The effect of oligosaccharide chains of different sizes on the maturation and physical properties of the G protein of vesicular stomatitis virus

Calcium homeostasis is required for contact-dependent helical and sinusoidal tip growth in Candida albicans hyphae

Hyphae of the dimorphic fungus, Candida albicans, exhibit directional tip responses when grown in contact with surfaces. On hard surfaces or in liquid media, the trajectory of hyphal growth is typically linear, with tip re-orientation events limited to encounters with topographical features (thigmotropism). In contrast, when grown on semisolid surfaces, the tips of C. albicans hyphae grow in an oscillatory manner to form regular two-dimensional sinusoidal curves and three-dimensional helices. We show that, like thigmotropism, initiation of directional tip oscillation in C. albicans hyphae is severely attenuated when Ca2+ homeostasis is perturbed. Chelation of extracellular Ca2+ or deletion of the Ca2+ transporters that modulate cytosolic [Ca2+] (Mid1, Cch1 or Pmr1) did not affect hyphal length but curve formation was severely reduced in mid1Delta and cch1Delta and abolished in pmr1Delta. Sinusoidal hypha morphology was altered in the mid1Delta, chs3Delta and heterozygous pmr1Delta/PMR1 strains. Treatments that affect cell wall integrity, changes in surface mannosylation or the provision of additional carbon sources had significant but less pronounced effects on oscillatory growth. The induction of two- and three-dimensional sinusoidal growth in wild-type C. albicans hyphae is therefore the consequence of mechanisms that involve Ca2+ influx and signalling rather than gross changes in the cell wall architecture.

The vesicular monoamine transporter 2 contains trafficking signals in both its N-glycosylation and C-terminal domains

The vesicular acetylcholine transporter (VAChT) and the vesicular monoamine transporter (VMAT) belong to the same transporter family that packages acetylcholine into synaptic vesicles (SVs) and biogenic amines into large dense core vesicles (LDCVs) and/or SVs, respectively. These transporters share similarities in sequence and structure with their N- and C-terminal domains located in the cytoplasm. When expressed in PC12 cells, VMAT2 localizes to LDCV, whereas VAChT is found mainly on synaptic-like microvesicles. Previous studies have shown that the cytoplasmic C-terminal domain of VAChT contains signals targeting this transporter to SVs. However, the targeting signals for VMAT have not been completely elucidated. To identify signals targeting VMAT2 to LDCV, the subcellular localization of VMAT2-VAChT chimeras was analyzed in PC12 cells. Chimeras having either the N-terminal region through transmembrane domain 2 of VMAT2 or the C-terminal domain of VMAT2 do not traffic to LDCV efficiently. In contrast, chimeras having both of these regions, or the luminal glycosylated loop in conjunction with transmembrane domains 1 and 2 and the C-terminal domain of VMAT2, traffic to LDCV. Treatment of PC12 cells with 1-deoxymannojirimycin, a specific alpha-mannosidase I inhibitor, causes VMAT2 to localize to synaptic-like microvesicles. The results indicate that both mature N-linked glycosylation and the C-terminus are important for proper trafficking of VMAT2 and that the locations of trafficking signals in VMAT2 and VAChT are surprisingly different.

Ligand-Modified Vesicular Stomatitis Virus Glycoprotein Displays a Temperature-Sensitive Intracellular Trafficking and Virus Assembly Phenotype

The production of potentially targetable VSV-G-pseudotyped retrovirus vectors has been hampered by inadequate understanding of the structure-function relationships of the VSV-G protein. In these studies we demonstrate assembly and production of MLV-based and HIV-1-based vector particles using VSV-G proteins modified by the insertion of a peptide ligand into a site corresponding to amino acid position 24 of the native VSV-G molecule. The inserted ligand represents the decapeptide encoding the collagen-binding domain of von Willebrand factor. We have used deconvolution microscopy to demonstrate that the modified VSV-G molecules sequester in perinuclear structures and are unavailable for assembly of infectious virus particles at the cell surface under standard tissue culture conditions at 37 degrees C. In contrast, at a lower permissive temperature of 30 degrees C, the modified VSV-G protein traffics appropriately to the cell surface and participates in useful titers. Furthermore, VSV-G-pseudotyped MLV-based and HIV-1-based vectors displaying the collagen-binding domain demonstrate a statistically significant increased attachment to a collagen matrix as indicated by an ELISA-like cell binding assay and by a focus transduction assay.

Chaperones and folding of MHC class I molecules in the endoplasmic reticulum

In this review we discuss the influence of chaperones on the general phenomena of folding as well as on the specific folding of an individual protein, MHC class I. MHC class I maturation is a highly sophisticated process in which the folding machinery of the endoplasmic reticulum (ER) is heavily involved. Understanding the MHC class I maturation per se is important since peptides loaded onto MHC class I molecules are the base for antigen presentation generating immune responses against virus, intracellular bacteria as well as tumours. This review discusses the early stages of MHC class I maturation regarding BiP and calnexin association, and differences in MHC class I heavy chain (HC) interaction with calnexin and calreticulin are highlighted. Late stage MHC class I maturation with focus on the dedicated chaperone tapasin is also discussed.

The glycoprotein G of rhabdoviruses

Rhabdoviruses show an RNA-containing helically-wound nucleocapsid either enclosed by or enclosing a membrane M protein, surrounded by a lipid bilayer through which dynamic protein trimers made up of non-covalently associated monomers of glycoprotein G (G) project outside. Mature monomeric rhabdoviral G has more than 500 amino acids, 2-6 potential glycosylation sites, 12-16 highly conserved cysteine residues, 2-3 stretches of a-d hydrophobic heptad-repeats, a removed amino terminal hydrophobic signal peptide, a close to the carboxy terminal hydrophobic transmembrane sequence and a carboxy terminal short hydrophylic cytoplasmic domain. Association-dissociation between monomers-trimers and displacement of the trimers along the plane of the lipid membrane, are induced by changes in the external conditions (pH, temperature, detergents, etc.). Throughout conformational changes the G trimers are responsible for the virus attachment to cell receptors, for low-pH membrane fusion and for reacting with host neutralizing monoclonal antibodies (MAbs). Antigenic differences could exist between monomers and trimers, which may have implications for future vaccine developments. The family Rhabdoviridae is made up of the Lyssavirus (rabies), the Vesiculovirus (vesicular stomatitis virus, VSV) and many rhabdoviruses infecting fish, plants, and arthropod insects. All these reasons make the G of rhabdoviruses an ideal subject to study comparative virology and to investigate new vaccine technologies.

Lec15 cells transfer glucosylated oligosaccharides to protein

B4-2-1 cells (Lec15 cells) are Chinese hamster ovary cells deficient in mannosylphosphoryldolichol synthase activity. They synthesize the truncated lipid intermediate Man5GlcNAc2-P-P-dolichol rather than the Glc3Man9GlcNAc2-P-P-dolichol synthesized by wild-type cells. In this report we present evidence that these cells did synthesize glucosylated Man5GlcNAc2-P-P-dolichol, but this species represented only a minor fraction of the labeled oligosaccharide-lipid. On the other hand, glucosylated oligosaccharides were a major species transferred to protein in these cells, showing that in vivo, glucosylated oligosaccharides are preferentially transferred to protein. The truncated oligosaccharides found in B4-2-1 cells were removed from the protein by N-glycanase treatment, since they were resistant to both endo-beta-N-acetylglucosaminidase H and F activity. B4-2-1 cells processed the glucosylated, truncated oligosaccharides transferred to G protein of vesicular stomatitis virus, leading to infectious virus.

Microheterogeneity of dopamine transporters in rat striatum and nucleus accumbens

Previously we have shown that the [125I]DEEP-labeled dopamine transporter from the rat nucleus accumbens has a higher apparent molecular weight than that from striatum. The present study confirms and extends these observations. Experiments with nucleus accumbens showed [125I]-DEEP to specifically bind to a protein with an apparent molecular weight of 76 kDa and with the pharmacological properties of the dopamine transporter. In exoglycosidase studies, treatment with neuraminidase, but not alpha-mannosidase, reduced the apparent molecular weight of the dopamine transporter from both the striatum and nucleus accumbens; however, a difference in the apparent molecular weight was still observed. N-Glycanase treatment, on the other hand, did reduce the apparent molecular weight of the dopamine transporters from the two regions to a similar value, approximately 56 kDa. In radioligand binding studies examining the effect of partial deglycosylation on striatal dopamine transporters, neuraminidase did not affect specific [3H]WIN 35,428 binding at 4 and 40 nM concentrations. In conclusion, the present study demonstrates that the difference in the apparent molecular weight of the dopamine transporter from these two regions is due to a difference in glycosylation and that the dopamine transporter from both regions contains similar amounts of sialic acid in their carbohydrate structure. Furthermore, the present data also indicate that the polypeptide portion of the dopamine transporter from both regions could be the same gene product.

Diabetic BB/Wor rat haptoglobin exhibits a probable structural abnormality in Asn-linked oligosaccharides

In this study we demonstrate that haptoglobin, a serum glycoprotein secreted by the liver, has altered structure in the BB/Wor diabetic rat. SDS-PAGE of haptoglobin (a tetramer composed of two glycosylated beta-chains each containing two sites for Asn-linked oligosaccharides connected by disulfide bonds with two nonglycosylated alpha-chains) clearly shows that the beta-chain of haptoglobin from diabetic rats is smaller than normal, with a molecular mass of 39 instead of 40 kDa. Both acute and chronic diabetic rats exhibit the defect. Defective haptoglobin appears in the serum within 4 days of onset of the disease, but insulin therapy prevents the defect. Removal of Asn-linked oligosaccharides with peptide: N-glycosidase F from Flavobacterium meningosepticum abolished the size difference between the beta-chains from normal and diabetic haptoglobin, with the molecular mass in both cases shifting to 30 kDa. Haptoglobin from both normal and diabetic rats was resistant to digestion by endoglycosidase H from Streptomyces griseus, which cleaves high mannose-type chains. Removal of sialic acid with neuraminidase treatment resulted in a reduction in the molecular mass in both cases, but without eliminating the size difference between the two. These results demonstrate that haptoglobin from diabetic BB/Wor rats contains a structural abnormality which correlates with onset of the disease. The defect is most likely due to an alteration in Asn-linked oligosaccharides, probably involving a change in the neutral sugars of complex-type oligosaccharide chains. This finding represents the first example of an altered Asn-linked oligosaccharides in diabetes.

Role of N-linked glycans of envelope glycoproteins in infectivity of human immunodeficiency virus type 1

We have shown that enzymatic removal of N-linked glycans from human immunodeficiency virus type 1 (HIV-1) recombinant envelope glycoproteins gp160 and gp120 produced in BHK-21 cells did not significantly reduce their ability to bind to CD4, the cellular receptor for the virus. Because recombinant proteins may behave differently from proteins present on virions, we investigated whether such viral envelope glycoproteins either in a purified form or present on viral particles could be deglycosylated by treatment with an endoglycosidase F-N-glycanase mixture which cleaves all accessible glycan moieties. Endoglycosidase analysis of the carbohydrate composition of purified viral gp120 (vgp120) indicated a glycosylation pattern similar to that for recombinant gp120 (rgp120), and treatment with endoglycosidase F-N-glycanase resulted in comparable molecular weight (MW) reduction for both molecules. Similarly, after immunoblotting of the deglycosylated viral preparation, the characteristic 160- and 120-kilodalton (kDa) bands were replaced by 90- and 60-kDa bands, respectively. The apparent MW of gp41 shifted to 35 kDa. These results are consistent with complete deglycosylation. The immunoreactive conformation of envelope glycoproteins remained unaltered after deglycosylation: they were recognized to the same extent by specific human polyclonal or mouse monoclonal antibodies, and no proteolysis of viral proteins occurred during enzymatic treatment. Deglycosylation of vgp120 resulted in a less than 10-fold reduction of the ability to bind to CD4, presented either in a soluble form or at the cell membrane. In addition, deglycosylation significantly reduced, but did not abolish, HIV-1 binding to and infectivity of CD4+ cells as determined, respectively, by an indirect immunofluorescence assay and a quantitative dose-response infection assay. Taken together, these results indicate that removal of glycans present on mature envelope glycoproteins of HIV-1 diminishes but does not abolish either virus binding to CD4 or its capacity to infect CD4+ cells.

The role of high-mannose and complex asparagine-linked glycans in the secretion and stability of glycoproteins

Suspension-cultured cells of sycamore (Acer pseudoplatanus L.) secrete a number of acid hydrolases and other proteins that have both highmannose and complex asparagine-linked glycans. We used affinity chromatography with concanavalin A and an antiserum specific for complex glycans in conjunction with in vivo-labeling studies to show that all of the secreted proteins carry glycans. The presence of complex glycans on secretory proteins indicates that they are passing through the Golgi complex on the way to the extracellular compartment. The sodium ionophore, monensin, did not block the transport of proteins to the extracellular medium, even though monensin efficiently inhibited the Golgi-mediated processing of complex glycans. The inhibition of N-glycosylation by tunicamycin reduced by 76% to 84% the accumulation of newly synthesized (i.e. radioactively labeled) protein that was secreted by the sycamore cells, while cytoplasmic protein biosynthesis was not affected by this antibiotic. However, in the presence of glycoprotein-processing inhibitors, such as castanospermine and deoxymannojirimycin, the formation of complex glycans was prevented but glycoprotein secretion was unchanged. These results support the conclusion that N-linked glycan processing is not necessary for sorting, but glycosylation is required for accumulation of secreted proteins in the extracellular compartment.

Dissecting glycoprotein biosynthesis by the use of specific inhibitors

It is possible to interfere with different steps in the dolichol pathway of protein glycosylation and in the processing of asparagine-linked oligosaccharides. Thus some clues about the role of protein-bound carbohydrate can be obtained by comparing the biochemical fates and functions of glycosylated proteins with their non-glycosylated counterparts, or with proteins exhibiting differences in the type of oligosaccharide side chains. Cells infected with enveloped viruses are good systems for studying both aspects of protein glycosylation, since they contain a limited number of different glycoproteins, often with well-defined functions. Tunicamycin, an antibiotic, as well as several sugar analogues have been found to act as inhibitors of protein glycosylation by virtue of their anti-viral properties. They interfere with various steps in the dolichol pathway resulting in a lack of functional lipid-linked oligosaccharide precursors. Compounds that interfere with oligosaccharide trimming represent a second generation of inhibitors of glycosylation. They are glycosidase inhibitors that interfere with the processing glucosidases and mannosidases and, as a result, the conversion of high-mannose into complex-type oligosaccharides is blocked. Depending upon the compound used, glycoproteins contain glucosylated-high-mannose, high-mannose or hybrid oligosaccharide structures instead of complex ones. The biological consequences of the alterations caused by the inhibitors are manifold: increased susceptibility to proteases, improper protein processing and misfolding of polypeptide chains, loss of biological activity and alteration of the site of virus-budding, to name but a few.

Effects of glucose starvation and puromycin treatment on lipid-linked oligosaccharide precursors and biosynthetic enzymes in Chinese hamster ovary cells in vivo and in vitro

Previous studies from several laboratories have demonstrated that glucose-starved Chinese hamster ovary (CHO) cells and other cells in culture switch from synthesis of the normal Glc3Man9GlcNAc2-P-P-Dol to Man5-GlcNAc2-P-P-Dol. In this study we have investigated this phenomenon in CHO cells in vitro and in vivo in order to determine the possible site of this block. Our results demonstrate that enzymatic activities responsible for Man9GlcNA2 synthesis in vitro are normal in glucose-starved cells. In vivo, however, the pool of GDP-[3H]Man is severely depleted, while [3H]mannose incorporation into lipid-linked and protein-bound Man5GlcNAc2 is increased. This result suggests that the available GDP-Man in starved cells is utilized to synthesize Man5GlcNAc2 preferentially, resulting in a reduction of Dol-P-Man and Man6-Man9 GlcNAc2 synthesis in vivo in glucose-starved cells. Conditions which prevent the depletion of GDP-[3H]Man in glucose-starved cells, such as puromycin or cycloheximide treatment, result in normal synthesis of Man9GlcNAc2 by glucose-starved cells. An unexpected finding in the course of this study is that puromycin or cycloheximide treatment of cells, which is known to inhibit lipid-linked oligosaccharide synthesis in glucose-fed cells, has no such inhibitory effect on glucose-starved cells.

A late developmental change in lysosomal enzyme sulfation specific to newly synthesized proteins in Dictyostelium discoideum

During development in Dictyostelium discoideum, several lysosomal glycosidases undergo changes in post-translational modification that are thought to involve differences in the extent of sulfation or phosphorylation, and appear to be required for the maintenance of cellular enzyme activity late in development. We have used monoclonal antibodies specific to the lysosomal enzyme alpha-mannosidase-1 to study the major late (12 hr) developmental change in the modification system. Pulse-chase experiments performed both early and late in development reveal that the substrate for the late form of modification is restricted to newly synthesized alpha-mannosidase-1 precursor protein. We have identified one modification difference between the two developmentally distinct isozymes of alpha-mannosidase-1: 35SO4 pulse-chase data show that the newly synthesized "late" enzyme precursor is significantly undersulfated in comparison with the enzyme synthesized early in development. This apparent lack of sulfation is associated with the lack of acquisition of endoglycosidase H resistance. By contrast, an aggregation-deficient mutant, which is defective with regard to the accumulation of alpha-mannosidase-1 activity late in development, synthesizes the "early" sulfated form of the enzyme throughout development. We conclude that the late developmental change in post-translational modification specifically involves one of the biochemical steps in which the N-linked oligosaccharide side chains of the newly synthesized alpha-mannosidase-1 precursor are modified by sulfation.

Inhibitory effects of tunicamycin on the mixed lymphocyte reaction and mitogen-induced lymphocyte blastogenesis

The role of asparagine(Asn)-linked saccharides on the surface of lymphocytes in cellular interactions was examined by performing studies on the effects of tunicamycin (TM), which inhibits the glycosylation of proteins N-glycosylated at asparagine residues, on the mixed lymphocyte reaction (MLR) and mitogen-induced lymphocyte blastogenesis of human lymphocytes by measuring 3H-thymidine (TdR) incorporation. Responses were expressed as percentages of that of the control (MLR without TM). The lymphocyte blastogenesis in the one- and two-way MLR were, respectively, 43.1% and 48.0% of the control at 0.1 microgram/ml of TM, and 5.5% and 7.2% at 1 microgram/ml of TM. The inhibitory effect of TM on the one-way MLR was shown using TM-pretreated stimulator cells, TM-pretreated responder cells or both. TM blocked lymphocyte blastogenesis in the secondary as well as in the primary MLR. The inhibitory effect of TM on the two-way MLR was observed when TM was added on day 0 to day 2, but not on day 4 of incubation. TM blocked mitogen-induced lymphocyte blastogenesis by phytohemagglutinin, concanavalin A or by pokeweed mitogen. As TM had no cytotoxic effect on cultured cells, these inhibitory effects of TM were thought to be due to the loss of Asn-linked saccharides from glycoprotein of the surface of lymphocytes. These findings indicated that Asn-linked saccharides of glycoprotein on the surface of lymphocytes were important in cellular interactions that are necessary for the cellular immune response.

Transport and processing of the glycosylated precursor of Concanavalin A in jack-bean

Concanavalin A (ConA) is a tetrameric lectin which is synthesized in the developing cotyledons of jack bean (Canavalia ensiformis L.) as a glycosylated precursor, pro-concanavalin A (pro-ConA). The processing of pro-ConA involves the excision of a small glycopeptide from the center of the pro-ConA molecule, and the ligation of the two polypeptides. In this paper, we show that pro-ConA is associated with the endoplasmic reticulum/Golgi fraction of the cells, and that the processing of pro-ConA occurs in the protein bodies. Processing is a complex process and different intermediate-sized polypeptides appear at different times during cotyledon development. The ConA-related polypeptides which accumulate during seed development may be the products of alternate processing events or breakdown products of ConA, rather than precursors of ConA. When glycosylation is prevented by tunicamycin, there is very little transport of pro-ConA out of the endoplasmic reticulum/Golgi system to the protein bodies; the unglycosylated pro-ConA which is transported is slowly processed. Tunicamycin does not prevent the transport of canavalin (a protein which is not glycosylated) or the transport and processing of the small amounts of glycosylated pro-ConA synthesized in the presence of the drug. This is, to our knowledge, the first demonstration that the transport of a glycoprotein in plant cells is dependent on the presence of the glycan.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.

Glycosylation allows cell-surface transport of an anchored secretory protein

We have previously described the construction and expression of a hybrid gene encoding a membrane-anchored form of rat growth hormone. This protein is anchored in cellular membranes by a carboxy-terminal extension composed of the transmembrane and cytoplasmic domains of the vesicular stomatitis virus glycoprotein. The protein is transported efficiently to the Golgi apparatus but not to the cell surface. To examine the possibility that N-linked glycosylation might be required for protein transport to the cell surface, we created two mutant proteins (using in vitro mutagenesis) in which single amino acids at two random sites in anchored growth hormone were changed to generate consensus sequences required for addition of N-linked carbohydrate. These mutant proteins, and a protein with both glycosylation sites, were glycosylated and were transported to the cell surface. These results suggest that N-linked glycosylation can serve as a signal for protein transport to the cell surface.

The effects of inhibitors of glucosidase I on the formation of Sindbis virus

We have examined the effects of deoxynojirimycin and castanospermine, compounds known to inhibit the removal of glucose from high mannose asparagine-linked oligosaccharides, on the formation of Sindbis virus. These drugs inhibited virion formation in baby hamster kidney (BHK) cells, 15B - the CHO cell line that lacks GlcNAc transferase activity, and chicken embryo fibroblasts, although our results with the latter cells were variable. We analyzed the [3H]mannose-labeled oligosaccharides from Sindbis virus infected 15B cells. Those from control cells were predominantly GlcNAc2Man5. Oligosaccharides from the treated cells were larger than the Man5 species and as expected, were partially resistant to alpha-mannosidase. The growth of Sindbis virus was inhibited to a much greater extent at 37 degrees C than at 30 degrees C in BHK cells treated with either deoxynojirimycin or castanospermine. Both of these compounds also inhibited the proteolytic cleavage of the viral glycoprotein precursor, PE2, to the virion glycoprotein, E2, but did not prevent the migration of the glycoprotein to the cell surface. These results, taken together with our earlier studies with vesicular stomatitis virus (Schlesinger et al., 1984) provide strong evidence that the removal of glucose residues during the processing of asparagine-linked oligosaccharides is critical for some proteins to achieve a functional conformation.

The common alpha subunit of bovine glycoprotein hormones: limited formation of native structure by the totally nonglycosylated polypeptide chain

The folding of the bovine glycoprotein hormone alpha subunit, synthesized in bacteria following insertion of the nucleotide sequence coding for this polypeptide, has been studied to determine the effect that a complete lack of carbohydrate has on this process. The bacterially derived alpha polypeptide (bac-alpha), extracted from E. coli in the presence of reductant and denaturant, had an estimated 0.2% native structure as determined by a conformationally sensitive radioimmunoassay. Upon reduction of disulfide bonds and reoxidation in air, the amount of native structure increased about 18-fold. Approximately 2% of the refolded bac-alpha preparation combines with the beta subunit of human chorionic gonadotropin (hCG beta) to form a complex that binds to the gonadotropin receptor and elicits a biological response. Since the correct folding (by immunological criteria) of bac-alpha (ca 3%) is significantly greater than expected from a random formation of disulfide bonds (0.1%), it appears that correct folding of alpha subunit can occur in the complete absence of carbohydrate, though in very low yield. Native bovine lutropin alpha subunit (LH alpha) and chemically deglycosylated LH alpha (which retains two asparagine-linked N-acetyl glucosamine residues per alpha oligosaccharide) were subjected to the same reduction/reoxidation regimen as the bacterially produced alpha subunit. As has been reported previously [Giudice LC, Pierce, JG, J Biol Chem 251: 6392, 1976] intact LH alpha fully regained its native structure. The partially deglycosylated LH alpha also refolds to a native-like structure in high yield as assessed by immunological assays and by its ability to combine with HCG beta to form a biologically active complex. The data show that carbohydrate, while not obligatory for correct folding, greatly facilitates the formation of functional alpha subunit.

Biological properties of a hemagglutinin mutant of influenza virus selected by host cells

Chick embryo fibroblast (CEF)-grown stocks of the WSN strain of influenza A(HINI) contain two variants which were designated F and C for fuzzy and clear plaque morphology on Madin-Darby bovine kidney (MDBK) cells. During growth in MDBK cells plaque-isolated F virus was completely replaced by C virus (L. Noronha-Blob and I.T. Schulze (1976), Virology 69, 314-322). The parental (F) and the mutant (C) viruses contain hemagglutinins which differ in their ability to bind to host cells. In addition, the host cells from which the purified viruses are obtained affect their binding properties. Thus, as compared to MDBK-grown F virus (FBK), MDBK-grown C virus (CBK) produced high amounts of mRNA and high virus yields in MDBK cells. CBK had greater affinity for SA alpha 2,3Gal and SA alpha 2,6Gal linkages on derivatized human erythrocytes than did FBK, independent of whether neuraminidase was present on the virions. CBK was also resistant to components of calf serum which inhibited FBK hemagglutination at 37 degrees. As compared to FBK, CBK had increased ability to bind to both MDBK cells and CEF at 37 degrees in the presence or absence of an inhibitor of neuraminidase. In addition, when cells with virus bound at 0 degrees were transferred to 37 degrees, CBK remained cell associated whereas about 80% of FBK dissociated from both cells. Thus, mutation from F to C increased the ability of the virus to associate with MDBK cell receptors. Studies carried out with F and C viruses from both cells indicated that the expression of the mutation depended in part on the host cells in which the virus was grown and in part on the cells used to measure the binding properties. A model relating these observations to selection of HA variants in nature is presented.

Studies on the role of glycosylation in the functions and antigenic properties of influenza virus glycoproteins

The biological and antigenic roles of glycosylation were investigated in the influenza hemagglutinin (HA) glycoprotein using the glycosylation inhibitor tunicamycin (TM). Under conditions where only the nonglycosylated form of HA was detected by immunoprecipitation and gel electrophoresis, the migration of glycoproteins to the cell surface was observed by immunofluorescence using either monospecific or monoclonal antibody to the HA polypeptide. Analysis of the surface fluorescence in TM-treated infected cells by a fluorescence-activated cell sorter (FACS) showed that all cells exhibited fluorescence in the complete absence of glycosylation. The relative amount of HA antigen on cell surfaces was found to be reduced by only 30-40% in TM-treated cells, and this reflected a similar reduction in intracellular synthesis. Electron microscopic studies using ferritin labeling also demonstrated that the nonglycosylated HA glycoprotein was present in significant amounts on surfaces of infected cells. Virions with nonglycosylated glycoproteins were purified, and were found to have an approximate 30-fold decrease in both hemagglutinin and neuraminidase specific activities. The possible role of oligosaccharides in antigenic variation among various H1N1 strains was investigated. Immunoprecipitation reactions involving five different monoclonal antibodies and five antigenic variants of A/USSR/90/77 revealed no major antigenic differences between the glycosylated and nonglycosylated forms of HA.

Physical properties of the glycoprotein of vesicular stomatitis virus measured by intrinsic fluorescence and aggregation

We have studied the denaturation and renaturation of the purified glycoprotein (G) of vesicular stomatitis virus by using intrinsic fluorescence spectroscopy and an aggregation assay. Our studies were carried out with G containing two complex oligosaccharide chains, with the asialo form of the protein, and for some experiments with G containing altered oligosaccharide structures. Fluorescence quenching using acrylamide showed no differences between the native and denatured states of G due to sialic acid content. Denaturation by guanidinium chloride (GdmCl) at 25 degrees C was reversible for the major transition region. The data analyzed by a two-state denaturation model gave a free energy of unfolding in the absence of denaturant of approximately 1.4 kcal/mol. For renaturation, two types of dialysis protocols were employed. The first (direct dialysis) involved dialysis against standard buffer [140 mM NaCl, 10 mM sodium phosphate, 1 mM disodium ethylenediaminetetraacetate, and 0.2% (w/v) poly(oxyethylene) 10-tridecyl ether, pH 7.4]. Recovery of the native emission maximum did not occur for any of the G proteins by using this procedure. The second (annealing dialysis) involved slow removal of GdmCl against decreasing concentrations of GdmCl in standard buffer over a period of 2-3 days. Only in this case was recovery of the native emission maximum and fluorescence intensity obtained. For those G proteins in which the oligosaccharide chains were decreased in size, this protocol led to extensive aggregation.

Application of high-field proton magnetic resonance spectroscopy in the structural determination of membrane-derived Sindbis virus glycopeptides

Sindbis virus membrane glycopeptides have ben purified in chemical quantities and their oligosaccharide structures analyzed by 1H NMR spectroscopy at 360 MHz. Interpretable spectra could be obtained with approximately 100 micrograms of oligosaccharide. Spectral analysis of the sialyl glycopeptides S1, S2, and S3 at high and low temperatures confirms their structures to be NANA alpha (2,3)Gal beta (1,4)-GlcNAc beta (1,2)Man alpha (1,6)-[NANA alpha (2,3)Gal beta (1,4)-GlcNAc beta (1,2)Man alpha (1,3)]-Man beta (1,4)GlcNAc beta (1,4)-[Fuc alpha-(1,6)]-GlcNAc beta 1-Asn. These are heterogeneous with respect to sialic acid (NANA). Spectra of two endo-beta-N-acetylglucosaminidase products of the S4 glycopeptides are reported. The interpretation of these spectra is consistent with Man5GlcNAc and Man7GlcNAc oligosaccharide structures. Their chemical shifts are essentially identical with those reported for ovalbumin glycopeptides of the same composition, with exception to the perturbations arising from their oligosaccharide nature.