Substitution of tyrosine 293 of GLUT1 locks the transporter into an outward facing conformation

Glucose transporters and sodium glucose co-transporters cooperatively import glucose into energy-demanding organs in carcinogenic liver fluke Clonorchis sinensis

BACKGROUND

The liver fluke Clonorchis sinensis imports large amounts of glucose to generate energy and metabolic intermediates through glycolysis. We hypothesized that C. sinensis absorbs glucose through glucose transporters and identified four subtypes of glucose transporter (CsGTP) and one sodium glucose co-transporter (CsSGLT) in C. sinensis.

METHODOLOGY/PRINCIPAL FINDINGS

Expressed sequence tags encoding CsGTPs were retrieved from the C. sinensis transcriptome database, and their full-length cDNA sequences were obtained by rapid amplification of cDNA ends (RACE). The tissue distribution of glucose transporters in C. sinensis adults was determined using immunohistochemical staining. Developmental expression was measured using RT-qPCR. The transport and distribution of glucose into living C. sinensis were monitored using confocal microscopy. Membrane topology and key functional residues of CsGTPs were homologous to their counterparts in animals and humans. CsGTP1, 2, and 4 were transcribed 2.4-5.5 times higher in the adults than metacercariae, while CsGTP3 was transcribed 2.1 times higher in the metacercariae than adults. CsSGLT transcription was 163.6 times higher in adults than in metacercariae. In adults, CsSGLT was most abundant in the tegument; CsGTP3 and CsSGLT were localized in the vitelline gland, uterine wall, eggs, mesenchymal tissue, and testes; CsGTP4 was found in sperm and mesenchymal tissue; and CsGTP1 was mainly in the sperm and testes. In C. sinensis adults, exogenous glucose is imported in a short time and is present mainly in the middle and posterior body, in which the somatic and reproductive organs are located. Of the exogenous glucose, 53.6% was imported through CsSGLT and 46.4% through CsGTPs. Exogenous glucose import was effectively inhibited by cytochalasin B and phlorizin.

CONCLUSIONS/SIGNIFICANCE

We propose that CsSGLT cooperates with CsGTPs to import exogenous glucose from the environmental bile, transport glucose across mesenchymal tissue cells, and finally supply energy-demanding organs in C. sinensis adults. Studies on glucose transporters may pave the way for the development of new anthelmintic drugs.

Enhanced Sampling Molecular Dynamics Simulations Reveal Transport Mechanism of Glycoconjugate Drugs through GLUT1

Glucose transporters GLUT1 belong to the major facilitator superfamily and are essential to human glucose uptake. The overexpression of GLUT1 in tumor cells designates it as a pivotal target for glycoconjugate anticancer drugs. However, the interaction mechanism of glycoconjugate drugs with GLUT1 remains largely unknown. Here, we employed all-atom molecular dynamics simulations, coupled to steered and umbrella sampling techniques, to examine the thermodynamics governing the transport of glucose and two glycoconjugate drugs (i.e., 6-D-glucose-conjugated methane sulfonate and 6-D-glucose chlorambucil) by GLUT1. We characterized the specific interactions between GLUT1 and substrates at different transport stages, including substrate recognition, transport, and releasing, and identified the key residues involved in these procedures. Importantly, our results described, for the first time, the free energy profiles of GLUT1-transporting glycoconjugate drugs, and demonstrated that H160 and W388 served as important gates to regulate their transport via GLUT1. These findings provide novel atomic-scale insights for understanding the transport mechanism of GLUT1, facilitating the discovery and rational design of GLUT1-targeted anticancer drugs.

The role of salt bridge networks in the stability of the yeast hexose transporter 1

BACKGROUND

The yeast S. cerevisiae preferably metabolizes glucose through aerobic glycolysis. Glucose transport is facilitated by multiple hexose transporters (Hxts), and their expression and activity are tightly regulated by multiple mechanisms. However, detailed structural and functional analyses of Hxts remain limited, largely due to the lack of crystal structure.

METHODS

Homology modeling was used to build a 3D structural model for the yeast glucose transporter Hxt1 and investigate the effects of site directed mutations on Hxt1 stability and glucose transport activity.

RESULTS

The conserved salt bridge-forming residues observed in the human Glut4 and the yeast glucose receptor Rgt2 were identified within and between the two 6-transmembrane spanning segments of Hxt1. Most of the RGT2 mutations that disrupt the salt bridge networks were known to cause constitutive signal generation, whereas the corresponding substitutions in HXT1 were shown to decrease Hxt1 stability. While substitutions of the two residues in the salt bridge 2 in Glut4-E329Q and E393D-were reported to abolish glucose transport, the equivalent substitutions in Hxt1 (D382Q and E454D) did not affect Hxt1 glucose transport activity.

CONCLUSIONS

Substitutions of equivalent salt bridge-forming residues in Hxt1, Rgt2, and Glut4 are predicted to lock them in an inward-facing conformation but lead to different functional consequences.

GENERAL SIGNIFICANCE

The salt bridge networks in yeast and human glucose transporters and yeast glucose receptors may play different roles in maintaining their structural and functional integrity.

An in vitro model of glucose transporter 1 deficiency syndrome at the blood-brain barrier using induced pluripotent stem cells

Glucose is an important source of energy for the central nervous system. Its uptake at the blood-brain barrier (BBB) is mostly mediated via glucose transporter 1 (GLUT1), a facilitated transporter encoded by the SLC2A1 gene. GLUT1 Deficiency Syndrome (GLUT1DS) is a haploinsufficiency characterized by mutations in the SLC2A1 gene, resulting in impaired glucose uptake at the BBB and clinically characterized by epileptic seizures and movement disorder. A major limitation is an absence of in vitro models of the BBB reproducing the disease. This study aimed to characterize an in vitro model of GLUT1DS using human pluripotent stem cells (iPSCs). Two GLUT1DS clones were generated (GLUT1-iPSC) from their original parental clone iPS(IMR90)-c4 by CRISPR/Cas9 and differentiated into brain microvascular endothelial cells (iBMECs). Cells were characterized in terms of SLC2A1 expression, changes in the barrier function, glucose uptake and metabolism, and angiogenesis. GLUT1DS iPSCs and iBMECs showed comparable phenotype to their parental control, with exception of reduced GLUT1 expression at the protein level. Although no major disruption in the barrier function was reported in the two clones, a significant reduction in glucose uptake accompanied by an increase in glycolysis and mitochondrial respiration was reported in both GLUT1DS-iBMECs. Finally, impaired angiogenic features were reported in such clones compared to the parental clone. Our study provides the first documented characterization of GLUT1DS-iBMECs generated by CRISPR-Cas9, suggesting that GLUT1 truncation appears detrimental to brain angiogenesis and brain endothelial bioenergetics, but maybe not be detrimental to iBMECs differentiation and barriergenesis. Our future direction is to further characterize the functional outcome of such truncated product, as well as its impact on other cells of the neurovascular unit.

Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS)

The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.

Functional characterization and tissue localization of the facilitative glucose transporter GLUT12

Facilitative glucose transporters (GLUTs) play crucial roles in glucose utilization and homeostasis. GLUT12 was initially isolated as a novel GLUT4-like transporter involved in insulin-dependent glucose transport. However, tissue distribution and biochemical properties of GLUT12 are not well understood. In this study, we investigated the basic kinetic properties and tissue distribution of GLUT12. Human GLUT12 and GLUT1 were overexpressed and purified using Ni-NTA column chromatography. Reconstituted proteoliposomes showed time-dependent d-glucose transport activity, which was inhibited by phloretin and dehydroascorbate. Dose dependence of glucose transport revealed a KM and Vmax values of 6.4 mM and 1.2 μmol/mg/min, respectively, indicating that GLUT12 is a high-affinity type GLUT. Glucose transport by GLUT12 was inhibited by ATP and glucose-1-phosphate, glucose-6-phosphate and disaccharides (properties similar to those of GLUT1). Indirect immunohistochemistry revealed the distribution of mouse GLUT12 in the apical region of distal tubules and collecting ducts in the kidney and epithelial cells of the jejunum. In addition to these cells, GLUT12 was present in chromaffin cells in the adrenal medulla, the anterior pituitary lobe, as well as the thyroid and pyloric glands. These tissue distributions suggest a unique function of GLUT12, besides that of an insulin-dependent glucose transport.

Structure, function and regulation of mammalian glucose transporters of the SLC2 family

The SLC2 genes code for a family of GLUT proteins that are part of the major facilitator superfamily (MFS) of membrane transporters. Crystal structures have recently revealed how the unique protein fold of these proteins enables the catalysis of transport. The proteins have 12 transmembrane spans built from a replicated trimer substructure. This enables 4 trimer substructures to move relative to each other, and thereby alternately opening and closing a cleft to either the internal or the external side of the membrane. The physiological substrate for the GLUTs is usually a hexose but substrates for GLUTs can include urate, dehydro-ascorbate and myo-inositol. The GLUT proteins have varied physiological functions that are related to their principal substrates, the cell type in which the GLUTs are expressed and the extent to which the proteins are associated with subcellular compartments. Some of the GLUT proteins translocate between subcellular compartments and this facilitates the control of their function over long- and short-time scales. The control of GLUT function is necessary for a regulated supply of metabolites (mainly glucose) to tissues. Pathophysiological abnormalities in GLUT proteins are responsible for, or associated with, clinical problems including type 2 diabetes and cancer and a range of tissue disorders, related to tissue-specific GLUT protein profiles. The availability of GLUT crystal structures has facilitated the search for inhibitors and substrates and that are specific for each GLUT and that can be used therapeutically. Recent studies are starting to unravel the drug targetable properties of each of the GLUT proteins.

Comprehensive in silico Study of GLUT10: Prediction of Possible Substrate Binding Sites and Interacting Molecules

OBJECTIVES

The Arterial Tortuosity Syndrome (ATS) is an autosomal recessive connective tissue disorder, mainly characterized by tortuosity and stenosis of the arteries with a propensity towards aneurysm formation and dissection. It is caused by mutations in the SLC2A10 gene that encodes the facilitative glucose transporter GLUT10. The molecules transported by and interacting with GLUT10 have still not been unambiguously identified. Hence, the study attempts to identify both the substrate binding site of GLUT10 and the molecules interacting with this site.

METHODS

As High-resolution X-ray crystallographic structure of GLUT10 was not available, 3D homology model of GLUT10 in open conformation was constructed. Further, molecular docking and bioinformatics investigation were employed.

RESULTS AND DISCUSSION

Blind docking of nine reported potential in vitro substrates with this 3D homology model revealed that substrate binding site is possibly made with PRO531, GLU507, GLU437, TRP432, ALA506, LEU519, LEU505, LEU433, GLN525, GLN510, LYS372, LYS373, SER520, SER124, SER533, SER504, SER436 amino acid residues. Virtual screening of all metabolites from the Human Serum Metabolome Database and muscle metabolites from Human Metabolite Database (HMDB) against the GLUT10 revealed possible substrates and interacting molecules for GLUT10, which were found to be involved directly or partially in ATS progression or different arterial disorders. Reported mutation screening revealed that a highly emergent point mutation (c. 1309G>A, p. Glu437Lys) is located in the predicted substrate binding site region.

CONCLUSION

Virtual screening expands the possibility to explore more compounds that can interact with GLUT10 and may aid in understanding the mechanisms leading to ATS.

Identification of druggable small molecule antagonists of the Plasmodium falciparum hexose transporter PfHT and assessment of ligand access to the glucose permeation pathway via FLAG-mediated protein engineering

Although the Plasmodium falciparum hexose transporter PfHT has emerged as a promising target for anti-malarial therapy, previously identified small-molecule inhibitors have lacked promising drug-like structural features necessary for development as clinical therapeutics. Taking advantage of emerging insight into structure/function relationships in homologous facilitative hexose transporters and our novel high throughput screening platform, we investigated the ability of compounds satisfying Lipinksi rules for drug likeness to directly interact and inhibit PfHT. The Maybridge HitFinder chemical library was interrogated by searching for compounds that reduce intracellular glucose by >40% at 10 μM. Testing of initial hits via measurement of 2-deoxyglucose (2-DG) uptake in PfHT over-expressing cell lines identified 6 structurally unique glucose transport inhibitors. WU-1 (3-(2,6-dichlorophenyl)-5-methyl-N-[2-(4-methylbenzenesulfonyl)ethyl]-1,2-oxazole-4-carboxamide) blocked 2-DG uptake (IC50 = 5.8 ± 0.6 μM) with minimal effect on the human orthologue class I (GLUTs 1-4), class II (GLUT8) and class III (GLUT5) facilitative glucose transporters. WU-1 showed comparable potency in blocking 2-DG uptake in freed parasites and inhibiting parasite growth, with an IC50 of 6.1 ± 0.8 μM and EC50 of 5.5 ± 0.6 μM, respectively. WU-1 also directly competed for N-[2-[2-[2-[(N-biotinylcaproylamino)ethoxy)ethoxyl]-4-[2-(trifluoromethyl)-3H-diazirin-3-yl]benzoyl]-1,3-bis(mannopyranosyl-4-yloxy)-2-propylamine (ATB-BMPA) binding and inhibited the transport of D-glucose with an IC50 of 5.9 ± 0.8 μM in liposomes containing purified PfHT. Kinetic analysis revealed that WU-1 acts as a non-competitive inhibitor of zero-trans D-fructose uptake. Decreased potency for WU-1 and the known endofacial ligand cytochalasin B was observed when PfHT was engineered to contain an N-terminal FLAG tag. This modification resulted in a concomitant increase in affinity for 4,6-O-ethylidene-α-D-glucose, an exofacially directed transport antagonist, but did not alter the Km for 2-DG. Taken together, these data are consistent with a model in which WU-1 binds preferentially to the transporter in an inward open conformation and support the feasibility of developing potent and selective PfHT antagonists as a novel class of anti-malarial drugs.

Chemical biology probes of mammalian GLUT structure and function

The structure and function of glucose transporters of the mammalian GLUT family of proteins has been studied over many decades, and the proteins have fascinated numerous research groups over this time. This interest is related to the importance of the GLUTs as archetypical membrane transport facilitators, as key limiters of the supply of glucose to cell metabolism, as targets of cell insulin and exercise signalling and of regulated membrane traffic, and as potential drug targets to combat cancer and metabolic diseases such as type 2 diabetes and obesity. This review focusses on the use of chemical biology approaches and sugar analogue probes to study these important proteins.

Isolation of state-dependent monoclonal antibodies against the 12-transmembrane domain glucose transporter 4 using virus-like particles

The insulin-responsive 12-transmembrane transporter GLUT4 changes conformation between an inward-open state and an outward-open state to actively facilitate cellular glucose uptake. Because of the difficulties of generating conformational mAbs against complex and highly conserved membrane proteins, no reliable tools exist to measure GLUT4 at the cell surface, follow its trafficking, or detect the conformational state of the protein. Here we report the isolation and characterization of conformational mAbs that recognize the extracellular and intracellular domains of GLUT4, including mAbs that are specific for the inward-open and outward-open states of GLUT4. mAbs against GLUT4 were generated using virus-like particles to present this complex membrane protein in its native conformation and using a divergent host species (chicken) for immunization to overcome immune tolerance. As a result, the isolated mAbs recognize conformational epitopes on native GLUT4 in cells, with apparent affinities as high as 1 pM and with specificity for GLUT4 across the human membrane proteome. Epitope mapping using shotgun mutagenesis alanine scanning across the 509 amino acids of GLUT4 identified the binding epitopes for mAbs specific for the states of GLUT4 and allowed the comprehensive identification of the residues that functionally control the GLUT4 inward-open and outward-open states. The mAbs identified here will be valuable molecular tools for monitoring GLUT4 structure, function, and trafficking, for differentiating GLUT4 conformational states, and for the development of novel therapeutics for the treatment of diabetes.

Molecular Dynamics Simulations of the Human Glucose Transporter GLUT1

Glucose transporters (GLUTs) provide a pathway for glucose transport across membranes. Human GLUTs are implicated in devastating diseases such as heart disease, hyper- and hypo-glycemia, type 2 diabetes and cancer. The human GLUT1 has been recently crystalized in the inward-facing open conformation. However, there is no other structural information for other conformations. The X-ray structures of E. coli Xylose permease (XylE), a glucose transporter homolog, are available in multiple conformations with and without the substrates D-xylose and D-glucose. XylE has high sequence homology to human GLUT1 and key residues in the sugar-binding pocket are conserved. Here we construct a homology model for human GLUT1 based on the available XylE crystal structure in the partially occluded outward-facing conformation. A long unbiased all atom molecular dynamics simulation starting from the model can capture a new fully opened outward-facing conformation. Our investigation of molecular interactions at the interface between the transmembrane (TM) domains and the intracellular helices (ICH) domain in the outward- and inward-facing conformation supports that the ICH domain likely stabilizes the outward-facing conformation in GLUT1. Furthermore, inducing a conformational transition, our simulations manifest a global asymmetric rocker switch motion and detailed molecular interactions between the substrate and residues through the water-filled selective pore along a pathway from the extracellular to the intracellular side. The results presented here are consistent with previously published biochemical, mutagenesis and functional studies. Together, this study shed light on the structure and functional relationships of GLUT1 in multiple conformational states.

The glucose transporter 1 -GLUT1- from the white shrimp Litopenaeus vannamei is up-regulated during hypoxia

During hypoxia the shrimp Litopenaeus vannamei accelerates anaerobic glycolysis to obtain energy; therefore, a correct supply of glucose to the cells is needed. Facilitated glucose transport across the cells is mediated by a group of membrane embedded integral proteins called GLUT; being GLUT1 the most ubiquitous form. In this work, we report the first cDNA nucleotide and deduced amino acid sequences of a glucose transporter 1 from L. vannamei. A 1619 bp sequence was obtained by RT-PCR and RACE approaches. The 5´ UTR is 161 bp and the poly A tail is exactly after the stop codon in the mRNA. The ORF is 1485 bp and codes for 485 amino acids. The deduced protein sequence has high identity to GLUT1 proteins from several species and contains all the main features of glucose transporter proteins, including twelve transmembrane domains, the conserved motives and amino acids involved in transport activity, ligands binding and membrane anchor. Therefore, we decided to name this sequence, glucose transporter 1 of L. vannamei (LvGLUT1). A partial gene sequence of 8.87 Kbp was also obtained; it contains the complete coding sequence divided in 10 exons. LvGlut1 expression was detected in hemocytes, hepatopancreas, intestine gills, muscle and pleopods. The higher relative expression was found in gills and the lower in hemocytes. This indicates that LvGlut1 is ubiquitously expressed but its levels are tissue-specific and upon short-term hypoxia, the GLUT1 transcripts increase 3.7-fold in hepatopancreas and gills. To our knowledge, this is the first evidence of expression of GLUT1 in crustaceans.

FGT-1 is the major glucose transporter in C. elegans and is central to aging pathways

Caenorhabditis elegans is widely used as a model for investigation of the relationships between aging, nutrient restriction and signalling via the DAF-2 (abnormal dauer formation 2) receptor for insulin-like peptides and AGE-1 [ageing alteration 1; orthologue of PI3K (phosphoinositide 3-kinase)], but the identity of the glucose transporters that may link these processes is unknown. We unexpectedly find that of the eight putative GLUT (glucose transporter)-like genes only the two splice variants of one gene have a glucose transport function in an oocyte expression system. We have named this gene fgt-1 (facilitated glucose transporter, isoform 1). We show that knockdown of fgt-1 RNA leads to loss of glucose transport and reduced glucose metabolism in wild-type worms. The FGT-1 glucose transporters of C. elegans thus play a key role in glucose energy supply to C. elegans. Importantly, knockdown of fgt-1 leads to an extension of lifespan equivalent, but not additive, to that observed in daf-2 and age-1 mutant worms. The results of the present study are consistent with DAF-2 and AGE-1 signalling stimulating glucose transport in C. elegans and this process being associated with the longevity phenotype in daf-2 and age-1 mutant worms. We propose that fgt-1 constitutes a common axis for the lifespan extending effects of nutrient restriction and reduced insulin-like peptide signalling.

Homology modeling of GLUT4, an insulin regulated facilitated glucose transporter and docking studies with ATP and its inhibitors

GLUT4 is a 12 transmembrane (TM) protein belonging to the Class I facilitated glucose transporter family that transports glucose into the cells in an insulin regulated manner. GLUT4 plays a key role in the maintenance of blood glucose homeostasis and inhibition of glucose transporter activity may lead to insulin resistance, hallmark of type 2 diabetes. No crystal structure data is available for any members of the facilitated glucose transporter family. Here, in this paper, we have generated a homology model of GLUT4 based on experimental data available on GLUT1, a Class I facilitated glucose transporter and the crystal structure data obtained from the Glycerol 3-phosphate transporter. The model identified regions in GLUT4 that form a channel for the transport of glucose along with the substrate interacting residues. Docking and electrostatic potential data analysis of GLUT4 model has mapped an ATP binding region close to the binding site of cytochalasin B and genistein, two GLUT4 inhibitors, and this may explain the mechanism by which these inhibitors could potentially affect the GLUT4 function.

Elucidation of the glucose transport pathway in glucose transporter 4 via steered molecular dynamics simulations

BACKGROUND

GLUT4 is a predominant insulin regulated glucose transporter expressed in major glucose disposal tissues such as adipocytes and muscles. Under the unstimulated state, GLUT4 resides within intracellular vesicles. Various stimuli such as insulin translocate this protein to the plasma membrane for glucose transport. In the absence of a crystal structure for GLUT4, very little is known about the mechanism of glucose transport by this protein. Earlier we proposed a homology model for GLUT4 and performed a conventional molecular dynamics study revealing the conformational rearrangements during glucose and ATP binding. However, this study could not explain the transport of glucose through the permeation tunnel.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the molecular mechanism of glucose transport and its energetic, a steered molecular dynamics study (SMD) was used. Glucose was pulled from the extracellular end of GLUT4 to the cytoplasm along the pathway using constant velocity pulling method. We identified several key residues within the tunnel that interact directly with either the backbone ring or the hydroxyl groups of glucose. A rotation of glucose molecule was seen near the sugar binding site facilitating the sugar recognition process at the QLS binding site.

CONCLUSIONS/SIGNIFICANCE

This study proposes a possible glucose transport pathway and aids the identification of several residues that make direct interactions with glucose during glucose transport. Mutational studies are required to further validate the observation made in this study.

Hexose transporter GLUT1 harbors several distinct regulatory binding sites for flavones and tyrphostins

The facilitative hexose transporter GLUT1 activity is blocked by tyrosine kinase inhibitors that include natural products such as flavones and isoflavones and synthetic compounds such as tyrphostins, molecules that are structurally unrelated to the transported substrates [Vera, et al. (2001) Biochemistry, 40, 777-790]. Here we analyzed the interaction of GLUT1 with quercetin (a flavone), genistein (an isoflavone), and tyrphostin A47 and B46 to evaluate if they share one common or have several binding sites on the protein. Kinetic assays showed that genistein, quercetin, and tyrphostin B46 behave as competitive inhibitors of equilibrium exchange and zero-trans uptake transport and noncompetitive inhibitors of net sugar exit out of human red cells, suggesting that they interact with the external surface of the GLUT1 molecule. In contrast, tyrphostin A47 was a competitive inhibitor of equilibrium exchange and zero-trans exit transport and a noncompetitive inhibitor of net sugar entry into red cells, suggesting that it interacts with the cytoplasmic surface of the transporter. Genistein protected GLUT1 against iodide-elicited fluorescence quenching and also decreased the affinity of d-glucose for its external binding site, while quercetin and tyrphostins B46 and A47 promoted fluorescence quenching and did not affect the external d-glucose binding site. These findings are explained by a carrier that presents at least three binding sites for tyrosine kinase inhibitors, in which (i) genistein interacts with the transporter in a conformation that binds glucose on the external surface (outward-facing conformation), in a site which overlaps with the external binding site for d-glucose, (ii) quercetin and tyrphostin B46 interact with the GLUT1 conformation which binds glucose by the internal side of the membrane (inward-facing conformation), but to a site accessible from the external surface of the protein, and (iii) the binding site for tyrphostin A47 is accessible from the inner surface of GLUT1 by binding to the inward-facing conformation of the transporter. These data provide groundwork for a molecular understanding of how the tyrosine kinase inhibitors directly affect glucose transport in animal cells.

Molecular dynamics simulation studies of GLUT4: substrate-free and substrate-induced dynamics and ATP-mediated glucose transport inhibition

Background

Glucose transporter 4 (GLUT4) is an insulin facilitated glucose transporter that plays an important role in maintaining blood glucose homeostasis. GLUT4 is sequestered into intracellular vesicles in unstimulated cells and translocated to the plasma membrane by various stimuli. Understanding the structural details of GLUT4 will provide insights into the mechanism of glucose transport and its regulation. To date, a crystal structure for GLUT4 is not available. However, earlier work from our laboratory proposed a well validated homology model for GLUT4 based on the experimental data available on GLUT1 and the crystal structure data obtained from the glycerol 3-phosphate transporter.

Methodology/Principal Findings

In the present study, the dynamic behavior of GLUT4 in a membrane environment was analyzed using three forms of GLUT4 (apo, substrate and ATP-substrate bound states). Apo form simulation analysis revealed an extracellular open conformation of GLUT4 in the membrane favoring easy exofacial binding of substrate. Simulation studies with the substrate bound form proposed a stable state of GLUT4 with glucose, which can be a substrate-occluded state of the transporter. Principal component analysis suggested a clockwise movement for the domains in the apo form, whereas ATP substrate-bound form induced an anti-clockwise rotation. Simulation studies suggested distinct conformational changes for the GLUT4 domains in the ATP substrate-bound form and favor a constricted behavior for the transport channel. Various inter-domain hydrogen bonds and switching of a salt-bridge network from E345-R350-E409 to E345-R169-E409 contributed to this ATP-mediated channel constriction favoring substrate occlusion and prevention of its release into cytoplasm. These data are consistent with the biochemical studies, suggesting an inhibitory role for ATP in GLUT-mediated glucose transport.

Conclusions/Significance

In the absence of a crystal structure for any glucose transporter, this study provides mechanistic details of the conformational changes in GLUT4 induced by substrate and its regulator.

Defining topological features of membrane proteins by nanoelectrospray ionisation mass spectrometry

The D-galactose-H(+) symport protein, GalP, of Escherichia coli is the bacterial homologue of the human glucose transport protein, GLUT1. Here we demonstrate that mass spectrometry can be used to map modification by covalently bound reagents, and also to detect structural changes in the GalP protein that occur upon substrate binding. The small thiol-group-specific reagent N-ethylmaleimide (NEM) was used to modify the cysteine residues in GalP(His)(6) both alone and in the presence of D-glucose, a known substrate. Employing a mixture of proteolysis and thermal degradation methods, the three cysteine residues were found to undergo sequential reactions with NEM, with Cys374 being modified first, followed by Cys389 and finally Cys19, thus indicating their different accessibilities within the three-dimensional structure of the protein. Prior binding of the substrate D-glucose to the protein protected Cys19 and Cys374 against NEM modification, but not Cys389. Cys374 had been expected to be shielded by D-glucose binding while Cys389 had been expected to be unaffected, consistent with their proposed respective locations in the vicinity of, and distant from, the sugar binding site. However, the inaccessibility of Cys19 was unexpected and suggests a structural change in the protein promoted by D-glucose binding which changes the proximity of Cys19 with respect to the D-glucose-binding site.

Functional studies of the T295M mutation causing Glut1 deficiency: glucose efflux preferentially affected by T295M

Glucose transporter type 1 (Glut1) deficiency syndrome (Glut1 DS, OMIM: #606777) is characterized by infantile seizures, acquired microcephaly, developmental delay, hypoglycorrhachia (CSF glucose <40 mg/dL), and decreased erythrocyte glucose uptake (56.1 +/- 17% of control). Previously, we reported two patients with a mild Glut1 deficiency phenotype associated with a heterozygous GLUT1 T295M mutation and normal erythrocyte glucose uptake. We assessed the pathogenicity of T295M in the Xenopus laevis oocyte expression system. Under zero-trans influx conditions, the T295M Vmax (590 pmol/min/oocyte) was 79% of the WT value and the Km (14.3 mM) was increased compared with WT (9.6 mM). Under zero-trans efflux conditions, both the Vmax (1216 pmol/min/oocyte) and Km (8.8 mM) in T295M mutant Glut1 were markedly decreased in comparison to the WT values (7443 pmol/min/oocyte and 90.8 mM). Western blot analysis and confocal studies confirmed incorporation of the T295M mutant protein into the plasma membrane. The side chain of M295 is predicted to block the extracellular "gate" for glucose efflux in our Glut-1 molecular model. We conclude that the T295M mutation specifically alters Glut1 conformation and asymmetrically affects glucose flux across the cell by perturbing efflux more than influx. These findings explain the seemingly paradoxical findings of Glut1 DS with hypoglycorrhachia and "normal" erythrocyte glucose uptake.

Glucose transporter 8 (GLUT8) from the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae)

We have cloned the fire ant glucose transporter 8 (GLUT8) cDNA providing the first molecular characterization of a GLUT8 in insects. Glucose is a poly-alcohol and, due to its high hydrophilicity, cannot move across cell membranes. GLUT8 is a putative facilitative transporter for the cellular import and export of glucose. The complete 2,974-bp cDNA encodes a 501-residue protein with a predicted molecular mass of 54.8 kDa. Transcripts were detected in the brain, midgut, hindgut, Malpighian tubule, fat body, ovary, and testis. The highest transcriptional expression was found in fat body. Northern blot analysis revealed different transcript sizes in mated queen brains, alate female ovaries, and male testes. We propose that four other sequences obtained from insect genome projects from the honey bee Apis mellifera (ENSAPMP00000006624), the malaria mosquito Anopheles gambiae (EAA11842), and the fruit fly Drosophila melanogaster (AAQ23604 and AAM52591) are likely the orthologues of the fire ant GLUT8. Phylogenetic relationships in insect glucose transporters are presented.

The p38 mitogen-activated protein kinase inhibitor SB203580 reduces glucose turnover by the glucose transporter-4 of 3T3-L1 adipocytes in the insulin-stimulated state

Insulin induces a profound increase in glucose uptake in 3T3-L1 adipocytes through the activity of the glucose transporter-4 (GLUT4). Apart from GLUT4 translocation toward the plasma membrane, there is also an insulin-induced p38 MAPK-dependent step involved in the regulation of glucose uptake. Consequently, treatment with the p38 MAPK inhibitor SB203580 reduces insulin-induced glucose uptake by approximately 30%. Pretreatment with SB203580 does not alter the apparent K(m) of GLUT4-mediated glucose uptake but reduces the maximum velocity by approximately 30%. Insulin-induced GLUT4 translocation and exposure of the transporter to the extracellular environment was not altered by pretreatment with SB203580, as evidenced by a lack of effect of the inhibitor on the amount of GLUT4 present in the plasma membrane, as assessed by subcellular fractionation, the amount of GLUT4 that is able to undergo biotinylation on intact adipocytes and the level of extracellular exposure of an ectopically expressed GLUT-green fluorescence protein construct with a hemagglutinin tag in its first extracellular loop. In contrast, labeling of GLUT4 after insulin stimulation by a membrane-impermeable, mannose moiety-containing, photoaffinity-labeling agent [2-N-4(1-azido-2,2,2-trifluoroethyl)benzoyl-1,3-bis(d-mannose-4-yloxy)-2-propylamine] that binds to the extracellular glucose acceptor domain was markedly reduced by SB203580, although photolabeling with this compound in the absence of insulin was unaffected by SB203580. These data suggest that SB203580 affects glucose turnover by the insulin-responsive GLUT4 transporter in 3T3-L1 adipocytes.

Molecular cloning of a member of the facilitative glucose transporter gene family GLUT11 (SLC2A11) and identification of transcription variants

We isolated a member of the facilitative glucose transporter (GLUT) gene family (GLUT11; SLC2A11 as a HGMW-approved symbol) based on the analysis of a human genomic BAC clone KB1125A3 located on band q11.2 of human chromosome 22. The gene GLUT11/SLC2A11 consists of 12 exons spanning over 29 kb in size and is located between two genes, SMARCB1 and MIF. The deduced amino acid sequence indicated the topological features of transmembrane helices and sequence motifs which are common to the GLUT protein family. The cDNA cloning revealed the presence of three types of variation in its transcripts. The first variation is caused by the existence of three distinct first exons (SLC2A11-a, -b, and -c). PCR analysis of multi-tissue-derived cDNA panels indicated the differential expression of these transcript variants. The second variation is caused by skipping over one exon (exon 6). The third variation is caused by the premature transcription termination at a site between exon 8 and exon 9. Both exon skipping and premature termination caused frameshift, resulting in the production of truncated GLUT11/SLC2A11 transcripts. These results suggested that transcription of GLUT11/SCL2A11 gene is controlled in a complex manner.

A three-dimensional model of the human facilitative glucose transporter Glut1

The human facilitative transporter Glut1 is the major glucose transporter present in all human cells, has a central role in metabolism, and is an archetype of the superfamily of major protein facilitators. Here we describe a three-dimensional structure of Glut1 based on helical packing schemes proposed for lactose permease and Glut1 and predictions of secondary structure, and refined using energy minimization, molecular dynamics simulations, and quality and environmental scores. The Ramachandran scores and the stereochemical quality of the structure obtained were as good as those for the known structures of the KcsA K(+) channel and aquaporin 1. We found two channels in Glut1. One of them traverses the structure completely, and is lined by many residues known to be solvent-accessible. Since it is delimited by the QLS motif and by several well conserved residues, it may serve as the substrate transport pathway. To validate our structure, we determined the distance between these channels and all the residues for which mutations are known. From the locations of sugar transporter signatures, motifs, and residues important to the transport function, we find that this Glut1 structure is consistent with mutagenesis and biochemical studies. It also accounts for functional deficits in seven pathogenic mutants.

Structural analysis of the GLUT1 facilitative glucose transporter (review)

The structure of the human erythrocyte facilitative glucose transporter (GLUT1) has been intensively investigated using a wide array of chemical and biophysical approaches. Despite the lack of a crystal structure for any of the facilitative monosaccharide transport proteins, detailed information regarding primary and secondary structure, membrane topology, transport kinetics, and functionally important residues has allowed the construction of a sophisticated working model for GLUT1 tertiary structure. The existing data support the formation of a central aqueous channel formed by the juxtaposition of several amphipathic transmembrane-spanning alpha-helices. The results of extensive mutational analysis of GLUT1 have elucidated many of the structural determinants of the glucose permeation pathway. Continued application of currently available technologies will allow further refinement of this working model. In addition to providing insights into the molecular basis of both normal and disordered glucose homeostasis, this detailed understanding of structure/function relationships within GLUT1 can provide a basis for understanding transport carried out by other members of the major facilitator superfamily.

GLUT8, a novel member of the sugar transport facilitator family with glucose transport activity

GLUT8 is a novel glucose transporter-like protein that exhibits significant sequence similarity with the members of the sugar transport facilitator family (29.4% of amino acids identical with GLUT1). Human and mouse sequence (86.2% identical amino acids) comprise 12 putative membrane-spanning helices and several conserved motifs (sugar transporter signatures), which have previously been shown to be essential for transport activity, e.g. GRK in loop 2, PETPR in loop 6, QQLSGVN in helix 7, DRAGRR in loop 8, GWGPIPW in helix 10, and PETKG in the C-terminal tail. An expressed sequence tag (STS A005N15) corresponding with the 3'-untranslated region of GLUT8 has previously been mapped to human chromosome 9. COS-7 cells transfected with GLUT8 cDNA expressed a 42-kDa protein exhibiting specific, glucose-inhibitable cytochalasin B binding (K(D) = 56.6 +/- 18 nm) and reconstitutable glucose transport activity (8.1 +/- 1. 4 nmol/(mg protein x 10 s) versus 1.1 +/- 0.1 in control transfections). In human tissues, a 2.4-kilobase pair transcript was predominantly found in testis, but not in testicular carcinoma. Lower amounts of the mRNA were detected in most other tissues including skeletal muscle, heart, small intestine, and brain. GLUT8 mRNA was found in testis from adult, but not from prepubertal rats; its expression in human testis was suppressed by estrogen treatment. It is concluded that GLUT8 is a sugar transport facilitator with glucose transport activity and a hormonally regulated testicular function.

Interaction between the critical aromatic amino acid residues Tyr(352) and Phe(504) in the yeast Gal2 transporter

Three critical aromatic sites have been identified in the yeast galactose transporter Gal2: Tyr(352) at the extracellular boundary of putative transmembrane segment (TM) 7, Tyr(446) in the middle of TM10 and Phe(504) in the middle of TM12. The relationship between these sites was investigated by random mutagenesis of each combination of two of the three residues. Galactose transport-positive clones selected by plate assays encoded Tyr(446) and specific combinations of aromatic residues at sites 352 and 504. Double-site mutants containing aromatic residues at these latter two positions showed either essentially full galactose transport activity (Phe(352)Trp(504) and Trp(352)Trp(504)) or no significant activity (Phe(352)Tyr(504) and Trp(352)Tyr(504)), whereas single-site mutants showed markedly reduced activity. These results are indicative of a specific interaction between sites 352 and 504 of Gal2.

Three aromatic amino acid residues critical for galactose transport in yeast Gal2 transporter

Tyr(446) in putative transmembrane segment 10 (TM10) of the yeast galactose transporter Gal2 has previously been identified as essential for galactose recognition. In the present study, alignment of the amino acid sequences of 63 sugar transporters or related proteins revealed 14 aromatic sites, including Tyr(446) of Gal2, that are conserved in >75% of these proteins. The importance of the remaining 13 conserved aromatic amino acids was examined individually by random mutagenesis using degenerate primers. Galactose transport-positive clones were identified by plate selection and subjected to DNA sequencing. For those transport-positive clones corresponding to Tyr(352), and Phe(504) mutants, all the amino acid substitutions comprised aromatic residues. The importance of the aromatic residues at these sites was further investigated by replacing them individually with each of the other 19 amino acids and measuring the galactose transport activity of the resulting mutants. Among both Tyr(352) and Phe(504) mutants, the other aromatic amino acids supported galactose transport; no other amino acids conferred high affinity transport activity. Thus, at least three aromatic sites are critical for galactose transport: one at the extracellular boundary of putative TM7 (Tyr(352)), one in the middle of putative TM10 (Tyr(446)), and one in the middle of putative TM12 (Phe(504)).

Cysteine-scanning mutagenesis of transmembrane segment 7 of the GLUT1 glucose transporter

The human erythrocyte facilitative glucose transporter (Glut1) is predicted to contain 12 transmembrane spanning alpha-helices based upon hydropathy plot analysis of the primary sequence. Five of these helices (3, 5, 7, 8, and 11) are capable of forming amphipathic structures. A model of GLUT1 tertiary structure has therefore been proposed in which the hydrophilic faces of several amphipathic helices are arranged to form a central aqueous channel through which glucose traverses the hydrophobic lipid bilayer. In order to test this model, we individually mutated each of the amino acid residues in transmembrane segment 7 to cysteine in an engineered GLUT1 molecule devoid of all native cysteines (C-less). Measurement of 2-deoxyglucose uptake in a Xenopus oocyte expression system revealed that nearly all of these mutants retain measurable transport activity. Over one-half of the cysteine mutants had significantly reduced specific activity relative to the C-less protein. The solvent accessibility and relative orientation of the residues within the helix was investigated by determining the sensitivity of the mutant transporters to inhibition by the sulfhydryl directed reagent p-chloromercuribenzene sulfonate (pCMBS). Cysteine replacement at six positions (Gln(282), Gln(283), Ile(287), Ala(289), Val(290), and Phe(291)), all near the exofacial side of the cell membrane, produced transporters that were inhibited by incubation with extracellular pCMBS. Residues predicted to be near the cytoplasmic side of the cell membrane were minimally affected by pCMBS. These data demonstrate that the exofacial portion of transmembrane segment 7 is accessible to the external solvent and provide evidence for the positioning of this alpha-helix within the glucose permeation pathway.

The Drosophila glucose transporter gene: cDNA sequence, phylogenetic comparisons, analysis of functional sites and secondary structures

Facilitative glucose transport is mediated by members of the glucose transporter (GLUT) protein family that belong to the large superfamily of twelve transmembrane segment transporters. We have cloned and sequenced a 2,168 base-pair cDNA from Drosophila melanogaster (termed Dmglut1: GenBank accession number AF064703) with strong homology to the mammalian Glut genes. The cDNA has an open reading frame encoding a protein of 480 amino acids which shows a similarity of 68% to the human GLUT1 protein. We have done a phylogenetic analysis of the cDNA and the deduced protein sequences and found a significant homology to a putative coding sequence (Ceglut1) in Caenorhabditis elegans. Here we report the results of analyses of functional sites and secondary structures of the proposed proteins and conclude that the Dmglut1 and Ceglut1 genes encode functional glucose transporters.

Tryptophan 388 in putative transmembrane segment 10 of the rat glucose transporter Glut1 is essential for glucose transport

The molecular mechanism of substrate recognition in membrane transport is not well understood. Two amino acid residues, Tyr446 and Trp455 in transmembrane segment 10 (TM10), have been shown to be important for galactose recognition by the yeast Gal2 transporter; Tyr446 was found to be essential in that its replacement by any of the other 19 amino acids abolished transport activity (Kasahara, M., Shimoda, E., and Maeda, M. (1997) J. Biol. Chem. 272, 16721-16724). The Glut1 glucose transporter of animal cells belongs to the same Glut transporter family as does Gal2 and thus might be expected to show a similar mechanism of substrate recognition. The role of the two amino acids, Phe379 and Trp388, in rat Glut1 corresponding to Tyr446 and Trp455 of Gal2 was therefore studied. Phe379 and Trp388 were individually replaced with each of the other 19 amino acids, and the mutant Glut1 transporters were expressed in yeast. The expression level of most mutants was similar to that of the wild-type Glut1, as revealed by immunoblot analysis. Glucose transport activity was assessed by reconstituting a crude membrane fraction of the yeast cells in liposomes. No significant glucose transport activity was observed with any of Trp388 mutants, whereas the Phe379 mutants showed reduced or no activity. These results indicate that the two aromatic amino acids in TM10 of Glut1 are important for glucose transport. However, unlike Gal2, the residue at the cytoplasmic end of TM10 (Trp388, corresponding to Trp455 of Gal2), rather than that in the middle of TM10 (Phe379, corresponding to Tyr446 of Gal2), is essential for transport activity.

Characterization of GLUT5 domains responsible for fructose transport

The domains responsible for the fructose specificity of GLUT5 were investigated by creating chimeras of GLUT5 with the selective glucose transporter GLUT3, which were expressed in Xenopus oocytes. 3-O-Methylglucose uptake of chimeric GLUT3-5 (M11; GLUT3 to the 11th transmembrane domain, GLUT5 to the carboxyl end) was similar to that of GLUT3, while fructose was not transported. Fructose uptake of chimeric GLUT5-3 (M3-5) to -5 (GLUT3 from the 3rd to 5th transmembrane domains, the rest GLUT5) was similar to that of GLUT5; no glucose was transported. Four chimeras transported neither fructose nor glucose: GLUT3-5 (M5; GLUT3 to the 5th transmembrane domain, GLUT5 to the carboxyl end), GLUT5-3 (M2; GLUT5 to the 2nd transmembrane domain, the rest GLUT3), GLUT5-3 (M3-11) to -5 (GLUT3 between the 3rd and 11th transmembrane domains, the rest GLUT5) and GLUT5-3 (M3-5) to -5-3 (M11; GLUT3 from the 3rd to 5th transmembrane domains and after the 11th transmembrane domain, the rest GLUT5). They, nevertheless, induced full-size proteins that were transported to the cell surface, as demonstrated by exofacial labeling with biotin. To conclude, the GLUT5 domain from the amino-terminus to the third transmembrane domain and that between the 5th and 11th transmembrane stretches seem to be necessary for fructose uptake.

Trinucleotide insertions, deletions, and point mutations in glucose transporters confer K+ uptake in Saccharomyces cerevisiae

Deletion of TRK1 and TRK2 abolishes high-affinity K+ uptake in Saccharomyces cerevisiae, resulting in the inability to grow on typical synthetic growth medium unless it is supplemented with very high concentrations of potassium. Selection for spontaneous suppressors that restored growth of trk1delta trk2delta cells on K+-limiting medium led to the isolation of cells with unusual gain-of-function mutations in the glucose transporter genes HXT1 and HXT3 and the glucose/galactose transporter gene GAL2. 86Rb uptake assays demonstrated that the suppressor mutations conferred increased uptake of the ion. In addition to K+, the mutant hexose transporters also conferred permeation of other cations, including Na+. Because the selection strategy required such gain of function, mutations that disrupted transporter maturation or localization to the plasma membrane were avoided. Thus, the importance of specific sites in glucose transport could be independently assessed by testing for the ability of the mutant transporter to restore glucose-dependent growth to cells containing null alleles of all of the known functional glucose transporter genes. Twelve sites, most of which are conserved among eukaryotic hexose transporters, were revealed to be essential for glucose transport. Four of these have previously been shown to be essential for glucose transport by animal or plant transporters. Eight represented sites not previously known to be crucial for glucose uptake. Each suppressor mutant harbored a single mutation that altered an amino acid(s) within or immediately adjacent to a putative transmembrane domain of the transporter. Seven of 38 independent suppressor mutations consisted of in-frame insertions or deletions. The nature of the insertions and deletions revealed a striking DNA template dependency: each insertion generated a trinucleotide repeat, and each deletion involved the removal of a repeated nucleotide sequence.

Functional analysis of the high affinity, Na(+)-dependent glutamate transporter GLAST-1 by site-directed mutagenesis

The reuptake of excitatory amino acids, such as glutamate, terminates excitatory signals and prevents the persistence of excitotoxic levels of glutamate in the synaptic cleft. The L-glutamate/L-aspartate transporter (GLAST-1) is the first member of the recently discovered glutamate transporter family, which includes GLT-1 and EAAC1. The neutral amino acid carrier ASCT1 is structurally closely related to this new family of membrane proteins. Transmembrane transport of neutral amino acids is expected to differ in its binding site from that of the acidic excitatory amino acids glutamate and aspartate. Three positively charged amino acid residues, Arg-122, Arg-280, Arg-479, and one polar Tyr-405 are conserved in all glutamate transporters. They are replaced by apolar amino acid residues in the ASCT1 sequence. We exchanged these residues in the GLAST-1-specific cDNA by site-directed mutagenesis. cRNAs of these mutants were expressed in the Xenopus oocyte system. The functional characterization of the mutants R122I and R280V and the double mutant R122I, R280V revealed that the mutations have no influence on the intrinsic properties and kinetics of glutamate transport but alter the Km-values for L-aspartate and the competitive inhibitor D,L-threo-3-hydroxy aspartate. Substitutions of Tyr-405 by Phe (Y405F) and Arg-479 (R479T) by Thr completely inactivate the glutamate transporter. Immunoprecipitations of [35S]methionine-labeled transporter molecules indicate similar expression levels of wild-type and mutant transporters. Immunostaining of oocyte sections clearly proves the correct targeting to and integration of the mutant GLAST-1 proteins in the plasma membrane. Our results suggest the pivotal function of the hydroxy group of the highly conserved Tyr-405 and the positively charged Arg-479 in the binding of the negatively charged acidic neurotransmitter glutamate.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Multifunctional transporter models: lessons from the transport of water, sugars, and ring compounds by GLUTs

Facilitative glucose transporters (GLUTs) have recently been shown to be multifunctional, transporting substrates other than sugars, such as water and ring compounds as large as nitrobenzene-diazol-aminoglucose. Other membrane proteins, including transporters and cystic fibrosis transmembrane conductance regulator, have also revealed a finite permeability to water. We compare the alpha-helical and beta-barrel models for the structure of GLUTs, discuss recent evidence, and argue that a beta-barrel fold explains it better. We show a model for GLUTs consisting of a relatively rigid beta-barrel translocation unit ("channel") of diameter ample enough to allow permeation of the above substrates (approximately 20 A) but gated shut by mobile loops at both ends. Such gates would open only after aromatic interactions would lead to binding of the ring substrates for GLUTs; water would, however, traverse crevices in the closed gates. Using the insights gained from GLUTs, we propose that other transporters may share with GLUTs the motif of a beta-barrel channel and would be permeable to water due to the presence of such channels together with similarly behaving gates.

Substrate recognition domain of the Gal2 galactose transporter in yeast Saccharomyces cerevisiae as revealed by chimeric galactose-glucose transporters

The Gal2 galactose transporter takes up galactose in yeast. A homologous glucose transporter from the same organism, Hxt2, was selected, and various chimeras between these two transporters were constructed by making use of homologous recombination in Escherichia coli. Comparison of the galactose transport activities of three series of chimeras enabled us to positively identify a crucial substrate recognition region of 101 amino acids that lies close to the carboxyl terminus of the Gal2 transporter.

Substitution of conserved tyrosine residues in helix 4 (Y143) and 7 (Y293) affects the activity, but not IAPS-forskolin binding, of the glucose transporter GLUT4

Six tyrosine residues (Y28, Y143, Y292, Y293, Y308, Y432(1)) which are conserved in all mammalian glucose transporters were substituted for phenylalanine by site-directed mutagenesis, and mutant glucose transporters were transiently expressed in COS-7 cells. Glucose transport activity as assessed by reconstitution of the solubilized transporters into lecithin liposomes was reduced by 70% in the mutant Y143F and appeared to be abolished in Y293F, but was not affected by substitution of Y28, Y292, Y308 and Y432. In contrast, covalent binding of the photolabel 125IAPS-forskolin was normal in all mutants. Stable expression of the mutants Y143F, Y293F, and Y292F in LTK cells yielded identical results. These data indicate that only two of the 6 conserved helical tyrosine residues, located in helices 4 and 7, are essential for full activity, but not for IAPS-forskolin binding of the GLUT4.