprotein glucosyltransferase in the endoplasmic reticulum.

Large-scale isolation of dolichol-linked oligosaccharides with homogeneous oligosaccharide structures: determination of steady-state dolichol-linked oligosaccharide compositions

The dolichol-linked oligosaccharide donor (Glc(3)Man(9)GlcNAc(2)-PP-Dol) for N-linked glycosylation of proteins is assembled in a series of reactions that initiate on the cytoplasmic face of the rough endoplasmic reticulum and terminate within the lumen. The biochemical analysis of the oligosaccharyltransferase and the glycosyltransferases that mediate assembly of dolichol-linked oligosaccharides (OS-PP-Dol) has been hindered by the lack of structurally homogeneous substrate preparations. We have developed an improved method for the preparative-scale isolation of dolichol-linked oligosaccharides from vertebrate tissues and yeast cells. Preparations that were highly enriched in either Glc(3)Man(9)GlcNAc(2)-PP-Dol or Man(9)GlcNAc(2)-PP-Dol were obtained from porcine pancreas and a Man(5)GlcNAc(2)-PP-Dol preparation was obtained from an alg3 yeast culture. Chromatography of the OS-PP-Dol preparations on an aminopropyl silica column was used to obtain dolichol-linked oligosaccharides with defined structures. A single chromatography step could achieve near-baseline resolution of dolichol-linked oligosaccharides that differed by one sugar residue. A sensitive oligosaccharyltransferase endpoint assay was used to determine the concentration and composition of the OS-PP-Dol preparations. Typical yields of Glc(3)Man(9)GlcNAc(2)-PP-Dol, Man(9)GlcNAc(2)-PP-Dol, and Man(5)GlcNAc(2)-PP-Dol ranged between 5 and 15 nmol per chromatographic run. The homogeneity of these preparations ranged between 85 and 98% with respect to oligosaccharide composition. Purification of dolichol-linked oligosaccharides from cultures of alg mutant yeast strains provides a general method to obtain authentic OS-PP-Dol assembly intermediates of high purity. The analytical methods described here can be used to accurately evaluate the steady-state dolichol-linked oligosaccharide compositions of wild-type and mutant cell lines.

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.

Regulation of the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol, feedback and product inhibition

The assembly of the core oligosaccharide region of asparagine-linked glycoproteins proceeds by means of the dolichol pathway. The first step of this pathway, the reaction of dolichol phosphate with UDP-GlcNAc to form N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-dolichol), is under investigation as a possible site of metabolic regulation. This report describes feedback inhibition of this reaction by the second intermediate of the pathway, N-acetylglucosaminyl-N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-GlcNAc-P-P-dolichol), and product inhibition by GlcNAc-P-P-dolichol itself. These influences were revealed when the reactions were carried out in the presence of showdomycin, a nucleoside antibiotic, present at concentrations that block the de novo formation of GlcNAc-GlcNAc-P-P-dolichol but not that of GlcNAc-P-P-dolichol. The apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol under basal conditions were 4.4 and 2.8 microM, respectively. Inhibition was also observed under conditions where mannosyl-P-dolichol (Man-P-dol) stimulated the biosynthesis of GlcNAc-P-P-dolichol; the apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol were 2.2 and 11 microM, respectively. Kinetic analysis of the types of inhibition indicated competitive inhibition by GlcNAc-P-P-dolichol toward the substrate UDP-GlcNAc and non-competitive inhibition toward dolichol phosphate. Inhibition by GlcNAc-GlcNAc-P-P-dolichol was uncompetitive toward UDP-GlcNAc and competitive toward dolichol phosphate. A model is presented for the kinetic mechanism of the synthesis of GlcNAc-P-P-dolichol. GlcNAc-P-P-dolichol also exerts a stimulatory effect on the biosynthesis of Man-P-dol, i.e. a reciprocal relationship to that previously observed between these two intermediates of the dolichol pathway. This network of inhibitory and stimulatory influences may be aspects of metabolic control of the pathway and thus of glycoprotein biosynthesis in general.

Segregation of glycosylphosphatidylinositol biosynthetic reactions in a subcompartment of the endoplasmic reticulum

Glycosylphosphatidylinositols (GPIs) are synthesized in the endoplasmic reticulum (ER) via the sequential addition of monosaccharides, fatty acid, and phosphoethanolamine(s) to phosphatidylinositol (PI). While attempting to establish a mammalian cell-free system for GPI biosynthesis, we found that the assembly of mannosylated GPI species was impaired when purified ER preparations were substituted for unfractionated cell lysates as the enzyme source. To explore this problem we analyzed the distribution of the various GPI biosynthetic reactions in subcellular fractions prepared from homogenates of mammalian cells. The results indicate the following: (i) the initial reaction of GPI assembly, i.e. the transfer of GlcNAc to PI to form GlcNAc-PI, is uniformly distributed in the ER; (ii) the second step of the pathway, i.e. de-N-acetylation of GlcNAc-PI to yield GlcN-PI, is largely confined to a subcompartment of the ER that appears to be associated with mitochondria; (iii) the mitochondria-associated ER subcompartment is enriched in enzymatic activities involved in the conversion of GlcN-PI to H5 (a singly mannosylated GPI structure containing one phosphoethanolamine side chain; and (iv) the mitochondria-associated ER subcompartment, unlike bulk ER, is capable of the de novo synthesis of H5 from UDP-GlcNAc and PI. The confinement of these GPI biosynthetic reactions to a domain of the ER provides another example of the compositional and functional heterogeneity of the ER. The implications of these findings for GPI assembly are discussed.

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.

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.

UDP-sugar pyrophosphatase of rat retina: subcellular localization and topography

Rat retinal tissue possesses as a developmentally regulated, highly active pyrophosphatase activity that hydrolyzes UDP-GalNAc and UDP-Gal but not CMP-NeuAc (Martina et al.: J Neurochem 62:1274-1280, 1995). We show here that this activity, measured with UDP-[3H]GalNAc as substrate, is associated to the membrane fraction of rat retinal homogenates and, upon subfractionation by isopycnic centrifugation in sucrose density gradients, is concentrated in fractions enriched in light Golgi membranes. We examined also the topographic disposition of the catalytic site of the enzyme in the transverse plane of the membranes by measuring the effect of protease treatment and of added EDTA on its activity. Pronase inhibited 50% of the translocation of UDP-[3H]GalNAc to the lumen of the Golgi vesicles but did not affect the enzyme activity either in the absence or in the presence of detergent. EDTA, a membrane-impermeant molecule, inhibited 90% of the activity of the enzyme but did not affect translocation of UDP-[3H]GalNAc and inhibited only 25% the incorporation of [3H]GalNAc into endogenous glycoconjugates. These results indicate that the translocation of UDP-[3H]GalNAc was not necessary for hydrolysis to occur and strongly suggest that the catalytic site of the UDP-sugar pyrophosphatase is oriented toward the cytosolic side of the Golgi vesicles. We speculate that this activity limits the availability of UDP-GalNAc to its specific translocator and, consequently, the luminal concentration of the nucleotide in the Golgi vesicles. In this way, by limiting the availability of UDP-GalNAc for the conversion of GM3 to GM2 by the GM3:N-acetyl-galactosaminyl transferase, it would contribute to the preferential use of GM3 for synthesis of GD3 and other "b" pathway gangliosides that are characteristic of the rat retina.

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.

Activation of GlcNAc-P-P-dolichol synthesis by mannosylphosphoryldolichol is stereospecific and requires a saturated alpha-isoprene unit

Exogenous mannosylphosphoryldolichol (Man-P-Dol) has previously been shown to stimulate UDP-GlcNAc:dolichyl phosphate N-acetylglucosamine 1-phosphate transferase (GPT1), the enzyme catalyzing the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-Dol). To define the structural specificity of the mannolipid-mediated activation of GPT1, the ability of a variety of mannosylphosphorylisoprenols to stimulate GlcNAc-lipid biosynthesis in microsomal preparations from retinas of the embryonic chick has been tested. For these comparisons several Man-P-isoprenols were synthesized enzymatically and chemically. The catalytic efficiency of activation expressed as the Vmax/Ka ratio was substantially higher for Man-P-Dol95 than for mannosylphosphorylpolyprenol95 (Man-P-Poly95), demonstrating that the saturated alpha-isoprene unit of the dolichyl moiety influences the mannolipid-enzyme interaction. The degree of activation increased with chain length and hydrophobicity of the dolichyl moiety when Man-P-dolichols containing 2, 11, and 19 isoprene units were evaluated. A strict stereospecificity was exhibited as beta-Man-P-Dol95 provided a 100-fold greater stimulation than the corresponding alpha-stereoisomer. The recognition of the saturated alpha-isoprene unit, the influence of chain length, and the strict stereospecificity of the interaction between beta-Man-P-Dol and GPT1 extend the description of the mannolipid-enzyme interaction and provide strong new evidence that Man-P-Dol levels can influence the rate of GlcNAc-P-P-Dol synthesis.

Functional characterization of carrier-mediated transport of uridine diphosphate N-acetylglucosamine across the endoplasmic reticulum membrane

Uptake and metabolism of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) by rough-endoplasmic-reticulum (rER)-derived vesicles was studied. Analysis of the molecular species, double-label experiments, cis-inhibition and trans-stimulation experiments revealed that uptake represented entry of intact UDP-GlcNAc into the microsomal lumen. The amount of vesicle-associated label at equilibrium uptake was directly proportional to the volume of the intravesicular space, and permeabilized microsomes were unable to retain UDP-GlcNAc. These findings indicate that uptake constituted effective translocation from the medium into the lumen of the vesicles. The microsomal uptake of UDP-GlcNAc met the criteria of bidirectional, carrier-mediated translocation. Transport was time and temperature dependent, saturable, selective, capable of trans-stimulation and operational against a concentration gradient. Uptake studies performed in membrane preparations that were highly enriched in either rER, smooth ER, or Golgi revealed that UDP-GlcNAc was taken up by the ER and by the Golgi apparatus.

A Mucor pusillus mutant defective in asparagine-linked glycosylation

A Mucor pusillus mutant defective in asparagine-linked glycosylation was found in our stock cultures. This mutant, designated 1116, secreted aspartic proteinase (MPP) in a less-glycosylated form than that secreted by the wild-type strain. Analysis of enzyme susceptibility, lectin binding, and carbohydrate composition indicated that this mutant secreted three glycoforms of MPPs, one of which contained no carbohydrate; the other two had truncated asparagine-linked oligosaccharide chains such as Man0-1GlcNAc2. Further analysis using oligosaccharide processing inhibitors, such as castanospermine, 1-deoxynojirimycin and N-methyldeoxynojirimycin, suggested that MPPs in the mutant were glycosylated through a transfer of the truncated lipid-linked oligosaccharides, Man0-1GlcNAc2, to the MPP protein but not through an aberrant processing. In addition, genetic studies with forced primary heterokaryons indicated that the mutation in strain 1116 was recessive.

Topography of basal and stimulated biosynthesis of GlcNAc-P-P-dolichol and (GlcNAc)2-P-P-dolichol: variation in effect of proteolysis

Evidence has recently appeared indicating the cytoplasmic orientation in microsomal vesicles of the GlcNAc-transferases that participate in the biosynthesis of GlcNAc-P-P-dolichol and (GlcNAc)2-P-P-dolichol. The topography of the stimulation of these activities brought about by dolichol-P-mannose and phosphatidylglycerol, however, is not known. The present report continues studies of the topography of these early reactions of the dolichol pathway, examined under basal and stimulatory conditions after proteolysis of intact microsomes isolated from livers of the embryonic chick. Under all conditions which were examined, the GlcNAc-transferase concerned with the biosynthesis of the chitobiosyl derivative was completely inhibited, consistent with its cytoplasmic orientation. The effect of proteolysis on the formation of GlcNAc-P-P-dolichol, however, varied with different proteolytic enzymes, ranging from no inhibition to over 90% inhibition. This variation in response suggests that the GlcNAc-transferase may be partially buried within the microsomal bilayer which only some preparations of proteolytic enzymes can penetrate. Evidence was obtained indicating the cytoplasmic orientation of the stimulation of GlcNAc-P-P-dolichol biosynthesis. These studies support the conclusion that both the catalytic and allosteric activating sites of the GlcNAc-transferase have a similar topography.

Monoclonal antibody to amphomycin. A tool to study the topography of dolichol monophosphate in the membrane

Understanding the topographical orientation of dolichol monophosphate (Dol-P) in the membrane of the endoplasmic reticulum (ER) is of utmost importance for studying the regulation of asparagine-linked protein glycosylation in eukaryotic cells. This was practically impossible due to the nonavailability of a suitable probe. Recent studies on the specific interaction between a lipopeptide, amphomycin, and Dol-P, provided an insight to develop a monospecific antibody to amphomycin which could recognize the amphomycin-Dol-P complex in order to detect Dol-P immunocytochemically in the ER membrane. We report herein the successful production of a monoclonal antibody to amphomycin. The antibody belongs to the IgG+IgM subclasses and is specific for amphomycin when analyzed by the enzyme-linked immunoassay and immunoblot procedures. The antibody recognizes with equal potency both the native amphomycin and also mild acid-hydrolyzed amphomycin from which N-terminal fatty acylated aspartic acid has been removed. Preincubation of amphomycin with the antibody partially reduced the inhibitory action of amphomycin on dolichol phosphate mannosyltransferase (EC 2.4.1.83). Furthermore, exposure of capillary endothelial cells to amphomycin, followed by the monoclonal antibody to amphomycin, followed sequentially by staining with FITC-conjugated goat anti-mouse IgG and examination under a fluorescent microscope gives intense fluorescence at the perinuclear region of the cell with a structure reminiscent of the ER.

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.