Lectin analysis of common glycoproteins detected on the surface of continuous microvascular endothelium in situ and in culture: identification of sialoglycoproteins

For many years, molecular interactions with vascular endothelium have been studied in vitro on cultured endothelial cells. Yet, it is clear that the different environmental conditions in vivo vs. in vitro may cause phenotypic drift and altered expression of cell surface molecules. In this study, we identify several endothelial surface proteins of similar apparent molecular mass by radioiodination of cultured microvascular cells and by intravascular radioiodination of rat heart endothelium in situ. The radioiodinated surface polypeptides detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) (followed by autoradiography) were subjected to lectin affinity chromatography in order to provide an additional screen for identifying common surface glycoproteins and a means for partial characterization of their glycans. With a battery of 18 lectins, seven major (gp140, gp120, gp100, gp85, gp75, gp60, gp47) and 6 minor (gp330, gp300, gp180, gp160, gp150, gp42) glycoproteins were identified on the cultured cells each with a different lectin binding profile. The lectin binding profiles of many endothelial glycoproteins in situ were similar to those of their counterparts in culture. A common set of seven major glycoproteins with the same apparent molecular masses was found in situ as well as in vitro. These common glycoproteins were characterized further using both sialidase digestion and sequential lectin affinity chromatography of cell lysates. Most of the glycoproteins appear to have both complex-type N-linked and O-linked glycans except for gp60 with only O-linked glycans, gp47 with only complex N-linked sugars, and gp42 with only simple N-linked sugars. A subset of sialoglycoproteins (gp140, gp120, gp100, gp60, gp47) was identified. One of them, gp120, is podocalyxin based on immunoprecipitation with specific antiserum and another one, gp60, is a recently identified albumin binding protein on the surface of cultured microvascular endothelial cells. This study shows that gp60 is indeed present on the surface of endothelium in situ and that it is a sialoglycoprotein with typical O-linked glycans. It is apparent that the continuous type of microvascular endothelium can indeed express in culture and in situ a common set of major glycoproteins.     

Transmembrane envelope glycoproteins of human immunodeficiency virus type 2 and simian immunodeficiency virus SIV-mac exist as homodimers.

An 80-kilodalton glycoprotein (gp80) was produced in human immunodeficiency virus type 2 (HIV-2)-infected cells along with three envelope glycoproteins that we have recently reported: the extracellular glycoprotein (gp125), the envelope glycoprotein precursor (gp140), and the transient dimeric form of the precursor (gp300). gp125 and gp80 were detectable after the synthesis of gp140 and the formation of gp300. Using a specific monoclonal antibody, we showed here that gp80 is a dimeric form of the transmembrane glycoprotein gp36 of HIV-2. Dimerization of the envelope glycoprotein precursor and dimeric forms of the transmembrane glycoproteins were also observed in cells infected with simian immunodeficiency virus (SIV-mac), a virus closely related to HIV-2. Under routine conditions of our experiments (i.e., extraction by 1% Triton X-100 before polyacrylamide gel electrophoresis in sodium dodecyl sulfate [SDS]), monomeric forms of the transmembrane glycoprotein of HIV-2 and SIV-mac were only seldomly observed. Dimeric forms of the envelope precursors and the transmembrane glycoproteins are probably stabilized by extraction in the nonionic detergent Triton X-100 since such dimeric forms resist dissociation during subsequent electrophoresis in the presence of the ionic detergent SDS. However, the dissociation of these dimeric forms might occur when samples are prepared by extraction directly in 1% SDS or by incubation of the purified dimers at acidic pH. Dimerization of the envelope precursor might be required for its processing to give the mature envelope proteins, whereas the transmembrane dimer might be essential for optimal structure of the virion and thus its infectivity.     

Lectin binding characterstics of mouse epididymal fluid and sperm extracts

Glycoproteins from luminal fluid of the mouse cauda epiciidymidis have been compared with glycoproteins from Triton X-100 extracts of mouse spermatozoa from varying regions of the epididymis, using lectins with specific affinity for different sugar residues. Concanavalin A recognizes 11 glycocomponents on Western blots of fractionated caudal fluid; wheat germ agglutinin (WGA) binds 12 proteins; Ulex europaeus agglutinin (UEA) binds seven; and Dolichos biflorus agglutinin (DBA) recognizes nine. Several of these glycoproteins display an affinity for more than one lectin, indicating a diversity in their exposed carbohydrate residues; whereas other proteins bind only one of the four lectins used. The results also show that some glycoproteins exhibit a higher affinity for particular lectins. Eight glycoproteins of similar mobility and lectin-binding characteristics are detected in Triton X-100 extracts of spermatozoa from different regions of the epididymis and in caudal fluid. The lectin affinity of some proteins appears or increases in spermatozoa from distal epididymal regions (54 kD, 32 kD), whereas the lectin affinity of others decreases (29 kD, 40 kD). There are differences in lectin affinities between proteins in sperm extracts and in caudal fluid. Some proteins show an affinity for three or four lectins in caudal fluid, but proteins of similar electrophoretic mobility in sperm extracts bind only one or two of the lectins. These data show that glycoproteins of similar mobility are present in caudal fluid and in Triton-X-100 sperm extracts, implying a potential interaction between caudal fluid components and epididymal sperm.     

Fluorochrome-coupled lectins reveal distinct cellular domains in human epidermis

The distribution of saccharide moieties in human interfollicular epidermis was studied with fluorochrome-coupled lectins. In frozen sections Concanavalin A (Con A), Lens culinaris agglutinin (LCA), Ricinus communis agglutinin I (RCAI), and wheat germ agglutinin (WGA) stained intensively both dermis and viable epidermal cell layers, whereas peanut agglutinin (PNA) bound only to living epidermal cell layers. Ulex europaeus agglutinin I (UEAI) bound to dermal endothelial cells and upper cell layers of the epidermis but left the basal cell layer unstained. Dolichos biflorus agglutinin (DBA) bound only to basal epidermal cells, whereas both soybean agglutinin (SBA) and Helix pomatia agglutinin (HPA) showed strong binding to the spinous and granular cell layers. On routinely processed paraffin sections, a distinctly different staining pattern was seen with many lectins, and to reveal the binding of some lectins a pretreatment with protease was required. All keratin-positive cells in human epidermal cell suspensions, obtained with the suction blister method, bound PNA, whereas only a fraction of the keratinocytes bound either DBA or UEAI. Such a difference in lectin binding pattern was also seen in epidermal cell cultures both immediately after attachment and in organized cell colonies. This suggests that in addition to basal cells, more differentiated epidermal cells from the spinous cell layer are also able to adhere and spread in culture conditions. Gel electrophoretic analysis of the lectin-binding glycoproteins in detergent extracts of metabolically labeled primary keratinocyte cultures revealed that the lectins recognized both distinct and shared glycoproteins. A much different lectin binding pattern was seen in embryonic human skin: fetal epidermis did not show any binding of DBA, whereas UEAI showed diffuse binding to all cell layers but gave a bright staining of dermal endothelial cells. This was in contrast to staining results obtained with a monoclonal cytokeratin antibody, which showed the presence of a distinct basal cell layer in fetal epidermis also. The results indicate that expression of saccharide moieties in human epidermal keratinocytes is related to the stage of cellular differentiation, different cell layers expressing different terminal saccharide moieties. The results also suggest that the emergence of a mature cell surface glycoconjugate pattern in human epidermis is preceded by the acquisition of cell layer-specific, differential keratin expression.     

Helix pomatia agglutinin binds specifically to the Golgi apparatus in cultured human fibroblasts and reveals two Golgi apparatus-specific glycoproteins

Fluorochrome-coupled Helix pomatia agglutinin (HPA), but not other lectin-conjugates with the same nominal specificity, bound specifically to the Golgi apparatus in cultured human fibroblasts, revealing a cytoplasmic juxtanuclear reticular structure. Unlike other Golgi-binding lectins the HPA-conjugates did not bind to the cell surface membrane or pericellular matrix. Experiments with 35S-methionine-labeled cells showed that HPA recognized two glycoproteins of Mr 170,000 and 400,000 among the secreted products of fibroblasts and two major cellular glycoproteins of Mr 40,000 and Mr 180,000 in Triton X-100 extracts of the cells. The two cellular HPA-binding polypeptides were also found in cells depleted of secretory products and in cells pulse-labeled shortly with 35S-methionine and then chased with methionine containing medium up to 12 h. These findings suggest that the two cellular glycoproteins recognized by HPA are retained in the Golgi apparatus and are therefore not precursors of secretory proteins. The results suggest that there are two endogenous, Golgi apparatus-specific glycoproteins in cultured human fibroblasts with terminal non-reducing O-glycosidic N-acetyl galactosaminyl residues.     

Concanavalin A and wheat germ agglutinin binding to sea urchin embryo basal laminae

Summary The basal laminae and inner extracellular matrices of Lytechinus pictus and Arbacia punctulata embryos were characterized on the basis of lectin binding. Developmental stage specific patterns of lectin binding were observed after microinjection of Con A-FITC and WGA-FITC. Lectin-specific patterns differed between control, sulfate free sea water (SFSW) and tunicamycin treated embryos. Con A injection resulted in the rounding-up of cells in the epithelium and was most pronounced in embryos cultured in the presence of tunicamycin. Basal laminae were isolated by Triton X-100 extraction of whole embryos. Proteins were separated by SDS-PAGE, electrophoretically transferred to nitrocellulose and incubated in biotinylated lectins. Lectin-binding glycoproteins were detected with avidin peroxidase. The electrophoretic pattern of Con A-binding proteins in early developmental stages of Arbacia was similar with several low molecular weight species appearing at gastrulation in control and SFSW embryos. WGA-binding in Arbacia and Lytechinus control embryos was limited to a 125,000 Mr glycoprotein (gp125). In addition to gp125, several high molecular weight WGA-binding glycoproteins were also detected in SFSW embryos. The evidence suggests that mesenchyme migration and gastrulation are correlated with changes in the molecular composition of the ECM.     

Characterization of a detergent-resistant surface lamina in cultured human fibroblasts

Treatment of cultured human fibroblasts with 0.5% Triton X-100 produces substratum-anchored cytoskeletal preparations consisting of cytoplasmic filaments, nucleus and a plasma membrane-derived surface lamina. The lamina was visualized in fluorescence microscopy with fluorochrome-coupled wheat germ agglutinin (WGA) as a lace-like structure, extending throughout the cell domain. It displayed a different organization at the ventral and dorsal surfaces of the cell, partially coaligning with bundles of actin and myosin filaments at the dorsal cell surface. At the ventral surface vinculin patches appeared to be included in the surface lamina. Polyacrylamide gel electrophoresis, combined with lectin reactivity studies and lectin affinity chromatography, revealed a 140 kD sialoglycoprotein as the major glycoprotein component of the surface lamina.     

Envelope polypeptides of Friend leukemia virus: purification and structural analysis.

Roughly 10% of surface glycoproteins in the envelope of mature Friend murine leukemia virus are coupled to membrane polypeptides by disulfide bridges. The remaining 90% of these glycoproteins are associated noncovalently. However, they could also be linked to membrane polypeptides by the treatment of purified Friend murine leukemia virus with 2,2'dithiobis(m-nitropyridine). These amphiphilic heterodimer polypeptides, gp84/86, were recovered almost quantitatively in the form of aggregates, termed rosettes, when prepared by solubilization of the viral membrane with Triton X-100 and subsequent velocity sedimentation. gp69/71 and p12(E)/15(E) were purified from these protein micelles after reduction of the disulfide bonds by gel chromatography. Electron micrographs of rosettes, as well as of purified p12(E)/15(E), showed structures different from native viral knobs. Isolated gp84/86 could be reassociated and then displayed more similarity to these viral surface projections. As shown by peptide mapping, the primary structures of the glycoproteins gp69/71 are highly related as are those of the membrane polypeptides p12(E) and p15(E). Furthermore, it was shown by two-dimensional polyacrylamide gel electrophoresis and re-electrophoresis of purified gp84/86 that the larger component, gp86, was composed of gp71 associated with p15(E) and p12(E), whereas the smaller component, gp84, was formed by gp69 bound only to p12(E).     

Synthesis by cultured human umbilical vein endothelial cells of two proteins structurally and immunologically related to platelet membrane glycoproteins IIb and IIIa

Human platelets participate in a number of adhesive interactions, including binding to exposed subendothelium after vascular injury, and platelet-platelet cohesion to form large aggregates. Platelet membrane glycoproteins (GP) IIb and IIIa constitute a receptor for fibrinogen that, together with fibrinogen and calcium, is largely responsible for mediating the formation of the primary hemostatic plug. Using highly specific polyclonal and monoclonal antibodies as probes, we could detect the presence of both of these glycoproteins in cultured human umbilical vein endothelial cells. Western-blot analysis showed that the endothelial cell analogues were similar in size to their platelet counterparts, and were present in cells that had been in culture for over 2 mo. Metabolic labeling of endothelium with [35S]methionine demonstrated that both GPIIb and GPIIIa were actively synthesized in culture. Using the technique of crossed immunoelectrophoresis, evidence was obtained that the endothelial cell forms of GPIIb and GPIIIa may exist complexed to one another after solubilization in Triton X-100. The presence of GPIIb-IIIa analogues in cultured endothelial cells may provide an opportunity to examine the structure, function, and synthesis of these two membrane glycoproteins, as well as provide a source of genetic material with which to begin detailed molecular genetic studies.     

Helix pomatia agglutinin binds specifically to the Golgi apparatus in cultured human fibroblasts and reveals two Golgi apparatus-specific glycoproteins

Summary Fluorochrome-coupled Helix pomatia agglutinin (HPA), but not other lectin-conjugates with the same nominal specificity, bound specifically to the Golgi apparatus in cultured human fibroblasts, revealing a cytoplasmic juxtanuclear reticular structure. Unlike other Golgi-binding lectins the HPA-conjugates did not bind to the cell surface membrane or pericellular matrix. Experiments with ³⁵S-methionine-labeled cells showed that HPA recognized two glycoproteins of M_r 170000 and 400000 among the secreted products of fibroblasts and two major cellular glycoproteins of M_r 40000 and M_r 180000 in Triton X-100 extracts of the cells. The two cellular HPA-binding polypeptides were also found in cells depleted of secretory products and in cells pulselabeled shortly with ³⁵S-methionine and then chased with methionine containing medium up to 12 h. These findings suggest that the two cellular glycoproteins recognized by HPA are retained in the Golgi apparatus and are therefore not precursors of secretory proteins. The results suggest that there are two endogenous, Golgi apparatus-specific glycoproteins in cultured human fibroblasts with terminal non-reducing O-glycosidic N-acetyl galactosaminyl residues.     

Identification and characteristics of concanavalin A-binding neurospecific glycoproteins in human brain and brain tumors

Soluble and membrane-bound neurospecific Con A-binding glycoproteins from human brain and tumours were identified and characterized, using a procedure which included stepwise extraction with low and high ionic strength buffers, buffered. Triton X-100 and sodium deoxycholate followed by ConA-Sepharose column chromatography, SDS-PAAG electrophoresis and immunoblotting. Adsorbed antisera against different types of neurospecific glycoproteins were used. The bulk of neurospecific glycoproteins (11 and 13) were revealed in protein fractions extracted with low ionic strength buffers and Triton X-100. In astrocytomas and glyoblastomas, some neurospecific glycoproteins were absent. Some glycoproteins were found in tumours, but were absent in brain tissue. Soluble, 77 kD glycoprotein, 11 and 16 kD glycoproteins solubilized with high ionic strength buffers and intrinsic membrane-bound 51, 57, 61, 74 and 77 kD glycoproteins can be viewed as stable neurospecific markers in malignant brain tumours.     

Stage-specific alterations in the apical membrane glycoproteins of endometrial epithelial cells related to implantation in rabbits

The rabbit endometrial epithelium undergoes differentiation prior to the time of blastocyst implantation, including loss of surface negativity and a change in glycocalyx morphology. Nonpregnant (estrous) and pseudopregnant rabbits were used to study specific alterations in proteins and saccharide composition of the luminal epithelial membrane and its glycocalyx related to the acquisition of receptivity to implantation. Pregnant animals were used to study further modification of the luminal surface by implanting blastocysts. The apical surface of luminal epithelial cells was solubilized by a 15-min intraluminal incubation of 1% Triton X-100 containing protease inhibitors. Proteins in extract solutions were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Three new polypeptides (24 kDa, 42 kDa and 58 kDa) were identified in uteri from receptive rabbits. Binding of succinyl Wheat Germ Agglutinin (sWGA) and Ricinus communis Agglutinin (RCA-I) lectins to the 24 kDa and 42 kDa components on Western blots of extracts separated by SDS-PAGE identified them as glycoproteins. Additionally, other polypeptides (26 kDa, 80-86 kDa and 145 kDa) showed changes in affinity for WGA, RCA-I or concanavalin A (Con A), depending on the hormonal state. Correlating with these findings was an increased binding of these lectins to intact nonciliated cells in uteri of receptive rabbits compared to estrous animals; ciliated cells bound Dolichos biflorus Agglutinin (DBA) specifically, regardless of the hormonal condition. Treatment of uteri from estrous animals, or Western blots of proteins from these animals, with neuraminidase prior to lectin exposure suggested the presence of glycoproteins having a sialic acid-D-galactose terminus in nonreceptive rabbits. Reduced binding of lectin to intact cells at implantation sites and to blots of proteins isolated from these sites, compared to nonimplantation sites, was noted. These results provide evidence for stage-specific alterations in protein and saccharide composition of the apical surface of endometrial epithelium prior to implantation, and indicate that implanting blastocysts further modify the luminal surface.     

Solubilization and enrichment of boar sperm membrane proteins

Boar sperm membranes are rather resistent to the solubilizing effect of some detergents. Deoxycholate, an ionic detergent, was efficient in solubilizing sperm proteins but some nonionic detergents like Triton X-100 displayed relatively poor capacity in rendering membrane proteins soluble. This may be due to sperm proteins being attached to submembraneous structures through bonds involving divalent cations, since mixtures of Triton X-100 and ethylenediamine tetraacetic acid (EDTA) were almost as efficient as deoxycholate in solubilizing membrane proteins. Since intact spermatozoa were directly treated with detergents the solubilized proteins comprised a mixture of intracellular and membrane components. To enrich for membrane proteins, affinity chromatography on columns containing different lectins was carried out. SDS polyacryiamide gel electrophoresis of sperm glycoproteins desorbed from the various lectin columns demonstrated that each lectin bound a unique set of components although most glycoproteins were recovered from two or more columns. Columns containing Lens culinaris hemagglutinin yielded more sperm glycoproteins than any of the other lectin columns examined. The predominant amount of the sperm proteins recovered from the Lens culinaris lectin column was membrane derived, as the majority of the proteins were integrated into liposomes. It is concluded that sperm membrane proteins are efficiently solubilized by detergent in the presence of a chelator and that most of the membrane glycoproteins can easily be enriched by affinity chromatography on a lectin column. Proteins obtained in this way should serve as excellent starting material for the isolation of individual sperm membrane proteins.     

Association of Rho(D) polypeptides with the membrane skeleton in Rho(D)-positive human red cells

The Rho(D) antigen was recently identified as a 28,000 to 33,000 m.w. polypeptide expressed on the surface of human Rho(D)+ cells. We now show that 70 to 80% of the Rho(D) polypeptides remain firmly associated with the membrane skeleton (detergent-insoluble matrix) obtained after treatment of isolated membranes with Triton X-100. The same treatment solubilized most of the major sialoglycoprotein, glycophorin A. The membrane skeleton-bound Rho(D) polypeptides were not solubilized by procedures that dissociated spectrin, actin, and glyceraldehyde-3-phosphate dehydrogenase from the membrane. Affinity-purified 125I-labeled anti-Rho(D) antibodies bound to intact Rho(D)+ cells, Rho(D)+ membranes, and isolated membrane skeletons from Rho(D)+ cells, but not to Rho(D)- cells. The binding to Rho(D)+ cells was competitively inhibited efficiently by Rho(D)+ membranes and weakly by Rho(D)- membranes. When isolated unsealed Rho(D)+ and Rho(D)- membranes were labeled by lactoperoxidase-catalyzed iodination and solubilized in Triton X-100, Rho(D) polypeptides were immune precipitated only from Rho(D)+ membranes.     

Solubilization and fractionation of glycoproteins and glycolipids of KB cell membranes

1. A fraction enriched in plasma membranes of human tumour KB cell line, a permissive cell for adenovirus type 5, was obtained. 2. Electrophoresis of the membranes in polyacrylamide gels with buffers containing sodium dodecyl sulphate showed that the membranes after reduction with 2-mercaptoethanol contained over 20 polypeptide species. Three polypeptides were glycosylated and had apparent mol.wts. of 92000, 72000 and 62000. 3. The glycoproteins and the specific receptors responsible for adenovirus adsorption to the membranes were readily extracted into solutions containing low concentrations of Triton X-100. Glycolipids and proteins were also made soluble. A membranous residue obtained after Triton X-100 extraction was enriched in several proteins that appeared to consist of polypeptides of lower molecular weight than the average of KB membrane polypeptides. 4. Sphingomyelin, cholesterol and triglycerides were similarly concentrated in the insoluble residue remaining after successive extractions of KB membranes with Triton X-100. Further, ceramide trihexoside was significantly less easily extracted from KB membranes than lactosyl ceramide. 5. The differences noted in the ease of extraction of membrane components are discussed. 6. The components of membranes made soluble by detergent extraction and containing the large part of the KB membrane glycoproteins were subjected to chromatography on Sepharose 6B and DEAE-cellulose and to isoelectric focusing in the presence of buffers containing Triton X-100. In general, the degree of separation into fractions enriched in individual glycoproteins was disappointing. Possible reasons for the poor fractionation of membrane components by chromatographic systems conveniently used for purification of proteins and glycoproteins of non-membranous origin are briefly discussed.     

岩藻糖糖链与肝癌细胞的迁移作用

通过凝集素印迹转移电泳和亲和层析技术,对岩藻糖糖基化蛋白在肝癌细胞中的作用进行了研究.在化学诱发的大鼠肝癌过程中, 分子质量在23 ku到40 ku范围内与荆豆凝集素(UEA)及扁豆凝集素(LCA)结合的岩藻糖糖基化蛋白显著减少, 诱癌至17～20周这些条带重新恢复,而分子质量为80 ku的条带却在诱癌过程中逐周增加.比较高、低转移性肝癌细胞的岩藻糖糖基化蛋白, 发现高转移性肝癌细胞具有多种增强的条带.利用橘果粉胞凝集素(AAL)和LCA亲和层析柱分离了这些岩藻糖基化糖蛋白, 并用这些糖蛋白直接作用于肝癌细胞,发现AAL-糖蛋白具有显著抑制肝癌细胞迁移的作用,迁移细胞数从对照的(100±4.9)%下降到(48.1±2.5)% (P＜0.01), LCA-糖蛋白也有类似作用.用胰酶和木瓜蛋白酶水解蛋白质部分后,形成的糖肽抑制肝癌细胞迁移的作用并不改变,甚至增强.此外直接用肝癌转移灶的组织测定了岩藻糖转移酶活性,发现α1,6岩藻糖基转移酶活性显著比正常肝组织高,而α1,3岩藻糖基转移酶活性没有显著的改变.用系列凝集素分析发现这些糖链主要能结合伴刀豆凝集素A, 也能结合E-型及L-型植物凝集素, 显示这种糖蛋白的糖链可能含有较多的高甘露糖型.这些结果提示糖链在诱癌过程中结构有了改变,使之在肝癌细胞的迁移和转移中起重要作用.     

Effect of Hypoxia on Endothelial Cell Surface Glycoprotein Expression: Modulation of Glycoprotein IIIa and Other Specific Surface Glycoproteins

Exposure to hypoxia alters many aspects of endothelial cell metabolism and function; however, changes in surface glycoconjugates under these conditions have not been extensively evaluated. In the current studies, we examined surface glycoproteins of cultured bovine aortic (BAEC) and pulmonary arterial (BPAEC) endothelial cells under standard culture conditions (21% oxygen) and following exposure to hypoxia (0% oxygen) for varying time periods (30 min to 18 h) using a system of biotinylation, lectin binding (concanavalin A, Con A; Griffonia simplicifolia , GSA; Arachis hypogaea, PNA; Ricinus communis, RCA; or Triticum vulgaris, WGA), subsequent strep-avidin binding, and staining. Using these methods, we identified differences in lectin binding between the two cell types cultured in 21% oxygen with all lectins except PNA. With exposure to 0% oxygen, there was no change in lectin binding to most surface glycoproteins. Several surface glycoproteins, including glycoprotein IIIa on both cell types, demonstrated a time-dependent decrease in lectin binding; in addition, there was an increase in lectin binding to a few specific surface glycoproteins on each cell type within 30-60 min of exposure to 0% oxygen. These changes in specific surface glycoproteins were confirmed in both cell types by ¹²⁵I labeling. Increased lectin binding was observed for Con A binding BAEC glycoproteins at molecular weight (MW) 116, 130, and 205 kDa, GSA binding BAEC glycoproteins at MW 120 and 205 kDa, and RCA binding BPAEC glycoproteins at MW 140 and 205 kDa. Increased binding of WGA or PNA was not observed during exposure to hypoxia. The specificity of lectin binding was further confirmed by competitive inhibition with the appropriate sugar. These studies demonstrate that there are baseline differences between BAEC and BPAEC cell surface glycoproteins and that exposure to hypoxia is associated with little change in lectin binding to most surface glycoproteins. There is, however, increased surface expression of a few glycoproteins that differ depending of the origin of the endothelial cell. Although the mechanism of this increase in lectin binding is not yet clear, subsequent studies suggested that it is due to increased availability of select carbohydrate moieties. The time course of these alterations suggests a possible role in the endothelial cell response to decreases in ambient oxygen tension.     

Identification of an octapeptide involved in homophilic interaction of the cell adhesion molecule gp80 of dictyostelium discoideum

During development of Dictyostelium discoideum, a surface glycoprotein of Mr 80,000 (gp80) is known to mediate EDTA-resistant cell-cell adhesion via homophilic interaction. Antibodies directed against a 13 amino acid sequence (13-mer) near the NH2 terminus of the protein were found to inhibit cell reassociation. This 13-mer also inhibited gp80-cell interaction and gp80-gp80 interaction. The cell binding site was mapped to the octapeptide sequence YKLNVNDS by using shorter peptide sequences to inhibit gp80 interaction. High salt concentrations inhibited homophilic interactions of both the 13-mer and gp80, suggesting that ionic interactions are involved in the forward binding reaction. Since disruption of homophilic interactions between the bound molecules required the presence of Triton X-100, hydrophobic interactions may occur after the initial ionic binding.     

Concanavalin A induces interactions between surface glycoproteins and the platelet cytoskeleton

We have measured the association of platelet surface membrane proteins with Triton X-100 (Triton)-insoluble residues in platelets surface labeled with 125I. In both concanavalin A (Con A)-stimulated and resting platelets, this fraction is composed largely of polypeptides with apparent molecular weights of 45,000, 200,000, and 250,000 which comigrate with authentic actin, myosin heavy chain, and actin binding protein, respectively, as judged by PAGE in SDS. Less than 10% of the two major 125I-labeled surface glycoproteins, GPiib and GPIII, were associated with the Triton residue in resting platelets. Within 45 s after Con A addition, 80-95% of these two glycoproteins became associated with the Triton residue and the amount of sedimentable actin doubled. No cosedimentation of GPIIb and III with the cytoskeletal protein-containing Triton residue was seen when Con A was added to a Triton extract of resting cells, indicating that the sedimentation of GPIIb and III seen in Con A-stimulated platelets was not due to precipitation of the glycoproteins by Con A after detergent lysis. Treatment of Triton extracts of Con A-stimulated platelets with DNase I (deoxyribonucleate 5'-oligonucleotidido-hydrolase [EC 3.1.4.5]) inhibited the sedimentation of actin and the two surface glycoproteins in a dose-dependent manner. This inhibition of cosedimentation was not due to an effect of DNase I on Con A-glycoprotein interactions since these two glycoproteins could be quantitatively recovered by Con A- Sepharose affinity absorption in the presence of DNase I. When the Con A bound to the Triton residue was localized ultrastructurally, it was associated with cell-sized structures containing filamentous material. In intact cells, there was simultaneous immunofluorescent coredistribution of surface-bound Con A and myosin under conditions which induced a redistribution of platelet myosin. These data suggest that Con A can, in the intact platelet, induce physical interactions between certain surface glycoproteins and the internal cytoskeleton.     

Assembly of glycoprotein-80 adhesion complexes in Dictyostelium. Receptor compartmentalization and oligomerization in membrane rafts

The phospholipid-anchored membrane glycoprotein (gp)-80 mediates cell-cell adhesion through a homophilic trans-interaction mechanism during Dictyostelium development and is enriched in a Triton X-100-insoluble floating fraction. To elucidate how gp80 adhesion complexes assemble in the plasma membrane, gp80-gp80 and gp80-raft interactions were investigated. A low density raft-like membrane fraction was isolated using a detergent-free method. It was enriched in sterols, the phospholipid-anchored proteins gp80, gp138, and ponticulin, as well as DdCD36 and actin, corresponding to components found in the Triton X-100-insoluble floating fraction. Chemical cross-linking revealed that gp80 oligomers were enriched in the raft-like membrane fraction, implicating stable oligomer-raft interactions. However, gp80 oligomers resisted sterol sequestration and were partially dissociated with Triton X-100, suggesting that compartmentalization in rafts was not solely responsible for their formation. The trans-dimer known to mediate adhesion was identified, but cis-oligomerization predominated and displayed greater accumulation during development. In fact, oligomerization was dependent on the level of gp80 expression and occurred among isolated gp80 extracellular domains, indicating that it was mediated by direct gp80-gp80 interactions. Rafts existed in gp80-null cells and such pre-existent membrane domains may provide optimal microenvironments for gp80 cis-oligomerization and the assembly of adhesion complexes.