Binding of cytochalasin B to a red cell membrane protein

Binding of [3H]ctyochalasin B and [3H]colchicine to isolated liver plasma membranes

The binding to isolated hepatocyte plasma membranes of radioactively labelled inhibitors of microfilamentous and microtubular protein function ([3H]cytochalasin B and [3H]colchicine, respectively) was studied as one means of assessing the degree of association of these proteins with cell surface membranes. [3H]Cytochalasin B which behaved identically to the unlabelled compound with respect to binding to these membranes was prepared by reduction of cytochalasin A with NaB3H4. The binding was rapid, readily reversible, proportional to the amount of membrane and relatively insensitive to changes of pH or ionic strength. At 10(-6) M [3H]cytochalasin B, glucose of p-chloromercuribenzoate, an inhibitor of glucose transport inhibited binding by about 20%; treatment of membranes with 0.6 M KI which depolymerizes F actin to G actin caused about 60% inhibition of binding. These two types of inhibition were additive indicating two separate classes of binding sites, one associated with sugar transport and one with microfilaments. Filamentous structures with the diameter of microfilaments (50 A) were seen in electron micrographs of thin sections of the membranes. At concentrations greater than 10(-5) M [3H]cytochalasin B, binding was proportional to drug concentration, characteristic of non-specific adsorption or partitioning. Intracellular membranes of the hepatocyte also bound [3H]cytochalasin B, those of the smooth endoplasmic reticulum to a greater extent than plasma membranes. [3H]Colchicine bound to plasma membranes in proportion to the amount of membrane and at a rate compatible with binding to tubulin. However, other properties of the binding including effects of temperature, drug concentration and antisera against tubulin were different from those of binding to tubulin. Hence, no evidence was obtained for association of microtubular elements with these membranes. Despite this there appeared to be an interdependence between microtubule and microfilament inhibitors: vinblastine sulfate stimulated [3H]cytochalasin B binding and cytochalasin B stimulated 3H colchicine binding. [3H]Colchicine also bound to intracellular membranes, especially smooth microsomes.

Toxic chemical agents as probes for permeation systems of the red blood cell

Chemical agents with different capacities to penetrate into the membrane and with different chemical reactivities can be used to gain information concerning the location of transport sites in the membrane structure and the particular functional ligands. If the agents are highly specific in their interactions and if their inhibitory effects are irreversible, they can also be used as probes to identify the transport components. Several examples are cited using the human red blood cells as a model. The anion transport system in particular has been studied by the use of nonpenetrating irreversible inhibitors, and more recently with a photoaffinity probe, NAP-taurine. In the dark the latter is transported in competition with the normal inorganic anions but after exposure to light, it becomes fixed in an irreversible bond that allows identification of the sites of its transport. It is proposed that anion transport involves a transmembrane protein of about 90,000 daltons that forms a channel through the lipid bilayer. The exchange of anions occurs via a gating mechanism containing a specific anion-binding site. Transport of water, cations and sugars may also involve similar transmembrane protein channels.

Reconstitution of D-glucose transport in vesicles composed of lipids and intrinsic protein (zone 4.5) of the human erythrocyte membrane

Elucidation of the mechanism of facilitated D-glucose transport in human erythrocytes is dependent on the identification and isolation of the membrane protein(s) mediating this process. Based on the fact that stereospecific D-glucose transport is reconstituted in liposomes prepared by sonication of a lipid suspension with ghosts or fractions derived from ghosts, a quantitative assay for the stereospecific D-glucose transport activity of these fractions was developed (Zala CA, Kahlenberg A: Biochem Biophys Res Commun 72:866, 1976). This assay was used to monitor the purification of ghosts. The solubilized membrane protein fraction was chromatographed on a column of diethylaminoethyl cellulose which was eluted stepwise with NaCl-phosphate buffers of increasing ionic strength. A fraction, eluted at an ionic strength of 0.1, displayed a 13- and 27-fold increase in reconstituted transport activity relative to ghosts and to the unfractionated Triton X-100 extract, respectively. This fraction, when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, consisted predominantly of the ghost proteins with an apparent molecular weight of 55,000, commonly designated as zone 4.5; periodic acid-Schiff-sensitive membrane glycoproteins 1-4 were absent. Transport reconstituted by this preparation of zone 4.5 membrane proteins was almost completely abolished by 1-fluoro-2,4-dinitrobenzene, mercuric chloride, and p-chloromercuribenzene sulfonate, but was unaffected by sodium iodoacetate. Extra- and intraliposomal phloretin and cytochalasin B, respectively, exhibited partial inhibition. The stereospecificity and inhibition characteristics of the reconstituted transport imply that all the components of the erythrocyte D-glucose transport system are contained in the zone 4.5 membrane protein preparation.

Fractionation of human erythrocyte membranes. Presence of the nucleoside transport complex in an insoluble residue

(1) Human erythrocyte membranes, when dialysed against water at pH 9.5, were partly solubilized, losing 80% of the membrane proteins and 65% of the membrane lipids. Sodium dodecyl sulphate gel electrophoresis of the particulate material revealed selective removal of proteins from the membrane. (2) The lipid-rich particulate material remaining retained the ability to bind specifically the nucleoside transport inhibitor, nitrobenzylthioinosine, previously shown to bind selectively to the nucleoside transport mechanism of whole erythrocytes and erythrocyte ghosts.

High-affinity cytochalasin B binding to normal and transformed BALB/3T3 cells

To study the molecular basis of changes in sugar uptake rate in cultured mouse fibroblasts with different physiological states, we have measured the high affinity binding of [3H] cytochalsin B, a potent sugar transport inhibitor, to actively growing and contact inhibited Balb/3T3 cells as well as to 3T12 and SV3T3 cells. Binding was the same whether the cells were detached from dishes with EDTA or trypsin. The amount of drug bound to intact cells measured with a centrifugation assay was essentially the same as that bound to cell sonicates measured with equilibrium dialysis. Cytochalasin B binding to intact cells was extremely rapid and reversible over a wide range of drug concentrations, and was not affected by 0.1 M D--glucose in the assay medium. Actively growing and contact inhibited 3T3 cells had a similar number of high affinity cytochalasin B binding sites per cell, while 3T12 and SV3T3 cells had one third to one fourth the number of sites per cell. However, the number of sites per mug cellular protein appeared to be similar for cells in all of the physiological states examined.

Current status of the thiol redox model for the regulation of hexose transport by insulin

Data obtained over the last two years pertinent to the thiol redox model for the modulation of hexose transport activity by insulin is summarized. The model proposes that activation of hexose transport in fat cells involves sulfhydryl oxidation to the disulfide form in a key protein component of the fat cell surface membrane. Theoretically, the rapid activation of transport by insulin may involve either the conversion of inactive membrane carriers to the active form as originally proposed, or the conversion of a low Vmax transport system to a high Vmax form. The present experiments showed that the percent inhibition of insulin-activated transport rates by submaximal levels of cytochalasin B was decreased compared to its effects on basal transport. Treatment of fat cells with N-ethylmaleimide inhibited cytochalasin B action but not transport activity. When insulin or the oxidant vitamin K5 was added to cells 5 minutes before the N-ethylmaleimide, the elevated transport activity was also resistant to the sulfhydryl reagent, but cytochalasin B retained its potent inhibitory effect on transport. The data demonstrate that unique properties characterize basal versus insulin-activated transport activity with respect to the sensitivity of cytochalasin B action to sulfhydryl blockade in isolated fat cells. The data are consistent with the concept that activation of transport activity reflects the conversion of a reduced (sulfhydryl) system characterized by a low Vmax to an oxidized (disulfide), high Vmax transport system.

Reconstitution of intramembrane particles in recombinants of erythrocyte protein band 3 and lipid: effects of spectrin-actin association

The integral membrane protein Band 3 of the human erythrocyte, either purified or in a crude Triton X-100 extract of ghosts, was combined with egg lecithin in a cholate solution. During dialysis to remove cholate, lipid bilayer vesicles formed in which Band 3 existed as a dimer and in which intramembrane particles indistinguishable from those in the native membrane were exposed by freeze-fracturing. The recombinant vesicles were stable in both high and low salt concentrations, sedimented at a density that increased in prportion to their protein content, and bound spectrin-actin extracted from erythrocyte ghosts. When spectrin-actin was associated with the vesicles, the behavior of the recombinant intramembrane particles simulated that of the erythrocyte ghost intramembrane particles: they were dispersed at pH 7.6 and aggregrated at pH 5-5.5. Thus, some of the characteristics of the native membrane have been reconstituted in the recombinant.

Cytochalasin B: evidence for a membrane-directed action in B-cells from rat pancreatic islets

In the presence of cytochalasin B (CCB) concentrations from 50 to 200 mug/ml there is a dose-dependant inhibition of insulin release from isolated rat pancreatic islets. Inhibition is unspecific with respect to glucose, leucine, tolbutamide or theophylline and is reversible. Production of 14CO2 from uniformly labeled D-glucose is decreased. Islets pretreated with an high (200 mug/ml) or low (10 mug/ml) CCB dose release more insulin in response to a subsequent glucose or leucine stimulus in a CCB free medium compared with controls. The data are compatible with a membrane-directed action of CCB.

Isolation of the major intrinsic transmembrane protein of the human erythrocyte membrane

The major intrinsic protein of the human erythrocyte membrane commonly referred to as "Band 3", was isolated by a multi-step procedure. Extraction of ghost membranes in dilute solutions of lithium diiodosalicylate removed most of the proteins considered to be extrinsic to the membrane. The resulting membrane fragments were solubilized in sodium dodecyl sulfate, and the major sialoglycoprotein (glycophorin A) was removed by wheat germ agglutinin-Sepharose affinity chromatography. Gel filtration in sodium dodecyl sulfate was used as the final step to yield the band 3 polypeptide in electrophoretically homogeneous form.

Proteolytic dissection of band 3, the predominant transmembrane polypeptide of the human erythrocyte membrane

Band 3 is the major, membrane-spanning, approximately90 000 dalton polypeptide of the human erythrocyte membrane. To facilitate the analysis of its structural integration into the membrane, we have cleaved this protein in situ into large fragments and ascertained their disposition. Digestion of intact cells with chymotrypsin yielded band 3 fragments with apparent molecular weights of 38 000 and 55 000. Both fragments resisted elution by NaOH and acetic acid, suggesting that they are anchored in the apolar core of the membrane. Both pieces communicate with the extracellular space, and the 55 000 dalton species extends to the cytoplasmic surface as well. Digestion of unsealed ghosts with chymotrypsin produced a hydrophobic 17 000 dalton species, a segment of the 55 000 dalton fragment, which spans and is firmly anchored in the core of the membrane. Trypsin and papain at low concentration generated integral band 3 fragments of 52 000 daltons and released major band 3 fragments of less than or equal to 41 000 daltons from the cytoplasmic side of the membrane. The latter water-soluble polypeptides remained associated in discrete complexes which retained the capacity to bind glyceraldehyde-3-phosphate dehydrogenase. An interchain disulfide bond, which can be induced only at the cytoplasmic surface, cross-linked intact band 3, and certain of its water-soluble fragments. Finally, fragments of 23 000 daltons were generated from the innersurface domain by reacting disulfide-linked band 3 dimers with cyanide or reduced polypeptides with 2-nitro-5-thiocyanobenzoate. A provisional ordering of these fragments is proposed.

Reconstitution of D-glucose transport catalyzed by a protein fraction from human erythrocytes in sonicated liposomes

A protein fraction was obtained from human erythrocyte ghosts by solubilization with Triton X-100 or octylglucoside. Triton X-100 was removed from the protein by Bio-Beads SM-2 and octylglucoside, by diafiltration. The solubilized protein fraction catalyzed D-glucose uptake when reconstituted in sonicated liposomes. The uptake was time dependent and inhibited by mercuric ions or cytochalasin B. The results indicate that the uptake represents transport of the sugar into the liposomes rather than binding to the reconstituted liposomes.

Subcellular localization of binding sites for cytochalasin D: evidence from activation energies

The activation energies for binding of tritiated cytochalasin D to HEp-2 cells and isolated plasma membrane were determined by Arrhenius plots. The higher value for intact cells (24 kcal/mol) compared to the plasma membrane fraction (4 kcal/mol at greater than 11.5 degrees C, 18 kcal/mol at less than 11.5 degrees C) was taken as evidence that [3H]cytochalasin D must penetrate the plasma membrane in order to reach its binding sites. The data support the conclusion that binding sites for [3H]cytochalasin D are intracellular, on the cytoplasmic face of the plasma membrane (rather than within the lipid bilayer), and on microsomes (endomembranes).