Function of the carbohydrate moieties of glycoproteins

Neurologic disease produced by Rathayibacter toxicus-derived corynetoxins

Corynetoxins, members of the tunicamycin group of antibiotics, are produced by the bacterium, Rathayibacter toxicus. They cause a severe neurologic disorder in domestic livestock, are hepatotoxins, and can damage retinal photoreceptors. For these toxins to be ingested by livestock, the bacterium must first be transported onto host plants by adhering to nematode larvae. In the infected seed heads, bacterial galls (gumma) then form. While corynetoxicity occurs most commonly in Australia, it has occurred sporadically in other countries and, due to the widespread global distribution of the bacterium, nematode, and host plants, there is great potential for further spread, particularly as the range of host plant species and nematode vectors identified for R. toxicus is increasing. Since many animal species are susceptible to corynetoxins poisoning, it is likely that humans would also be vulnerable if exposed to these potent, lethal toxins.

Diastereoselective synthesis of furanose and pyranose substituted glycine and alanine derivatives via proline-catalyzed asymmetric α-amination of aldehydes

A concise organocatalytic route toward the synthesis of furanose and pyranose substituted glycine and alanine derivatives is reported. These compounds are core structural units of some of the naturally available antibiotics and antifungal agents. Proline-catalyzed asymmetric α-amination of aldehydes derived from sugars is used as the key reaction to synthesize twelve sugar amino acid derivatives. The asymmetric transformations proceeded in good yields and with good to excellent diastereoselectivity. The application of the synthesized amino acids is demonstrated by synthesizing a tripeptide containing one of them.

Canonical pathways and networks regulated by estrogen in the bovine mammary gland

Many attempts have been made to identify estrogen-responsive genes using high-throughput approaches such as microarray, serial analysis of gene expression (SAGE), and in silico prediction. However, few studies have systematically analyzed regulatory networks and pathways affected by estrogen. In this report, we analyzed transcript profiles obtained from 16 prepubertal heifers in a 2 x 2 factorial experiment, with ovarian status (intact or ovariectomized) as the first factor and estrogen treatment as the second (control or estradiol). After 54 h of estrogen treatment, gene expression was evaluated in the parenchyma and fat pad of the bovine mammary gland using a high-density oligonucleotide microarray. The genes significantly regulated by estrogen were subject to pathway and regulatory network analysis using Ingenuity Pathways Analysis software. Approximately 2,344 genes responded significantly to estrogen treatment. Of these, 1016 genes were influenced by estrogen regardless of tissue or ovarian status, while the remaining genes were significant in one of four specific effects of tissue or ovarian status. The canonical pathways significantly regulated by estrogen (P < 0.05) included protein ubiquitination, G2/M cell cycle control, IGF1 signaling, N-glycan biosynthesis, sterol biosynthesis, and oxidative phosphorylation. A total of 23 regulatory networks were identified as estrogen responsive. The results provide insight into the molecular mechanisms through which estrogen regulates bovine mammary gland growth and development, supporting the concept that interaction between tissues within the mammary gland promotes mammary epithelial growth.

Fibronectin fragments and their role in inflammatory arthritis

OBJECTIVES

To identify potential immunopathogenic links between fibronectin (Fn) fragmentation and the inflammatory response in chronic joint disease.

METHODS

Scientific papers involving studies of Fn fragments and inflammatory processes important in the pathogenesis of arthritis, including chondrolysis, synoviocyte growth and adhesion, polymorphonuclear leukocyte (PMN) and monocyte function, proteolysis, and immune complex activation were reviewed. In addition, reports identifying Fn fragments in synovial fluid (SF) were assessed.

RESULTS

A series of Fn fragments have been identified in arthritic SF by several investigators. Fn and fragments ranging from 30 to 200 kd are present in elevated concentrations in inflammatory SF. SF Fn fragments display reduced affinity for fibrin and collagen. The 29- and 50-kd amino terminal fragments mediate release of proteoglycan from articular cartilage by RGD-independent mechanisms. Fn fragments can induce fibroblast gene expression of metalloproteinases or can act as proteinases themselves. A 90-kd plasmin generated fragment possesses homology with streptokinase. Fragments mediate PMN chemotaxis and enhance proliferation of CD4+ lymphocytes as well as binding to the C1q component of complement and influencing the behavior of immune complexes.

CONCLUSIONS

Fn fragments can be functionally and biochemically characterized in diseased SF. Modification of fragment formation and inhibition of fragment function may have potential therapeutic value in the interruption of chronic synovial inflammation.

Carbohydrate analysis of a chimeric recombinant monoclonal antibody by capillary electrophoresis with laser-induced fluorescence detection

A general method for the analysis of asparaginyl-linked (N-linked) carbohydrate moieties of an IgG1 monoclonal antibody is described here. The antibody, rituximab, is a mouse/human chimeric antibody to human CD20 antigen. The glycans present on rituximab are neutral complex biantennary oligosaccharides with zero, one, and two terminal galactose residues (G0, G1, and G2, respectively). To monitor the variation of the glycosylation during manufacture, the glycans were first enzymatically released from the antibody via digestion with peptide-N-glycosidase F, then derivatized with a charged fluorophore, 8-aminopyrene-1,3,6-trisulfonic acid and further separated by capillary electrophoresis with laser-induced fluorescence detection. All observed glycans were fully resolved, including the positional isomers of G1. The exact nature of the isomers in terms of the location of the terminal galactose was further characterized via multiple enzymatic digestion steps including mannosidase with activity toward specific Man(alpha 1,3) linkage. The optimization and several key parameters, i.e., enzymatic digestion and derivatization, in the assay development will be discussed. Moreover, to ensure that the assay can be used in routine lot release testing, the assay was validated and found to be accurate and precise. The analytical approach described is suitable for characterization as well as routine testing of the N-linked glycan content in any IgG1 monoclonal antibody and glycoproteins in general.

Detection of carbohydrates in capillary electrophoresis

This review focuses on recent developments in sensitive detection modes for carbohydrates after separation by capillary electrophoretic methods. To bring detection sensitivity for carbohydrates analysis in line with current methods in protein sequencing, concentration detection limits of 10(-6) molar or better are required. A discussion of mass detection limits and concentration detection limits is followed by an overview of detection modes for natural and labeled carbohydrates. Amperometric detection and UV and laser-induced fluorescence detection after reductive amination, in particular with 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS), are discussed in more detail. Finally, the paper outlines developments to be expected in the near future, focusing on the needs in glycobiology such as improved sensitivity and selectivity.

Long-term ethanol exposure impairs glycosylation of both N- and O-glycosylated proteins in rat liver

Carbohydrate residues of glycoproteins play important roles in their functions. We have previously shown that long-term ethanol treatment in rats alters the normal glycosylation pattern of plasma transferrin and apolipoprotein (apo) E. Glycosylation of proteins is a posttranslational process that is regulated by both glycosyltransferases and glycosidases, the resident enzymes of hepatic subcellular organelles. In this investigation using rat transferrin and apo E as model N- and O-glycosylated proteins, respectively, we have explored the effects of long-term ethanol treatment on the (1) incorporation of various labeled sugar precursors into these specific glycoproteins, (2) activities of mannosyltransferase, galactosyltransferase, and sialytransferases, and (3) hepatic synthetic rate of N-acetyl glucosamine (GlcNAc) alpha 2,6-sialyltransferase (2,6-ST). The relative ratio of labeled sugar to leucine incorporation (glycosylation index) showed a 43% (P < .01) decrease for relative mannosylation of transferrin molecule at both the microsomal and Golgi level in the ethanol group (AN) versus the control group (CN). For apo E, relative mannosylation was reduced by 48.9% (P < .01) and 46.9% (P < .01), respectively, at the microsomal and Golgi level in the AN versus CN. More importantly, relative sialation of transferrin was reduced by 86% (P < .001) in AN as compared with CN. Relative sialation of apo E was reduced by 35% (P < .01) in AN as compared with CN.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and solution structure of an S-glycosylated cyclic hexapeptide. Evidence for conformational change induced by glycosylation

Synthesis and conformational analysis of the S-glycosylated cyclic hexapeptide cyclo(-D-Pro1-Phe2-Cys3(tetra-O-acetyl-beta-D-galactopyranosyl)-++ +Trp4-Lys(Z)5- Phe6-) I was carried out to examine the influence of a saccharide residue in position i of a standard beta-turn on the formation of reverse turns and on the biological activity. Synthesis was carried out in the liquid phase employing a galactosylated cysteine building block. The cyclization reagents DPPA/NaHCO3 avoided high dilution conditions. Spectroscopic data were extracted from homo- and heteronuclear 2D-NMR techniques (TOCSY, NOESY, HMQC, HMQC-TOCSY, HMBCS-270). For structural refinement restrained molecular dynamics (MD) simulations in vacuo and with explicit DMSO as solvent were performed. Finally, simulations in DMSO without experimental restraints provided insight in stability and dynamics of the structural model. A comparison of the S-glycosylated Cys3 peptide with the analogous Thr3 peptide exhibits a similar overall conformation of the hexapeptide [beta II' D-Pro-Phe and another beta-turn about Trp4-Lys5(Z)]. However, the latter shows a distinct dynamic flip beta I, beta II in the glycopeptide, whereas the Thr-analogue only populates beta I. This influence is attributed to a beta I stabilizing effect of a hydrogen bridge of Thr-O gamma in position i to the NH of the amino acid in position i + 2, which is lacking in the glycosylated compound.

Synthesis and conformational analysis of N-glycopeptides. II. CD, molecular dynamics, and NMR spectroscopic studies on linear N-glycopeptides

The comprehensive structural analysis reported herein of eight N-glycopeptides, in three different solvents, is based on quantitative CD experiments, homonuclear nuclear Overhauser effect measurements, and molecular dynamics (MD) calculations. Although several orientations of the two amide planes attached to the carbohydrate pyranose ring are possible, according to NOE, CD data, and MD simulations, of all of the glycopeptide models, regardless of the type of the carrier peptide, only one dominant conformer population was found. This conformer is characterized by a nearly trans orientation of the CH and NH hydrogens of both acetamido groups. This finding is in perfect agreement with x-ray crystallographic data on the solid state conformation of the 1-N-acetyl- and 1-N-(beta-aspartyl)-2-acetamido-2-deoxy-beta-D-glucopyranosyla min e. The precise identification of this dominant conformer of N-glycopeptides in solution was the major question addressed herein by the structural analyses. A "CD additivity" experiment was carried out using an equimolar solution of Boc-Pro-Asp-NHCH3 and 1-N-acetyl-3,4,6- tri-O-acetyl-2-acetamido-2-deoxy-beta-D-glucopyranosylamine at ambient temperature in acetonitrile. The CD spectrum obtained from the equimolar solution of the above two molecules (the "spectroscopic sum") was identical with the CD curve obtained from the algebraic summation of the individually recorded CD spectra of the peptide and the carbohydrate moiety ("mathematical sum"). The global picture of the CD spectral analyses of the eight parent peptides with the eight N-glycopeptides revealed that in trifluoroethanol and acetonitrile, the side-chain modification of the Asn models (natural N-glycopeptide analogues) by N-glycosylation has a significant effect on the conformation of the carrier peptide, resulting in a decrease in the original type I beta-turn content. Simultaneously, the type II beta-turn conformational percentage increased to approximately 20%. Such a conformational ratio change seems to be larger than the expected errors arising from the CD analyses, and agrees with the results of MD calculations. N-glycosylation of Asn residues causes perturbations, not only through the covalent bond, but also through specific hydrogen bonds between the backbone and side chain atoms. CD spectroscopy, augmented by efficient CD curve deconvolution techniques, has proved to be a useful tool for studying multicomponent conformer mixtures of small linear peptides in solution and changes of conformational equilibria caused by N-glycosylation.

Thickening of the glycocalyx during methotrexate induced small intestinal mucosal atrophy in the rat

In this study, the morphology of the glycocalyx of small intestinal enterocytes in the rat was investigated after the induction of mucosal atrophy by methotrexate (MTX) in 18 Wistar rats. On the 2nd, 4th and 6th day after the intraperitoneal administration of MTX, a group of 6 rats was sacrificed and tissue specimens taken at the level of the Treitz' ligament were post fixed in a mixture of 2.0% watery osmiumtetroxide and 1.5% potassium-ferrocyanide to enhance visibility of glycoconjugates and processed for electron microscopy. In the control rats, the glycocalyx was hardly visible on the villous enterocytes, but rather conspicuous on the enterocytes in the crypts. On the second day after injection of MTX, the glycocalyx of the villous enterocytes was more pronounced but on the crypt enterocytes it resembled that of control animals. On the 4th day after MTX administration, a striking reduction and shortening of microvilli of villous enterocytes was seen, together with very pronounced glycocalyces, while crypt cells possessed well developed cell coats. On the 6th day after the injection of MTX, the glycocalyx of both villous and crypt cells appeared almost normal again.

Cooperativity of carbohydrate moiety orientation and beta-turn stability is determined by intramolecular hydrogen bonds in protected glycopeptide models

The 2,3,4,6-Tetra-O-acetyl-beta-D-gluco-, and beta-D-galactopyranosides, as well as approximately 4:1 anomeric mixtures of alpha- and beta-mannopyranosides of Boc-X-Y-NHCH3 dipeptides (X-Y = Pro-Ser, Pro-D-Ser, Val-Ser, Val-D-Ser, and Gly-Ser) have been synthesized. CD and ir spectroscopic studies were performed to characterize the conformation of the glycosylated peptide backbone and examine the possible formation of intrapeptide and glycopeptide intramolecular H-bonds. It was found that O-glycosylated peptides containing a D-serine residue are likely to adopt a type II beta-turn while those with the Pro-Ser or Val-Ser sequence feature a type I (III) beta-turn in solution. Glycosylation also increases the magnitude of the CD bands, characteristic of the given type of beta-turns, which can be interpreted as an indication of the stabilization of the folded backbone conformation. Infrared data showed that in nonpolar solutions the peracetyl glycopeptides adopt both single- and double H-bonded conformations whose ratio, in some cases, depends on the position at C-2' of the H-bond acceptor acetoxy group. These data suggest that five-, seven-, or ten-membered glyco-turns may play an important role in fixing the steric orientation of the carbohydrate antennae systems in glycoproteins.

Regulation of glycolipid synthesis during differentiation of clonal murine muscle cells

The two clonal murine muscle cell lines G7 and G8, originally derived from the M114 line, represent unique models for comparative studies of myogenesis. Glycolipid synthesis was examined during differentiation using [3H]-galactose and [3H]-glucosamine as precursors. Upon G7 contact glucosylceramide labeling increased and nLcOse5Cer labeling stopped. During membrane fusion, glucosylceramide labeling stopped and lactosylceramide became the major synthetic product. G8 cells presented a different pattern, with increased labeling of GbOse3Cer during myogenesis. The major ganglioside synthesized by both myoblasts was GM3, and more complex structures were observed following completion of myotube formation. Total glycopeptide labeling increased when G8 myoblasts fused and remained elevated in myotubes, whereas no differences during fusion of G7 cells were noted. Upon comparison of the two clonal lines, the only consistent observation was a significant increase in the synthesis of total gangliosides and neutral glycolipid during cell contact and membrane fusion (p less than 0.02). The results suggest that changes in the synthesis of specific glycolipid structures during myogenesis are unique to each muscle cell line examined. However, transient increases in synthesis of total myoblast gangliosides and neutral glycolipids may be a more general phenomenon, possibly by curbing proliferation or by altering myoblast membrane fluidity characteristics during differentiation.

Role of glycosylation in the transport of recombinant glycoproteins through the secretory pathway of lepidopteran insect cells

Cell lines established from the Lepidopteran insect Spodoptera frugiperda (e.g., Sf9) are used routinely as hosts for the expression of foreign proteins by baculovirus vectors. Previously, we showed that human tissue plasminogen activator (t-PA) was expressed, N-glycosylated, and secreted by Sf9 cells infected with a recombinant baculovirus (Jarvis DL, Summers MD: Mol Cell Biol 9:214-223, 1989). We also showed that t-PA secretion was blocked by tunicamycin (TM), an inhibitor of N-glycosylation, but not by castanospermine (CS) or N-methyldeoxynojirimycin, inhibitors of the initial steps in N-linked oligosaccharide processing. This suggested that the addition, but not the processing, of N-linked oligosaccharides is required for the secretion of recombinant t-PA from baculovirus-infected Sf9 cells. In this study, we present a more generalized evaluation of the role of N-glycosylation in the transport of recombinant glycoproteins through the Sf9 cell secretory pathway. Several different secretory or membrane-bound glycoproteins were expressed in control, TM-treated, or CS-treated Sf9 cells, and their appearance in the medium or on the cell surface was measured. The results showed that TM blocked the transport of some, but not all, of these proteins, whereas CS did not block the transport of any. This suggests that N-glycosylation is sometimes required for the transport of recombinant glycoproteins through the Sf9 secretory pathway, while processing of the oligosaccharides is not. At least two other proteins, p80 and p31, consistently coimmunoprecipitated with the nonglycosylated precursors of recombinant glycoproteins expressed in TM-treated Sf9 cells. Neither was antigenically related to any of the recombinant proteins. Relatively larger amounts of p80 and p31 were coprecipitated when transport was completely blocked by TM compared to when transport was only reduced or was unaffected. These results suggest that p80 and p31 block the transport of some nonglycosylated glycoprotein precursors in TM-treated Sf9 cells by binding to them and producing transport-incompetent heterooligomeric complexes. If this speculation is correct, then p80 and p31 are functionally analogous to the mammalian immunoglobulin heavy chain binding/glucose-regulated 78 kilodalton protein (BiP/GRP78).

Glycosylation and secretion of human tissue plasminogen activator in recombinant baculovirus-infected insect cells

Cell lines established from the lepidopteran insect Spodoptera frugiperda (fall armyworm; Sf9) are used routinely as hosts for the expression of foreign proteins by recombinant baculovirus vectors. We have examined the pathway of protein glycosylation and secretion in these cells, using human tissue plasminogen activator (t-PA) as a model. t-PA expressed in Sf9 cells was both N glycosylated and secreted. At least a subset of the N-linked oligosaccharides in extracellular t-PA was resistant to endo-beta-N-acetyl-D-glucosaminidase H, which removes immature, high-mannose-type oligosaccharides. This refutes the general conclusion from previous studies that Sf9 cells cannot process immature N-linked oligosaccharides to an endo-beta-N-acetyl-D-glucosaminidase H-resistant form. A nonglycosylated t-PA precursor was not detected in Sf9 cells, even with very short pulse-labeling times. This suggests that the mammalian signal sequence of t-PA is efficiently recognized in Sf9 cells and that it can mediate rapid translocation across the membrane of the rough endoplasmic reticulum, where cotranslational N glycosylation takes place. However, t-PA was secreted rather slowly, with a half-time of about 1.6 h. Thus, a rate-limiting step(s) in secretion occurs subsequent to translocation and N glycosylation of the t-PA polypeptide. Treatment of Sf9 cells with tunicamycin, but not with inhibitors of oligosaccharide processing, prevented the appearance of t-PA in the extracellular medium. This suggests that N glycosylation per se, but not processing of the N-linked oligosaccharides, is required directly or indirectly in baculovirus-infected Sf9 cells for the secretion of t-PA. Finally, the relative efficiency of secretion decreased dramatically with time of infection, suggesting that the Sf9 host cell secretory pathway is compromised during the later stages of baculovirus infection.

Segment W of Campoletis sonorensis virus: expression, gene products, and organization

Campoletis sonorensis virus (CsV, Polydnaviridae) is a segmented double-stranded DNA virus which has an apparently symbiotic relationship with the parasitic wasp, Campoletis sonorensis. CsV replicates in the oviducts of the parasitic wasp and is injected into the wasp's host, Heliothis virescens (Lepidoptera; Noctuiidae), during oviposition. In the parasitized lepidopteran host, the virus has a dramatic effect on host physiology and viral gene products are believed to play an essential role in the survival of the parasitic wasp's egg and larva. In the current study, we used Northern blot analyses to examine expression from segment W in the parasitized host and in the parasitic wasp. Segment W hybridized primarily to two relatively abundant mRNAs (1.6 and 1.0 kb) from the parasitized host. These 1.6- and 1.0-kb mRNAs, which were previously shown to be transcribed from two closely related genes (WHv1 and WHv2) on segment W (G. W. Blissard, O. P. Smith, and M. D. Summers, 1987, Virology 160, 120-134) increased in relative abundance between 2 and 24 hr postparasitization (pp) and were detected throughout parasitization (8 days). To study the proteins encoded by these closely related genes, the open reading frame from each of the related genes was cloned into a baculovirus expression vector. By pulse labeling in the presence and absence of tunicamycin, we examined secretion and glycosylation of these CsV proteins in infected lepidopteran cells (Spodoptera frugiperda). Expression of segment W in the oviducts of the female wasp was also examined. Segment W hybridized to at least five CsV mRNAs on Northern blots of poly(A) mRNA from C. sonorensis oviducts. To identify specific CsV mRNAs and map putative viral genes expressed in wasp oviduct tissues, segment W was used to screen a cDNA library of C. sonorensis oviduct mRNAs. Three cDNAs were used to identify CsV mRNAs by Northern blot analyses and to map the locations of three putative CsV genes on segment W. Cross-hybridization within the CsV genome was examined with cloned segment W and with the three cloned cDNAs.

The effect of tunicamycin and monensin on the association of Trypanosoma cruzi with resident macrophages

The effect of incubation of parasites (epimastigote and trypomastigote forms of Trypanosoma cruzi) or macrophages in the presence of tunicamycin (TM) or monensin (M) on the parasite-macrophage association was analysed. Treatment of the parasites with TM, a drug which interferes with the process of N-glycosylation of proteins, increased by about 70% and decreased by about 27% the infection of epimastigote and trypomastigote forms, respectively. Treatment of the macrophages with TM increased by about 65% and reduced by about 45% the ingestion of epimastigote and trypomastigote forms, respectively. Treatment of the parasites or the macrophages with monensin, a drug which interferes with the sorting of membrane proteins, significantly reduced the ingestion of epimastigote and trypomastigote forms by the macrophages. The effects of both drugs were reversible. Treatment of the macrophages with trypsin followed by their incubation in fresh medium containing tunicamycin or monensin was used to analyse further the effects of these two drugs on the macrophages. The effects of the drugs on macrophages and parasites were controlled by electron microscopy. The results obtained suggest that the N-glycosylated proteins exposed on the surface of both cells are involved in the process of parasite-macrophage interaction and that interference in the process of sorting of membrane components alters the interaction.

Suppression of macrophage lysosomal enzymes after Leishmania donovani infection

In order to have an insight into the role of host lysosomal enzymes in the intracellular survival of Leishmania parasites, the activities of beta-galactosidase, alpha-mannosidase, and N-acetyl-beta-D-glucosaminidase were studied in peritoneal macrophages of hamsters infected with L. donovani. There was a significant decrease of all three lysosomal enzymes after infection. Heat-killed or formalin-treated parasites failed to inhibit the enzymes, instead a slight stimulation was observed. Purified excreted factor from promastigotes had no effect on the enzymes except beta-galactosidase which was inhibited up to 20%. Inhibition of enzymes was not due to increased secretion after infection. The absence of induction of any endogenous macrophage inhibitor was confirmed by mixed experiments. The levels of 5'-nucleotidase and lactate dehydrogenase remained unchanged after infection. Thus, the inhibition of lysosomal enzymes appears to be the effect of infection process and reflects to actua decrease rather than increased secretion or the action of any inhibitors present in Leishmania promastigotes.

Secretion of acid phosphatase by axenic Entamoeba histolytica NIH-200 and properties of the extracellular enzyme

Entamoeba histolytica (NIH-200) secreted large amounts of acid phosphatase in its external environment when grown axenically in modified TPS-II medium. Fractionation by DEAE-cellulose chromatography of the precipitate obtained from the cell-free medium at 60% ammonium sulfate saturation yielded 3 distinct peaks of enzyme activity. The enzyme in all the peaks showed resistance to tartrate but was inhibited by fluoride, cupric chloride, ethylene diamine-tetra acetic acid, ammonium molybdate and cysteine; however, enzyme associated with different peaks differed in its polyacrylamide gel electrophoretic profiles and behavior towards concanavalin A.

Glycosylation and secretion of human tissue plasminogen activator in recombinant baculovirus-infected insect cells

Cell lines established from the lepidopteran insect Spodoptera frugiperda (fall armyworm; Sf9) are used routinely as hosts for the expression of foreign proteins by recombinant baculovirus vectors. We have examined the pathway of protein glycosylation and secretion in these cells, using human tissue plasminogen activator (t-PA) as a model. t-PA expressed in Sf9 cells was both N glycosylated and secreted. At least a subset of the N-linked oligosaccharides in extracellular t-PA was resistant to endo-beta-N-acetyl-D-glucosaminidase H, which removes immature, high-mannose-type oligosaccharides. This refutes the general conclusion from previous studies that Sf9 cells cannot process immature N-linked oligosaccharides to an endo-beta-N-acetyl-D-glucosaminidase H-resistant form. A nonglycosylated t-PA precursor was not detected in Sf9 cells, even with very short pulse-labeling times. This suggests that the mammalian signal sequence of t-PA is efficiently recognized in Sf9 cells and that it can mediate rapid translocation across the membrane of the rough endoplasmic reticulum, where cotranslational N glycosylation takes place. However, t-PA was secreted rather slowly, with a half-time of about 1.6 h. Thus, a rate-limiting step(s) in secretion occurs subsequent to translocation and N glycosylation of the t-PA polypeptide. Treatment of Sf9 cells with tunicamycin, but not with inhibitors of oligosaccharide processing, prevented the appearance of t-PA in the extracellular medium. This suggests that N glycosylation per se, but not processing of the N-linked oligosaccharides, is required directly or indirectly in baculovirus-infected Sf9 cells for the secretion of t-PA. Finally, the relative efficiency of secretion decreased dramatically with time of infection, suggesting that the Sf9 host cell secretory pathway is compromised during the later stages of baculovirus infection.

On the utility of 13C-n.m.r. spectroscopy in the identification of the primary structures of manno-oligosaccharides and glycopeptides

The utility of 13C-n.m.r. spectroscopy in the identification of the primary structures of mannose-containing glycans is investigated. Unlike 1H resonances where the chemical shifts reflect multiple short- and long-range effects, the chemical shifts of 13C resonances are dependent largely upon short-range effects classified as glycosylation (linkage) and substitution effects. These effects are parametized for glycans composed of mannose and encoded in a FORTRAN algorithm. Applications of this program to "unknown" sets of experimental chemical shifts for the resonances of anomeric carbons gave the following conclusions. (1) This program can be used to produce a sub-set of possible structures inclusive of the "known" structure. (2) For other than simple oligosaccharides, it is unlikely that a single structure is consistent with the data for anomeric carbons alone, even when the linkage composition of the glycan has been assessed from other spectral data. (3) When used in conjunction with other chemical techniques, this program can provide a powerful tool for primary analysis of the structure of mannose-containing glycans.

Acid protease activity of a major surface membrane glycoprotein (gp63) from Leishmania mexicana promastigotes

A unique protease with activity optimal at pH 4.0 and trailing toward the alkaline pH spectrum was detected with intact glutaraldehyde-fixed promastigotes of Leishmania mexicana amazonensis, indicating surface localisation of the enzyme. That this surface protease may be a virulence factor is suggested by its apparent roles in multiple steps during leishmanial infections of macrophages. Indeed, its specific activity was 2-2.5 fold higher on virulent cells than on avirulent cells. Several lines of evidence indicate that this acid protease activity is expressed by the major surface glycoprotein (gp63) of L. m. amazonensis. Monoclonal antibody affinity purified gp63 degraded serum albumin, hemoglobin, complement C3, immunoglobulin G and purified rat liver lysosomal proteins in their native forms. The specific activity is about 20-fold higher at pH 4.0 than at pH 7.5 and is about four-fold higher at the body temperature of the mammalian host (37 degrees C) than at that of the insect host (27 degrees C). The protease activity is sodium dodecyl sulphate-sensitive. Among various protease inhibitors tested, only heavy metal ions (1 mM), 1,10-orthophenanthroline (1 mM) and bestatin (100 ng ml-1) significantly inhibited gp63 acid protease activity by up to 80%. N-linked oligosaccharides of gp63 appear to be important for the stability of this molecule, possibly by preventing its autodegradation. Purified gp63 effected limited proteolysis of human complement C3 molecules at the physiological serum pH of 7.5 in a manner, which supports the idea of its participation in complement-receptor mediated endocytosis of promastigotes by macrophages.

Membrane glycoproteins are involved in the differentiation of the BC3H1 muscle cell line

The nonfusing muscle cell line BC3H1 expresses a family of muscle-specific proteins when the fetal bovine serum (FBS) concentration is reduced from 20 to 1%. We have used a series of glycosylation inhibitors to assess the role played by glycoproteins in the initiation of differentiation in this cell line. Tunicamycin (TNM) and 2-deoxy-D-glucose, added to cells when the FBS concentration was reduced, blocked creatine phosphokinase (CPK) induction by 70-95%. These effects were dose dependent and reversible. TNM and 2-deoxy-D-glucose also reversed CPK induction in differentiated cells. Leupeptin and N-acetylglucosamine did not reverse these effects. 1-Deoxynojirimycin, 1-deoxymannojirimycin, and swainsonine have no effect on induced CPK expression, whereas castanospermine, a glucosidase I inhibitor, blocked its induction completely. As attempts to use conditioned medium from cells grown in 1 or 20% FBS have no effect on this differentiation process we conclude that high mannose structures, but not complex form glycoproteins, bound to the surface of BC3H1 cells play a role in transducing signals for differentiation and are probable mediators of cell/cell contact.

Carbohydrate heterogeneity of fibronectins. Synovial fluid fibronectin resembles the form secreted by cultured synoviocytes but differs from the plasma form

Large quantities of fibronectin (Fn) are present in inflammatory synovial fluid. Inflammatory synovial fluid Fn, while indistinguishable from plasma Fn on the basis of reactivity to polyclonal antibodies, displays alterations in molecular size and charge. Since biochemical differences between plasma and synovial fluid fibronectins might be in part due to differences in glycosylation we have compared the carbohydrate composition of plasma Fn, synovial fluid Fn, and Fn from synoviocyte conditioned medium by biochemical assay, glycopeptide analysis, and binding to a series of lectins. Synovial fluid Fn has a greater carbohydrate content but contains less sialic acid when compared with plasma Fn. Glycopeptides formed from synovial fluid Fn are smaller than plasma Fn glycopeptides. These data suggest the presence of an additional N-linked oligosaccharide chain on synovial fluid Fn. In addition, synovial fluid Fn contains N-acetyl galactosamine indicating the presence of O-linked oligosaccharides. Synovial fluid Fn and Fn isolated from rheumatoid synoviocyte-conditioned medium display strong reactivity with the lectins wheat germ agglutinin (WGA) and peanut agglutinin (PNA), whereas normal and rheumatoid plasma Fn react weakly. The PNA reactivity of synovial fluid Fn is mediated by terminal beta-galactose residues on the gelatin-binding domain, whereas the enhanced WGA reactivity of synovial Fn is mediated by a sialic acid containing oligosaccharide located on a 27-kD C-terminal fragment. These data demonstrate domain-specific biochemical differences between plasma and synovial fluid fibronectins. These differences suggest a local origin for synovial fluid Fn and may contribute to functional differences between these forms of the protein.

Uptake and processing of glycoproteins by rat hepatic mannose receptor

A linear compartmental model has been developed for the in vivo metabolism of glycoproteins. The model is applied to the interpretation of dynamic data from the rat on agalactoorosomucoid (AGOR), an N-acetylglucosamine (GlcNAc-)-terminated glycoprotein, and three neoglycoproteins terminating in mannose [mannose36-bovine serum albumin (Man-BSA)] or glucose [maltose29-BSA (Mal29-BSA) and maltose8-BSA (Mal8-BSA)]. All of these proteins are taken up by the Man/GlcNAc receptor on hepatic sinusoidal cells. The rate of uptake was found to be determined by sugar type (Man-BSA, 0.78 min-1 greater than Mal29-BSA, 0.13 min-1), sugar density (Mal29-BSA greater than Mal8-BSA), and the geometry of the sugar display (AGOR, 0.51 min-1 greater than Mal29-BSA). Intracellular transport from the cell membrane to the lysosomes was slower for Man-BSA (approximately 3 min) than for the other ligands (approximately 0 min), suggesting that receptor-ligand uncoupling was slower for Man-BSA for which the receptor had the highest affinity or that extralysosomal catabolism of the other ligands occurred. Catabolism was also determined by the carbohydrate moiety of the ligand; it was greater for Mal29-BSA and Mal8-BSA (greater than or equal to 0.8 min-1) than for Man-BSA (0.27 min-1), and AGOR, with a complex oligosaccharide, was most resistant to degradation (0.14 min-1). An understanding of these structural features of glycoproteins that influence hepatic uptake, transport, and catabolism will be of value in drug targeting and for enzyme replacement in lysosomal storage disorders.(ABSTRACT TRUNCATED AT 250 WORDS)

Tunicamycin-resistant Leishmania mexicana amazonensis: expression of virulence associated with an increased activity of N-acetylglucosaminyltransferase and amplification of its presumptive gene

Tunicamycin at 10 micrograms/ml inhibits the growth and infectivity of the parasitic protozoan Leishmania mexicana amazonensis. Tunicamycin-resistant variants of this parasite were produced by gradual acclimatization of cells to increasing concentrations of the drug up to 80 micrograms/ml and a single-step selection of ethyl methanesulfonate-pretreated or differentiating leishmanias with the drug at 10 micrograms/ml. Prolonged exposure to the drug increases stability of drug resistance of those resistant to 10 micrograms/ml. Tunicamycin-resistant cells contain amplified DNA, which hybridizes in proportion to the cells' degree of drug resistance with Alg 7, a cloned DNA probe apparently encoding yeast N-acetylglucosaminyltransferase. This enzyme from all variants remained sensitive to inhibition by tunicamycin, but its specific activity was up to 15-fold higher than that of the wild type. Thus, amplification of the gene encoding this enzyme appears to result in its overproduction in the variants, accounting for their resistance to tunicamycin. The tunicamycin-resistant cells are more virulent to mice than their parental wild type. Thus, leishmanial virulence may be related to amplification or expression of gene(s) encoding enzymes involved in the regulation of N-glycosylation of parasite proteins.

p185, a product of the neu proto-oncogene, is a receptorlike protein associated with tyrosine kinase activity

The neu oncogene was originally identified in cell lines derived from rat neuroectodermal tumors. neu is related to but distinct from the c-erbB gene, which encodes the epidermal growth factor (EGF) receptor. neu encodes a protein, designated p185, that is serologically related to the EGF receptor. Identification of the normal homolog of p185 encoded by the neu proto-oncogene enabled us to compare the product of the neu proto-oncogene with the mutated version specified by the neu oncogene and with the EGF receptor. The normal form of p185 was structurally similar to its transforming counterpart, indicating that activation of the neu oncogene did not cause major structural alterations in the gene product. Both normal and transforming forms of p185 were associated with tyrosine kinase activity, supporting the idea that normal p185 functions as a growth factor receptor. p185 differed both structurally and functionally from the EGF receptor. p185 and the EGF receptor had distinct electrophoretic mobilities when synthesized under normal culture conditions or in the presence of tunicamycin. EGF did not stimulate increased turnover of p185 and did not bind quantitatively to p185. A number of other growth factors failed to stimulate degradation of p185 or tyrosine phosphorylation of p185 and are therefore unlikely to be ligands for p185.

Effect of tunicamycin on the extracellular acid phosphatase of Leishmania donovani promastigotes

Inhibition of replication of Leishmania donovani promastigotes in axenic culture medium by tunicamycin, an inhibitor of asparagine linked protein glycosylation, depends upon the cell density of the culture at the time of addition of tunicamycin as well as upon the concentration of tunicamycin itself. Parasite multiplication in cultures with initial densities of less than or equal to 1 X 10(6) cells ml-1 and a tunicamycin concentration of 1 microgram ml-1 was limited to 2-3 replications, but this limitation was not observed in cultures with initial densities greater than or equal to 2 X 10(6) cells ml-1. Under conditions in which tunicamycin inhibited parasite growth and protein synthesis by only 15% and 6%, respectively, there was a greater than 90% reduction in the level of secreted acid phosphatase activity in comparison to control cultures. The extracellular acid phosphatase activity remaining in tunicamycin treated cultures was electrophoretically distinct from that found in control cultures. No significant decrease in the amount of [35S]methionine incorporated into acid insoluble products in the supernatant of tunicamycin treated cultures was observed, and a radiolabeled protein with an electrophoretic Mr of 97,000 was immunoprecipitated from this supernatant by an anti extracellular acid phosphatase antiserum. It was concluded that the L. donovani extracellular acid phosphatase, previously shown to be a mannose containing glycoprotein, contains N-linked oligosaccharides which are necessary for maintenance of its catalytic activity, but not its secretion.

The effect of tunicamycin on Leishmania brasiliensis. Glycosylation and the cell surface components

Culture conditions of Leishmania cells were developed to allow the study of the effect of tunicamycin (TM) on glycosylation and on the cell surface components. Leishmania incorporate [14C]-mannose and [35S]-methionine in vitro. The incorporation of [14C]-mannose is linear for 150 min and is inhibited by TM (2 micrograms/ml) in a time dependent effect which reaches a plateau of 45% inhibition at 36 h. Under the same experimental conditions [35S]-methionine incorporation into protein is slightly affected. This is reflected by an almost identical polypeptide pattern for TM treated and non-treated cells when analyzed on SDS-PAGE. On the contrary, strong differences were detected on the labeled compounds analyzed on SDS-PAGE followed by autoradiography when the precursor used was [14C]-mannose. A shift in the electrophoretic mobility of most of the glycopeptides synthesized in the presence of TM was observed, which is also reflected in the structure of the main Leishmania cell surface components. The findings are discussed in the light of biological implications.

HTLV-III/LAV-neutralizing antibodies to an E. coli-produced fragment of the virus envelope

Immunization with either an Escherichia coli recombinant segment of the human T-cell lymphotropic virus (HTLV-III/LAV) envelope protein (gp 120) or with deglycosylated gp 120 envelope protein produced antibodies that neutralize HTLV-III/LAV infection in vitro. Virus neutralization titers of these antisera were equivalent to those obtained with purified native gp120 as immunogen. This localizes at least one class of neutralizing epitopes to the carboxyl-terminal half of the molecule. In addition, native gp120 prevented HTLV-III/LAV--mediated cell fusion, whereas the recombinant gp120 fragment did not. This shows that although glycosylation is not required for induction of neutralizing antibodies, it may be important for interaction with CD4, the virus receptor. A segment of the HTLV-III/LAV envelope produced in E. coli may be an important ingredient of a vaccine for acquired immune deficiency syndrome.

The heterogeneity of rat androgen binding protein (rABP) in the vascular compartment differs from that in the testicular tubular lumen. Further evidence for bidirectional secretion of rABP

Fractionation of testicular extracts and serum on a Concanavalin A-Sepharose column resolved two peaks of immunoreactive rat androgen binding protein. The rat androgen binding protein in the first peak, designated Form I, was present in the void volume; the other, designated Form II rat androgen binding protein, was bound by the column and specifically eluted by alpha-methylmannoside. In the course of studying the heterogeneity of rat androgen binding protein on Concanavalin A-Sepharose, it was observed that the distribution of the two forms of this protein was similar in the fluid obtained by micropuncture from the seminiferous tubule and the rete testis, that is, the ratios of Form I to Form II were 1:1 and 1:1.8, respectively. By contrast, Form I rat androgen binding protein in blood, interstitial fluid, and thoracic duct lymph of adult rats was reduced relative to Form II; the ratios of Form I:Form II in these fluids were 1:4.4, 1:3.1, and 1:4.6, respectively. since previous studies indicated that the reduced amount of Form I relative to Form II observed in the blood of adult rats was not the result of more rapid clearance of Form I, these results suggest that Form I rat androgen binding protein is preferentially secreted into the lumen of the seminiferous tubule rather than into the interstitial fluid and blood. We conclude that Sertoli cells in adult rats may partition rat androgen binding protein between the interstitial and luminal compartments of the testis based on the carbohydrate composition of this protein.

p185, a product of the neu proto-oncogene, is a receptorlike protein associated with tyrosine kinase activity

The neu oncogene was originally identified in cell lines derived from rat neuroectodermal tumors. neu is related to but distinct from the c-erbB gene, which encodes the epidermal growth factor (EGF) receptor. neu encodes a protein, designated p185, that is serologically related to the EGF receptor. Identification of the normal homolog of p185 encoded by the neu proto-oncogene enabled us to compare the product of the neu proto-oncogene with the mutated version specified by the neu oncogene and with the EGF receptor. The normal form of p185 was structurally similar to its transforming counterpart, indicating that activation of the neu oncogene did not cause major structural alterations in the gene product. Both normal and transforming forms of p185 were associated with tyrosine kinase activity, supporting the idea that normal p185 functions as a growth factor receptor. p185 differed both structurally and functionally from the EGF receptor. p185 and the EGF receptor had distinct electrophoretic mobilities when synthesized under normal culture conditions or in the presence of tunicamycin. EGF did not stimulate increased turnover of p185 and did not bind quantitatively to p185. A number of other growth factors failed to stimulate degradation of p185 or tyrosine phosphorylation of p185 and are therefore unlikely to be ligands for p185.

Biochemical and immunologic studies on the native and deglycosylated forms of gamma-glutamyltranspeptidase of rat kidney

We have deglycosylated the enzyme gamma-glutamyl transpeptidase by treatment of the protein with anhydrous hydrofluoric acid at 0 degree C. After deglycosylation, the heavy and light subunits showed a molecular weight of 43 and 23 Kd respectively. Whereas the antiserum against the native enzyme recognized both proteins, the antiserum against the deglycosylated enzyme failed to recognize the native enzyme, indicating that some of the determinants of the native enzyme are masked by the carbohydrate moiety.

Inhibition of in vivo and in vitro infectivity of Leishmania donovani by tunicamycin

Leishmania donovani 2S strain promastigotes were rendered non-infectious to mice and mouse peritoneal macrophages by treatment with tunicamycin, an inhibitor of N-linked protein glycosylation. Concentrations of tunicamycin (1-10 micrograms ml-1) that reduced promastigote infectivity to 2% or less of control levels had little or no measurable effect on the in vitro growth of the promastigotes. Tunicamycin has no apparent effect on the entry of promastigotes into macrophages. These results indicate that the sugar residues of glycoproteins are important to the promastigote during the early stages of macrophage infection.

A role for glycosylation of the alpha subunit in transduction of biological signal in glycoprotein hormones

The biological properties of recombinants of glycoprotein hormones in which the alpha and beta subunits were differentially deglycosylated have been investigated. Specific deglycosylation of the alpha subunit generated a recombinant that had more receptor-binding activity but did not produce hormone response in the target cells. The deglycosylated alpha + beta recombinant was also an antagonist of the action of the native hormone. Thus, the carbohydrates in the alpha subunit play a dominant role in the transduction of the hormone signal into the cell.

Swainsonine toxicosis mimics lectin histochemistry of mannosidosis

Cells affected by locoweed (Astragalus lentiginosus) and Swainsona galegifolia toxicosis or mannosidosis exhibit similarities in their catabolism of N-linked glycoproteins and accumulation of cytoplasmic vacuoles. We used nine different biotinylated lectins as histochemical markers for specific sugars and avidin-biotin-peroxidase complex as a visualant to study the cells affected with these conditions. Since locoweed and Swainsona spp block mannosidase activity, we expected a similar lectin staining pattern in cells under these conditions as that seen in mannosidosis. Concanavalia ensiformis agglutinin, wheat germ agglutinin and succinyl wheat germ agglutinin stained the undegraded glycoproteins and oligosaccharides stored in the lysosomes of affected cells in all three conditions. Bandeirea simplicifolia-I, Dolichos biflorus agglutinin, peanut agglutinin, Ricinus communis agglutinin-I, soybean agglutinin and Ulex europaeus agglutinin-I did not stain any of these cells. These results indicate that in all three conditions there is an accumulation of undegraded oligosaccharides that contain alpha-mannosyl and beta-N-acetyl glucosamine residues which are revealed by lectin staining in the vacuoles of all affected cells.

O-linked oligosaccharides of mouse egg ZP3 account for its sperm receptor activity

Previously, we reported that ZP3, one of three different glycoproteins present in the mouse egg's zona pellucida, serves as a sperm receptor. Furthermore, small glycopeptides derived from egg ZP3 retain full sperm receptor activity, suggesting a role for carbohydrate, rather than polypeptide chain in receptor function. Here, we report that removal of O-linked oligosaccharides from ZP3 destroys its sperm receptor activity, whereas removal of N-linked oligosaccharides has no effect. A specific size class of O-linked oligosaccharides, recovered following mild alkaline hydrolysis and reduction of ZP3, is shown to possess sperm receptor activity and to bind to sperm. The results presented strongly suggest that mouse sperm bind to eggs via O-linked oligosaccharides present on ZP3.

Effector functions of a monoclonal aglycosylated mouse IgG2a: binding and activation of complement component C1 and interaction with human monocyte Fc receptor

Aglycosylated monoclonal anti-DNP mouse IgG2a produced in the presence of tunicamycin was compared with the native monoclonal IgG2a with respect to its ability to interact with the first component of complement, C1, and to compete with human IgG for binding to human monocyte Fc receptors. The aglycosylated IgG2a was found to bind subcomponent C1q with an equivalent capacity to the native IgG2a, but the dissociation constant was found to be increased three-fold. When activation of C1 by the glycosylated and aglycosylated IgG2a was compared, the rate of C1 activation by the aglycosylated IgG2a was reduced approximately three-fold. In contrast aglycosylation was accompanied by a large decrease (greater than or equal to 50-fold) in the apparent binding constant of monomeric IgG2a to human monocytes. The data suggest that the aglycosylated IgG2a has a structure which differs in the CH2 domain from the native IgG2a, and that the heterogeneous N-linked oligosaccharides of this monoclonal IgG2a which occur at a conserved position in the CH2 domain play a role in maintaining the integrity of its monocyte-binding site. This lack of monocyte binding may result either from a localized conformational change occurring in a single CH2 domain or from an alteration in the CH2-CH2 cross-domain architecture which is normally structured by a pair of opposing and interacting oligosaccharides. The minimal changes in C1q binding and C1 activation suggest that the oligosaccharides are, at most, indirectly involved in these events.

An ultrastructural study of complex carbohydrates in the posterior chamber and vitreous base of the mouse

The fibrillar and mucoid extracellular matrix of the posterior chamber and vitreous base was studied in the mouse by electron microscopy using fixation and staining methods that demonstrated complex carbohydrates. These methods, including block-staining with Alcian Blue, allowed globular and filamentous hyaluronic acid, finely filamentous oligosaccharides, laminated glycolipids or lipophilic glycoproteins and stellate proteoglycan monomers to be identified tentatively. There was much less globular hyaluronic acid along the basement membrane of the peripheral retina and ciliary body than has been observed in the posterior fundus. A finely filamentous network on the basement membrane interconnected with a similar network covering individual collagen fibrils, zonules and meridional fibrillar laminae as well as with a branching fibrillar network that was seen in the posterior chamber and vitreous base. This interconnected system of fibrillar proteins and complex carbohydrates was also connected to the anterior hyaloid membrane. The infoldings of the ciliary epithelium contained stellate densities with characteristics of proteoglycan monomers similar to those reported in the matrix of cartilage. The complex carbohydrates of the posterior chamber and vitreous base are of several types known to affect protein function, provide water binding and assist in mechanical stability.

Modification of simian virus 40 large tumor antigen by glycosylation

The SV40-encoded transforming protein, large tumor antigen (T-ag), is multifunctional. Chemical modifications of the T-ag polypeptide may be important for its multifunctional capacity. T-ag is additionally modified by glycosylation. T-ag was metabolically labeled in SV40-infected cells with tritiated galactose or glucosamine, but not with mannose or fucose. The identity of glycosylated T-ag was established by immunoprecipitation with a variety of T-ag-specific antisera, including monoclonal antibodies. Incorporation of labeled sugar into T-ag was inhibited in the presence of excess unlabeled sugars, but not in the presence of excess unlabeled amino acids. Labeled monosaccharides could be preferentially removed from T-ag with a mixture of glycosidic enzymes. In addition, galactose was removed from purified T-ag by acid hydrolysis and identified as such by thin-layer chromatography. T-ag oligosaccharides were resistant to treatment with EndoH, and glycosylation was not inhibited by tunicamycin. Together, these data strongly suggest that T-ag is glycosylated. Several characteristics, including lack of mannose labeling, EndoH resistance, and tunicamycin resistance, suggest that T-ag is not an N-linked glycoprotein. Rather, these properties are more consistent with the identification of T-ag as an O-linked glycoprotein.

Membrane morphogenesis in retinal rod outer segments: inhibition by tunicamycin

Isolated Xenopus laevis retinas were incubated with 3H-labeled mannose or leucine in the presence or absence of tunicamycin (TM), a selective inhibitor of dolichyl phosphate-dependent protein glycosylation. At a TM concentration of 20 micrograms/ml, the incorporation of [3H]mannose and [3H]leucine into retinal macromolecules was inhibited by approximately 66 and 12-16%, respectively, relative to controls. Cellular uptake of the radiolabeled substrates was not inhibited at this TM concentration. Polyacrylamide gel electrophoresis revealed that TM had little effect on the incorporation of [3H]leucine into the proteins of whole retinas and that labeling of proteins (especially opsin) in isolated rod outer segment (ROS) membranes was negligible. The incorporation of [3H]mannose into proteins of whole retinas and ROS membranes was nearly abolished in the presence of TM. Autoradiograms of control retinas incubated with either [3H]mannose or [3H]leucine exhibited a discrete concentration of silver grains over ROS basal disc membranes. In TM-treated retinas, the extracellular space between rod inner and outer segments was dilated and filled with numerous heterogeneously size vesicles, which were labeled with [3H]leucine but not with [3H]mannose. ROS disc membranes per se were not labeled in the TM-treated retinas. Quantitative light microscopic autoradiography of retinas pulse-labeled with [3H]leucine showed no differences in labeling of rod cellular compartments in the presence or absence of TM as a function of increasing chase time. These results demonstrate that TM can block retinal protein glycosylation and normal disc membrane assembly under conditions where synthesis and intracellular transport of rod cell proteins (e.g., opsin) are not inhibited.

Solubilization and characterization of striatal dopamine receptors

Dopamine receptor binding proteins were solubilized with the detergent 3-(3-cholamidopropyl) dimethylammonio-2-hydroxy-1-propanesulfonate (CHAPSO) from bovine and rat striatal membranes. The binding of the dopamine antagonist [3H]spiroperidol [( 3H]Spi) to the solubilized dopamine receptors was determined by the polyethyleneglycol method. The CHAPSO-solubilized dopamine receptor binding proteins remain in the supernatant fraction following centrifugation at 100,000 X g for 2 h. The CHAPSO-solubilized dopamine receptor proteins, as well as the prelabeled [3H]Spi-receptor protein complex, bind specifically to wheat germ agglutinin (WGA)-agarose columns, which is consistent with an identification as glycoproteins, HPLC analysis of the CHAPSO-solubilized, prelabeled [3H]Spi-receptor protein complex (CHAPSO preparation) reveals association with a high molecular weight form, indicating the formation of aggregates and/or micelles. Treatment of the WGA-agarose-bound [3H]Spi-receptor protein complex with digitonin (CHAPSO-digitonin preparation) results in dissociation of the high molecular weight form into lower molecular weight forms. The HPLC profile of the prelabeled [3H]Spi-receptor complex in the CHAPSO-digitonin preparation reveals two radioactive peaks. The major peak had a retention time of 16 min, corresponding to an apparent MW of 175,000, whereas the minor peak had a retention time of 21 min, corresponding to an apparent MW of 49,000. The CHAPSO-solubilized dopamine receptor binding proteins are sensitive to modulation by GTP, indicating that the association with the GTP binding component is preserved in the "soluble" state. The potencies of dopamine antagonists and agonists for inhibiting the binding of [3H]Spi to CHAPSO-solubilized dopamine receptor proteins are similar to those for membrane-bound proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Integrity of glycoprotein complex sugars is required for homing but not for several other membrane-mediated functions

In order to correlate the biochemistry of cell surface carbohydrates with cell function, we have treated cells with swainsonine and followed the biochemical and functional modifications induced by this compound. After treatment with swainsonine, surface glycoproteins had a lower apparent molecular weight and a higher isoelectric point. This is compatible with the replacement of complex carbohydrates by hybrid or high-mannose carbohydrates. Several functional tests were unaffected. However, swainsonine-treated cells displayed an altered pattern of in vivo homing, suggesting that carbohydrates play a role in this process.