Transmembrane Segment 12 of the Glut1 Glucose Transporter Is an Outer Helix and Is Not Directly Involved in the Transport Mechanism

Metabolic gatekeepers: Dynamic roles of sugar transporters in insect metabolism and physiology

Sugars play multiple critical roles in insects, serving as energy sources, carbon skeletons, osmolytes and signalling molecules. The transport of sugars from source to sink via membrane proteins is essential for the uptake, distribution and utilization of sugars across various tissues. Sugar supply and distribution are crucial for insect development, flight, diapause and reproduction. Insect sugar transporters (STs) share significant structural and functional similarities with those in mammals and other higher eukaryotes. However, they exhibit unique characteristics, including differential regulation, substrate selectivity and kinetics. Here, we have discussed structural diversity, evolutionary trends, expression dynamics, mechanisms of action and functional significance of insect STs. The sequence and structural diversity of insect STs, highlighted by the analysis of conserved domains and evolutionary patterns, underpins their functional differentiation and divergence. The review emphasizes the importance of STs in insect metabolism, physiology and stress tolerance. It also discusses how variations in transporter regulation, expression, selectivity and activity contribute to functional differences. Furthermore, we have underlined the potential and necessity of studying these mechanisms and roles to gain a deeper understanding of insect glycobiology. Understanding the regulation and function of sugar transporters is vital for comprehending insect metabolism and physiological potential. This review provides valuable insights into the diverse functionalities of insect STs and their significant roles in metabolism and physiology.

Transporter tandems: precise tools for normalizing active transporter in the plasma membrane

The transport efficiency (TE) describes the performance of a transport protein for a specific substrate. To compare the TE of different transporters, the number of active transporters in the plasma membrane must be monitored, as it may vary for each transporter and experiment. Available methods, like LC-MS quantification of tryptic peptides, fail to discriminate inactive intracellular transporters or, like cell-surface biotinylation followed by affinity chromatography and Western blotting, are imprecise and very laborious. We wanted to normalize active transporters by the activity of a second transporter. A transporter tandem, generated by joining two transporter cDNAs into a single open reading frame, should guarantee a 1 : 1 stoichiometry. Here we created a series of tandems with different linkers between the human ergothioneine (ET) transporter ETT (gene symbol SLC22A4) and organic cation transporter OCT2 (SLC22A2). The linker sequence strongly affected the expression strength. The stoichiometry was validated by absolute peptide quantification and untargeted peptide analysis. Compared with wild-type ETT, the normalized ET clearance of the natural variant L503F was higher (f = 1.34); G462E was completely inactive. The general usefulness of the tandem strategy was demonstrated by linking several transporters with ETT; every construct was active in both parts. Transporter tandems can be used - without membrane isolation or protein quantification - as precise tools for transporter number normalization, to identify, for example, relevant transporters for a drug. It is necessary, however, to find suitable linkers, to check the order of transporters, and to verify the absence of functional interference by saturation kinetics.

GLUT Characterization Using Frog Xenopus laevis Oocytes

Xenopus laevis oocytes are a useful heterologous expression system for expressing glucose transporters (GLUTs) and examining their functions. In this chapter, we provide a detailed protocol on oocyte extraction and preparation for GLUT9 protein expression. Furthermore, we describe the determination of GLUT9 overexpression level by biotinylation and Western blotting analysis. Finally, we also describe how GLUT9-expressing oocytes can be used to measure urate kinetics by radioisotopes as well as two-microelectrode voltage clamping techniques.

Residues in the eighth transmembrane domain of the proton-coupled folate transporter (SLC46A1) play an important role in defining the aqueous translocation pathway and in folate substrate binding

The proton-coupled folate transporter (PCFT-SLC46A1) is required for intestinal folate absorption and folate transport across the choroid plexus. This report addresses the structure/function of the 8th transmembrane helix. Based upon biotinylation of cysteine-substituted residues by MTSEA-biotin, 14 contiguous exofacial residues to Leu316 were accessible to the extracellular compartment of the 23 residues in this helix (Leu303-Leu325). Pemetrexed blocked biotinylation of six Cys-substituted residues deep within the helix implicating an important role for this region in folate binding. Accessibility decreased at 4°C vs RT. The influx K_t, K_i and V_max were markedly increased for the P314C mutant, similar to what was observed for Y315A and Y315P mutants. However, the K_t, alone, was increased for the P314Y mutant. To correlate these observations with PCFT structural changes during the transport cycle, homology models were built for PCFT based upon the recently reported structures of bovine and rodent GLUT5 fructose transporters in the inward-open and outward- open conformations, respectively. The models predict substantial structural alterations in the exofacial region of the eighth transmembrane helix as it cycles between its conformational states that can account for the extended and contiguous aqueous accessibility of this region of the helix. Further, a helix break in one of the two conformations can account for the critical roles Pro314 and Tyr315, located in this region, play in PCFT function. The data indicates that the 8th transmembrane helix of PCFT plays an important role in defining the aqueous channel and the folate binding pocket.

Advances in understanding the pathogenesis of the red cell volume disorders

Genetic defects of erythrocyte transport proteins cause disorders of red blood cell volume that are characterized by abnormal permeability to the cations Na(+) and K(+) and, consequently, by changes in red cell hydration. Clinically, these disorders are associated with chronic haemolytic anaemia of variable severity and significant co-morbidities, such as iron overload. This review provides an overview of recent insights into the molecular basis of this group of rare anaemias involving cation channels and transporters dysfunction. To date, a total of 5 different membrane proteins have been reported to be responsible for volume homeostasis alteration when mutated, 3 of them leading to overhydrated cells (AE1 [also termed SLC4A1], RHAG and GLUT1 [also termed SCL2A1) and 2 others to dehydrated cells (PIEZO1 and the Gardos Channel). These findings are not only of basic scientific interest, but also of direct clinical significance for improving diagnostic procedures and identify potential approaches for novel therapeutic strategies.

Glucose transporters: cellular links to hyperglycemia in insulin resistance and diabetes

Abnormal expression and/or function of mammalian hexose transporters contribute to the hallmark hyperglycemia of diabetes. Due to different roles in glucose handling, various organ systems possess specific transporters that may be affected during the diabetic state. Diabetes has been associated with higher rates of intestinal glucose transport, paralleled by increased expression of both active and facilitative transporters and a shift in the location of transporters within the enterocyte, events that occur independent of intestinal hyperplasia and hyperglycemia. Peripheral tissues also exhibit deregulated glucose transport in the diabetic state, most notably defective translocation of transporters to the plasma membrane and reduced capacity to clear glucose from the bloodstream. Expression of renal active and facilitative glucose transporters increases as a result of diabetes, leading to elevated rates of glucose reabsorption. However, this may be a natural response designed to combat elevated blood glucose concentrations and not necessarily a direct effect of insulin deficiency. Functional foods and nutraceuticals, by modulation of glucose transporter activity, represent a potential dietary tool to aid in the management of hyperglycemia and diabetes.

Comparative Sequence-Function Analysis of the Major Facilitator Superfamily: The "Mix-and-Match" Method

The major facilitator superfamily (MFS) is a diverse group of secondary transporters with members found in all kingdoms of life. The paradigm for MFS is the lactose permease (LacY) of Escherichia coli, which has been the test bed for the development of many methods applied for the analysis of transport proteins. X-ray structures of an inward-facing conformation and the most recent structure of an almost occluded conformation confirm many conclusions from previous studies. One fundamentally important problem for understanding the mechanism of secondary active transport is the identification and physical localization of residues involved in substrate and H(+) binding. This information is exceptionally difficult to obtain with the MFS because of the broad sequence diversity among the members. The increasing number of solved MFS structures has led to the recognition of a common feature: inverted structure-repeat, formed by fused triple-helix domains with opposite orientation in the membrane. The presented method here exploits this feature to predict functionally homologous positions of known relevant positions in LacY. The triple-helix motifs are aligned in combinatorial fashion so as to detect substrate and H(+)-binding sites in symporters that transport substrates, ranging from simple ions like phosphate to more complex disaccharides.

Mechanisms of expression and translocation of major fission yeast glucose transporters regulated by CaMKK/phosphatases, nuclear shuttling, and TOR

Glucose transporters play a pivotal role in glucose homeostasis. The fission yeast high-affinity glucose transporter Ght5 is regulated with regard to transcription and localization via CaMKK and TOR pathways. These results clarify the evolutionarily conserved mechanisms underlying glucose homeostasis that prevent hyperglycemia in humans.

Hexose transporters are required for cellular glucose uptake; thus they play a pivotal role in glucose homeostasis in multicellular organisms. Using fission yeast, we explored hexose transporter regulation in response to extracellular glucose concentrations. The high-affinity transporter Ght5 is regulated with regard to transcription and localization, much like the human GLUT transporters, which are implicated in diabetes. When restricted to a glucose concentration equivalent to that of human blood, the fission yeast transcriptional regulator Scr1, which represses Ght5 transcription in the presence of high glucose, is displaced from the nucleus. Its displacement is dependent on Ca²⁺/calmodulin-dependent kinase kinase, Ssp1, and Sds23 inhibition of PP2A/PP6-like protein phosphatases. Newly synthesized Ght5 locates preferentially at the cell tips with the aid of the target of rapamycin (TOR) complex 2 signaling. These results clarify the evolutionarily conserved molecular mechanisms underlying glucose homeostasis, which are essential for preventing hyperglycemia in humans.

Functional architecture of MFS D-glucose transporters

The Major Facilitator Superfamily (MFS) is a diverse group of secondary transporters with over 10,000 members, found in all kingdoms of life, including Homo sapiens. One objective of determining crystallographic models of the bacterial representatives is identification and physical localization of residues important for catalysis in transporters with medical relevance. The recently solved crystallographic models of the D-xylose permease XylE from Escherichia coli and GlcP from Staphylococcus epidermidus, homologs of the human D-glucose transporters, the GLUTs (SLC2), provide information about the structure of these transporters. The goal of this work is to examine general concepts derived from the bacterial XylE, GlcP, and other MFS transporters for their relevance to the GLUTs by comparing conservation of functionally critical residues. An energy landscape for symport and uniport is presented. Furthermore, the substrate selectivity of XylE is compared with GLUT1 and GLUT5, as well as a XylE mutant that transports D-glucose.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

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.

Sequence determinants of GLUT1 oligomerization: analysis by homology-scanning mutagenesis

The human blood-brain barrier glucose transport protein (GLUT1) forms homodimers and homotetramers in detergent micelles and in cell membranes, where the GLUT1 oligomeric state determines GLUT1 transport behavior. GLUT1 and the neuronal glucose transporter GLUT3 do not form heterocomplexes in human embryonic kidney 293 (HEK293) cells as judged by co-immunoprecipitation assays. Using homology-scanning mutagenesis in which GLUT1 domains are substituted with equivalent GLUT3 domains and vice versa, we show that GLUT1 transmembrane helix 9 (TM9) is necessary for optimal association of GLUT1-GLUT3 chimeras with parental GLUT1 in HEK cells. GLUT1 TMs 2, 5, 8, and 11 also contribute to a less abundant heterocomplex. Cell surface GLUT1 and GLUT3 containing GLUT1 TM9 are 4-fold more catalytically active than GLUT3 and GLUT1 containing GLUT3 TM9. GLUT1 and GLUT3 display allosteric transport behavior. Size exclusion chromatography of detergent solubilized, purified GLUT1 resolves GLUT1/lipid/detergent micelles as 6- and 10-nm Stokes radius particles, which correspond to GLUT1 dimers and tetramers, respectively. Studies with GLUTs expressed in and solubilized from HEK cells show that HEK cell GLUT1 resolves as 6- and 10-nm Stokes radius particles, whereas GLUT3 resolves as a 6-nm particle. Substitution of GLUT3 TM9 with GLUT1 TM9 causes chimeric GLUT3 to resolve as 6- and 10-nm Stokes radius particles. Substitution of GLUT1 TM9 with GLUT3 TM9 causes chimeric GLUT1 to resolve as a mixture of 6- and 4-nm particles. We discuss these findings in the context of determinants of GLUT oligomeric structure and transport function.

Ligand-induced movements of inner transmembrane helices of Glut1 revealed by chemical cross-linking of di-cysteine mutants

The relative orientation and proximity of the pseudo-symmetrical inner transmembrane helical pairs 5/8 and 2/11 of Glut1 were analyzed by chemical cross-linking of di-cysteine mutants. Thirteen functional di-cysteine mutants were created from a C-less Glut1 reporter construct containing cysteine substitutions in helices 5 and 8 or helices 2 and 11. The mutants were expressed in Xenopus oocytes and the sensitivity of each mutant to intramolecular cross-linking by two homobifunctional thiol-specific reagents was ascertained by protease cleavage followed by immunoblot analysis. Five of 9 mutants with cysteine residues predicted to lie in close proximity to each other were susceptible to cross-linking by one or both reagents. None of 4 mutants with cysteine substitutions predicted to lie on opposite faces of their respective helices was susceptible to cross-linking. Additionally, the cross-linking of a di-cysteine pair (A70C/M420C, helices 2/11) predicted to lie near the exoplasmic face of the membrane was stimulated by ethylidene glucose, a non-transported glucose analog that preferentially binds to the exofacial substrate-binding site, suggesting that the binding of this ligand stimulates the closure of helices at the exoplasmic face of the membrane. In contrast, the cross-linking of a second di-cysteine pair (T158C/L325, helices 5/8), predicted to lie near the cytoplasmic face of the membrane, was stimulated by cytochalasin B, a glucose transport inhibitor that competitively inhibits substrate efflux, suggesting that this compound recruits the transporter to a conformational state in which closure of inner helices occurs at the cytoplasmic face of the membrane. This observation provides a structural explanation for the competitive inhibition of substrate efflux by cytochalasin B. These data indicate that the binding of competitive inhibitors of glucose efflux or influx induce occluded states in the transporter in which substrate is excluded from the exofacial or endofacial binding site.

Stomatin-deficient cryohydrocytosis results from mutations in SLC2A1: a novel form of GLUT1 deficiency syndrome

The hereditary stomatocytoses are a series of dominantly inherited hemolytic anemias in which the permeability of the erythrocyte membrane to monovalent cations is pathologically increased. The causative mutations for some forms of hereditary stomatocytosis have been found in the transporter protein genes, RHAG and SLC4A1. Glucose transporter 1 (glut1) deficiency syndromes (glut1DSs) result from mutations in SLC2A1, encoding glut1. Glut1 is the main glucose transporter in the mammalian blood-brain barrier, and glut1DSs are manifested by an array of neurologic symptoms. We have previously reported 2 cases of stomatin-deficient cryohydrocytosis (sdCHC), a rare form of stomatocytosis associated with a cold-induced cation leak, hemolytic anemia, and hepatosplenomegaly but also with cataracts, seizures, mental retardation, and movement disorder. We now show that sdCHC is associated with mutations in SLC2A1 that cause both loss of glucose transport and a cation leak, as shown by expression studies in Xenopus oocytes. On the basis of a 3-dimensional model of glut1, we propose potential mechanisms underlying the phenotypes of the 2 mutations found. We investigated the loss of stomatin during erythropoiesis and find this occurs during reticulocyte maturation and involves endocytosis. The molecular basis of the glut1DS, paroxysmal exercise-induced dyskinesia, and sdCHC phenotypes are compared and discussed.

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.

Transmembrane helix 7 in the Na+/dicarboxylate cotransporter 1 is an outer helix that contains residues critical for function

Citric acid cycle intermediates, including succinate and citrate, are absorbed across the apical membrane by the NaDC1 Na+/dicarboxylate cotransporter located in the kidney and small intestine. The secondary structure model of NaDC1 contains 11 transmembrane helices (TM). TM7 was shown previously to contain determinants of citrate affinity, and Arg-349 at the extracellular end of the helix is required for transport. The present study involved cysteine scanning mutagenesis of 26 amino acids in TM7 and the associated loops. All of the mutants were well expressed on the plasma membrane, but many had low or no transport activity: 6 were inactive and 7 had activity less than 25% of the parental. Three of the mutants had notable changes in functional properties. F336C had increased transport activity due to an increased Vmax for succinate. The conserved residue F339C had very low transport activity and a change in substrate selectivity. G356C in the putative extracellular loop was the only cysteine mutant that was affected by the membrane-impermeant cysteine reagent, MTSET. However, direct labeling of G356C with MTSEA-biotin gave a weak signal, indicating that this residue is not readily accessible to more bulky reagents. The results suggest that the amino acids of TM7 are functionally important because their replacement by cysteine had large effects on transport activity. However, most of TM7 does not appear to be accessible to the extracellular fluid and is likely to be an outer helix in contact with the lipid bilayer.

Sugar transporters of the major facilitator superfamily in aphids; from gene prediction to functional characterization

Analysis of the pea aphid (Acyrthosiphon pisum) genome using signatures specific to the Major Facilitator Superfamily (Pfam Clan CL0015) and the Sugar_tr family (Pfam Family PF00083) has identified 54 genes encoding potential sugar transporters, of which 38 have corresponding ESTs. Twenty-nine genes contain the InterPro IPR003663 hexose transporter signature. The protein encoded by Ap_ST3, the most abundantly expressed sugar transporter gene, was functionally characterized by expression as a recombinant protein. Ap_ST3 acts as a low-affinity uniporter for fructose and glucose that does not depend on Na(+) or H(+) for activity. Ap_ST3 was expressed at elevated levels in distal gut tissue, consistent with a role in gut sugar transport. The A. pisum genome shows evidence of duplications of sugar transporter genes.

Trivalent arsenicals and glucose use different translocation pathways in mammalian GLUT1

Rat glucose transporter isoform 1 or rGLUT1, which is expressed in neonatal heart and the epithelial cells that form the blood-brain barrier, facilitates uptake of the trivalent arsenicals arsenite as As(OH)₃ and methylarsenite as CH₃As(OH)₂. GLUT1 may be the major pathway for arsenic uptake into heart and brain, where the metalloid causes cardiotoxicity and neurotoxicity. In this paper, we compare the translocation properties of GLUT1 for trivalent methylarsenite and glucose. Substitution of Ser(66), Arg(126) and Thr(310), residues critical for glucose uptake, led to decreased uptake of glucose but increased uptake of CH₃As(OH)₂. The K(m) for uptake of CH₃As(OH)₂ of three identified mutants, S66F, R126K and T310I, were decreased 4-10 fold compared to native GLUT1. The osmotic water permeability coefficient (P(f)) of GLUT1 and the three clinical isolates increased in parallel with the rate of CH₃As(OH)₂ uptake. GLUT1 inhibitors Hg(II), cytochalasin B and forskolin reduced uptake of glucose but not CH₃As(OH)₂. These results indicate that CH₃As(OH)₂ and water use a common translocation pathway in GLUT1 that is different to that of glucose transport.

Purification and characterization of mammalian glucose transporters expressed in Pichia pastoris

The major bottleneck to the application of high-resolution techniques such as crystallographic X-ray diffraction and spectroscopic analyses to resolve the structure of mammalian membrane proteins has been the ectopic expression and purification of sufficient quantities of non-denatured proteins. This has been especially problematic for members of the major facilitator superfamily, which includes the family of mammalian glucose transporters. A simple and rapid method is described for the purification of milligram quantities of recombinant GLUT1 and GLUT4, two of the most intensively studied GLUT isoforms, after ectopic expression in Pichia pastoris. The proteins obtained were >95% pure and exhibited functional transport and ligand-binding activities.

Model of the exofacial substrate-binding site and helical folding of the human Glut1 glucose transporter based on scanning mutagenesis

Transmembrane helix 9 of the Glut1 glucose transporter was analyzed by cysteine-scanning mutagenesis and the substituted cysteine accessibility method (SCAM). A cysteine-less (C-less) template transporter containing amino acid substitutions for the six native cysteine residues present in human Glut1 was used to generate a series of 21 mutant transporters by substituting each successive residue in predicted transmembrane segment 9 with a cysteine residue. The mutant proteins were expressed in Xenopus oocytes, and their specific transport activities were directly compared to that of the parental C-less molecule whose function has been shown to be indistinguishable from that of native Glut1. Only a single mutant (G340C) had activity that was reduced (by 75%) relative to that of the C-less parent. These data suggest that none of the amino acid side chains in helix 9 is absolutely required for transport function and that this helix is not likely to be directly involved in substrate binding or translocation. Transport activity of the cysteine mutants was also tested after incubation of oocytes in the presence of the impermeant sulfhydryl-specific reagent, p-chloromercuribenzene sulfonate (pCMBS). Only a single mutant (T352C) exhibited transport inhibition in the presence of pCMBS, and the extent of inhibition was minimal (11%), indicating that only a very small portion of helix 9 is accessible to the external solvent. These results are consistent with the conclusion that helix 9 plays an outer stabilizing role for the inner helical bundle predicted to form the exofacial substrate-binding site. All 12 of the predicted transmembrane segments of Glut1 encompassing 252 amino acid residues and more than 50% of the complete polypeptide sequence have now been analyzed by scanning mutagenesis and SCAM. An updated model is presented for the outward-facing substrate-binding site and relative orientation of the 12 transmembrane helices of Glut1.

Analysis of glucose transporter topology and structural dynamics

Homology modeling and scanning cysteine mutagenesis studies suggest that the human glucose transport protein GLUT1 and its distant bacterial homologs LacY and GlpT share similar structures. We tested this hypothesis by mapping the accessibility of purified, reconstituted human erythrocyte GLUT1 to aqueous probes. GLUT1 contains 35 potential tryptic cleavage sites. Fourteen of 16 lysine residues and 18 of 19 arginine residues were accessible to trypsin. GLUT1 lysine residues were modified by isothiocyanates and N-hydroxysuccinimide (NHS) esters in a substrate-dependent manner. Twelve lysine residues were accessible to sulfo-NHS-LC-biotin. GLUT1 trypsinization released full-length transmembrane helix 1, cytoplasmic loop 6-7, and the long cytoplasmic C terminus from membranes. Trypsin-digested GLUT1 retained cytochalasin B and d-glucose binding capacity and released full-length transmembrane helix 8 upon cytochalasin B (but not D-glucose) binding. Transmembrane helix 8 release did not abrogate cytochalasin B binding. GLUT1 was extensively proteolyzed by alpha-chymotrypsin, which cuts putative pore-forming amphipathic alpha-helices 1, 2, 4, 7, 8, 10, and 11 at multiple sites to release transmembrane peptide fragments into the aqueous solvent. Putative scaffolding membrane helices 3, 6, 9, and 12 are strongly hydrophobic, resistant to alpha-chymotrypsin, and retained by the membrane bilayer. These observations provide experimental support for the proposed GLUT1 architecture; indicate that the proposed topology of membrane helices 5, 6, and 12 requires adjustment; and suggest that the metastable conformations of transmembrane helices 1 and 8 within the GLUT1 scaffold destabilize a sugar translocation intermediate.

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.

Transmembrane segment 6 of the Glut1 glucose transporter is an outer helix and contains amino acid side chains essential for transport activity

Experimental data and homology modeling suggest a structure for the exofacial configuration of the Glut1 glucose transporter in which 8 transmembrane helices form an aqueous cavity in the bilayer that is stabilized by four outer helices. The role of transmembrane segment 6, predicted to be an outer helix in this model, was examined by cysteine-scanning mutagenesis and the substituted cysteine accessibility method using the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzene-sulfonate (pCMBS). A fully functional Glut1 molecule lacking all 6 native cysteine residues was used as a template to produce a series of 21 Glut1 point mutants in which each residue along helix 6 was individually changed to cysteine. These mutants were expressed in Xenopus oocytes, and their expression levels, functional activities, and sensitivities to inhibition by pCMBS were determined. Cysteine substitutions at Leu(204) and Pro(205) abolished transport activity, whereas substitutions at Ile(192), Pro(196), Gln(200), and Gly(201) resulted in inhibition of activity that ranged from approximately 35 to approximately 80%. Cysteine substitutions at Leu(188), Ser(191), and Leu(199) moderately augmented specific transport activity relative to the control. These results were dramatically different from those previously reported for helix 12, the structural cognate of helix 6 in the pseudo-symmetrical structural model, for which none of the 21 single-cysteine mutants exhibited reduced activity. Only the substitution at Leu(188) conferred inhibition by pCMBS, suggesting that most of helix 6 is not exposed to the external solvent, consistent with its proposed role as an outer helix. These data suggest that helix 6 contains amino acid side chains that are critical for transport activity and that structurally analogous outer helices may play distinct roles in the function of membrane transporters.

Ins and outs of major facilitator superfamily antiporters

The major facilitator superfamily (MFS) represents the largest group of secondary active membrane transporters, and its members transport a diverse range of substrates. Recent work shows that MFS antiporters, and perhaps all members of the MFS, share the same three-dimensional structure, consisting of two domains that surround a substrate translocation pore. The advent of crystal structures of three MFS antiporters sheds light on their fundamental mechanism; they operate via a single binding site, alternating-access mechanism that involves a rocker-switch type movement of the two halves of the protein. In the sn-glycerol-3-phosphate transporter (GlpT) from Escherichia coli, the substrate-binding site is formed by several charged residues and a histidine that can be protonated. Salt-bridge formation and breakage are involved in the conformational changes of the protein during transport. In this review, we attempt to give an account of a set of mechanistic principles that characterize all MFS antiporters.

A highly conserved hydrophobic motif in the exofacial vestibule of fructose transporting SLC2A proteins acts as a critical determinant of their substrate selectivity

The substrate specificity of the facilitated hexose transporter, GLUT, family, (gene SLC2A) is highly varied. Some appear to be able to translocate both glucose and fructose, while the ability to handle 2-deoxyglucose and galactose does not necessarily correlate with the other two hexoses. It has become generally accepted that a central substrate binding/translocation site determines which hexoses can be transported. However, a recent study showed that a single point mutation of a hydrophobic residue in GLUTs 2, 5 & 7 removed their ability to transport fructose without affecting the kinetics of glucose permeation. This residue is in the 7th transmembrane helix, facing the aqueous pore and lies close to the opening of the exofacial vestibule. This study expands these observations to include the other class II GLUTs (9 & 11) and shows that a three amino acid motif (NXI/NXV) appears to be critical in determining if fructose can access the translocation mechanism. GLUT11 can also transport fructose, but it has the motif DSV at the same position, which appears to function in the same manner as NXI and when all three residues are replaced with NAV fructose transport lost. These results are discussed in relation to possible roles for hydrophobic residues lining the aqueous pore at the opening of the exofacial vestibule. Finally, the possibility that the translocation binding site may not be the sole determinant of substrate specificity for these proteins is examined.

Facilitated hexose transporters: new perspectives on form and function

The recent sequencing of the human genome has resulted in the addition of nine new hGLUT isoforms to the SLC2A family, many of which have widely varying substrate specificity, kinetic behavior, and tissue distribution. This review examines some new hypotheses related to the structure and function of these proteins.

GLUT1 deficiency syndrome--2007 update

GLUT1 deficiency syndrome (GLUT1DS, OMIM 606777) is a treatable epileptic encephalopathy resulting from impaired glucose transport into the brain. The essential biochemical finding is a low glucose concentration in the cerebrospinal fluid (CSF; hypoglycorrhachia; mean 1.7 [SD 0.3mmol/L]) in the setting of normoglycaemia. CSF lactate is normal. Patients present with an early-onset epilepsy resistant to anticonvulsants, developmental delay, and a complex movement disorder. Hypotonic, ataxic, and dystonic features are most prominent. Speech is often severely affected. Some patients develop spasticity and secondary microcephaly. The phenotype is highly variable ranging from severe impairment to children without seizures. Electroencephalography (EEG) may show 2.5-4Hz spike-waves improving on food intake. Neuroimaging is uninformative. Most patients carry heterozygous de novo mutations in the GLUT1 gene (OMIM 138140, gene map locus 1p35-31.3). Autosomal dominant transmission and several mutational hot spots have been identified, but phenotype-genotype correlations are not yet apparent. Homozygous GLUT1 mutations presumably are lethal. The ketogenic diet is the treatment of choice as it provides an alternative fuel to the brain. It should be introduced early and maintained into puberty. Seizures are effectively controlled with the onset of ketosis, but might recur and require comedication. The effect on neurodevelopment appears less impressive. The increasing number of patients, molecular and biochemical analysis, recent research into ketogenic diet mechanisms, and the development of animal models for GLUT1DS have brought substantial insights in disease manifestations and mechanisms. This review summarizes data on 84 published cases and highlights recent advances in understanding this entity.

Structural basis of GLUT1 inhibition by cytoplasmic ATP

Cytoplasmic ATP inhibits human erythrocyte glucose transport protein (GLUT1)–mediated glucose transport in human red blood cells by reducing net glucose transport but not exchange glucose transport (Cloherty, E.K., D.L. Diamond, K.S. Heard, and A. Carruthers. 1996. Biochemistry. 35:13231–13239). We investigated the mechanism of ATP regulation of GLUT1 by identifying GLUT1 domains that undergo significant conformational change upon GLUT1–ATP interaction. ATP (but not GTP) protects GLUT1 against tryptic digestion. Immunoblot analysis indicates that ATP protection extends across multiple GLUT1 domains. Peptide-directed antibody binding to full-length GLUT1 is reduced by ATP at two specific locations: exofacial loop 7–8 and the cytoplasmic C terminus. C-terminal antibody binding to wild-type GLUT1 expressed in HEK cells is inhibited by ATP but binding of the same antibody to a GLUT1–GLUT4 chimera in which loop 6–7 of GLUT1 is substituted with loop 6–7 of GLUT4 is unaffected. ATP reduces GLUT1 lysine covalent modification by sulfo-NHS-LC-biotin by 40%. AMP is without effect on lysine accessibility but antagonizes ATP inhibition of lysine modification. Tandem electrospray ionization mass spectrometry analysis indicates that ATP reduces covalent modification of lysine residues 245, 255, 256, and 477, whereas labeling at lysine residues 225, 229, and 230 is unchanged. Exogenous, intracellular GLUT1 C-terminal peptide mimics ATP modulation of transport whereas C-terminal peptide-directed IgGs inhibit ATP modulation of glucose transport. These findings suggest that transport regulation involves ATP-dependent conformational changes in (or interactions between) the GLUT1 C terminus and the C-terminal half of GLUT1 cytoplasmic loop 6–7.