An ATP-modulated specific association of glyceraldehyde-3-phosphate dehydrogenase with human erythrocyte glucose transporter

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

Aromatic hydrocarbons upregulate glyceraldehyde-3-phosphate dehydrogenase and induce changes in actin cytoskeleton. Role of the aryl hydrocarbon receptor (AhR)

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme involved in several cellular functions including glycolysis, membrane transport, microtubule assembly, DNA replication and repair, nuclear RNA export, apoptosis, and the detection of nitric oxide stress. Therefore, modifications in the regulatory ability and function of GAPDH may alter cellular homeostasis. We report here that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and beta-naphthoflavone, which are well-known ligands for the aryl hydrocarbon receptor (AhR), increase GAPDH mRNA levels in vivo and in vitro, respectively. These compounds fail to induce GAPDH transcription in an AhR-null mouse model, suggesting that the increase in GAPDH level is dependent upon AhR activation. To analyse the consequences of AhR ligands on GAPDH function, mice were treated with TCDD and the level of liver activity of GAPDH was determined. The results showed that TCDD treatment increased GAPDH activity. On the other hand, treatment of Hepa-1 cells with beta-naphthoflavone leads to an increase in microfilament density when compared to untreated cultures. Collectively, these results suggest that AhR ligands, such as polycyclic hydrocarbons, can modify GAPDH expression and, therefore, have the potential to alter the multiple functions of this enzyme.

GAPDH binds GLUT4 reciprocally to hexokinase-II and regulates glucose transport activity

Dietary glucose is taken up by skeletal muscle through GLUT4 (glucose transporter 4). We recently identified by MS proteins displaying insulin-dependent co-precipitation with Myc-tagged GLUT4 from L6 myotubes, including GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and HKII (hexokinase-II). In the present paper we explored whether GAPDH and HKII interact directly with cytoplasmic regions of GLUT4 and their possible inter-relationship. Endogenous and recombinant GAPDH and HKII bound to a chimeric protein linearly encoding all three cytosolic domains of GLUT4 [GST (glutathione-transferase)-GLUT4-cyto]. Both proteins bound to a lesser extent the middle cytosolic loop but not individual N- or C-terminal domains of GLUT4. Purified GAPDH and HKII competed for binding to GST-GLUT4-cyto; ATP increased GAPDH binding and decreased HKII binding to this construct. The physiological significance of the GAPDH-GLUT4 interaction was explored by siRNA (small interfering RNA)-mediated GAPDH knockdown. Reducing GAPDH expression by 70% increased HKII co-precipitation with GLUT4-Myc from L6 cell lysates. GAPDH knockdown had no effect on surface-exposed GLUT4-Myc in basal or insulin-stimulated cells, but markedly and selectively diminished insulin-stimulated 3-O-methyl glucose uptake and GLUT4-Myc photolabelling with ATB-BMPA {2-N-[4-(1-azitrifluoroethyl)benzoyl]-1,3-bis-(D-mannos-4-yloxy)-2-propylamine}, suggesting that the exofacial glucose-binding site was inaccessible. The results show that GAPDH and HKII reciprocally interact with GLUT4 and suggest that these interactions regulate GLUT4 intrinsic activity in response to insulin.

Glyceraldehyde 3-phosphate dehydrogenase is a cellular target of the insulin mimic demethylasterriquinone B1

This study was undertaken to identify cellular proteins that bind an orally active natural product insulin mimic. Phage display cloning was used with a biotinylated derivative of this molecule as bait. Among the proteins identified was glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which has recently been shown to affect insulin receptor signaling. Binding data support a role for human GAPDH as another target of the insulin mimic, which could explain its action as a selective insulin receptor modulator.

Glycolysis and glutamate accumulation into synaptic vesicles. Role of glyceraldehyde phosphate dehydrogenase and 3-phosphoglycerate kinase

Glucose is the major source of brain energy and is essential for maintaining normal brain and neuronal function. Hypoglycemia causes impaired synaptic transmission. This occurs even before significant reduction in global cellular ATP concentration, and relationships among glycolysis, ATP supply, and synaptic transmission are not well understood. We demonstrate that the glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate kinase (3-PGK) are enriched in synaptic vesicles, forming a functional complex, and that synaptic vesicles are capable of accumulating the excitatory neurotransmitter glutamate by harnessing ATP produced by vesicle-bound GAPDH/3-PGK at the expense of their substrates. The GAPDH inhibitor iodoacetate suppressed GAPDH/3-PGK-dependent, but not exogenous ATP-dependent, [(3)H]glutamate uptake into isolated synaptic vesicles. It also decreased vesicular [(3)H]glutamate content in the nerve ending preparation synaptosome; this decrease was reflected in reduction of depolarization-induced [(3)H]glutamate release. In contrast, oligomycin, a mitochondrial ATP synthase inhibitor, had minimal effect on any of these parameters. ADP at concentrations above 0.1 mm inhibited vesicular glutamate and dissipated membrane potential. This suggests that the coupled GAPDH/3-PGK system, which converts ADP to ATP, ensures maximal glutamate accumulation into presynaptic vesicles. Together, these observations provide insight into the essential nature of glycolysis in sustaining normal synaptic transmission.

HSP90, HSP70, and GAPDH directly interact with the cytoplasmic domain of macrophage scavenger receptors

The macrophage scavenger receptor (MSR) is a trimeric membrane protein which binds to modified low-density lipoprotein (LDL) and has been indicated in the development of atherosclerosis. It has recently been demonstrated that the N-terminal cytoplasmic domain of MSR has an important role in the efficient internalization and cell-surface expression of the receptor. This study shows that the N-terminal cytoplasmic domain in bovine was constructed using a peptide architecture technique in which the peptide chain was bundled at their C-terminus to yield a trimeric form and that this did not form an ordered structure. Furthermore, the binding proteins to the cytoplasmic domain of MSR were determined for the first time using a peptide affinity column. Sequence analyses of the specific binding proteins in bovine revealed that heat shock protein 90 (HSP90), heat shock protein 70 (HSP70), leucine aminopeptidase (LAP), adenocylhomocysteinase, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were included. GST-pull-down assay and immunoprecipitation analyses on HSP90, HSP70, and GAPDH showed that all these proteins could bind to the cytoplasmic domain of MSR in vitro and in vivo. These proteins interact with the cytoplasmic domain directly and may have an effect on the functions of MSR such as internalization, cell-surface expression, and signal transduction.

The predicted ATP-binding domains in the hexose transporter GLUT1 critically affect transporter activity

The glucose transporter GLUT1 has three short amino acid sequences (domains I-III) with homology to typical ATP-binding domains. GLUT1 is a facilitative transporter, however, and transports its substrates down a concentration gradient without a specific requirement for energy or hydrolysis of ATP. Therefore, we assessed the functional role of the predicted ATP-binding domains in GLUT1 by site-directed mutagenesis and expression in Xenopus oocytes. For each mutant, we determined the level of protein expression and the kinetics of transport under zero-trans influx, zero-trans efflux, and equilibrium exchange conditions. Although all five mutants were expressed at levels similar to that of the wild-type GLUT1, each single amino acid change in domains I or III profoundly affected GLUT1 function. The mutants Gly116-->Ala in domain I and Gly332-->Ala in domain III exhibited only 10-20% of the transport activity of the wild-type GLUT1. The mutants Gly111-->Ala in domain I and Leu336-->Ala in domain III showed altered kinetic properties; neither the apparent Km nor the Vmax for 3-methylglucose transport were increased under equilibrium exchange conditions, and they did not show the expected level of countertransport acceleration. The mutant Lys117-->Arg in domain I showed a marked increase in the apparent Km for 3-methylglucose transport under zero-trans efflux and equilibrium exchange conditions while maintaining countertransport acceleration. These results indicate that the predicted ATP-binding domains I and III in GLUT1 are important components of the region in GLUT1 involved in transport of the substrate and that their integrity is critical for maintaining the activity and kinetic properties of the transporter.

Direct inhibition of the hexose transporter GLUT1 by tyrosine kinase inhibitors

The facilitative hexose transporter GLUT1 is a multifunctional protein that transports hexoses and dehydroascorbic acid, the oxidized form of vitamin C, and interacts with several molecules structurally unrelated to the transported substrates. Here we analyzed in detail the interaction of GLUT1 with a group of tyrosine kinase inhibitors that include natural products of the family of flavones and isoflavones and synthetic compounds such as the tyrphostins. These compounds inhibited, in a dose-dependent manner, the transport of hexoses and dehydroascorbic acid in human myeloid HL-60 cells, in transfected Chinese hamster ovary cells overexpressing GLUT1, and in normal human erythrocytes, and blocked the glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts. Kinetic analysis of transport data indicated that only tyrosine kinase inhibitors with specificity for ATP binding sites inhibited the transport activity of GLUT1 in a competitive manner. In contrast, those inhibitors that are competitive with tyrosine but not with ATP failed to inhibit hexose uptake or did so in a noncompetitive manner. These results, together with recent evidence demonstrating that GLUT1 is a nucleotide binding protein, support the concept that the inhibitory effect on transport is related to the direct interaction of the inhibitors with GLUT1. We conclude that predicted nucleotide-binding motifs present in GLUT1 are important for the interaction of the tyrosine kinase inhibitors with the transporter and may participate directly in the binding transport of substrates by GLUT1.

Characterization and partial purification of liver glucose transporter GLUT2

GLUT2, the major facilitative glucose transporter isoform expressed in hepatocytes, pancreatic beta-cells, and absorptive epithelial cells, is unique not only with its low affinity and broad substrate specificity as a glucose transporter, but also with its implied function as a glucose-sensor. As a first essential step toward structural and biochemical elucidation of these unique, GLUT2 functions, we describe here the differential solubilization and DEAE-column chromatography of rat hepatocyte GLUT2 protein and its reconstitution into liposomes. The reconstituted GLUT2 bound cytochalasin B in a saturable manner with an apparent dissociation constant (K(d)) of 2.3 x 10(-6) M and a total binding capacity (B(T)) of 8.1 nmol per mg protein. The binding was completely abolished by 2% mercury chloride, but not affected by cytochalasin E. Significantly, the binding was also not affected by 500 mM D-glucose or 3-O-methyl D-glucose (3OMG). The purified GLUT2 catalyzed mercury chloride-sensitive 3OMG uptake, and cytochalasin B inhibited this 3OMG uptake. The inhibition was dose-dependent with respect to cytochalasin B, but was independent of 3OMG concentrations. These findings demonstrate that our solubilized GLUT2 reconstituted in liposomes is at least 60% pure and functional, and that GLUT2 is indeed unique in that its cytochalasin B binding is not affected by its substrate (D-glucose) binding. Our partially purified GLUT2 reconstituted in vesicles will be useful in biochemical and structural elucidation of GLUT2 as a glucose transporter and as a possible glucose sensor.

ATP-dependent substrate occlusion by the human erythrocyte sugar transporter

Human erythrocyte sugar transport presents a functional complexity that is not explained by existing models for carrier-mediated transport. It has been suggested that net sugar uptake is the sum of three serial processes: sugar translocation, sugar interaction with an intracellular binding complex, and the release from this complex into bulk cytosol. The present study was carried out to identify the erythrocyte sugar binding complex, to determine whether sugar binding occurs inside or outside the cell, and to determine whether this binding complex is affected by cytosolic ATP or transporter quaternary structure. Sugar binding assays using cells and membrane protein fractions indicate that sugar binding to erythrocytes is quantitatively accounted for by sugar binding to the hexose transport protein, GluT1. Kinetic analysis of net sugar fluxes indicates that GluT1 sugar binding sites are cytoplasmic. Intracellular ATP increases GluT1 sugar binding capacity from 1 to 2 mol of 3-O-methylglucose/mol GluT1 and inhibits the release of bound sugar into cytosol. Reductant-mediated, tetrameric GluT1 dissociation into dimeric GluT1 is associated with the loss of ATP and 3-O-methylglucose binding. We propose that sugar uptake involves GluT1-mediated, extracellular sugar translocation into an ATP-dependent cage formed by GluT1 cytoplasmic domains. Caged or occluded sugar has three possible fates: (1) transport out of the cell (substrate cycling); (2) interaction with sugar binding sites within the cage, or (3) release into bulk cytosol. We show how this hypothesis can account for the complexity of erythrocyte sugar transport and its regulation by cytoplasmic ATP.

Glucose transporters and insulin action: some insights into diabetes management

Insulin stimulates glucose uptake in muscle and adipose cells primarily by recruiting GLUT4 from an intracellular storage pool to the plasma membrane. Dysfunction of this process known as insulin resistance causes hyperglycemia, a hallmark of diabetes and obesity. Thus the understanding of the mechanisms underlying this process at the molecular level may give an insight into the prevention and treatment of these health problems. GLUT4 in rat adipocytes, for example, constantly recycles between the cell surface and an intracellular pool by endocytosis and exocytosis, each of which is regulated by an insulin-sensitive and GLUT4-selective sorting mechanism. Our working hypothesis has been that this sorting mechanism includes a specific interaction of a cytosolic protein with the GLUT4 cytoplasmic domain. Indeed, a synthetic peptide of the C-terminal cytoplasmic domain of GLUT4 induces an insulin-like GLUT4 recruitment when introduced in rat adipocytes. Relevance of these observations to a novel euglycemic drug design is discussed.

Protein interactions with the glucose transporter binding protein GLUT1CBP that provide a link between GLUT1 and the cytoskeleton

Subcellular targeting and the activity of facilitative glucose transporters are likely to be regulated by interactions with cellular proteins. This report describes the identification and characterization of a protein, GLUT1 C-terminal binding protein (GLUT1CBP), that binds via a PDZ domain to the C terminus of GLUT1. The interaction requires the C-terminal four amino acids of GLUT1 and is isoform specific because GLUT1CBP does not interact with the C terminus of GLUT3 or GLUT4. Most rat tissues examined contain both GLUT1CBP and GLUT1 mRNA, whereas only small intestine lacked detectable GLUT1CBP protein. GLUT1CBP is also expressed in primary cultures of neurons and astrocytes, as well as in Chinese hamster ovary, 3T3-L1, Madin-Darby canine kidney, Caco-2, and pheochromocytoma-12 cell lines. GLUT1CBP is able to bind to native GLUT1 extracted from cell membranes, self-associate, or interact with the cytoskeletal proteins myosin VI, alpha-actinin-1, and the kinesin superfamily protein KIF-1B. The presence of a PDZ domain places GLUT1CBP among a growing family of structural and regulatory proteins, many of which are localized to areas of membrane specialization. This and its ability to interact with GLUT1 and cytoskeletal proteins implicate GLUT1CBP in cellular mechanisms for targeting GLUT1 to specific subcellular sites either by tethering the transporter to cytoskeletal motor proteins or by anchoring the transporter to the actin cytoskeleton.

Regulation of glyceraldehyde-3-phosphate dehydrogenase in differentiating HD3 cells

Red blood cells usually replenish their ATP pools by glycolysis, fueled by glucose imported via the cell membrane. Mature red cells of some species (e.g. pig, chicken) have, however, been reported to show very low glucose transport. The subject of this study was the possible dependency of the level of a key glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAD) on glucose transporter activity during the maturation of chicken red cells. The chicken pronormoblast cell line, HD3, was used as a model system. These cells contain higher levels of GAD and glucose transporter activities than normal chicken bone marrow cells, but reduce their levels during maturation. In an attempt to assess whether the decrease in GAD activity is regulated by the glucose transport, the chicken GLUT3 expressed under the control of viral promoter was introduced into HD3 cells by retroviral infection (pDOL-cGT3). Upon cell differentiation and maturation, both cells with and without the exogenous transporter decreased GAD activity. Butyric acid did not affect the regulation of GAD activity upon differentiation. These results show that the development of chicken red cells is manifested by reduction of their GAD activity and that this is not affected by their sugar transporter activity. The very low GAD activity in embryonic chicken red cells is thus due to a loss of this activity at an early stage in their development. Because of the very low glucose transport and GAD activities in mature chicken red cells, rates of glycolysis are likely to be low and suggesting an alternative pathway for ATP production in these cells.

A synthetic peptide corresponding to the GLUT4 C-terminal cytoplasmic domain causes insulin-like glucose transport stimulation and GLUT4 recruitment in rat adipocytes

In rat epididymal adipocytes, practically all of the major glucose transporter isoform GLUT4 is constitutively sequestered in intracellular membranes and moves to the plasma membrane in response to insulin, whereas about half of GLUT1, the minor isoform, is constitutively functional at the plasma membrane and thus less affected by insulin. Transfection studies using cells whose glucose transport is normally not regulated by insulin have suggested that the C-terminal cytoplasmic domain of GLUT4 is responsible for its constitutive intracellular sequestration. To test if this was also the case in a classical insulin target cell, we introduced synthetic peptides corresponding to the C-terminal cytoplasmic domain of GLUT4 and GLUT1 (GLUT4C and GLUT1C, respectively) into rat adipocytes and studied their effects on the glucose transport activity and the steady state GLUT4 and GLUT1 distribution between the plasma membrane and intracellular membranes in host cells. GLUT4C introduced into basal adipocytes caused a large (up to 4.5-fold) and dose-dependent increase in the plasma membrane GLUT4, with a proportional reduction in microsomal GLUT4, without affecting GLUT1 distribution. GLUT4C incorporation also caused a large (up to 3-fold) dose-dependent stimulation of 3-O-methyl D-glucose (3OMG) flux in host cells. GLUT4C caused little if any GLUT4 or GLUT1 redistribution and changes in 3OMG flux in insulin-stimulated adipocytes. GLUT1C, on the other hand, did not affect GLUT1 or GLUT4 targeting and 3OMG flux in host cells. These findings not only underscore the importance of the C-terminal cytoplasmic domain of GLUT4 in its constitutive intracellular sequestration in a classical insulin target cell but also suggest the existence of a regulatory protein in adipocytes that interacts with GLUT4 at its cytoplasmic domain, thus participating in the constitutive intracellular sequestration of GLUT4.

Glutathione conjugates recognize the Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase

Leukotriene (LT) C4 and other glutathione conjugates are synthesized intracellularly and then move to the plasma membrane for export. The intracellular proteins that bind these molecules and the significance of these interactions are poorly understood. To identify the binding sites of membrane-associated proteins that recognize these molecules, we utilized photoaffinity probes to label the inner leaflet of erythrocytes. The predominant molecule labeled with S-(p-nitrobenzyl)glutathione-[125I]4-azidosalicylic acid (PNBG-[125I]ASA) or LTC4-[125I]4-azidosalicylic acid (LTC4-[125I]ASA) was 38 kDa. The protein was labeled with PNBG-[125I]ASA, electroblotted to polyvinylidene difluoride membranes, digested in situ with lysyl endopeptidase, and two radiolabeled peptides isolated by reverse phase-high performance liquid chromatography. These contained an identity of 7/11 with amino acids 119-129, and 11/11 with amino acids 67-77 of human liver glyceraldehyde-3-phosphate dehydrogenase (GAPDH), respectively. Photoaffinity labeling with PNBG-[125I]ASA was blocked completely by 100 microM ATP and greater than 50% with 100 microM NAD+. LTC4-[125I]ASA binding to the NAD+ site was confirmed by V8 protease digestion of purified GAPDH labeled with LTC4-[125I]ASA or PNBG-[125I]ASA, with both labels localized to the 6.8-kDa N-terminal fragment. Photoaffinity labeling of HL-60 cells with LTC4-125I-ASA identified GAPDH as the predominant cytoplasmic binding protein in these cells. These data indicate that GAPDH is a membrane-associated and cytoplasmic protein which binds glutathione conjugates including LTC4.

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

Binding of cytosolic proteins to myofibrils in ischemic rat hearts

Myofibrillar proteins (MPs) were extracted from isolated and perfused rat hearts subjected to different periods of ischemia to investigate the occurrence of protein degradation and/or the association of cytosolic proteins with the myofibrillar pellet. A 23-kD band was detected by SDS-PAGE of MPs after 5 minutes of ischemia, with its density gradually increasing to a plateau after 20 minutes. Longer periods of ischemia were associated with the appearance of a 39-kD band. Irrespective of the duration of ischemia, both these bands persisted during reperfusion. A partial proteolytic degradation of troponin T (TnT) and troponin I (TnI) has been claimed to be responsible for the generation of these peptides. However, the N-terminal sequence of the 39-kD band was identical to that of GAPDH, whereas Edman sequencing after pepsin digestion showed that the 23 kD is alpha B-crystallin. The binding of the two cytosolic proteins to myofibrils was confirmed by immunofluorescence analysis on cryosections of ischemic hearts. In vitro studies showed that acidosis was sufficient to induce the binding of alpha B-crystallin, whereas the inhibition of ATP depletion prevented the binding of GAPDH. Thiol oxidation is unlikely to promote GAPDH binding, since perfusion with iodoacetate under aerobic conditions or treatment of homogenates with N-ethylmaleimide or diamide failed to induce GAPDH association with the myofibrils. These changes of the myofibrillar proteins could be considered as intracellular markers of the evolution of the ischemic damage. In addition, the binding of the 23-kD peptide might be involved in alterations of contractility.

Modulation of GLUT1 intrinsic activity in clone 9 cells by inhibition of oxidative phosphorylation

Brief (1-2 h) exposure of Clone 9 cells to inhibitors of oxidative phosphorylation such as azide is known to markedly increase glucose uptake. Clone 9 cells express GLUT1 but not GLUT2, -3, and -4, and the azide effect was not accompanied by any increase in cellular or plasma membrane GLUT1 level. To identify the molecular event underlying this apparent increase in GLUT1 intrinsic activity, we studied the acute effects of azide on the substrate binding activity of GLUT1 in Clone 9 cells by measuring glucose-sensitive cytochalasin B binding. The glucose-displaceable, cytochalasin B binding activity was barely detectable in membranes isolated from Clone 9 cells under control conditions but was readily detectable after a 60-min incubation of cells in the presence of 5 mM azide showing a 3-fold increase in binding capacity with no change in binding affinity. Furthermore, the cytochalasin B binding activity of purified human erythrocyte GLUT1 reconstituted in liposomes was significantly reduced in the presence of cytosol derived from azide-treated Clone 9 cells but not in the presence of cytosol from control cells; this effect was heat-labile and abolished by the presence of the peptide corresponding to the GLUT1 COOH-terminal sequence. These results suggest that a cytosolic protein in Clone 9 cells binds to GLUT1 at its COOH-terminal domain and inhibits its substrate binding and that azide-induced metabolic alteration releases GLUT1 from this inhibitory interaction. Studying the binding of cytosolic proteins derived from 35S-labeled Clone 9 cells to glutathione S-transferase fusion protein containing glucose transporter COOH-terminal sequences, we identified 28- and 70-kDa proteins that bind specifically to the cytoplasmic domain of GLUT1 and GLUT4 in vitro. We also found a 32P-labeled, 85-kDa protein that binds to GLUT4 but not to GLUT1 and only in cytosol derived from azide-treated cells. The roles, if any, of these glucose transporter-binding proteins in the azide-sensitive modulation of GLUT1 substrate binding activity in Clone 9 cells are yet to be determined.

Interaction of facilitative glucose transporter with glucokinase and its modulation by ADP and glucose-6-phosphate

Bacterial glucokinase (GK) binds to purified, human erythrocyte glucose transporter (GT) reconstituted in vesicles. The binding is largely abolished if GT is predigested with trypsin, indicating that GK binds to the cytoplasmic domain of GT. The binding is a saturable function of GK concentration showing two distinct affinities with apparent KD of 0.33 and 5.1 microM. The binding is stimulated by an increasing concentration of ADP with the 50% maximal effect at 5 mM. Glucose-6-phosphate (G6P) also stimulates the binding with a distinct optimum at 25 mM. The binding is stimulated only slightly by ATP. D-glucose has no affect on the binding. KCl enhances the binding with the maximal effect at physiological intracellular concentrations. The binding is sensitive to changes in pH with an optimum at pH 4. The binding causes no detectable functional change in GT. However, the enzymatic activity of GK measured at nanomolar concentrations of GK is significantly greater in the presence of GT vesicles than in its absence or in the presence of protein-free vesicles, indicating that GK interacts with GT at this low concentration range with an apparent KD of 10 mM. Although its physiological significance is not known, the GK-GT interaction in vitro described here suggests that these two proteins may also interact in the cell and regulate carbohydrate metabolism.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Regulation of glucose uptake by stressed cells

Lactate production by BHK cells is stimulated by arsenite, azide, or by infection with Semliki Forest virus (SFV). In the case of arsenite or SFV infection, the increase correlates approximately with the increase in glucose transport as measured by uptake of [3H] deoxy glucose (dGlc); in the case of azide, the increase in lactate production exceeds that of glucose transport. Hence glucose utilization by BHK cells and its stimulation by anaerobic and other types of cellular stress is controlled at least in part at the level of glucose transport. The glucose uptake by BHK cells is also stimulated by serum and by glucose deprivation. In these circumstances, as with arsenite, stimulation is reversible, with t1/2 of 1-2 hours; stimulation is compatible with a translocation of the glucose transporter protein between an intracellular site and the plasma membrane (shown here for serum and previously for arsenite). The surface binding and rate of internalization of [125I]-labelled transferrin and [125I] alpha 2-macroglobulin was studied to determine whether changes in glucose transport are accompanied by changes in the surface concentration or rate of internalization of membrane proteins. The findings indicate that changes in glucose transport do not reflect a consistent and general redistribution of membrane receptors. Taken together, the results are compatible with the proposal that BHK cells exposed to stimuli like insulin or serum, or to stresses like arsenite, azide, SFV infection, or deprivation of glucose, respond in the same manner: namely, by an increased capacity to transport glucose brought about by reversible and specific translocation of the transporter protein from an (inactive) intracellular site to the plasma membrane.

Uniporters and anion antiporters

The past year has seen a flurry of activity in the area of protein-mediated hexose uniport. Topics of interest covered here include: structure-function studies; the interaction of glucose carriers with glycolytic enzymes; regulation of cell surface glucose-carrier concentrations by insulin and the signalling mechanisms involved; and the role of the glucose-carrier isoform, GLUT2, in pancreatic beta-cell glucose-dependent insulin secretion. Nucleoside uniport and Glu-Asp antiport are also discussed briefly.

The isoelectric point of the human red cell glucose transporter

The isoelectric point (pI) of the human red cell glucose transporter (Glut 1) was determined. Inconsistent values of 6.0, 6.4-6.5 and 8 have been reported earlier. Integral membrane proteins from human red cells were analyzed by two-dimensional electrophoresis with isoelectric focusing and sodium dodecyl sulfate gel electrophoresis (2D-PAGE). A zone of monomeric Glut 1 was found at pH 8.7, but most of the Glut 1 focused at pH 6-7 together with the anion transporter and other components. Purified Glut 1 focused only at pH 8.5 +/- 0.2 (S.D., n = 12) and deglycosylated purified Glut 1 only at pH 8.4 +/- 0.1 (n = 5), as shown by 2D-PAGE. The absence of Glut 1 below pH 8 in the latter cases was confirmed by immunoblotting with a monoclonal antibody. Furthermore, Glut 1 was photoaffinity-labelled with [3H]cytochalasin B and subjected to isoelectric focusing in one dimension. The pI of the labelled Glut 1 was 8.6 +/- 0.3 (n = 11). A pI of 9.1 was calculated for the Glut 1 polypeptide on the basis of amino acid composition and pKa values for amino acid side groups. The sialic acid content of the glycosylated transporter from fresh red cells was determined at approximately 2.1 sialic acid residues per transporter, which corresponds to a calculated pI of 8.8. The pI values of other human glucose transporter polypeptides of the facilitative diffusion type (Glut 2, 3, 4 and 5) were calculated at 8.4, 7.4, 7.1 and 6.2, respectively.