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

Microneedle Patches Loaded with Nanovesicles for Glucose Transporter-Mediated Insulin Delivery

Glucose-responsive insulin delivery systems that mimic insulin secretion activity in the pancreas show great potential to improve clinical therapeutic outcomes for people with type 1 and advanced type 2 diabetes. Here, we report a glucose-responsive insulin delivery microneedle (MN) array patch that is loaded with red blood cell (RBC) vesicles or liposome nanoparticles containing glucose transporters (GLUTs) bound with glucosamine-modified insulin (Glu-Insulin). In hyperglycemic conditions, high concentrations of glucose in interstitial fluid can replace Glu-Insulin via a competitive interaction with GLUT, leading to a quick release of Glu-Insulin and subsequent regulation of blood glucose (BG) levels in vivo. To prolong the effective glucose-responsive insulin release from MNs, additional free Glu-Insulin, which serves as "stored insulin", is loaded after RBC vesicles or liposome nanoparticles bound with Glu-Insulin. In the streptozotocin (STZ)-induced type 1 diabetic mouse model, this smart GLUT-based insulin patch can effectively control BG levels without causing hypoglycemia.

Slc25a17 acts as a peroxisomal coenzyme A transporter and regulates multiorgan development in zebrafish

Slc25a17 is known as a peroxisomal solute carrier, but the in vivo role of the protein has not been demonstrated. We found that the zebrafish genome contains two slc25a17 genes that function redundantly, but additively. Notably, peroxisome function in slc25a17 knockdown embryos is severely compromised, resulting in an altered lipid composition. Along the defects found in peroxisome-associated phenotypic presentations, we highlighted that development of the swim bladder is also highly dependent on Slc25a17 function. As Slc25a17 showed substrate specificity towards coenzyme A (CoA), injecting CoA, but not NAD⁺ , rescued the defective swim bladder induced by slc25a17 knockdown. These results indicated that Slc25a17 acts as a CoA transporter, involved in the maintenance of functional peroxisomes that are essential for the development of multiple organs during zebrafish embryogenesis. Given high homology in protein sequences, the role of zebrafish Slc25a17 may also be applicable to the mammalian system.

Drop-in-well chamber for droplet interface bilayer with built-in electrodes

Various methods have been developed for the formation of planar lipid bilayers, and recent techniques using water-in-oil droplets, such as droplet interface bilayer (DIB) and contact bubble bilayer (CBB) methods, allow the ready formation of bilayers with arbitrary lipid compositions. Here, we developed a simple and portable DIB system using drop-in-wells, shaping two merging wells for settling electrolyte droplets. An aliquot of the electrolyte solution (1μL) is dropped into an organic solvent, and the droplet sinks to the drop-in-well at the bottom, where two monolayer-lined droplets come in contact to form the bilayer. Pre-installed electrodes allow electrophysiological measurements. The detailed drop-in-well method is presented, and some variations of the method, such as the use of microelectrodes and a sheet with a small hole for low-noise recordings, are extended. Examples of single channel current recordings of the KcsA potassium channel are demonstrated.

Lipid Bilayers Manipulated through Monolayer Technologies for Studies of Channel-Membrane Interplay

Fluidity and mosaicity are two critical features of biomembranes, by which membrane proteins function through chemical and physical interactions within a bilayer. To understand this complex and dynamic system, artificial lipid bilayer membranes have served as unprecedented tools for experimental examination, in which some aspects of biomembrane features have been extracted, and to which various methodologies have been applied. Among the lipid bilayers involving liposomes, planar lipid bilayers and nanodiscs, recent developments of lipid bilayer methods and the results of our channel studies are reviewed herein. Principles and techniques of bilayer formation are summarized, which have been extended to the current techniques, where a bilayer is formed from lipid-coated water-in-oil droplets (water-in-oil bilayer). In our newly developed method, termed the contact bubble bilayer (CBB) method, a water bubble is blown from a pipette into a bulk oil phase, and monolayer-lined bubbles are docked to form a bilayer through manipulation by pipette. An asymmetric bilayer can be readily formed, and changes in composition in one leaflet were possible. Taking advantage of the topological configuration of the CBB, such that the membrane's hydrophobic interior is contiguous with the surrounding bulk organic phase, oil-dissolved substances such as cholesterol were delivered directly to the bilayer interior to perfuse around the membrane-embedded channels (membrane perfusion), and current recordings in the single-channel allowed detection of immediate changes in the channels' response to cholesterol. Chemical and mechanical manipulation in each monolayer (monolayer technology) allows the examination of dynamic channel-membrane interplay.

Synthetic beta cells for fusion-mediated dynamic insulin secretion

Generating artificial pancreatic beta cells by using synthetic materials to mimic glucose-responsive insulin secretion in a robust manner holds promise for improving clinical outcomes in people with diabetes. Here, we describe the construction of artificial beta cells (AβCs) with a multicompartmental 'vesicles-in-vesicle' superstructure equipped with a glucose-metabolism system and membrane-fusion machinery. Through a sequential cascade of glucose uptake, enzymatic oxidation and proton efflux, the AβCs can effectively distinguish between high and normal glucose levels. Under hyperglycemic conditions, high glucose uptake and oxidation generate a low pH (<5.6), which then induces steric deshielding of peptides tethered to the insulin-loaded inner small liposomal vesicles. The peptides on the small vesicles then form coiled coils with the complementary peptides anchored on the inner surfaces of large vesicles, thus bringing the membranes of the inner and outer vesicles together and triggering their fusion and insulin 'exocytosis'.

Structural Biology of the Major Facilitator Superfamily Transporters

The ancient and ubiquitous major facilitator superfamily (MFS) represents the largest secondary transporter family and plays a crucial role in a multitude of physiological processes. MFS proteins transport a broad spectrum of ions and solutes across membranes via facilitated diffusion, symport, or antiport. In recent years, remarkable advances in understanding the structural biology of the MFS transporters have been made. This article reviews the history, classification, and general features of the MFS proteins; summarizes recent structural progress with a focus on the sugar porter family transporters exemplified by GLUT1; and discusses the molecular mechanisms of substrate binding, alternating access, and cotransport coupling.

Excited state proton transfer dynamics of an eminent anticancer drug, ellipticine, in octyl glucoside micelle

Photophysics and proton transfer dynamics of an eminent anticancer drug, ellipticine (EPT), have been investigated inside a biocompatible octyl-β-D-glucoside (OBG) micellar medium using steady state and time-resolved fluorescence spectroscopic techniques. EPT exists as protonated form in aqueous solution of pH 7. When EPT molecules are encapsulated in OBG micelles, protonated form is converted to neutral form in the ground state due to the hydrophobic effect of the micellar environment. Interestingly, steady state fluorescence results indicate the existence of both neutral and protonated forms of EPT in the excited state, even though neutral molecules are selectively excited, and it is attributed to the conversion of neutral to protonated form of EPT by the excited state proton transfer (ESPT) process. A clear isoemissive point in the time-resolved area normalized emission spectra (TRANES) further supports the excited state conversion of neutral to protonated form of EPT. Notably, this kind of proton transfer dynamics is not observed in other conventional micelles, such as, SDS, Triton-X and CTAB. Therefore, the observed ESPT dynamics is believed to be an outcome of combined effects of the local dielectric constant felt by EPT and the local proton concentration at the OBG micellar surface.

Transport of sugars

Soluble sugars serve five main purposes in multicellular organisms: as sources of carbon skeletons, osmolytes, signals, and transient energy storage and as transport molecules. Most sugars are derived from photosynthetic organisms, particularly plants. In multicellular organisms, some cells specialize in providing sugars to other cells (e.g., intestinal and liver cells in animals, photosynthetic cells in plants), whereas others depend completely on an external supply (e.g., brain cells, roots and seeds). This cellular exchange of sugars requires transport proteins to mediate uptake or release from cells or subcellular compartments. Thus, not surprisingly, sugar transport is critical for plants, animals, and humans. At present, three classes of eukaryotic sugar transporters have been characterized, namely the glucose transporters (GLUTs), sodium-glucose symporters (SGLTs), and SWEETs. This review presents the history and state of the art of sugar transporter research, covering genetics, biochemistry, and physiology-from their identification and characterization to their structure, function, and physiology. In humans, understanding sugar transport has therapeutic importance (e.g., addressing diabetes or limiting access of cancer cells to sugars), and in plants, these transporters are critical for crop yield and pathogen susceptibility.

Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics

Proteoliposomes represent a suitable and up to date tool for studying membrane transporters which physiologically mediate absorption, excretion, trafficking and reabsorption of nutrients and metabolites. Using recently developed reconstitution strategies, transporters can be inserted in artificial bilayers with the same orientation as in the cell membranes and in the absence of other interfering molecular systems. These methodologies are very suitable for studying kinetic parameters and molecular mechanisms. After the first applications on mitochondrial transporters, in the last decade, proteoliposomes obtained with optimized methodologies have been used for studying plasma membrane transporters and defining their functional and kinetic properties and structure/function relationships. A lot of information has been obtained which has clarified and completed the knowledge on several transporters among which the OCTN sub-family members, transporters for neutral amino acid, B0AT1 and ASCT2, and others. Transporters can mediate absorption of substrate-like derivatives or drugs, improving their bioavailability or can interact with these compounds or other xenobiotics, leading to side/toxic effects. Therefore, proteoliposomes have recently been used for studying the interaction of some plasma membrane and mitochondrial transporters with toxic compounds, such as mercurials, H₂O₂ and some drugs. Several mechanisms have been defined and in some cases the amino acid residues responsible for the interaction have been identified. The data obtained indicate proteoliposomes as a novel and potentially important tool in drug discovery.

A peroxisomal carrier delivers NAD⁺ and contributes to optimal fatty acid degradation during storage oil mobilization

The existence of a transport protein that imports cytosolic NAD(+) into peroxisomes has been controversially discussed for decades. Nevertheless, the biosynthesis of NAD(+) in the cytosol necessitates the import of NAD(+) into peroxisomes for numerous reduction/oxidation (redox) reactions. However, a gene encoding such a transport system has not yet been identified in any eukaryotic organism. Here, we describe the peroxisomal NAD(+) carrier in Arabidopsis. Our candidate gene At2g39970 encodes for a member of the mitochondrial carrier family. We confirmed its peroxisomal localization using fluorescence microscopy. For a long time At2g39970 was assumed to represent the peroxisomal ATP transporter. In this study, we could show that the recombinant protein mediated the transport of NAD(+) . Hence, At2g39970 was named PXN for peroxisomal NAD(+) carrier. The loss of PXN in Arabidopsis causes defects in NAD(+) -dependent β-oxidation during seedling establishment. The breakdown of fatty acid released from storage oil was delayed, which led to the retention of oil bodies in pxn mutant seedlings. Based on our results, we propose that PXN delivers NAD(+) for optimal fatty acid degradation during storage oil mobilization.

A CRAC-like motif in BAX sequence: relationship with protein insertion and pore activity in liposomes

Several proteins that interact with cholesterol have a highly conserved sequence, corresponding to the cholesterol recognition/interaction amino acid consensus. Since cholesterol has been proposed to modulate both oligomerization and insertion of the pro-apoptotic protein BAX, we investigated the existence of such a motif in the BAX sequence. Residues 113 to 119 of the recombinant BAX α5-helix, LFYFASK, correspond with the sequence motif described for the consensus pattern, -L/V-(X)(1-5)-Y-(X)(1-5)-R/K. Functional characterization of the point mutations, K119A, Y115F, and L113A in BAX, was performed in liposomes supplemented with cholesterol, comparing binding, integration, and pore forming activities. Our results show that the mutations Y115F and L113A changed the cholesterol-dependent insertion observed in the wild type protein. In addition, substitutions in the BAX sequence modified the concentration dependency of carboxyfluorescein release in liposomes, although neither pore activity of the wild type or of any of the mutants significantly increased in cholesterol-enriched liposomes. Thus, while it is likely that the putative CRAC motif in BAX accounts for its enhanced insertion in cholesterol-enriched liposomes; the pore forming properties of BAX did not depend on cholesterol content in the membranes, albeit those mutations changed the pore channeling activity of the protein.

4F2hc stabilizes GLUT1 protein and increases glucose transport activity

Glucose transporter 1 (GLUT1) is widely distributed throughout various tissues and contributes to insulin-independent basal glucose uptake. Using a split-ubiquitin membrane yeast two-hybrid system, we newly identified 4F2 heavy chain (4F2hc) as a membrane protein interacting with GLUT1. Though 4F2hc reportedly forms heterodimeric complexes between amino acid transporters, such as LAT1 and LAT2, and regulates amino acid uptake, we investigated the effects of 4F2hc on GLUT1 expression and the associated glucose uptake. First, FLAG-tagged 4F2hc and hemagglutinin-tagged GLUT1 were overexpressed in human embryonic kidney 293 cells and their association was confirmed by coimmunoprecipitation. The green fluorescent protein-tagged 4F2hc and DsRed-tagged GLUT1 showed significant, but incomplete, colocalization at the plasma membrane. In addition, an endogenous association between GLUT1 and 4F2hc was demonstrated using mouse brain tissue and HeLa cells. Interestingly, overexpression of 4F2hc increased the amount of GLUT1 protein in HeLa and HepG2 cells with increased glucose uptake. In contrast, small interfering RNA (siRNA)-mediated 4F2hc gene suppression markedly reduced GLUT1 protein in both cell types, with reduced glucose uptake. While GLUT1 mRNA levels were not affected by overexpression or gene silencing of 4F2hc, GLUT1 degradation after the addition of cycloheximide was significantly suppressed by 4F2hc overexpression and increased by 4F2hc siRNA treatment. Taken together, these observations indicate that 4F2hc is likely to be involved in GLUT1 stabilization and to contribute to the regulation of not only amino acid but also glucose metabolism.

The targeted expression of nucleotide sugar transporters to the E. coli inner membrane

The heterologous expression of functional mammalian integral membrane proteins still represents a significant hurdle towards evaluating the relationship between their structure and function. We have therefore utilised the OmpA signal sequence to deliberately target the expression of a mammalian nucleotide sugar transporter, the murine CMP-sialic acid transporter, to the E. coli inner membrane. The functionality of the recombinant CMP-sialic acid transporter could then be evaluated either following the spheroplasting of E. coli cells or through the isolation of the E. coli inner membrane and the formation of mixed phosphatidylcholine-inner membrane proteoliposomes.

Peroxisomal ATP import is essential for seedling development in Arabidopsis thaliana

Several recent proteomic studies of plant peroxisomes indicate that the peroxisomal matrix harbors multiple ATP-dependent enzymes and chaperones. However, it is unknown whether plant peroxisomes are able to produce ATP by substrate-level phosphorylation or whether external ATP fuels the energy-dependent reactions within peroxisomes. The existence of transport proteins that supply plant peroxisomes with energy for fatty acid oxidation and other ATP-dependent processes has not previously been demonstrated. Here, we describe two Arabidopsis thaliana genes that encode peroxisomal adenine nucleotide carriers, PNC1 and PNC2. Both proteins, when fused to enhanced yellow fluorescent protein, are targeted to peroxisomes. Complementation of a yeast mutant deficient in peroxisomal ATP import and in vitro transport assays using recombinant transporter proteins revealed that PNC1 and PNC2 catalyze the counterexchange of ATP with ADP or AMP. Transgenic Arabidopsis lines repressing both PNC genes were generated using ethanol-inducible RNA interference. A detailed analysis of these plants showed that an impaired peroxisomal ATP import inhibits fatty acid breakdown during early seedling growth and other beta-oxidation reactions, such as auxin biosynthesis. We show conclusively that PNC1 and PNC2 are essential for supplying peroxisomes with ATP, indicating that no other ATP generating systems exist inside plant peroxisomes.

A cell-free translation and proteoliposome reconstitution system for functional analysis of plant solute transporters

We describe here a novel proteoliposome reconstitution system for functional analysis of plant membrane transporters that is based on a modified wheat germ cell-free translation system. We established optimized conditions for the reconstitution system with Arabidopsis thaliana phosphoenolpyruvate/phosphate translocator 1 (AtPPT1) as a model transporter. A high activity of AtPPT1 was achieved by synthesis of the protein in the presence of both a detergent such as Brij35 and liposomes. We also determined the substrate specificities of three putative rice PPT homologs with this system. The cell-free proteoliposome reconstitution system provides a valuable tool for functional analysis of transporter proteins.

Purification and characterization of the L-fucose transporter

L-Fucose is a monosaccharide present in low levels in the serum. It is, however, a common structural component of glycoproteins. L-Fucose is accumulated in eukaryotic cells by a specific, facilitative diffusion transport system which has been designated the fucose transporter. In this study, purification of the transporter from mouse brain was performed by detergent extraction followed by ion-exchange and reactive dye ligand column chromatography. Purification was followed using a transport assay into reconstituted liposomes. A 111-fold purification with 5% yield was achieved from the crude homogenate. The apparent molecular weight of the protein was 57 kDa. Transport was found to be saturable. The K(m) and V(max) values are estimated at 3 microM and 275 pmol/min/mg, respectively. The tissue distribution of fucose transport was examined in liver, kidney, heart, lung, spleen, brain, muscle, adipose, ovary, pancreas, and thymus. Some fucose transport was found in all tissues examined. Very low levels were observed in the liver relative to all other tissues examined. The only monosaccharide which could inhibit the uptake of L-[5,6-(3)H]fucose was fucose itself.

The structural basis of substrate translocation by the Escherichia coli glycerol-3-phosphate transporter: a member of the major facilitator superfamily

The major facilitator superfamily represents the largest group of secondary active membrane transporters in the cell. The 3.3A resolution structure of a member of this protein superfamily, the glycerol-3-phosphate transporter from the Escherichia coli inner membrane, reveals two domains connected by a long central loop. These N- and C-terminal domains, each containing a six-helix bundle, are related by pseudo-twofold symmetry. A substrate translocation pore is located between the two domains and is open to the cytoplasm. Two arginines at the closed end of the pore comprise the substrate-binding site. Biochemical experiments show that, upon substrate binding, the protein adopts a more compact conformation. The crystal structure suggests that the transporter operates through a single binding site, alternating access mechanism via a rocker-switch type of movement of the N- and C-terminal domains. The structure and mechanism of the glycerol-3-phosphate transporter form a paradigm for other members of the major facilitator superfamily.

A high-throughput assay for rat liver golgi and Saccharomyces cerevisiae-expressed murine CMP-N-acetylneuraminic acid transport proteins

Rat liver Golgi and Saccharomyces cerevisiae-expressed CMP-Neu5Ac transport protein were reconstituted in phosphatidylcholine liposomes and transport of CMP-Neu5Ac into these proteoliposomes was determined. The separation of transported substrate from free substrate was performed using Multiscreen minicolumns loaded with Sephadex G-50 resin (fine). The CMP-Neu5Ac transport characteristics of the rat liver Golgi and S. cerevisiae-expressed transporters, determined using this separation system, were very similar to those previously reported. Inhibition studies, utilizing the above procedure, revealed that the main structural features required for recognition of glycosyl nucleosides by the rat liver Golgi CMP-Neu5Ac transport protein were the nature of the nucleoside base and the anomeric configuration of the associated carbohydrate. In general, pyrimidine-based glycosyl nucleosides were found to inhibit transport to a far greater extent than purine-based glycosyl nucleosides, an observation that is in good agreement with previous reports. These results indicate that the reconstitution procedure, in conjunction with Multiscreen minicolumns, is an effective high-throughput method for the determination of CMP-Neu5Ac transport.

A reaction-induced fourier transform-infrared spectroscopic study of the lactose permease. A transmembrane potential perturbs carboxylic acid residues

In chemiosmotic coupling, a transmembrane ion gradient is used as the source of energy to drive reactions. This process occurs in all cells, but the microscopic mechanism is not understood. Here, Escherichia coli lactose permease was used in a novel spectroscopic method to investigate the mechanism of chemiosmotic coupling in secondary active transporters. To provide a light-triggered electrochemical gradient, bacteriorhodopsin was co-reconstituted with the permease, and reaction-induced Fourier transform-infrared spectra were obtained from the co-reconstituted samples. The bacteriorhodopsin contributions were subtracted from these data to give spectra reflecting permease conformational changes that are induced by an electrochemical gradient. Positive bands in the 1765-1730 cm(-1) region are attributable to carboxylic acid residues in the permease and are consistent with changes of pK(a), protonation state, or environment. This is the first direct information concerning gradient-induced structural changes in the permease at the single amino acid level. Ultimately, these structural changes facilitate galactoside binding and may be involved in the storage of free energy.

Carbonyl cyanide phenylhydrazones as probes of the anionic activator site of the human erythrocyte glutathione adduct transport ATPase

We have previously shown that the ATPase activity associated with the erythrocyte glutathione adduct transporter is also stimulated by 2,4-dinitrophenol and p-trifluoromethoxy carbonylcyanide phenylhydrazone, both well-known anionic and lipophilic uncouplers of oxidative phosphorylation by mitochondria [C. G. Winter, D. C. DeLuca, and H. Szumilo (1994) Arch. Biochem. Biophys. 314, 17-22]. In this paper, we report the testing of a series of ring-substituted carbonylcyanide phenylhydrazones as activators of the ATPase. All of the compounds tested stimulated the ATPase to similar extents, based on Vmax values. The K0.5 for stimulation of the ATPase depended on the electron-withdrawing characteristics of the ring substituents, resulting in a Hammett linear free energy relationship for the m- and p-substituted derivatives. The slope of this relationship, with lower K0.5 values for electron-withdrawing substituents, suggests that an anionic residue in the active site partially discourages binding of this class of activators. ortho-Substituted carbonylcyanide phenylhydrazones do not follow this relationship, but show lower apparent affinities than expected from their pKa values. This finding suggests that steric effects in that region of the binding site negatively influence the affinity.

Glucose uptake occurs by facilitated diffusion in procyclic forms of Trypanosoma brucei

The glucose transporter of Trypanosoma brucei procyclic forms was characterized and compared with its bloodstream form counterpart. Measuring the glucose consumption enzymatically, we determined a saturable uptake process of relatively high affinity (Km = 80 microM, Vmax = 4 nmol min-1 10(-8) cells), which showed substrate inhibition at glucose concentrations above 1.5 mM (Ki = 21 mM). Control experiments measuring deoxy-D-[3H]Glc uptake under zero-trans conditions indicated that substrate inhibition occurred on the level of glycolysis. Temperature-dependent kinetics revealed a temperature quotient of Q10 = 2.33 and an activation energy of Ea = 64 kJ mol-1. As shown by trans-stimulation experiments, glucose uptake was stereospecific for the D isomer, whereas L-glucose was not recognized. Inhibitor studies using either the uncoupler carbonylcyanide-4-(trifluoromethoxy)phenylhydrazone (5 microM), the H+/ATPase inhibitor N,N'-dicyclohexylcarbodiimide (20 microM), the ionophor monensin (1 microM), or the Na+/K+-ATPase inhibitor ouabain (1 mM) showed insignificant effects on transport efficiency. The procyclic glucose transporter was subsequently enriched in a plasma-membrane fraction and functionally reconstituted into proteoliposomes. Using Na+-free conditions in the absence of a proton gradient, the specific activity of D-[14C]glucose transport was determined as 2.9 nmol min-1 (mg protein)-1 at 0.2 mM glucose. From these cumulative results, we conclude that glucose uptake by the procyclic insect form of the parasite occurs by facilitated diffusion, similar to the hexose-transport system expressed in bloodstream forms. However, the markedly higher substrate affinity indicates a differential expression of different transporter isoforms throughout the lifecycle.

Properties of the Peribacteroid Membrane ATPase of Pea Root Nodules and Its Effect on the Nitrogenase Activity

Peribacteroid membrane vesicles from pea (Pisum sativum) root nodules were isolated from membrane-enclosed bacteroids by an osmotic shock. The ATPase activity associated with this membrane preparation was characterized, and its electrogenic properties were determined. The pH gradient was measured as a change of the fluorescence intensity of 9-amino-6-chloro-2-methoxyacridine and the membrane potential as a shift of absorbance of bis-(3-propyl-5-oxoisoxazol-4-yl)pentamethine oxonol. It was demonstrated that the ATPase generates a pH gradient as well as a membrane potential across the peribacteroid membrane. The reversibility of the ATPase was demonstrated by a light-dependent ATP synthesis by peribacteroid membrane vesicles fused with bacteriorhodopsin-phospholipid vesicles. The light-driven ATP synthesis by the peribacteroid membrane ATPase was completely inhibited by a proton-conducting ionophore. The proton-pumping activity of the peribacteroid membrane ATPase could also be demonstrated with peribacteroid membrane-enclosed bacteroids, and effects on nitrogenase activity were established. At pH values below 7.5, an active peribacteroid membrane ATPase inhibited the nitrogenase activity of peribacteroid membrane-enclosed bacteroids. At pH values above 8, at which whole cell nitrogenase activity was inhibited, the protonpumping activity of the peribacteroid membrane ATPase could partially reverse the pH inhibition. Vanadate, an inhibitor of plasma membrane and peribacteroid membrane ATPases, stimulated nodular nitrogenase activity. It will be proposed that the proton-pumping activity of the peribacteroid membrane ATPase in situ is a possible regulator of nodular nitrogenase activity.

Glucose transport and glycolytic enzyme activities in erythrocytes of two-year-old thoroughbreds undergoing training exercise

D-Glucose transport and cytosolic enzyme activities were measured in erythrocytes from 2-year-old thoroughbreds under continuous training exercise (race horses) and compared with those from untrained horses of various ages (sires, mares and untrained 2-year-old thoroughbreds). The activities of the glucose transport and glycolytic enzymes, hexokinase and pyruvate kinase, in the race horses' erythrocytes were elevated to 2-3.5 times above those of untrained horses. There were no significant differences in plasma glucose, triglyceride or IRI concentrations between the horses in training and untrained horses. The increases in glucose transport and glycolytic enzyme activities in their erythrocytes are considered to reflect an increased metabolic activity in the race horses resulting from the training exercises.

Functional reconstitution of the Trypanosoma brucei plasma-membrane D-glucose transporter

The D-glucose transporter of Trypanosoma brucei was solubilized from the plasma membrane and reconstituted into proteoliposomes. Using the reconstitution of D-glucose transport as the assay and non-specific L-glucose uptake as control, we have purified a membrane protein fraction from T. brucei bloodstream-form ghosts by EDTA/alkali treatment and solubilization with the detergents octylglucoside or octylthioglucoside. Upon removal of the detergent by dialysis, the solubilized protein fraction was reconstituted in sonicated liposomes by a freeze/thaw-sonication step. The reconstituted transporter catalyzed specific D-glucose uptake and was compared in several characteristics with the native facilitated-diffusion transporter as present in live trypanosomes [Seyfang, A. & Duszenko, M. (1991) Eur. J. Biochem. 202, 191-196]. As in vivo, the uptake is time dependent and Na+ independent. Transporter substrate affinity and inhibitor specificity are completely retained and it is inhibited by mercuric ions, phloretin and cytochalasin B, but only partially inhibited by phlorizin. The reconstituted transporter also demonstrates trans-stimulation properties indicative of the carrier-mediated transport of D-glucose. In contrast to the human erythrocyte-type glucose transporter, in T. brucei D-fructose uptake was also catalyzed by the same reconstituted protein fraction and specific D-glucose or D-fructose transport were mutually competitive. Both the inhibitor studies and the fructose transport capacity in the reconstituted system are in good agreement with the native transport in live trypanosomes. The specific activity of D-glucose transport was 1.9 +/- 0.3 nmol.min-1.mg protein-1 at 0.2 mM D-glucose and the yield was about 0.8% of total ghost protein after removal of the variant-surface-glycoprotein coat. The successful functional reconstitution of a protozoan glucose transporter represents an important step towards its purification and detailed characterization. This is especially interesting since bloodstream-form trypanosomes depend entirely upon glycolysis for their ATP production.

Role of liver-type glucose transporter (GLUT2) in transport across the basolateral membrane in rat jejunum

To obtain information on the regulation of glucose transport across the basolateral membrane (BLM) of intestinal epithelial cells, we measured the number of [3H]cytochalasin B binding sites and the level of liver-type glucose transporter (GLUT2) protein in the BLM in the jejunum of rats (i) with diabetes (ii) given a high-carbohydrate diet or (iii) with experimental hyperglycemia (12 h infusion of a high-glucose solution). A glucose uptake and the number of D-glucose inhibitable [3H]cytochalasin B binding sites in BLM vesicles were significantly increased in all three conditions. Western blot analysis showed that the amount of GLUT2 protein in BLM vesicles was increased in rats with diabetes and those given a high-carbohydrate diet, but not in those with experimental hyperglycemia. These results suggest that there is a mechanism for rapid regulation of glucose transport in the BLM that does not depend on change in the amount of GLUT2.

Reconstitution of sodium channels in large liposomes formed by the addition of acidic phospholipids and freeze-thaw sonication

Phosphatidylcholine (PC) alone or with phosphatidylethanolamine (PE) are sufficient for the reconstitution of Na+ channels in planar lipid bilayers. However, when Na+ channels were first reconstituted into liposomes using the freeze-thaw-sonication method, addition of acidic phospholipids, such as phosphatidylserine (PS), to the neutral phospholipids was necessary to obtain a significant toxin-modulated 22Na uptake. To further investigate the acidic phospholipid effect on reconstitution into liposomes, Na+ channels purified from Electrophorus electricus electrocytes were reconstituted into liposomes of different composition by freeze-thaw sonication and the effect of batrachotoxin and tetrodotoxin on the 22Na flux was measured. The results revealed that, under our experimental conditions, the presence of an acidic phospholipid was also necessary to obtain a significant neurotoxin-modulated 22Na influx. Though neurotoxin-modulated 22Na fluxes have been reported in proteoliposomes made with purified Na+ channels and PC alone, the 22Na fluxes were smaller than those found using lipid mixtures containing acidic phospholipids. Electron microscopy of negatively stained proteoliposomes prepared with PC, PC/PS (1:1 molar ratio), and PS revealed that the acidic phospholipid increases the size of the reconstituted proteoliposomes. The increment in size caused by the acidic phospholipid, due to the associated increase in internal volume for 22Na uptake and in area for Na+ channel incorporation, appears to be responsible for the large neurotoxin-modulated 22Na fluxes observed.

The anionic conjugates of bilirubin and bile acids stimulate ATP hydrolysis by S-(dinitrophenyl)glutathione ATPase of human erythrocyte

These studies demonstrate that bilirubin-ditaurate (an analog of bilirubin-diglucuronide), lithocholic acid 3-O-sulfate, and lithocholic acid 3-O-glucuronide, which are believed to be transported from liver into bile through an active transport process stimulate ATP hydrolysis by purified dinitrophenylglutathione ATPase of human erythrocytes. The Km and Vmax values of the enzyme for these substrates are similar to those for dinitrophenylglutathione indicating the transport mechanisms for bilirubin conjugates, and anionic bile acid-conjugates from hepatocytes to bile and transport of GSH-conjugates from erythrocytes may be mediated by similar mechanisms.

Membrane fluidity changes of liposomes in response to various odorants. Complexity of membrane composition and variety of adsorption sites for odorants

Three kinds of liposomes prepared from phosphatidylcholine (PC), azolectin, and azolectin-containing membrane proteins of the canine erythrocytes were used as models for olfactory cells. To explore properties of the adsorption sites of odorants, membrane fluidity changes in response to various odorants were measured with various fluorescence dyes which monitor the fluidity at different depths and different regions of the membranes. (a) Application of various odorants changed the membrane fluidity of azolectin liposomes. The patterns of membrane fluidity changes in response to odorants having a similar odor were similar to each other and those in response to odorants having different odors were different from each other. These results suggested that odorants having a similar odor are adsorbed on a similar site and odorants having different odors are adsorbed on different sites. (b) Such variation of the pattern was not seen in liposomes of a simple composition (PC liposome). (c) In the proteoliposomes whose composition was more complex than that of azolectin liposomes, the patterns of membrane fluidity changes varied among odorants having a similar odor. It was concluded that liposomes of complex membrane composition have the variety of adsorption sites for odorants.

A Cl(-)-translocating adenosinetriphosphatase in Acetabularia acetabulum. 2. Reconstitution of the enzyme into liposomes and effect of net charges of liposomes on chloride permeability and reconstitution

The Mono Q-III fraction, a Mg2(+)-ATPase, isolated from Acetabularia acetabulum was reconstituted into liposomes of various net charges prepared by the reversed-phase method and tested for a Cl(-)-translocating activity. The liposomes from a mixture of egg lecithin, dicetyl phosphate, and cholesterol (63:18:9 mole ratio, negative liposomes) and from a mixture of egg lecithin and cholesterol (63:9 mole ratio, neutral liposomes) were less leaky than positive liposomes from asolectin, and from a mixture of egg lecithin, stearylamine, and cholesterol (63:18:9 mole ratio). A significant increase in 36Cl- efflux from the negative and neutral liposomes was observed by addition of ATP in the presence of valinomycin after incorporation of the enzyme by short-term dialysis. The ATP-driven 36Cl- efflux was inhibited by addition of azide, an inhibitor of the ATPase. The preincubation of the enzyme with phenylglyoxal, an arginine-modifying reagent, inactivated ATP-mediated 36Cl- efflux, but the ATPase activity of the preparation was not affected. When chloride was replaced by 35SO4(2)-, no ATP-dependent 35SO4(2)- efflux was detectable from the proteoliposomes. Proton-translocating activity of the enzyme was also tested, and no fluorescent quenching of 9-ACMA was observed.

ATP dependent primary active transport of xenobiotic-glutathione conjugates by human erythrocyte membrane

We have demonstrated the presence of a dinitrophenyl glutathione (Dnp-SG) stimulated ATPase in human erythrocyte membranes. This ATPase mediates the active transport of glutathione-xenobiotic conjugate such as Dnp-SG from erythrocytes into the plasma. It is suggested that this transport system is distinct from the system which actively transports oxidized glutathione from the erythrocytes.

Purification and reconstitution of chloride channels from kidney and trachea

Chloride channels mediate absorption and secretion of fluid in epithelia, and the regulation of these channels is now known to be defective in cystic fibrosis. Indanyl-oxyacetic acid 94 (IAA-94) is a high-affinity ligand for the chloride channel, and an affinity resin based on that structure was developed. Solubilized proteins from kidney and trachea membranes were applied to the affinity matrix, and four proteins with apparent molecular masses of 97, 64, 40, and 27 kilodaltons were eluted from the column by excess IAA-94. A potential-dependent 36Cl- uptake was observed after reconstituting these proteins into liposomes. Three types of chloride channels with single-channel conductances of 26, 100, and 400 picosiemens were observed after fusion of these liposomes with planar lipid bilayers. Similar types of chloride channels have been observed in epithelia.

Characteristics of glucose transport in neuronal cells and astrocytes from rat brain in primary culture

Glucose transport systems in cultured neuronal cells and astrocytes of rats were characterized by measuring the uptake of 2-deoxy-D-[3H]glucose ([3H]2-DG) into the cells. Various sugars inhibited 2-DG uptake by neuronal cells and astrocytes similarly, a finding indicating that the substrate specificities of the transporters in the two types of cells were almost the same. However, the Km values for 2-DG of neuronal cells and astrocytes were 1.7 and 0.36 mM, respectively. The uptake of 2-DG was strongly inhibited by cytochalasin B. Nucleosides, such as adenosine, inosine, and uridine, inhibited 2-DG uptake competitively in both neuronal cells and astrocytes. The uptake by both types of cells were also inhibited by forskolin, but not by cyclic AMP, an observation suggesting that forskolin bound directly to the transporters to cause inhibition. Its inhibition was competitive in astrocytes and noncompetitive in neuronal cells. Astrocytes contained a glucose transporter with a subunit molecular weight of 45K, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after photoaffinity labeling using [3H]cytochalasin B as a probe.

Reconstitution of Golgi vesicle CMP-sialic acid and adenosine 3'-phosphate 5'-phosphosulfate transport into proteoliposomes

We have previously shown that Golgi apparatus vesicles transport nucleotide sugars and nucleotide sulfate into their lumen. These transport activities are organelle and substrate specific and are characterized by apparent Km for nucleotide derivatives in the low micromolar range. As part of our goal of purifying and characterizing the above transport proteins, we have reconstituted a protein extract from rat liver Golgi membranes into phosphatidylcholine liposomes. The resulting proteoliposomes transport CMP-N-acetylneuraminic acid (CMP-AcNeu) and adenosine 3'-phosphate 5'-phosphosulfate with very similar affinity and inhibition characteristics as intact Golgi vesicles. Sialic acid and sodium sulfate, which are transported only very slowly into the lumen of Golgi vesicles, are transported at low rates by the reconstituted proteoliposomes. Neither rough endoplasmic reticulum-derived vesicles nor proteoliposomes made from proteins of the rough endoplasmic reticulum transport CMP-AcNeu. The above results demonstrate that this reconstituted system can be used for further purification and characterization of nucleotide sugar and nucleotide sulfate translocator proteins. This approach should also be useful to study membrane transport proteins of lysosomes and endosomes.

Expression of the human erythrocyte glucose transporter in Escherichia coli

The gene encoding the human erythrocyte glucose transporter, cloned from HepG2 hepatoma cells, was expressed in Escherichia coli by introducing a prokaryote-type ribosome binding site, subcloning the gene into the T7 promoter/T7 polymerase expression system, and transforming a strain that is defective in glucose transport. Cells bearing plasmids with the transporter gene take up 2-deoxy-D-glucose and D-glucose, unlike cells bearing plasmids without the transporter gene. Moreover, 2-deoxy-D-glucose uptake is inhibited by unlabeled D-glucose, cytochalasin B, or mercuric chloride but not by L-glucose. The glucose transport protein is inserted into the membrane of E. coli, as evidenced by immunoblotting experiments with two site-directed polyclonal antibodies, one directed against the COOH terminus of the glucose transporter and the other directed against a synthetic peptide containing amino acid residues 225-238. As detected with both antibodies, the protein migrates with apparent molecular mass of 34 kDa in sodium dodecyl sulfate/12% polyacrylamide, a size similar to that of the unglycosylated glucose-transport protein synthesized in vitro.

A novel dinitrophenylglutathione-stimulated ATPase is present in human erythrocyte membranes

Vesicles prepared from human erythrocyte membranes were found to catalyze ATP hydrolysis that was stimulated by dinitrophenylglutathione (Dnp-SG). This activity was dependent on temperature and Mg2+ and independent of ion pump ATPases present in erythrocyte membranes. The activity was a linear function of protein and time up to 60 min. The Km values of ATPase for Dnp-SG and ATP were found to be 49 microM and 1.67 mM, respectively. This suggests that in erythrocytes, the transport of Dnp-SG requires direct enzymatic hydrolysis of ATP and both Dnp-SG-stimulated ATPase activity and the ATP-dependent efflux of Dnp-SG from erythrocytes represent different activities of the same protein.

The sodium channel of excitable and non-excitable cells

Excitation and conduction in the majority of excitable cells, as originally described in the squid axon, are initiated by a transient and highly selective increase of the membrane Na conductance, which allows this ion to move passively down its electrochemical gradient (Hodgkin & Katz, 1949; Hodgkin & Huxley, 1952). The term ‘Na channel’ was introduced to describe the mechanism involved in this conductance change (Hodgkin & Keynes, 1955).

A proton NMR study of the mechanism of the erythrocyte glucose transporter

A generalizable 1H NMR technique is developed and used to monitor beta-D-glucose binding to glucose transport sites on erythrocyte membranes. This technique provides resolution of beta-D-glucose binding sites on opposite sides of the membrane, thereby enabling study of recruitment of transport sites from one side of the membrane to the other. Cytochalasin B, which competitively and specifically inhibits glucose binding to the inward-facing glucose transport site, recruits all glucose transport sites on both sides of the membrane to the inward-facing conformation. This result strongly supports a one-site model in which a single transport site alternates between distinct inward- and outward-facing conformations. The rate-limiting step in the transport process is translocation of the transport site between the two conformations, since the beta-D-glucose binding and dissociation events at both the inward- and outward-facing transport sites are shown to be fast compared to the known turnover rate of the glucose transport cycle. A model is presented for the transport machinery in which the glucose molecule binds in a cleft between channel-forming transmembrane helices, and during the transport event a sliding barrier moves past the transport site, thereby exposing the site to the opposite solution compartment.