Effect of tunicamycin on the biosynthesis of the major human red cell sialoglycoprotein, glycophorin A, in the leukemia cell line K562

Small molecule inhibitors of mammalian glycosylation

Glycans are one of the fundamental biopolymers encountered in living systems. Compared to polynucleotide and polypeptide biosynthesis, polysaccharide biosynthesis is a uniquely combinatorial process to which interdependent enzymes with seemingly broad specificities contribute. The resulting intracellular cell surface, and secreted glycans play key roles in health and disease, from embryogenesis to cancer progression. The study and modulation of glycans in cell and organismal biology is aided by small molecule inhibitors of the enzymes involved in glycan biosynthesis. In this review, we survey the arsenal of currently available inhibitors, focusing on agents which have been independently validated in diverse systems. We highlight the utility of these inhibitors and drawbacks to their use, emphasizing the need for innovation for basic research as well as for therapeutic applications.

GZD824 Inhibits GCN2 and Sensitizes Cancer Cells to Amino Acid Starvation Stress

Eukaryotic initiation factor 2α (eIF2α) kinase general control nonderepressible 2 (GCN2) drives cellular adaptation to amino acid limitation by activating the integrated stress response that induces activating transcription factor 4 (ATF4). Here, we found that a multikinase inhibitor, GZD824, which we identified using a cell-based assay with ATF4 immunostaining, inhibited the GCN2 pathway in cancer cells. Indeed, GZD824 suppressed GCN2 activation, eIF2α phosphorylation, and ATF4 induction during amino acid starvation stress. However, at lower nonsuppressive concentrations, GZD824 paradoxically stimulated eIF2α phosphorylation and ATF4 expression in a GCN2-dependent manner under unstressed conditions. Such dual properties conceivably arose from a direct effect on GCN2, as also observed in a cell-free GCN2 kinase assay and shared by a selective GCN2 inhibitor. Consistent with the GCN2 pathway inhibition, GZD824 sensitized certain cancer cells to amino acid starvation stress similarly to ATF4 knockdown. These results establish GZD824 as a multikinase GCN2 inhibitor and may enhance its utility as a drug under development. SIGNIFICANCE STATEMENT: GZD824, as a direct general control nonderepressible 2 (GCN2) inhibitor, suppresses activation of the integrated stress response during amino acid limitation, whereas it paradoxically stimulates this stress-signaling pathway at lower nonsuppressive concentrations. The pharmacological activity we identify herein will provide the basis for the use of GZD824 to elucidate the regulatory mechanisms of GCN2 and to evaluate the potential of the GCN2-activating transcription factor 4 pathway as a target for cancer therapy.

BCR-ABL tyrosine kinase inhibition induces metabolic vulnerability by preventing the integrated stress response in K562 cells

The integrated stress response (ISR) is a cellular process that is characterized by activation of eukaryotic initiation factor (eIF)2α kinases and subsequent induction of activating transcription factor (ATF)4. The ISR plays an important role in protecting cells from tumor-related metabolic stresses, such as nutrient deprivation and perturbed proteostasis. Here, we showed that disruption of the ISR, together with increased cellular stress vulnerability, was produced by pharmacological inhibition of BCR-ABL, the oncogenic driver in chronic myeloid leukemia (CML). Treatment of CML-derived K562 cells with BCR-ABL tyrosine kinase inhibitors, including imatinib, dasatinib, nilotinib and ponatinib, prevented activation of eIF2α kinases, protein kinase-like endoplasmic reticulum kinase (PERK) and general control nonderepressible 2, and downstream ATF4 induction during metabolic stress. Prevention of ATF4 induction likely occurred as a result of the combinatorial suppression of the eIF2α kinase and phosphoinositide 3-kinase/mammalian target of rapamycin signaling pathways. In addition, we found that pharmacological inhibition of PERK mimicked BCR-ABL inhibition to enhance apoptosis induction under stress conditions. These findings indicate that the ISR is under the control of BCR-ABL and may foster adaptation to tumorigenic stresses in CML cells.

A novel splice variant of the protein tyrosine phosphatase PTPRJ that encodes for a soluble protein involved in angiogenesis

PTPRJ is a receptor protein tyrosine phosphatase with tumor suppressor activity. Very little is known about the role of PTPRJ ectodomain, although recently both physiological and synthetic PTPRJ ligands have been identified. A putative shorter spliced variant, coding for a 539 aa protein corresponding to the extracellular N-terminus of PTPRJ, is reported in several databases but, currently, no further information is available.

Here, we confirmed that the PTPRJ short isoform (named sPTPRJ) is a soluble protein secreted into the supernatant of both endothelial and tumor cells. Like PTPRJ, also sPTPRJ undergoes post-translational modifications such as glycosylation, as assessed by sPTPRJ immunoprecipitation. To characterize its functional activity, we performed an endothelial cell tube formation assay and a wound healing assay on HUVEC cells overexpressing sPTPRJ and we found that sPTPRJ has a proangiogenic activity. We also showed that sPTPRJ expression down-regulates endothelial adhesion molecules, that is a hallmark of proangiogenic activity. Moreover, sPTPRJ mRNA levels in human high-grade glioma, one of the most angiogenic tumors, are higher in tumor samples compared to controls. Further studies will be helpful not only to clarify the way sPTPRJ works but also to supply clues to circumvent its activity in cancer therapy.

Asparagine-linked oligosaccharide-independent secretion of egg envelope glycoprotein ZPC of the Japanese quail (Coturnix japonica)

In avian species, a glycoprotein homologous to mammalian ZPC is synthesized in the granulosa cells of developing follicles. We have previously reported that the newly synthesized ZPC (proZPC) in the granulosa cells is cleaved at the consensus furin cleavage site to generate mature ZPC prior to secretion. In the present study, we examined the role of asparagine (N)-linked oligosaccharides in the proteolytic processing of proZPC and the subsequent secretion of ZPC by using site-directed mutagenesis of the consensus sequence for N-glycosylation, and tunicamycin, an inhibitor for N-glycosylation of glycoprotein. Western blot analysis demonstrated that tunicamycin did not block either proteolytic cleavage of proZPC or the subsequent ZPC secretion. Moreover, a site-directed mutant that possesses a mutated sequence for N-glycosylation was efficiently secreted from the cells. These results indicate that proteolytic cleavage of proZPC, and the subsequent ZPC secretion occur in the absence of N-linked oligosaccharides. Therefore, the addition of N-glycans to ZPC polypeptide is not required for quail ZPC secretion.

Host cell-induced differences in O-glycosylation of herpes simplex virus gC-1. I. Structures of nonsialylated HPA- and PNA-binding carbohydrates

Lectins with narrow oligosaccharide specificities were established as probes to study the host cell influence on the biosynthesis of O-linked oligosaccharides of the herpes simplex virus type 1 (HSV-1)-specified glycoprotein C (gC-1). We found that only gC-1 and no other glycoprotein bound to the peanut lectin (PNA), with main specificity for Gal(beta 1-3)GalNAc. Previously, we have shown that only gC-1 binds to the Helix pomatia lectin (HPA), with main specificity for terminal GalNAc. The O-linked oligosaccharides binding to PNA and HPA were released by alkaline borohydride treatment and characterized. A structural determination of these oligosaccharides showed that the HPA-binding carbohydrates were monosaccharides (GalNAc), and that the PNA-binding oligosaccharides were disaccharides with the structure Gal-GalNAc. The PNA- and HPA-binding oligosaccharides were arranged as Pronase-resistant clusters on gC-1, consisting of about seven individual, adjacent oligosaccharides. In addition to these disaccharides, Pronase-resistant PNA-binding glycopeptides of gC-1 also contained neutral trisaccharides. Larger O-linked oligosaccharides, binding to the wheat germ lectin, were found in gC-1, but not in proximity to the PNA-binding ones. It was concluded that the lectins mentioned should be useful probes in screening HSV-infected cells of different lineages for differences in processing of O-linked oligosaccharides.

Immunogenicity and antigenicity of immunoglobulins: detection of human immunoglobulin light-chain carbohydrate, using concanavalin A

Concanavalin A binding to glycoprotein bands on nitrocellulose blots was used to detect mannose, sorbose, N-acetylgalactosamine and/or glucose residues on 100% (31/31) of human Bence Jones protein light chains, following sodium dodecyl sulphate-polyacrylamide gel electrophoresis. All (20/20) light chains form IgG myeloma proteins and light chains from a preparation of normal polyclonal human IgG were also bound by concanavalin A. The specificity of concanavalin A for glycoproteins was demonstrated by its binding to human Fc fragments and a human monoclonal anti-Rhesus D antibody (REG-A), but not to human albumin pFc' fragments and aglycosylated REG-A derived from cells grown in the presence of the glycosylation inhibitor tunicamycin. These results suggest that all Bence Jones proteins and light chains from myeloma IgG proteins contain mono- or oligosaccharides linked O-glycosidically to serine or threonine residues.

Glycophorins of human erythroleukemic K562 cells

Glycophorins related to alpha glycophorin, of the human erythrocyte membrane, were isolated from human erythroleukemic K562 cells. The glycophorins were purified using sodium dodecyl sulfate (SDS)/trichloroacetic acid fractionation and Folch and hot phenol extractions. 0.1-0.2 micrograms was obtained/10(8) cells, or approximately a 15% yield. SDS-gel electrophoresis revealed a pattern similar to erythrocyte alpha glycophorin except for the slower mobility of the glycophorin monomer. Two populations of K562 glycophorins, present in nearly equivalent amounts, were distinguished by their binding to Lens culinaris lectin agarose. The two populations exhibited similar gel electrophoretic patterns except for the presence of delta-like glycophorin exclusively in the population that did not bind to L. culinaris lectin. Immunoblotting revealed a lack of reaction of the major alpha and delta-like glycophorin bands in all K562 glycophorins with M or N erythrocyte glycophorin-specific monoclonal antibodies. Only minor species of intermediate electrophoretic mobility in glycophorins not binding to L. culinaris showed a reaction with these antibodies. Both populations of glycophorins incorporated radiolabeled glucosamine, mannose, and fucose and contained O-glycosidically linked tri- and tetrasaccharides, present in a ratio of approximately 1:1 indicating a significant degree of hyposialylation when compared to erythrocyte alpha glycophorin. No precursor/product relationship was demonstrated between the major forms of two populations. K562 cell surface labeling with lactoperoxidase revealed that only the glycophorins that exhibited binding to L. culinaris were accessible to iodination and could be the only species expressed at the cell surface.

Membrane antigen expression during hemopoietic differentiation

The pattern of certain groups of antigens expressed on the surface of hemopoietic cells changes either during the course of differentiation from pluripotent stem cells to mature functional cells or as a function of the proliferative state of the cells. A map of these changes is emerging and is providing valuable information for selecting and purifying rare stem cells and for classifying the acute leukemias. This knowledge is also beginning to provide insights into physiological and pathological cellular interactions affecting the early stages of hemopoiesis, and is being exploited to remove T lymphocytes from allogeneic bone marrow grafts in order to prevent graft-vs.-host disease as well as leukemic cells from bone marrow before autologous reinfusion. In this article I will briefly review the cellular basis of hemopoiesis and then discuss the methods used to determine the presence of antigens on normal hemopoietic cells. I will then summarize the pattern of membrane antigens expressed during differentiation and conclude by discussing the biological and therapeutic implications.

Correlation of tunicamycin-sensitive surface glycoproteins from Trypanosoma cruzi with parasite interiorization into mammalian cells

Trypomastigote forms of Trypanosoma cruzi lose infectivity to cultured mammalian cells when exposed to tunicamycin. Upon reincubation into fresh medium, parasites recover their full penetration capacity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of [35S]methionine-labeled polypeptides showed that tunicamycin-treated parasites present several components with altered electrophoretic mobility when compared with controls. Immunoprecipitation with rabbit hyperimmune and human chagasic sera indicated that the surface antigens of approximate molecular masses of 175-180, 120-125, 90-95 and 85 kDa are not encountered in tunicamycin-treated trypomastigotes. By affinity chromatography on wheat germ agglutinin-Sepharose, it was observed that the trypomastigote-specific 85 kDa glycoprotein (Tc-85) is affected by the drug. The other affected components are glycoproteins with affinity for concanavalin A. The results suggest that tunicamycin-sensitive surface glycoproteins from T. cruzi are involved in the parasite interiorization into mammalian cells.

The cell specificity and biosynthesis of mouse glycophorins studied with monoclonal antibodies

Murine erythropoiesis represents a favourable system in which to investigate the coordinate regulation of gene expression due to the availability of erythroid precursor cells at various stages of differentiation. In this report, we investigate the biosynthesis and cell specificity of two characteristic murine RBC membrane glycoproteins that resemble the human RBC glycophorins: a major component of apparent molecular mass 31 kD (glycophorin MA) and a minor 46 kD component (glycophorin MB). Both glycophorins bind to wheat germ lectin and share a common protein antigenic determinant recognised by a monoclonal antibody (GP 29.4), but they differ significantly in their carbohydrate components: whilst both glycophorins contain mainly O-linked sugars, glycophorin MA contains in addition at least one N-linked carbohydrate residue and terminal sialic acid residues. Pulse-chase in vivo labelling experiments combined with in vitro translations of glycophorin mRNAs show that the initial precursor to glycophorin MA is a 24.5 kD polypeptide which is subsequently processed and glycosylated to give the mature 31 kD molecule via a 21.5 kD polypeptide intermediate. Both glycophorins MA and MB are synthesized most actively in early to mid erythroblasts (e.g., Friend cells induced for 3 days with DMSO) but their synthesis is considerably reduced by the reticulocyte stage. However, of the other cell types tested (neuroblastoma, myeloma, fibroblasts, epithelial cells and T-lymphoma cells), none synthesizes glycophorin with the possible exception of a low level in thymus tissue. Thus murine glycophorins, in contrast to the RBC cytoskeletal proteins (spectrin, ankyrin, band 4.1) seem to be restricted to the erythroid cell lineage like human glycophorin.

Identification of the major human sialoglycoprotein from red cells, glycophorin AM, as the receptor for Escherichia coli IH 11165 and characterization of the receptor site

The pyelonephritogenic Escherichia coli strain 1 H 11165 specifically agglutinates human erythrocytes carrying the M blood group antigen. The polymorphic forms of this antigen, M and N, are located in the NH2-terminal region of the major human red-cell sialoglycoprotein, glycophorin A. Radioactively labeled glycophorin A from M cells specifically bound to the bacteria. Purified glycophorin AM, but not glycophorin AN, efficiently inhibited for binding. Mild periodate treatment oxidized the NH2-terminal serine in glycophorin AM and this resulted in loss of binding to the bacteria. High concentrations of serine and alkali-labile oligosaccharides derived from glycophorin AM inhibited the binding, whereas the synthetic M-specific NH2-terminal pentapeptide Ser-Ser-Thr-Thr-Gly did not. Neuraminidase treatment of glycophorin AM did not destroy the binding. The most efficient inhibition of binding was observed with the N-terminal glyco-octapeptide obtained from glycophorin AM by CNBr cleavage. This peptide contains both the essential serine residue and the alkali-labile oligosaccharides, which both are recognized by the bacterium.

Analysis of the hemagglutinin glycoprotein from mutants of vaccinia virus that accumulates on the nuclear envelope

We isolated mutants whose vaccinia hemagglutinin (HA) accumulates on nuclear envelopes and the rough endoplasmic reticulum. Mutant HA must be blocked at a pre-Golgi step because it has high-mannose-type carbohydrates but no fucose. Neither N- nor O-glycosidically linked carbohydrates are involved in the transport defect of the mutant HA, because tunicamycin, an inhibitor of N-type glycosylation, has no effect, and O-type glycosylation takes place in the Golgi organelle. The unglycosylated form of the mutant HA synthesized in the presence of tunicamycin is 3000 daltons larger than the wild type. This higher molecular weight is related to the transport defect. HAs translated in vitro also show this difference, evidence that it reflects mutation in the HA structural gene. Portions of HAs that project into the cytoplasm seem to account for this weight difference. Thus the cytoplasmic tail of glycoprotein has an important function in transport out of the rough endoplasmic reticulum.

Effect of tunicamycin on polarized membrane functions of an established kidney epithelial cell line

We have explored the relationship between glycoprotein biosynthesis, cell proliferation and function of a differentiated polarized membrane assessed by dome formation in the MDCK epithelial cell line. At 0.1 microgram/ml tunicamycin, complete inhibition of cell proliferation was observed in either serum-containing or serum-free, hormone-supplemented growth medium. By contrast, no inhibition of either spontaneous dome formation or that triggered by inducers of cell differentiation such as hexamethylene bisacetamide was observed at 0.5 microgram/ml tunicamycin, although total glycosylation of cellular proteins was inhibited by 75%. Our results suggest that the polarized sorting out of epithelial membrane proteins to apical and basolateral surfaces and their functions related to vectorial transepithelial fluid transport, monitored by dome formation, can persist unimpaired despite considerable underglycosylation of cellular glycoproteins and inhibition of cell proliferation.

K562 cell anion exchange differs markedly from that of mature red blood cells

Human K562 leukemic cells exhibit several erythroid properties, including synthesis and expression of the major red blood cell sialoglycoprotein, glycophorin. This has led us to ask if these cells express a functional anion transport system analogous to that which is associated with the other major erythrocyte glycoprotein, band 3. The chloride-36 exchange flux in K562 cells is less than 0.6% of that which would be expected in mature erythrocytes under similar conditions. Unlike red blood cells, K562 cells do not exhibit a high chloride-sulfate selectivity, and various agents that inhibit red blood cell chloride exchange are all much less effective in K562 cells. On the basis of these flux measurements, K562 cells probably contain less than 600 fully functional red blood cell-like band 3 molecules per cell, in contrast to about a million molecules in the mature red blood cell. The possible-existence of greatly altered band 3 molecules with a reduced turnover rate and/or a reduced affinity for chloride and for various inhibitors is unlikely but cannot be completely excluded. Anion transport was also measured in K562 cells that had been induced to increase hemoglobin synthesis by various chemical agents. Even under these conditions, chloride fluxes indicated no substantial increase in the number of functional anion transport sites or their chloride transport rate.

Glycoprotein biosynthesis in peripheral nervous system myelin: effect of tunicamycin

The effect of an inhibitor of N-glycosylation of glycoproteins, tunicamycin, on synthesis of PNS myelin proteins was investigated in vitro by using chopped sciatic nerves or spinal roots of 21-day-old Wistar rats. Tunicamycin when incubated with these nerves in the presence of 3H-labeled fucose, mannose, or glucosamine inhibited the uptake of radioactivity into myelin proteins including some high-molecular-weight proteins, P0, 23K protein, and 19K protein by amounts ranging from 42 to 79%. Uptake of 14C amino acid mixture was inhibited much less by tunicamycin, but a new radioactive protein peak appeared when the protein mixtures had been separated by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. This protein ran directly in front of the P0 peak, did not correspond to any bands stained by Fast green, and was not labeled by fucose. This peak appeared in increasing larger proportions with progressive time of incubation of nerves with 3H amino acids in the presence of tunicamycin. The new protein, which cross-reacts with P0 antiserum, was tentatively identified as a nonglycosylated P0 protein that appears to be almost as well incorporated as P0 into the subcellular fraction containing myelin. At this time it is not possible to determine whether the unglycosylated P0 is actually assembled into a site and configuration like that of P0.

Role of sialic acid in the mobility of membrane proteins containing O-linked oligosaccharides on polyacrylamide gel electrophoresis in sodium dodecyl sulfate

The major sialoglycoprotein of the human red-cell membrane, glycophorin A, contains 15 O-glycosidically linked oligosaccharides and one N-glycosidic oligosaccharide. The protein shows a decreased mobility on polyacrylamide gel electrophoresis in sodium dodecyl sulfate after neuraminidase treatment of the non-denatured protein. The molecular mechanism behind this phenomenon has been elucidated. Neuraminidase treatment of glycophorin A in intact cells or after solubilization in buffers containing Triton X-100 resulted in conversion of the predominant tetrasaccharide N-acetylneuraminosyl alpha 2-3galactosyl beta 1-3(N-acetylneuraminosyl alpha 2-6)-N-acetylgalactosamine to the trisaccharide galactosyl beta 1-3(N-acetylneuraminosyl alpha 2-6)-N-acetylgalactosamine and the disaccharide galactosyl beta 1-3-N-acetylgalactosamine. After denaturation with sodium dodecyl sulfate, Vibrio cholerae neuraminidase also liberated the N-acetylgalactosamine-bound sialic acids. Such treatment resulted in increased electrophoretic mobility. The results show that distal sialic acids linked to galactose are readily available to neuraminidase, and that their negative charge gives an increased electrophoretic mobility in polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. In contrast, most of the N-acetylgalactosamine-linked sialic acids of glycophorin A are not liberated by neuraminidase without denaturation of the substrate. Like sialic acids of complex-type oligosaccharides the decreased electrophoretic mobility caused by them is exclusively due to their mass while no significant contribution by the charge was seen.

Coronavirus glycoprotein E1, a new type of viral glycoprotein

The carbohydrate contents of coronavirus glycoproteins E1 and E2 have been analyzed. E2 has complex and mannose-rich-type oligosaccharide side-chains, which are attached by N -glycosidic linkages to the polypeptide. Glycosylation of E2 is initiated at the co-translational level, and it is inhibited by tunicamycin, 2-deoxy-glucose, and 2-deoxy-2-fluoro-glucose. Thus, E2 belongs to a glycoprotein type found in many other enveloped viruses. E1, in contrast, represents a different class of glycoprotein. The following observations indicate that its carbohydrate side-chains have 0 -glycosidic linkage. (1) The constituent sugars of E1 are N -acetylglucosamine, N -acetylgalactosamine, galactose, and neuraminic acid; mannose and fucose are absent. (2) The side-chains can be removed by β-elimination. (3) Glycosylation of E1 is not sensitive to the compounds interfering with N -glycosylation. E1 is the first viral glycoprotein analyzed that contains only 0 -glycosidic linkages. Coronaviruses are therefore a suitable model system to study biosynthesis and processing of this type of glycoprotein.

Studies on the biosynthesis of the variant surface glycoproteins of Trypanosoma brucei: sequence of glycosylation

The variant surface glycoprotein (VSG) of several Trypanosoma brucei clones has been metabolically labeled with [35S]methionine in parasites grown in the presence or in the absence of tunicamycin. Pulse and chase experiments followed by immunoprecipiation with anti-VSG sera and polyacrylamide gel electrophoresis of the immunoprecipitates, have helped to elucidate two steps of the sequence of glycosylation of VSG. Immediately after, or concomitantly with the synthesis of the protein, a first type of oligosaccharide side chain is also synthesized. Tunicamycin, a specific inhibitor of asparagine glycosylation, completely inhibits this reaction. The total amount of VSG found in trypanosomes grown in the presence of tunicamycin is less than in controls. The unglycosylated molecule has an apparent molecular weight 5-10% smaller than the mature form. A subsequent step, occurring after translation, is the synthesis of carbohydrate side chains which contain a cross-reacting antigenic determinant detected in all VSGs so far studied by immunoprecipitations with heterologous antisera. The percentage of total VSG bearing sugars involved in cross-reactions is variable in different clones, increases with time and subsequently decreases, suggesting that some of the carbohydrate might undergo trimming. Polyacrylamide gel electrophoresis of immunoprecipitates suggests that the absence or the incomplete synthesis of this oligosaccharide does not alter the apparent molecular weight of VSG. In four out of the five clones studied, the sugar(s) responsible for cross-reactions seem to be located within oligosaccharides linked to the protein through both N-glycosidic and other unidentified types of linkages. This is suggested by the partial effect of tunicamycin on the extent of VSGs cross-reactivities measured by immunoprecipitations with heterologous antiserum.

Mutant cells that abnormally process plasma membrane glycoproteins encoded by murine leukemia virus

Wild-type normal rat kidney fibroblasts infected with the Friend strain of murine leukemia virus (MuLV) contain two virus-encoded glycoproteins on the outer surfaces of their plasma membranes: an envelope glycoprotein with an apparent molecular weight of 70,000 (gp70), and a glycoprotein that reacts with antisera to the major virion internal core proteins p30, p15, p12 and p10 and has an apparent molecular weight of 93,000 (gp93gag). To analyze the functions of these glycoproteins and to develop a model system for studying genetics of membrane synthesis, we used an immunoselection method to isolate variant cell clones defective in processing these glycoproteins into their plasma membranes. Several lines of evidence, including complementation of glycoprotein processing defects by fusion with uninfected wild-type cells, indicate that the immunoselected variants have stably inherited membrane synthesis abnormalities that are encoded by cellular rather than by viral genes. The H-4 cell line, which was selected by use of antiserum to gp70, has metabolic defects that interfere with processing of both gp70 and gp93gag into its plasma membranes. Nevertheless, this cell line releases noninfectious MuLV. Furthermore, two cell lines (2 and 5), which were selected by use of antiserum to the virion core protein p30, specifically lack detectable cell surface or intracellular gp93gag but contain cell surface gp70 and release infectious MuLV. These results suggest that MuLV particles can bud efficiently from cells that lack known virus-encoded plasma membrane constituents.

Characterization of Glycophorin A and band 3 from Tn polyagglutinable erythrocytes

The 2 types of erythrocytes from a person with persistent mixed-field polyagglutinability (Tn abnormality) were separated from each other by preparative cell electrophoresis. Surface labelling using the galactose oxidase/NaB3H4 technique followed by polyacrylamide gel electrophoresis showed a strong labelling in the glycophorin A region of Tn positive erythrocytes indicating exposed galactosyl N-acetyl/galactosaminyl residues. Tn positive cell membranes were labelled by the galactose oxidase/NaB3H4 technique and solubilized in non-ionic detergent. After chromatography on Helix pomatia lectin-linked Sepharose, glycophorin A was immunoprecipitated from the sugar eluate using specific antiserum. Glycophorin A from Tn negative cells and normal red blood cells did not bind to Helix pomatia lectin but to Lens culinaris lectin-Sepharose. Glycophorin A and band 3 were isolated by preparative gel electrophoresis from normal cells and the two red cell populations of the Tn individual. Pronase treatment of labelled glycophorin A followed by gel filtration revealed a more efficient proteolysis in molecules isolated from Tn positive cells. Mild alkaline treatment of galactose oxidase/NaB3H4 or periodate/NaB3H4 labelled glycophorin A liberated 3 different oligosaccharides from Tn positive cells. No significant difference was found between the oligosaccharides of band 3 protein from normal and Tn positive cells and the amounts of glycophorin A were identical in both cell types when determined by radioimmunoassay.

Cell-free synthesis and glycosylation of the major human-red-cell sialoglycoprotein, glycophorin A

The human erythroid cell line, K562, synthesizes the major red cell sialoglycoprotein, glycophorin A. We have isolated an mRNA fraction which codes for glycophorin A from K562 cells and studied the synthesis of the sialoglycoprotein in a rabbit reticulocyte cell-free system. In the absence of membranes a precursor form of glycophorin A was synthesized. This was identified using specific anti-(glycophorin A) serum. The apparent molecular weight of the carbohydrate-free precursor of glycophorin A was 19 500. This exceeds the molecular weight of the glycophorin A apoprotein by approximately 5000. In the presence of membranes from dog pancreas, the synthesized glycophorin A precursor was N-glycosylated and probably also O-glycosylated. The oligosaccharide chains remained incomplete and the glycoprotein synthesized in vitro corresponded to the glycosylated precursor of glycophorin A obtained in intact cells.