Glycoprotein biosynthesis in the developing rat brain. II. Microsomal galactosaminyltransferase utilizing endogenous and exogenous protein acceptors

Polypeptide N-acetylgalactosaminyltransferase activity in tracheal epithelial microsomes

Pig tracheal epithelium, a site of extensive mucin biosynthesis, contained polypeptide N-acetylgalactosaminyltransferase activity directed towards L-threonine residues. The enzyme preparation was broadly similar in properties to preparations from other tissues, e.g. pig and bovine submaxillary glands, bovine colostrum, BW5147 mouse lymphoma and baby-hamster kidney cells. Enzyme was membrane-bound and was released from microsomal preparations by extraction with Triton X-100. Extracted enzyme had a pH optimum of 7.5, had a requirement for Mn2+ (10 mM) and was inhibited by Na2EDTA. The Km for UDP-N-acetylgalactosamine was 110 microM and that for an octapeptide acceptor (VTPRTPPP) was 3.0 mM at 37 degrees C. Using a range of synthetic peptides of known structure related to TPPP it was established that L-threonine residues were specifically O-glycosylated probably in the alpha-configuration. Synthetic peptides containing the TPPP sequence required a peptide length of five or more for significant acceptor activity. In VTPRTPPP the two threonine residues were similarly glycosylated, as revealed by tryptic cleavage of the glycosylated product and separation of the 3H-labelled fragments. The enzyme preparation also specifically catalysed the transfer of N-acetylgalactosaminyl residues from UDP-N-acetyl[1-3H]galactosamine to bovine submaxillary mucin core protein and to myelin basic protein.

The attachment of UDP-hexosamines to the ribosomes isolated from rat liver

The binding of UDP-N-acetylhexosamines with purified ribosomes was studied and it was found that the radioactive nucleotides can be attached to these particles. The radioactivity of the purified ribosomal pellet depends on the amounts of ribosomes and UDP-N-acetylhexosamines. Some characteristics of the binding system indicate that the attachment of UDP-sugar to ribosome does not require the participation of glycosyltransferases. The results of the competition experiment would suggest that there are specific sites on ribosomes for the binding of UDP-N-acetylglucosamine.

Site of addition of N-acetyl-galactosamine to the E1 glycoprotein of mouse hepatitis virus-A59

By pulse-chase labeling with [35S]methionine and long-term labeling with 3H-sugars, the E1 glycoprotein of coronavirus MHV-A59 has been shown to acquire O-linked oligosaccharides in a two-step process. About 10 min after synthesis of the E1 protein, N-acetyl-galactosamine was added. This was followed approximately 10 min later by the addition of both galactose and sialic acid to give the mature oligosaccharides. This sequence of additions was confirmed by analyzing the 3H-labeled oligosaccharides bound to each of the E1 forms using gel filtration on P4 columns. The intracellular location of the first step was determined by exploiting the temperature sensitivity of virus release. The virus normally buds first into a smooth membrane compartment lying between the rough endoplasmic reticulum and the cis side of the Golgi stack (Tooze et al., 1984). At 31 degrees C the virus is assembled but does not appear to enter the Golgi stacks. The addition of N-acetyl-galactosamine is unaffected although the addition of galactose and sialic acid is inhibited. These results strongly suggest that addition of N-acetyl-galactosamine occurs in this budding compartment, the morphology of which is similar to that of transitional elements and vesicles.

Two enzymes involved in the synthesis of O-linked oligosaccharides are localized on membranes of different densities in mouse lymphoma BW5147 cells

Microsomal membranes from mouse lymphoma BW5147 cells were fractionated on a continuous sucrose gradient and assayed for two enzymes involved in the synthesis of O-linked oligosaccharides. Both enzymes were recovered in membranes that were less dense than the membranes containing the endoplasmic reticulum marker enzymes, glucosidase I and II. UDP-Gal:N-acetylgalactosamine-beta 1, 3-galactosyltransferase had a distribution that coincided with that of the galactosyltransferase that acts on asparagine-linked oligosaccharides. This latter enzyme has been immunolocalized to the trans Golgi elements. The UDP-GalNAc:polypeptide N-acetylgalactosaminyl-transferase was recovered in a membrane fraction of intermediate density, between the endoplasmic reticulum and trans Golgi markers. These findings are consistent with the assembly of O-linked oligosaccharides occurring in at least two different Golgi compartments.

Cytochemical localization of terminal N-acetyl-D-galactosamine residues in cellular compartments of intestinal goblet cells: implications for the topology of O-glycosylation

The O-linked oligosaccharides of mucin-type glycoproteins contain N-acetyl-D-galactosamine (GalNAc) that is not found in N-linked glycoproteins. Because Helix pomatia lectin interacts with terminal GalNAc, we used this lectin, bound to particles of colloidal gold, to localize such sugar residues in subcellular compartments of intestinal goblet cells. When thin sections of low temperature Lowicryl K4M embedded duodenum or colon were incubated with Helix pomatia lectin-gold complexes, no labeling could be detected over the cisternal space of the nuclear envelope and the rough endoplasmic reticulum. A uniform labeling was observed over the first and several subsequent cis Golgi cisternae and over the last (duodenal goblet cells) or the two last (colonic goblet cells) trans Golgi cisternae as well as forming and mature mucin droplets. However, essentially no labeling was detected over several cisternae in the central (medial) region of the Golgi apparatus. The results strongly suggest that core O-glycosylation takes place in cis Golgi cisternae but not in the rough endoplasmic reticulum. The heterogenous labeling for GalNAc residues in the Golgi apparatus is taken as evidence that termination of certain O-oligosaccharide chains by GalNAc occurs in trans Golgi cisternae.

Studies on the biosynthesis of cartilage proteoglycan in a model system of cultured chondrocytes from the Swarm rat chondrosarcoma

Biosynthesis of cartilage proteoglycan was examined in a model system of cultured chondrocytes from a transplantable rat chondrosarcoma. Extensive modification with the addition of chondroitin sulfate glycosaminoglycan, N-linked oligosaccharide, and O-linked oligosaccharide is required to convert a newly synthesized core protein precursor into a proteoglycan. Kinetic analyses revealed the presence of a large pool of core protein precursor (t 1/2 approximately 90 min) awaiting completion into proteoglycan. The large t 1/2 of this pool allowed kinetic labeling experiments with a variety of radioactive precursors to distinguish between early biosynthetic events associated primarily with the rough endoplasmic reticulum from late events associated primarily with the Golgi apparatus. The results of a series of experiments indicated that the addition of N-linked oligosaccharide chains occurs early in the biosynthetic process in association with the rough endoplasmic reticulum, whereas the initiation and completion of O-linked oligosaccharides occurs much later, at about the same time as chondroitin sulfate synthesis. This also indicated that keratan sulfate chains, when present in the completed molecule, are added in the Golgi apparatus, as they are probably built on oligosaccharide primers closely related to the O-oligosaccharide chains. Furthermore, when 3H-glucose was used as the precursor, the entry of label into xylose, the linkage sugar between the core protein and the chondroitin sulfate chain, was found to occur within 5 min of the entry of label into galactose and galactosamine in the remainder of the chondroitin sulfate chain. This indicated that the initiation and completion of the chondroitin sulfate chain occurs late in the pathway probably entirely in the Golgi apparatus. Thus, proteoglycan synthesis can be described as occurring in two stages in this system, translation and N-glycosylation of a core protein precursor which has a long half-life in the rough endoplasmic reticulum, followed by extensive rapid modification in the Golgi complex in which the majority of glycosaminoglycan and oligosaccharide chains are added to the core protein precursor with subsequent rapid secretion into the extracellular matrix.

Studies on subcellular fractions which are involved in myelin membrane assembly: isolation from developing mouse brain and characterization by enzyme markers, electron microscopy, and electrophoresis

An extensive scheme for the subcellular fractionation of myelinating mouse brain is presented. Several centrifugation procedures for the separation of membranes involved in myelinogenesis are critically appraised, and guidelines for selection of centrifugation conditions are given. Characteristics of subcellular fractions are presented in the form of electron micrographs; also presented are distribution of RNA and protein; electrophoretic profiles of membrane proteins, and verification of the myelin-specific basic proteins, proteolipid protein, and glycoprotein by the immuno-electroblot technique; and the distribution of eight marker enzyme activities. Myelin-related membranes were found to differ both qualitatively and quantitatively in their complement of myelin-specific proteins. These myelin-containing fractions appear to represent different stages of myelination that coexist in developing mouse brain. These results provide the fundamental methodologies and background information for kinetic radioisotope analysis of intracellular events in the assembly of myelin presented in a companion article.

O-linked oligosaccharides are acquired by herpes simplex virus glycoproteins in the Golgi apparatus

The O-linked oligosaccharides on mature forms of herpes simplex virus type 1 (HSV1) glycoproteins were characterized, and were found to account largely for the lower electrophoretic mobilities of these forms relative to the mobilities of immature forms. Other posttranslational modifications of HSV1 glycoproteins (designated gB, gC, gD and gE) were related temporally to the discrete shifts in electrophoretic mobilities that signal acquisition of the O-linked oligosaccharides. Fatty acid acylation (principally of gE) could be detected just prior to the shifts, whereas conversion of high-mannose-type N-linked oligosaccharides to the complex type occurred coincident with the shifts. The addition of O-linked oligosaccharides did not occur in cells treated with the ionophore monensin or in a ricin-resistant cell line defective in the processing of N-linked oligosaccharides. We conclude that extension of O-linked oligosaccharide chains on HSV1 glycoproteins, and probably also attachment of the first O-linked sugars, occurs as a late posttranslational modification in the Golgi apparatus.

Characterization of postmortem human brain proteins by two-dimensional gel electrophoresis

The proteins of membrane and cytosol fractions from frozen human postmortem brain were analyzed by two-dimensional gel electrophoresis (isoelectric range: 5.1-6.0) and both Coomassie-blue and ammoniacal silver staining. Cytosol preparations were analyzed from six different postmortem brains from patients with various neurologic diagnoses and immediate causes of death. Intervals between death and brain freezing (-70 degrees C) ranged from 2 to 20 h. The vast majority of proteins detected in these cytosol fractions had identical molecular weights and isoelectric points in each of six human brains examined. However, in some tissue samples tubulin was either quantitatively decreased or undetectable. The possibility that this partial or complete depletion of tubulin was related to postmortem interval and/or brain freezing was studied using rat forebrain tissue. Rat brain incubated at room temperature for up to 24 h did not reproduce the changes seen in the region of human cytosol tubulin. However, other changes seen in the two-dimensional electrophoretic pattern of rat cytosol proteins did relate to postmortem interval, brain freezing, or both. Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum were prepared from three human brains, with highly reproducible two-dimensional patterns. Protein analysis of these membrane fractions revealed that human RER contained significant amounts of tubulin, in contrast to rat RER which contained no detectable tubulin. This discrepancy was elucidated by allowing rat brains to remain at room temperature for 24 h before freezing; gels of rat RER prepared from this tissue showed that tubulin subunits were present.

Substrate specificity of cerebral GDP-fucose: glycoprotein fucosyltransferase

Solubilized sheep brain fucosyltransferase was shown to transfer fucose from GDP-fucose onto glycoprotein and glycopeptide acceptors, such as asialofetuin, asialotransferrin, their glycopeptides and glycopeptides from ovalbumin, but not on to monosaccharides and disaccharides such as galactose, N-acetylglucosamine and lactose. Competition studies between asialofetuin and glycopeptide V from ovalbumin provided evidence that both substrates compete for a common enzyme active site. The position of the fucosyl linkage was then investigated. Endo-beta-N-glucosaminidase D digestion of fucosylated and acetylated glycopeptide V showed that fucose is not linked to asparagine-linked N-acetylglucosamine. Hydrazinolysis and nitrous acid deamination performed on asialofetuin and glycopeptide V proved that fucose is not linked to external galactose or N-acetylglucosamine either. Thus we assume that fucose is linked to the oligomannochitobiosyl core of the glycan, and probably to the second N-acetylglucosamine.

Effect of hypoxia on the superficially exposed carbohydrates of subcellular fractions of guinea pig cerebral cortex

Experimental hypoxia led to changes in carbohydrate-containing macromolecules located on the microsomal and synaptosomal membrane surface. It was found that hypoxia reduced the external membrane-bound sialic acid content in microsomal and synaptosomal fractions. On the other hand, a decrease in fucose content of proteins in hypoxic synaptosomal fraction was found. Similarly, a decrease was observed in the amount of 3H-labeled concanavalin A bound to the microsomal and synaptosomal glycoprotein fractions. Treatment of microsomal and synaptosomal fractions with galactose oxidase followed by reduction with tritiated potassium borohydride resulted in a significantly higher incorporation rate of tritium into the hypoxic membranes. This was accompanied by an increase in the endogenous glycoprotein galactose transferase activity both in microsomal and synaptosomal fractions.

The characterization of tubulin in CNS membrane fractions

Rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER), and a plasma membrane (PM) fraction enriched in synaptic membranes were isolated from rat forebrain. The proteins in these membrane fractions were analyzed by two-dimensional gel electrophoresis (2DGE) in the isoelectric range of 5.1 to 6.0 by a modification of the O'Farrell procedure. Proteins were detected by Coomassie Brilliant Blue staining of the electrophoretograms. The results of these analyses were compared with 2DGE analysis of cytosol proteins, with particular attention given to tubulin subunits and actin. The RER contained one major protein (53K 5.4) in the beta-tubulin region with a molecular weight of 53,000 and an isoelectric point of 5.4. The SER contained at least two major proteins in the alpha-tubulin region; one with a migration identical to 53K 5.4 and other proteins with slightly higher apparent molecular weights and more acidic isoelectric points (54K, 5.4 to 5.3), identical to cytoplasmic beta-tubulin. The PM fraction also contained multiple overlapping proteins (54K, 5.4 to 5.3) in the beta-tubulin area and a trace amount of the 53K 5.4 protein. The proteins in the beta-tubulin region were removed from the 2DGE electrophoretogram and digested by Staphylococcus aureus V8 protease, and the peptides separated on one-dimensional polyacrylamide gels. The peptide patterns of 53K 5.4 protein from RER and SER were almost identical and differed significantly from the cytoplasmic beta-tubulin pattern; however, the peptide maps of the PM and SER beta-tubulin region were identical to the cytoplasmic beta-tubulin. The 2DGE analysis of RER did not contain proteins in the region of cytoplasmic alpha-tubulin. SER and PM contained proteins in the alpha-tubulin region with a similar, but not identical, peptide analysis to cytoplasmic alpha-tubulin. Significant amounts of actin were detected in 2DGE analysis of SER and PM, and the peptide analysis of the actin was identical to the cytoplasmic actin analysis. The RER fraction contained only trace amounts of actin. The cytosol and all membrane fractions contained a protein (68K 5.6) found among microtubule-associated proteins, as judged by molecular weight and isoelectric point. Several proteins present in all membrane fractions (61K 5.1 and 58K 5.1) bound to concanavalin A agarose.

Enzymatic O-glycosylation of kappa-caseinomacropeptide by ovine mammary Golgi membranes

The results of subcellular fractionation of sheep mammary gland membranes indicate that N-acetylgalactosaminyl polypeptide transferase and galactosyl-N-acetylgalactosaminyl transferase, which are involved in the assembly of disaccharide units of kappa-casein, are localized chiefly in Golgi membranes. The glycosyltransferase activities incorporating N-acetyl [1-14C] galactosamine and [U-14C] galactose from uridine diphosphate N-acetyl [1-14C] galactosamine and uridine diphosphate [U-14C] galactose, respectively, were measured after membrane solubilization with Triton X-100 either with unglycosylated caseinomacropeptide, or with this polypeptide containing the N-acetylgalactosamine side chain residues (desialylated and degalactosylated caseinomacropeptide). Radioactive N-acetylgalactosamine was incorporated in the unglycosylated acceptor peptide, and the glycosidic bonds in the product were alkali labile, suggesting that they were linked to the hydroxyamino acid residues. In addition radioactive N-acetylgalactosamine was released after alpha N-acetyl-D-galactosaminidase treatment of labelled caseinomacropeptide. [U-14C] galactose was incorporated in the desialylated and degalactosylated acceptor peptide. Reductive alkaline treatment of [U-14C] galactose peptide resulted in the release of a major product, the chromatographic properties of which in TLC were identical with authentic galactosyl (1 leads to 3) N-acetylgalactosaminitol. The structure of the labelled disacchariditol determined after periodate oxidation (two equivalents) by gas liquid chromatography-mass spectrometry revealed that the [U-14C] galactose was linked to position C-3 on the N-acetylgalactosaminyl-residue. The anomery of the galactose, as determined by a chemical method, indicates unambiguously a beta configuration.