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

Biosynthesis and processing of ribophorins in the endoplasmic reticulum

Ribophorins are two transmembrane glycoproteins characteristic of the rough endoplasmic reticulum, which are thought to be involved in the binding of ribosomes. Their biosynthesis was studied in vivo using lines of cultured rat hepatocytes (clone 9) and pituitary cells (GH 3.1) and in cell-free synthesis experiments. In vitro translation of mRNA extracted from free and bound polysomes of clone 9 cells demonstrated that ribophorins are made exclusively on bound polysomes. The primary translation products of ribophorin messengers obtained from cultured hepatocytes or from regenerating livers co-migrated with the respective mature proteins, but had slightly higher apparent molecular weights (2,000) than the unglycosylated forms immunoprecipitated from cells treated with tunicamycin. This indicates that ribophorins, in contrast to all other endoplasmic reticulum membrane proteins previously studied, contain transient amino-terminal insertion signals which are removed co-translationally. Kinetic and pulse-chase experiments with [35S]methionine and [3H]mannose demonstrated that ribophorins are not subjected to electrophoretically detectable posttranslational modifications, such as proteolytic cleavage or trimming and terminal glycosylation of oligosaccharide side chain(s). Direct analysis of the oligosaccharides of ribophorin l showed that they do not contain the terminal sugars characteristic of complex oligosaccharides and that they range in composition from Man8GlcNAc to Man5GlcNAc. These findings, as well as the observation that the mature proteins are sensitive to endoglycosidase H and insensitive to endoglycosidase D, are consistent with the notion that the biosynthetic pathway of the ribophorins does not require a stage of passage through the Golgi apparatus.

Processing of N-linked oligosaccharides from precursor- to mature-form herpes simplex virus type 1 glycoprotein gC

Immature and mature forms of glycoprotein gC were purified by immunoadsorbent from herpes simplex virus type 1-infected BHK cells labeled with [3H]mannose for a 20-min pulse or for 11 h followed by a 3-h chase. The nature of N-asparagine-linked oligosaccharides carried by the immature form, pgC (molecular weight = 92,000), and the mature gC (molecular weight = 120,000) has been investigated. All pronase-digested glycopeptides of pgC were susceptible to endo-beta-N-acetylglucosaminidase H treatment; thus they have a high-mannose structure. Using thin-layer chromatography to separate endo-beta-N-acetylglucosaminidase H-cleaved oligosaccharides, polymannosyl chains of different sizes, ranging from Man9GlcNAc to Man5GlcNAc, were separated. The major components were Man8GlcNAc and Man7GlcNAc, suggesting that pgC labeled in a 20-min pulse represents the form of glycoprotein already routed to the Golgi apparatus. Analysis of glycopeptides of mature gC showed that the majority (95%) of N-linked glycans were converted to complex-type glycans. Ion-exchange chromatography and affinity chromatography on concanavalin A-Sepharose and leucoagglutinin-agarose revealed that diantennary and triantennary glycans predominated, whereas tetrantennary chains were not present. Parts of the di- and triantennary chains were not fully sialylated. The high heterogeneity of complex-type chains found in mature gC may be related to the high number of N-glycosylation sites of the glycoprotein as predicted by DNA sequencing studies (Frink et al., J. Virol. 45:634-647, 1983).

Evidence for translational regulation of herpes simplex virus type 1 gD expression

We compared the rates of synthesis of herpes simplex virus type 1 glycoproteins C and D and quantitated the accumulation of translatable mRNA for each glycoprotein at various times after infection. The rate of synthesis of gD increased sharply early in the infection, peaked by 4 to 6 h after infection, and declined late in the infection. In contrast, the rate of synthesis of gC increased steadily until at least 15 h after infection. The levels of mRNA for both of these glycoproteins, as detected by hybridization and by translation in vitro, continued to increase until at least 15 or 16 h after infection. Synthesis of both gC and gD and their respective mRNAs was found to be sensitive to inhibition of viral DNA replication with phosphonoacetic acid. The finding that reduced amounts of gD were synthesized late in the replicative cycle, whereas gD mRNA continued to accumulate in the cytoplasm, argues that the synthesis of gD is regulated, in part, at the level of translation.

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.

The carbohydrates of mouse hepatitis virus (MHV) A59: structures of the O-glycosidically linked oligosaccharides of glycoprotein E1

Two size classes of O-glycosidically linked oligosaccharides were liberated from glycoprotein E1 of mouse hepatitis virus (MHV) A59 by reductive beta-elimination and separated by h.p.l.c. The structures of the reduced oligosaccharides were determined by successive exoglycosidase digestions and by methylation analyses involving combined capillary gas chromatography-mass spectrometry and mass fragmentography after chemical ionization with ammonia. Oligosaccharide A (Neu5Ac alpha 2----3 Gal beta 1----3 GalNAc) comprised 35% of the total carbohydrate side chains, while the remaining 65% of the oligosaccharides of E1 had the branched structure B: Neu5Ac alpha 2----3 Gal beta 1----3 (Neu5Ac alpha 2----6) GalNAc. Both oligosaccharides were linked to the E1 polypeptide via N-acetylgalactosamine, and 20% of the sialic acids present in E1 glycopeptides were found to consist of N-acetyl-9-mono-O-acetylneuraminic acid. The reported structures of the O-linked glycans are discussed in the context of the amino acid sequence of E1, which exhibits a cluster of four hydroxyamino acids (Ser-Ser-Thr-Thr) as potential O-glycosylation sites at the amino terminus. Oligosaccharides with identical structures and an identical O-glycosylated tetrapeptide sequence are present in the blood group M-active glycophorin A of the human erythrocyte membrane.

Limited proteolysis of herpes simplex virus glycoproteins that occurs during their extraction from vero cells

Herpes simplex virus glycoproteins extracted from infected Vero cells can be smaller in apparent size than the same viral products extracted from infected HEp-2 cells. Here we show that the differences in size result primarily from limited proteolysis, during or after their extraction, of the viral glycoproteins made in Vero cells. In the absence of appropriate protease inhibitors, both mature and immature forms of four different glycoproteins specified by herpes simplex virus type 2 were significantly smaller (based on electrophoretic mobilities in acrylamide gels) when extracted from Vero cells than when extracted from HEp-2 cells. Inclusion of certain protease inhibitors in the extraction buffer, however, permitted isolation of immature forms from Vero cells that were indistinguishable in size from the immature forms extracted from HEp-2 cells. Under these conditions, the mature forms of glycoproteins B and E were also indistinguishable by electrophoretic sizing from those made in HEp-2 cells, whereas the mature forms of glycoproteins D and F were smaller, indicating the possibility of differences between Vero and HEp-2 cells in the post-translational processing of glycoproteins D and F.

Monensin prevents terminal glycosylation of the N- and O-linked oligosaccharides of the HLA-DR-associated invariant chain and inhibits its dissociation from the alpha-beta chain complex

In B-lymphoblastoid cells, the HLA-DR-associated invariant chain is processed to a form containing O-linked as well as N-linked oligosaccharides. After neuraminidase treatment, the O-linked carbohydrate is susceptible to digestion with an endoglycosidase (endo-beta-N-acetylgalactosaminidase) that cleaves glycans with the structure Gal(beta 1----3)-GalNAc-Ser/Thr, and sialic acid can be added back to this core oligosaccharide by specific sialyltransferases. Treatment of cells with the sodium ionophore monensin markedly affects the post-translational processing of the invariant chain, although that of associated alpha and beta chains is minimally affected. Only a small portion of the N-linked carbohydrate on the invariant chain is processed to an endoglycosidase-H-resistant form. The sialic acid residues normally found on the O-linked glycans are not added, but at least the first residue, GalNAc, is added. In addition to the changes in glycosylation, an intracellular accumulation of HLA-DR antigens also occurs in monensin-treated cells. The accumulation of HLA-DR antigens and the overall slower turnover rates of the alpha, beta, and invariant polypeptides observed after monensin treatment probably reflects the build-up of newly synthesized proteins in Golgi apparatus-derived vacuoles coupled with a decrease in normal degradation in lysosomes.

Nucleotide sequence specifying the glycoprotein gene, gB, of herpes simplex virus type 1

The nucleotide sequence thought to specify the glycoprotein gene, gB, of the KOS strain of herpes simplex virus type 1 (HSV-1) has been determined. A 3.1-kilobase (kb), viral-specified RNA was mapped to the left half of the BamHI-G fragment (0.345 to 0.399 map units). TATA, CAT-box, and possible mRNA start sequences characteristic of HSV-1 genes are found near 0.368 map units. The first available ATG codon is at 0.366 and the first in-phase chain terminator at 0.348 map units. A polyA-addition signal (AATAAA) occurs 17 nucleotides past the chain terminator. Translation of these sequences would yield a 100.3-kilodalton (kDa) polypeptide characterized by a 5' signal sequence, nine N-linked saccharide addition sites, a strongly hydrophobic membrane-spanning sequence, and a highly charged 3' cytoplasmic anchor sequence. Two mutants of KOS, tsJ12 and tsJ20, that are temperature-sensitive for viral growth and for the production of gB, have been physically mapped to 0.357 to 0.360 and 0.360 to 0.364 map units, respectively (DeLuca et al., in preparation). The nucleotide sequence of the mutants was determined in these regions. In both cases a single amino acid replacement within the 100.3-kDa polypeptide is predicted from the sequence analysis.

Assembly in vitro of a spanning membrane protein of the endoplasmic reticulum: the E1 glycoprotein of coronavirus mouse hepatitis virus A59

The E1 glycoprotein of coronavirus mouse hepatitis virus A59 was synthesized in vitro by translation of viral mRNA in the presence of dog pancreatic microsomes. Its disposition in the membrane was investigated by digestion with proteases and by selective NH2-terminal labeling. The protein spans the membrane, but only small portions from the NH2 and COOH terminus are exposed respectively in the lumenal and cytoplasmic domains; the bulk of the molecule is apparently buried in the membrane. The protein lacks a cleavable leader sequence and does not acquire its characteristic O-linked oligosaccharides in rough microsomes. It may enter the membrane at any stage during synthesis of the first 150 amino acid residues. These unusual features of the protein might help to explain why it is not transported to the cell surface in vivo but remains in intracellular membranes, causing the virus to bud there.

Polymorphism of the human C3b/C4b receptor. Identification of a third allele and analysis of receptor phenotypes in families and patients with systemic lupus erythematosus

We have isolated C3bR from surface-labeled erythrocytes of 180 normal individuals and 45 patients with SLE. These studies have identified a previously unrecognized C3bR molecule on E with a Mr of approximately 160,000 daltons on nonreduced SDS-polyacrylamide gels. A similar receptor phenotype is also found on other C3bR-bearing peripheral blood leukocytes. Family studies demonstrate that this approximately 160,000-dalton molecule represents a third allele that is inherited in a codominant fashion at the same locus as the two previously described C3bR alleles. In unrelated normal donors a common allele (A) determines an approximately 190,000-dalton C3bR (gene frequency 0.83), a second allele (B) determines an approximately 220,000-dalton C3bR (gene frequency = 0.16), and a third rare allele (C) determines an approximately 160,000-dalton C3bR (gene frequency = 0.01). There were no major differences in gene frequencies among Caucasians and blacks or normal individuals and patients with SLE. However, compared with normal individuals, heterozygous C3bR-AC patients with SLE express large amounts of the approximately 160,000-dalton C3bR on E. Expression of C3bR molecules among heterozygous siblings is similar, suggesting that an inherited factor controls expression of the two molecules in heterozygous donors. These observations constitute an instructive example of a structural polymorphism of an integral membrane glycoprotein and provide a structural and genetic basis for further molecular and functional analyses of C3bR in normal and patient populations.

Virus-specific glycoproteins associated with the nuclear fraction of herpes simplex virus type 1-infected cells

Monospecific antisera to herpes simplex virus type 1 (HSV-1) glycoproteins gB, gC, and gD were used to identify the HSV-1-specific glycoproteins associated with the nuclear fraction as compared with those associated with cytoplasmic fraction, whole-cell lysates, and purified virions. The results indicate that a predominance of HSV glycoprotein precursors pgC(105), pgB(110), and pgD(52) is associated with the nuclear fraction. Treatment of the nuclear fraction with the enzyme endo-beta-N-acetylglucosaminidase H indicated that the lower-molecular-weight glycoproteins are sensitive to this endoglycosidase. These results suggest that in the nuclear fraction of HSV-1-infected cells virus-specific glycoproteins gB, gC, and gD are predominately in the high-mannose precursor form; however, detectable amounts of the fully glycosylated forms of gC and gD were also found.

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.

Detection of antibodies to herpes simplex virus with a continuous cell line expressing cloned glycoprotein D

The gene for glycoprotein D of herpes simplex virus type 1 (HSV-1) was expressed in stable mammalian cell lines. Glycoprotein D produced in these cells has a number of antigenic determinants in common with the native glycoprotein. Cell lines expressing glycoprotein D were used in an enzyme-linked immunosorbent assay to detect human antibodies to glycoprotein D. This strategy should prove useful in determining the extent to which the immune response to HSV-1 is directed toward glycoprotein D.

Processing of herpes simplex virus-1 glycans in cells defective in glycosyl transferases of the Golgi system: relationship to cell fusion and virion egress

We studied herpes simplex virus-1 (HSV-1) glycan structure and the expression of HSV-1 functions regulated by viral glycoproteins in Ric21 cells (P. VISCHER and R. C. HUGHES, Eur. J. Bioch. 117, 275-284, 1981). This is a line of ricin-resistant mutant BHK cells defective in the enzymes of the Golgi system which add terminal sugars to N-linked glycans. Two kinds of alterations were observed in the glycosylation of HSV glycoproteins in Ric21 cells. First, there was a defective processing of complex glycans leading to a reduction of biantennary and triantennary species and an increase of incompletely processed monosialylated oligosaccharides. Second, there was an overall reduction in the accumulation of HSV-1 glycoproteins. We found that (i) the release of herpesvirions from Ric21 cells was markedly lower than that from BHK cells, possibly reflecting reduced terminal sugar addition which, in turn, might affect the intracellular transport of glycoproteins. (ii) HSV-1 (MP)-infected Ric21 cells fused with a low efficiency. Furthermore, polycaryocytosis was reduced or abolished in BHK and in Ric21 cells exposed to neuraminidase, indicating that the presence of sialic acid residues in the cell surface glycans is essential for cells to interact in a fashion that brings cell fusion. (iii) Although capsid assembly was comparable, the rate of accumulation of infectious virus decreased in Ric21 cells. Infectivity of released virions from Ric21 and BHK cells was similar, in agreement with previous studies showing that complex-type glycans do not appear to be required for herpesvirion infectivity. The decrease in infectious HSV-1 yield seems to correlate with overall reduced ability to synthesize glycoproteins.

N-acetylgalactosaminyltransferase activity involved in O-glycosylation of herpes simplex virus type 1 glycoproteins

We report on N-acetylgalactosaminyltransferase (UDPacetylgalactosamine--protein acetylgalactosaminyltransferase; EC 2.4.1.41) activity in herpes simplex virus type 1 (HSV-1)-infected BHK and RicR14 cells, a line of ricin-resistant BHK cells defective in N-acetylglucosaminyltransferase I. The enzyme catalyzed the transfer of [14C]N-acetylgalactosamine (GalNAc) from UDP-[14C]GalNAc into HSV glycoproteins, as identified by immunoprecipitation. The sugar was selectively incorporated into the immature forms of herpesvirus glycoproteins pgC, pgD, and gA-pgB, which are known to contain N-linked glycans of the high-mannose type. The high incorporation of [14C]GalNAc into endogenous acceptors of HSV-1-infected RicR14 cells was consistent with the accumulation of immature forms of HSV glycoproteins which occurs in these cells. Mild alkaline borohydride treatment of glycoproteins labeled via GalNAc transferase showed that the transferred GalNAc was O-linked and represented the first sugar added to the peptide backbone.

Characterization of a herpes simplex virus type 2 75,000-molecular-weight glycoprotein antigenically related to herpes simplex virus type 1 glycoprotein C

Evidence is presented that the herpes simplex virus type 2 glycoprotein previously designated gF is antigenically related to herpes simplex virus type 1 gC (gC-1). An antiserum prepared against type 1 virion envelope proteins immunoprecipitated gF of type 2 (gF-2), and competition experiments revealed that the anti-gC-1 component of the antiserum was responsible for the anti-gF-2 cross-reactivity. An antiserum prepared against fully denatured purified gF-2, however, and three anti-gF-2 monoclonal antibodies failed to precipitate any type 1 antigen, indicating that the extent of cross-reactivity between gC-1 and gF-2 may be limited. Several aspects of gF-2 synthesis and processing were investigated. Use of the enzymes endo-beta-N-acetylglucosaminidase H and alpha-D-N-acetylgalactosaminyl oligosaccharidase revealed that the fully processed form of gF-2 (about 75,000 [75K] apparent molecular weight) had both complex-type N-linked and O-linked oligosaccharides, whereas newly synthesized forms (67K and 69K) had only high-mannose N-linked oligosaccharides. These last two forms were both reduced in size to 54K by treatment with endo-beta-N-acetylglucosaminidase H and therefore appear to differ only in the number of N-linked chains. Neutralization tests and radioiodination experiments revealed that gF-2 is exposed on the surfaces of virions and that the 75K form of gF-2 is exposed on cell surfaces. The similarities and differences of gF-2 and gC-1 are discussed in light of recent mapping results which suggest collinearity of their respective genes.

O-glycosylation in Saccharomyces cerevisiae is initiated at the endoplasmic reticulum

The first mannose of O-linked oligomannose chains in S. cerevisiae is transferred to Ser/Thr residues via dolichylphosphate mannose. Only this reaction (and not the subsequent reactions requiring GDP-Man) proceeds at the endoplasmic reticulum.

Biosynthesis and molecular nature of the T3 antigen of human T lymphocytes

Immunoprecipitates of the T3 antigen prepared from HPB-ALL cells by using the monoclonal antibody UCH-T1 were analysed by SDS-polyacrylamide gel electrophoresis. Cells which had been biosynthetically labelled for up to 4 h gave a major polypeptide of mol. wt. 19 000 plus two weaker, more diffuse bands of mol. wts. 21 000 and 23 000, whereas surface labelled cells gave a prominent band of mol. wt. 19 000, a major band of 21 000 and a weaker diffuse band of approximately 26 000. As judged from their sensitivity to proteinase-K digestion, all the above polypeptides possess a transmembrane orientation. Digestion with endoglycosidases H and F (endo-H and endo-F), and tunicamycin treatment indicate that all the polypeptides, except that of 19 000 mol. wt. are N-glycosylated. The 21 000 and 23 000 mol. wt. chains possess both immature and mature oligosaccharide units, whereas the 26 000 mol. wt. band apparently has mature units only. Pulse chase experiments combined with digestion by endo-F and endo-H suggest that the N-glycosylated polypeptides are derived from two polypeptides of mol. wts. 14 000 and 16 000. It is concluded that the T3 antigen is derived from three different non-glycosylated polypeptides two of which are subsequently N-glycosylated to give the 21 000, 23 000 and 26 000 forms. The cell surface T3 antigen most probably comprises at least two distinct, non-covalently associated polypeptides, but the number and types of polypeptides giving rise to the whole molecule and whether different complexes exist is at present unclear.

The confined function model of the Golgi complex: center for ordered processing of biosynthetic products of the rough endoplasmic reticulum

The organized and characteristic elements of the Golgi complex (GC) are the stacked smooth-surfaced cisternae, which are found in the centrosphere of all eukaryotic cells. These cisternae, in conjunction with other associated smooth-surfaced membranes, are responsible for executing net unidirectional intracellular transport (ICT) from the rough endoplasmic reticulum (RER) toward more distally located structures. This chapter focuses on the broad range of accessory activities that occur during transport, the family of “posttranslational modifications.” These events are, in all likelihood, not essential for the “primary” function of the GC yet they are crucial in allowing the cell to tailor its biosynthetic products for its own needs and the needs of the organism as a whole. In addition to modifying products of the rough endoplasmic reticulum, the GC may be involved in processing events because of its participation in other routes of vesicular traffic—for example, centripetal traffic from the cell surface. Various nonequivalent criteria have been used to ascribe processing events to the GC-autoradiography, preparative or analytic subcellular fractionation, interruption by ICT inhibitors, and delay in the impact of cycloheximide.