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Salhany JM, Sloan RL, Schopfer LM. Characterization of the stilbenedisulphonate binding site on band 3 Memphis variant II (Pro-854-->Leu). Biochem J 1996; 317 ( Pt 2):509-14. [PMID: 8713079 PMCID: PMC1217516 DOI: 10.1042/bj3170509] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Band 3 Memphis variant II is a mutant anion-exchange protein associated with the Diego a+ blood group antigen. There are two mutations in this transporter: Lys-56-->Glu within the cytoplasmic domain, and Pro-854-->Leu within the membrane-bound domain. The Pro-854 mutation, which is thought to give rise to the antigenicity, is located within the C-terminal subdomain of the membrane-bound domain. Yet, there is an apparent enhancement in the rate of covalent binding of H2DIDS (4,4'-di-isothiocyanatodihydro-2, 2'-stilbenedisulphonate) to 'lysine A' (Lys-539) in the N-terminal subdomain, suggesting widespread conformational changes. In this report, we have used various kinetic assays which differentiate between conformational changes in the two subdomains, to characterize the stilbenedisulphonate site on band 3 Memphis variant II. We have found a significantly higher H2DIDS (a C-terminal-sensitive inhibitor) affinity for band 3 Memphis variant II, due to a lower H2DIDS 'off' rate constant, but no difference was found between mutant and control when DBDS (4,4'-dibenzamido-2,2'-stilbenedisulphonate) (a C-terminal-insensitive inhibitor) 'off' rates were measured. Furthermore, there were no differences in the rates of covalent binding to lysine A, for either DIDS (4,4'-di-isothiocyanato-2,2'-stilbenedisulphonate) or H2DIDS. However, the rate of covalent intrasubunit cross-linking of Lys-539 and Lys-851 by H2DIDS was abnormally low for band 3 Memphis variant II. These results suggest that the Pro-854-->Leu mutation causes a localized conformational change in the C-terminal subdomain of band 3.
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MESH Headings
- 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid/chemistry
- 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid/metabolism
- 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid/analogs & derivatives
- 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid/chemistry
- 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid/metabolism
- Anion Exchange Protein 1, Erythrocyte/chemistry
- Anion Exchange Protein 1, Erythrocyte/metabolism
- Binding Sites
- Cross-Linking Reagents
- Humans
- Kinetics
- Mutation
- Protein Conformation
- Spectrophotometry, Ultraviolet
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Affiliation(s)
- J M Salhany
- Veterans Administration Medical Center, Omaha, NE, USA
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2
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Moriyama R, Tomida M, Hoshino F, Makino S. Local structural difference between human and bovine band 3 in the anion transport inhibitor-binding region. Int J Biochem Cell Biol 1995; 27:575-83. [PMID: 7671135 DOI: 10.1016/1357-2725(95)00022-h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
We have examined molecular properties of inhibitor-complexed human and bovine band 3, an anion transport protein of erythrocyte membrane, in order to demonstrate the structural characteristics of the inhibitor binding region. Band 3 modified with DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonate), a potent anion transport inhibitor, generated a positive circular dichroic band at a wavelength of 345 nm, corresponding to a DIDS chromophore. The dichroic spectra of human band 3-DIDS complex and its bovine counterpart differed markedly in their ellipticity. Under the conditions that H2DIDS (the dihydro-derivative of DIDS) cross-linked two chymotryptic fragments of human band 3, the reagent failed to cross-link the equivalent bovine fragments. The inhibitory effect of PLP (pyridoxal 5'-phosphate), a substrate and affinity label, on phosphate influx into red blood cells was more pronounced for human band 3 than for bovine band 3. The residue Lys-562 of human band 3 was found to be modified with PLP, while the corresponding residue of bovine band 3 was devoid of reactivity with PLP.
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Affiliation(s)
- R Moriyama
- Department of Applied Biological Sciences, School of Agricultural Sciences, Nagoya University, Japan
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3
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Azzarolo AM, Ritchie G, Quamme G. Inhibition of sodium-phosphate cotransport in renal brush-border membranes with the stilbenedisulfonate, H2-DIDS. BIOCHIMICA ET BIOPHYSICA ACTA 1991; 1069:70-6. [PMID: 1932052 DOI: 10.1016/0005-2736(91)90105-h] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Membrane proteins involved with sodium/phosphate cotransport across the renal brush border provide the sensitive control for phosphate homeostasis. The present study describes the inhibition of sodium/phosphate cotransport with the stilbenedisulfonate derivatives, DIDS and H2-DIDS. Preincubation of the rat brush-border membrane vesicles with H2-DIDS led to the inhibition of sodium-dependent phosphate uptake with a half maximal concentration, IC50, of about 10 microM. The inhibition was irreversible supporting the notion that H2-DIDS forms covalent bonds with the cotransporter. The cotransporter could be protected by excess sodium phosphate but not sodium chloride, sodium sulfate, sodium succinate, sodium bicarbonate, nor sodium phosphonoformate. These observations suggest that the stilbenedisulfonates may be useful in labeling the sodium/phosphate cotransporter within renal brush-border membranes.
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Affiliation(s)
- A M Azzarolo
- Department of Medicine, University of British Columbia Hospital, Vancouver, Canada
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4
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Edwards EH, Sprague EA, Schwartz CJ. Low-density lipoprotein endocytosis. I. Influence of the multivalent ligand cationized ferritin on normal and receptor-negative human fibroblasts. Exp Mol Pathol 1988; 48:353-72. [PMID: 3371459 DOI: 10.1016/0014-4800(88)90071-8] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Based upon the observation that the multivalent ligand cationized ferritin (CF) alters the cell surface distribution of anionic domains and significantly enhances the adsorptive endocytosis of 125I-labeled human serum albumin, these studies were undertaken to probe the influence of CF on receptor-mediated low-density lipoprotein (LDL) endocytosis and the nature of the mechanisms involved. A brief 1-min exposure of normal receptor upregulated fibroblasts to CF (0.2 mg/ml) resulted in a significant decrease (P less than 0.001) in the subsequent internalization and degradation of 125I-LDL. Studies with receptor downregulated normal fibroblasts indicated that CF pretreatment did not measurably influence 125I-LDL internalization and only slightly inhibited its degradation (P less than 0.05). In contrast, CF pretreatment of FH receptor-negative mutant skin fibroblasts resulted in a modest but significant increase in both 125I-LDL internalization and degradation (P less than 0.05). Scatchard analyses of binding data indicated that CF-pretreated upregulated normal fibroblasts exhibit a single class of LDL binding sites with an affinity, Kd = 24.7 +/- 4.1 nM, almost 10-fold lower than the affinity of binding sites in untreated controls, Kd = 3.2 +/- 0.06 nM. Increasing either the concentration or the duration of CF exposure resulted in additional inhibition of LDL internalization and degradation associated primarily with a decrease in the number of LDL binding sites without any further change in binding affinity. Total cellular LDL receptor-mediated binding, measured using an octylglucoside solubilization-filtration assay, confirmed the CF-induced decrease in high-affinity LDL binding. Pulse-chase experiments showed that CF had no direct influence on LDL degradation, nor did it influence targeting of the LDL-containing endosome toward exocytosis. Further, restoration of LDL receptor function to control values after CF pretreatment required de novo protein synthesis. The normal feedback inhibition of HMG-CoA reductase activity was nearly abolished by CF pretreatment. Additionally, CF pretreatment was found to induce not only a redistribution of surface anionic sites, but also a very rapid internalization of surface components labeled with 4,4'-[3H]diisothiocyano-1,2..diphenylethane-2,2'-disulfonic acid. It is concluded that the inhibitory influence of CF on LDL endocytosis is mediated via a decrease in the affinity and in the number of functional LDL receptors.
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Affiliation(s)
- E H Edwards
- Department of Pathology, University of Texas Health Science Center, San Antonio 78284
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5
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Jessen F, Sjøholm C, Hoffmann EK. Identification of the anion exchange protein of Ehrlich cells: a kinetic analysis of the inhibitory effects of 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) and labeling of membrane proteins with 3H-DIDS. J Membr Biol 1986; 92:195-205. [PMID: 3783658 DOI: 10.1007/bf01869388] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
In Ehrlich ascites tumor cells 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) inhibits the chloride exchange both reversibly and irreversibly. The reversible inhibition is practically instantaneous and of a competitive nature with Ki about 2 microM at zero chloride concentration. This is succeeded by a slow irreversible binding of DIDS to the transporter, with a chloride dependence suggesting binding to the same site as for reversible DIDS binding/inhibition. To identify the membrane protein involved in anion exchange, cells were labeled with 3H-DIDS. Incubation of cells for 10 min with 25 microM DIDS at pH 8.2 leads to more than 95% inhibition of the DIDS-sensitive chloride exchange flux when the chloride concentration is low (15 mM). This condition was used for the 3H-DIDS-labeling experiments. After incubation the cells were disrupted, the membranes isolated and solubilized, and the proteins separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The distribution of the 3H-activity in the gel showed only one major peak, which could be related to protein with a mol wt of about 30,000 Daltons. The number of transport sites was estimated at about 400,000 per cell, and from the DIDS-sensitive chloride flux under steady-state conditions we calculate a turnover number of 340 ions per sec per site.
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6
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Low PS. Structure and function of the cytoplasmic domain of band 3: center of erythrocyte membrane-peripheral protein interactions. BIOCHIMICA ET BIOPHYSICA ACTA 1986; 864:145-67. [PMID: 2943319 DOI: 10.1016/0304-4157(86)90009-2] [Citation(s) in RCA: 346] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
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7
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Hoffmann EK. Anion transport systems in the plasma membrane of vertebrate cells. BIOCHIMICA ET BIOPHYSICA ACTA 1986; 864:1-31. [PMID: 3521744 DOI: 10.1016/0304-4157(86)90014-6] [Citation(s) in RCA: 141] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
In the case of the red blood cell, anion transport is a highly specific one-for-one exchange catalyzed by a major membrane protein known as band 3 or as capnophorin. This red cell anion-exchange system mediates the Cl-(-)HCO3- exchange responsible for most of the bicarbonate transport capacity of the blood. The rapidly expanding knowledge of the molecular biology and the transport kinetics of this specialized transport system is very briefly reviewed in Section III. Exchange diffusion mechanisms for anions are found in many cells other than erythrocytes. The exchange diffusion system in Ehrlich cells has several similarities to that in red cells. In several cell types (subsection IV-B), there is evidence that intracellular pH regulation depends on Cl-(-)HCO3- exchange processes. Anion exchange in other single cells is described in Section IV, and its role in pH regulation is described in Section VII. Anion exchange mechanism operating in parallel with, and only functionally linked to Na+-H+ or K+-H+ exchange mechanisms can also play a role in cell volume regulation as described in Section VII. In the Ehrlich ascites cell and other vertebrate cells, electroneutral anion transfer has been found to occur also by a cotransport system for cations and chloride operating in parallel with the exchange diffusion system. The cotransport system is capable of mediating secondary active chloride influx. In avian red cells, the cotransport system has been shown to be activated by adrenergic agonists and by cyclic AMP, suggesting that the cotransport is involved in regulatory processes (see subsection V-A.). In several cell types, cotransport systems are activated and play a role during volume regulation, as described in Section V and in Section VII. It is also likely that this secondary active cotransport of chloride plays a significant role for the apparently active extrusion of acid equivalents from certain cells. If a continuous influx of chloride against an electrochemical gradient is maintained by a cotransport system, the chloride disequilibrium can drive an influx of bicarbonate through the anion exchange mechanism, as described in Section VII. Finally, even the electrodiffusion of anions is shown to be regulated, and in Ehrlich cells and human lymphocytes an activation of the anion diffusion pathway plays a major role in cell volume regulation as described in Section VI and subsection VII-B.(ABSTRACT TRUNCATED AT 250 WORDS)
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8
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Donovan JA, Jennings ML. N-hydroxysulfosuccinimido active esters and the L-(+)-lactate transport protein in rabbit erythrocytes. Biochemistry 1986; 25:1538-45. [PMID: 3707891 DOI: 10.1021/bi00355a012] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Esters of N-hydroxysulfosuccinimide strongly inhibit L-(+)-lactate transport in rabbit erythrocytes, probably by acylating amino groups on the transport protein. Lactate transport studies using bis(sulfosuccinimido) suberate (BS3), bis(sulfosuccinimido) adipate (BS2A), bis(sulfosuccinimido) dithiobis(propionate), and a variety of monocarboxylate esters suggest that an exofacial amino group of the lactate transport protein is essential for lactate transport. Also, reductive methylation studies show that even when positive charge is preserved in modified amino groups, the transport is strongly inhibited. At pH less than 6, band 3 mediated inorganic anion transport is enhanced in BS3-treated cells, while at pH greater than 6, it is inhibited. BS3-induced inhibition of L-(+)-lactate transport does not have this pH dependence. BS3 reduces the labeling of a 40-50-kDa membrane polypeptide (band R) by tritiated 4,4'-diisothiocyanato-2,2-dihydrostilbenedisulfonate ([3H]H2DIDS) and by tritiated bis(sulfosuccinimido) adipate ([3H]BS2A). Tritiated sulfosuccinimido acetate (S2[3H]acetate) also labels band R, over a range of concentrations where lactate transport is inhibited in a dose-dependent manner by S2 acetate. BS3 is a known impermeant protein cross-linker. S2 acetate permeates rabbit red cell membranes by an H2DIDS-inhibitable mechanism. BS3 cross-links the proteolytic fragments of rabbit band 3 produced by extracellular chymotrypsin. These labeling experiments support an association between band R and specific monocarboxylate transport.
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9
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Donovan JA. Protein-mediated chloride-phosphate and lactate-lactate exchange in cytoskeleton-free vesicles budded from rabbit erythrocytes. BIOCHIMICA ET BIOPHYSICA ACTA 1985; 816:68-76. [PMID: 4005240 DOI: 10.1016/0005-2736(85)90394-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Spectrin-free budded vesicles from rabbit erythrocytes (Leonards, K.S. and Ohki, S. (1983) Biochim. Biophys. Acta 728, 383-393) exchange intravesicular L-[14C]lactate for extravesicular L-lactate and intravesicular [36C]chloride for extravesicular phosphate with inhibitor sensitivity consistent with what is seen in intact cells. The time-course of these fluxes is faster than for intact cells, but is somewhat slower than predicted from surface to volume ratios. Labelling with tritiated 4,4'-diisothiocyanyl-2,2'-dihydrostilbenedisulfonate (H2DIDS) at concentrations which selectively inhibit inorganic anion exchange or specific lactate exchange supports the involvement of a 93-110 kDa (band 3) polypeptide in anion transport and a 40-50 kDa polypeptide in lactate transport across these vesicle membranes. Since the budded vesicles have a markedly simplified protein profile on electrophoresis, their isolated membranes represent a preliminary stage in the purification of these transport proteins in which structure and function appear to be preserved.
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10
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Jennings ML, Adams-Lackey M, Denney GH. Peptides of human erythrocyte band 3 protein produced by extracellular papain cleavage. J Biol Chem 1984. [DOI: 10.1016/s0021-9258(17)43096-1] [Citation(s) in RCA: 31] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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11
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Jennings ML, Adams-Lackey M. A rabbit erythrocyte membrane protein associated with L-lactate transport. J Biol Chem 1982. [DOI: 10.1016/s0021-9258(18)33595-6] [Citation(s) in RCA: 33] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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12
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Stimulus-response coupling in the human neutrophil. The role of anion fluxes in degranulation. J Biol Chem 1982. [DOI: 10.1016/s0021-9258(18)34516-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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13
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Zoccoli MA, Hoopes RR, Karnovsky ML. Identification of a rat liver microsomal polypeptide involved in the transport of glucose 6-phosphate. Labeling with 4,4'-diisothiocyano-1,2-diphenyl[3H]ethane-2,2'-disulfonic acid. J Biol Chem 1982. [DOI: 10.1016/s0021-9258(18)34870-1] [Citation(s) in RCA: 30] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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14
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Aull F. Inhibition of Ehrlich ascites cell anion transport by 1-isothiocyanate-4-benzenesulfonic acid. BIOCHIMICA ET BIOPHYSICA ACTA 1980; 599:580-6. [PMID: 7407108 DOI: 10.1016/0005-2736(80)90201-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The effects of 1-isothiocyanate-4-benzene sulfonic acid on steady state Cl- and SO24(4 transport in Ehrlich mouse ascites tumor cells were investigated. At 10 mM, 1-isothiocyanate-4-benzenesulfonic acid reduced SO24(-) exchange by 94% but Cl- exchange was reduced by only 37%; Cl- exchange was not further inhibited by as much as 60 min of preincubation with 1-isothiocyanate-4-benzenesulfonic acid. Inhibition of Cl- exchange was completely reversible following 30-45 min of contact with 1-isothiocyanate-4-benzenesulfonic acid whereas under the same conditions, inhibition of SO24(-) exchange was irreversible. The effect o 1-isothiocyanate-4-benzenesulfonic acid on SO24(-) transport could be reversed, however, when exposure to 1-isothiocyanate-4-benzene-sulfonic acid lasted for only 2 min. In these respects the action of 1-isothiocyanate-4-benzenesulfonic acid resembles that of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid; the results are compatible with separate membrane sites for Cl- and SO24(-) transport. The Ki for reversible inhibition of SO24(-) transport, determined from a Dixon plot, was 4.8 mM and the inhibition appeared to be non-competitive.
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Levinson C. Transport of anions in Ehrlich ascites tumor cells: effects of disulfonic acid stilbene in relation to transport mechanism. Ann N Y Acad Sci 1980; 341:482-93. [PMID: 6930841 DOI: 10.1111/j.1749-6632.1980.tb47193.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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16
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Pan YT, Schmitt JW, Sanford BA, Elbein AD. Adherence of bacteria to mammalian cells: inhibition by tunicamycin and streptovirudin. J Bacteriol 1979; 139:507-14. [PMID: 378976 PMCID: PMC216897 DOI: 10.1128/jb.139.2.507-514.1979] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Group B streptococci were labeled either by growing the cells in [14C]fructose or by using the surface label 4,4'-[3H]diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid, which reacts with amino groups. A quantitative assay was developed by using these labeled bacteria to study the adherence of streptococci to canine kidney epithelial cells. The bacteria adhered to kidney cells that had been infected with influenza A virus, but did not adhere to uninfected cells. The binding of 3H-labeled group B streptococci was proportional to the number of bacteria added and showed saturation kinetics. The binding was blocked by the addition of unlabeled group B streptococci but was not affected by addition of streptococci from other groups. It was also blocked by mixing the 3H-labeled streptococci with influenza A virus before adding the bacteria to the kidney cells. When the kidney cells were infected with influenza virus in the presence of either tunicamycin or streptovirudin, these antibiotics inhibited the appearance of viral hemagglutinin in the kidney cells and also prevented the release of mature virus. In these experiments, the adherence of 3h-labeled streptococci was also inhibited. Tunicamycin was shown to block the incorporation of [14C]mannose into lipid-linked oligosaccharides and glycoprotein in both normal and virus-infected kidney cells. These data give strong support to the notion that adherence of streptococci to mammalian cells involves recognition of viral hemagglutinin, a glycoprotein whose synthesis is blocked by certain antibiotics.
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Smith TC, Levinson C. Ehrlich ascites tumor cell surface labeling and kinetics of glycocalyx release. JOURNAL OF SUPRAMOLECULAR STRUCTURE 1979; 12:115-25. [PMID: 544932 DOI: 10.1002/jss.400120109] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Ehrlich ascites tumor cells spontaneously release cell surface material (glycocalyx) into isotonic saline medium. Exposure of these cells to tritium-labeled 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (3H2DIDS) at 4 degrees C leads to preferential labeling of the cell surface coat. We have combined studies of the kinetics of 3H2DIDS-label release, the effects of enzymatic treatment, and cell electrophoretic mobility to characterize the 3H2DIDS-labeled components of the cell surface. Approximately 73% of the cell-associated radioactivity is spontaneously released from the cells after 5 h at 23 degrees C. The kinetics of release is consistent with the first-order loss of two fractions; a slow (tau 1/2 = 360 min) component representing 33% of the radioactivity and a fast (tau 1/2 = 20 min) component representing 26%. The remaining 14% of the labile binding may reflect mechanically induced surface release. Trypsin (1 microgram/ml) also removes approximately 73% of the labeled material within 30 min and converts the kinetics of release to that of a single component (tau 1/2 = 5.5 min). The specific activity (SA) of material released by trypsin immediately after labeling is 83% of the SA of the material spontaneously lost in 1 h. However, trypsinization following a 2-h period of spontaneous release yields material of reduced (43%) SA. Neither 3H2DIDS labeling nor the initial spontaneous loss of labeled material alters cell electrophoretic mobility. However, extended spontaneous release is accompanied by a significant decrease in surface charge density. Trypsinization immediately following labeling or after spontaneous release (2 h) reduces mobility by 32%. We have tentatively identified the slowly released compartment as contributing to cell surface negativity.
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