Cysteine scanning mutagenesis of helices 2 and 7 in GLUT1 identifies an exofacial cleft in both transmembrane segments

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.

Mechanistic Insights into Protein Stability and Self-aggregation in GLUT1 Genetic Variants Causing GLUT1-Deficiency Syndrome

Human sodium-independent glucose cotransporter 1 (hGLUT1) has been studied for its tetramerization and multimerization at the cell surface. Homozygous or compound heterozygous mutations in hGLUT1 elicit GLUT1-deficiency syndrome (GLUT1-DS), a metabolic disorder, which results in impaired glucose transport into the brain. The reduced cell surface expression or loss of function have been shown for some GLUT1 mutants. However, the mechanism by which deleterious mutations affect protein structure, conformational stability and GLUT1 oligomerization is not known and require investigation. In this review, we combined previous knowledge of GLUT1 mutations with hGLUT1 crystal structure to analyze native interactions and several natural single-point mutations. The modeling of native hGLUT1 structure confirmed the roles of native residues in forming a range of side-chain interactions. Interestingly, the modeled mutants pointed to the formation of a variety of non-native novel interactions, altering interaction networks and potentially eliciting protein misfolding. Self-aggregation of the last part of hGLUT1 was predicted using protein aggregation prediction tool. Furthermore, an increase in aggregation potential in the aggregation-prone regions was estimated for several mutants suggesting increased aggregation of misfolded protein. Protein stability change analysis predicted that GLUT1 mutant proteins are unstable. Combining GLUT1 oligomerization behavior with our modeling, aggregation prediction, and protein stability analyses, this work provides state-of-the-art view of GLUT1 genetic mutations that could destabilize native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation in a disease state.

Molecular machinery of starch digestion and glucose absorption along the midgut of Musca domestica

Until now there is no molecular model of starch digestion and absorption of the resulting glucose molecules along the larval midgut of Musca domestica. For addressing to this, we used RNA-seq analyses from seven sections of the midgut and carcass to evaluate the expression level of the genes coding for amylases, maltases and sugar transporters (SP). An amylase related protein (Amyrel) and two amylase sequences, one soluble and one with a predicted GPI-anchor, were identified. Three highly expressed maltase genes were correlated with biochemically characterized maltases: one soluble, other glycocalyx-associated, and another membrane-bound. SPs were checked as being apical or basal by proteomics of microvillar preparations and those up-regulated by starch were identified by real time PCR. From the 9 SP sequences with high expression in midgut, two are putative sugar sensors (MdSP4 and MdSP5), one is probably a trehalose transporter (MdSP8), whereas MdSP1-3, MdSP6, and MdSP9 are supposed to transport glucose into cells, and MdSP7 from cells to hemolymph. MdSP1, MdSP7, and MdSP9 are up-regulated by starch. Based on the data, starch is at first digested by amylase and maltases at anterior midgut, with the resulting glucose units absorbed at middle midgut. At this region, low pH, lysozyme, and cathepsin D open the ingested bacteria and fungi cells, freeing sugars and glycogen. This and the remaining dietary starch are digested by amylase and maltases at the end of middle midgut and up to the middle part of the posterior midgut, with resulting sugars being absorbed along the posterior midgut.

Computational simulation and modeling of the blood-brain barrier pathology

In silico methods and models in the pathology of the blood-brain barrier (BBB) or also called BBB "computational pathology", are based on using mathematical approaches together with complex, high-dimensional experimental data to evaluate and predict disease-related impacts on the CNS. These computational methods and tools have been designed to deal with BBB-linked neuropathology at the molecular, cellular, tissue, and organ levels. The molecular and cellular levels mainly include molecular docking and molecular dynamics simulations (atomistic and coarse-grain) of mutated or misfolded tight junction proteins, receptors, and various BBB transporters. The tissue and organ levels encompass the mechanistic and pharmacokinetic models as well as finite-element method and pathway analyses enriched with multiple sources of raw data (e.g., in vitro and in vivo, histopathological records, "-omics", and imaging data). Overall, this review discusses comprehensive computational techniques and strategies at different levels of complexity, providing new insights and future directions for diagnosis, treatment improvement, and a deeper understanding of BBB-related neuropathological events.

Substituted-cysteine accessibility and cross-linking identify an exofacial cleft in the 7th and 8th helices of the proton-coupled folate transporter (SLC46A1)

The proton-coupled folate transporter (PCFT-SLC46A1) is required for folate transport across the apical membrane of the small intestine and across the choroid plexus. This study focuses on the structure/function of the 7th transmembrane domain (TMD), and its relationship to the 8th TMD as assessed by the substituted cysteine accessibility method (SCAM) and dicysteine cross-linking. Nine exofacial residues (I278C; H281C-L288C) of 23 residues in the 7th TMD were accessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate (MTSEA-biotin). Pemetrexed, a high-affinity substrate for PCFT, decreased or abolished biotinylation of seven of these residues consistent with their location in or near the folate binding pocket. Homology models of PCFT based on Glut5 fructose transporter structures in both inward- and outward- open conformations were constructed and predicted that two pairs of residues (T289-I304C and Q285-Q311C) from the 7th and 8th TMDs should be in sufficiently close proximity to form a disulfide bond when substituted with cysteines. The single Cys-substituted mutants were accessible to MTSEA-biotin and functional with and without pretreatment with dithiotreitol. However, the double mutants were either not accessible at all, or accessibility was markedly reduced and function markedly impaired. This occurred spontaneously without inclusion of an oxidizing agent. Dithiotreitol restored accessibility and function consistent with disulfide bond disruption. The data establish the proximity of exofacial regions of the 7th and 8th TMDs and their role in defining the aqueous translocation pathway and suggest that these helices may be a component of an exofacial cleft through which substrates enter the protein binding pocket in its outward-open conformation.

An Insulin-Like Modular Basis for the Evolution of Glucose Transporters (GLUT) with Implications for Diabetes

Glucose transporters (GLUT) are twelve-transmembrane spanning proteins that contain two pores capable of transporting glucose and dehydroascorbate in and out of cells. The mechanism by which transport is effected is unknown. An evolutionarily-based hypothesis for the mechanism of glucose transport is presented here based on reports that insulin has multiple binding sites for glucose. It is proposed that insulin-like peptides were incorporated as modular elements into transmembrane proteins during evolution, resulting in glucose transporting capacity. Homology searching reveals that all GLUT contain multiple copies of insulin-like regions. These regions map onto a model of GLUT in positions that define the glucose transport cores. This observation provides a mechanism for glucose transport involving the diffusion of glucose from one insulin-like glucose-binding region to another. It also suggests a mechanism by which glucose disregulation may occur in both type 1 and type 2 diabetes: insulin rapidly self-glycates under hyperglycemic conditions. Insulin-like regions of GLUT may also self-glycate rapidly, thereby interfering with transport of glucose into cells and disabling GLUT sensing of blood glucose levels. All aspects of the hypothesis are experimentally testable.

Hypoglycemic activity of 6-bromoembelin and vilangin in high-fat diet fed-streptozotocin-induced type 2 diabetic rats and molecular docking studies

AIMS

This paper investigates the hypoglycemic activity of two derivatives of embelin (1) viz. 6-bromoembelin (2) and vilangin (3), in high-fat diet - STZ induced diabetic rats.

MAIN METHODS

The effects of 6-bromoembelin (2) and vilangin (3) on insulin resistance, β-cell dysfunction and glucose transport in high-fat diet (HFD) fed-streptozotocin (STZ) (40mg/kg) induced type 2 diabetic rats were evaluated. The binding modes of 6-bromoembelin (2) and vilangin (3) into PPARγ, PI3K, Akt, and GLUT4 were also studied using Autodock 4.2 and ADT 1.5.6 programs.

KEY FINDINGS

At the dose of 30mg/kg, the plasma glucose, plasma insulin and body weight were reduced by both embelin derivatives in diabetic rats. Additionally the altered lipid profiles and hexokinase, glucose-6-phosphatase and fructose-1,6-bisphosphatase levels were brought to normal. Compared to diabetic control group, there was a significant increase in the expression of PPARγ in epididymal adipose tissue. Inhibition of adipogenic activity and mild activation of PPARγ levels in the skeletal muscle and liver were observed. In epididymal adipose tissue, the compounds increased the insulin-mediated glucose uptake through the activation and translocation of GLUT4 in PI3K/p-Akt signaling cascade.

SIGNIFICANCE

The derivatives of embelin such as 6-bromoembelin (2) and vilangin (3) may be useful in the prevention and treatment of obesity-linked type 2 diabetes mellitus.

Mechanistic Study of Human Glucose Transport Mediated by GLUT1

The glucose transporter 1 (GLUT1) belongs to the major facilitator superfamily (MFS) and is responsible for the constant uptake of glucose. However, the molecular mechanism of sugar transport remains obscure. In this study, homology modeling and molecular dynamics (MD) simulations in lipid bilayers were performed to investigate the combination of the alternate and multisite transport mechanism of glucose with GLUT1 in atomic detail. To explore the substrate recognition mechanism, the outward-open state human GLUT1 homology model was generated based on the template of xylose transporter XylE (PDB ID: 4GBZ), which shares up to 29% sequence identity and 49% similarity with GLUT1. Through the MD simulation study of glucose across lipid bilayer with both the outward-open GLUT1 and the GLUT1 inward-open crystal structure, we investigated six different conformational states and identified four key binding sites in both exofacial and endofacial loops that are essential for glucose recognition and transport. The study further revealed that four flexible gates consisting of W65/Y292/Y293-M420/TM10b-W388 might play important roles in the transport cycle. The study showed that some side chains close to the central ligand binding site underwent larger position changes. These conformational interchanges formed gated networks within an S-shaped central channel that permitted staged ligand diffusion across the transporter. This study provides new inroads for the understanding of GLUT1 ligand recognition paradigm and configurational features which are important for molecular, structural, and physiological research of the MFS members, especially for GLUT1-targeted drug design and discovery.

Identification of residues important for substrate uptake in a glucose transporter from the filamentous fungus Trichoderma reesei

The glucose transporter is an important player in cell metabolism that mediates the intracellular uptake of glucose. Here, we characterized the glucose transporter Stp1 from the filamentous fungus Trichoderma reesei. The individual substitution of several conserved residues for Ala in Stp1 corresponding to those interacting with D-glucose in the xylose/H(+) symporter XylE inflicted contrasting effects on its ability to support the growth of an hxt-null yeast on glucose. The targeted change of Phe 50, proximal to the substrate-binding site, was also found to exert a profound effect on the activity of Stp1. In contrast with the charged residues, the substitution of Phe 50 with either the hydrophilic residues Asn and Gln or the small residues Gly and Ala significantly enhanced the transport of glucose and its fluorescent analogue, 2-NBDG. On the other hand, a variant with the three substitutions I115F, F199I and P214L displayed remarkably improved activity on glucose and 2-NBDG transport. Further analysis indicated that the combined mutations of Ile 115 and Pro 214, positioned on the lateral surface of the Stp1 N-domain, fully accounted for the enhanced transport activity. These results provide insight into the structural basis for glucose uptake in fungal sugar transporters.

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.

Pathogenic mutations causing glucose transport defects in GLUT1 transporter: The role of intermolecular forces in protein structure-function

Two families of glucose transporter - the Na(+)-dependent glucose cotransporter-1 (SGLT family) and the facilitated diffusion glucose transporter family (GLUT family) - play a crucial role in the translocation of glucose across the epithelial cell membrane. How genetic mutations cause life-threatening diseases like GLUT1-deficiency syndrome (GLUT1-DS) is not well understood. In this review, we have combined previous functional data with our in silico analyses of the bacterial homologue of GLUT members, XylE (an outward-facing, partly occluded conformation) and previously proposed GLUT1 homology model (an inward-facing conformation). A variety of native and mutant side chain interactions were modeled to highlight the potential roles of mutations in destabilizing protein-protein interaction hence triggering structural and functional defects. This study sets the stage for future studies of the structural properties that mediate GLUT1 dysfunction and further suggests that both SGLT and GLUT families share conserved domains that stabilize the transporter structure/function via a similar mechanism.

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.

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.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

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.

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.

Functional analysis of the glucose transporters-1a, [corrected] -6, and -13.1 expressed by zebrafish epithelial cells

The hexose supply and subsequent metabolism are crucial for the operations of the iono- and osmoregulatory mechanisms in fish, but how hexose is transported and supplied to cells of the ionoregulatory epithelia is unknown. Three zebrafish glucose transporters (zGLUTs), zGLUT1a, -13.1, and -6, were previously found to respectively be expressed by ionocytes (Na(+)-K(+)-ATPase-rich, Na(+)-Cl(-) cotransporter-expressing, and H(+)-ATPase-rich cells) and adjacent energy-depositing cells [glycogen-rich (GR) cells] in zebrafish skin and gills (32). The present study aimed to test if the transport kinetics of these three zGLUTs differ, and if the transport functional differences are of physiological relevance to the respective functions of epithelial cells. The three zGLUTs expressed by Xenopus laevis oocytes revealed different d-glucose transport kinetics; zGLUT13.1 showed the lowest Michaelis constant (K(m)), whereas zGLUT6 had the highest K(m) and maximal velocity. In morpholino injection experiments, translational knockdown of zGLUT1a and -13.1, respectively, impaired Cl(-)/Ca(2+) and Na(+)/Ca(2+) uptake, but loss-of-function of zGLUT6 did not cause a significant effect on ion uptake functions in zebrafish. Based on these results, zGLUT1a and -13.1 appear to be superior to zGLUT6 in competing for glucose under a situation of low blood glucose due to extensive energy consumption, whereas, in a high blood glucose situation, zGLUT6 is able to absorb the excess glucose for energy deposition. The timely and sufficient supply of energy to ionocytes so that they can carry out ion regulation is definitely a more important event than storing energy in GR cells, particularly when acute environmental change disturbs the ion balance in zebrafish.

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.

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.

Identification of a key residue determining substrate affinity in the human glucose transporter GLUT1

Asn(331) in transmembrane segment 7 of the yeast Saccharomyces cerevisiae transporter Hxt2 has been identified as a single key residue for high-affinity glucose transport by comprehensive chimera approach. The glucose transporter GLUT1 of mammals belongs to the same major facilitator superfamily as Hxt2 and may therefore show a similar mechanism of substrate recognition. The functional role of Ile(287) in human GLUT1, which corresponds to Asn(331) in Hxt2, was studied by its replacement with each of the other 19 amino acids. The mutant transporters were individually expressed in a recently developed yeast expression system for GLUT1. Replacement of Ile(287) generated transporters with various affinities for glucose that correlated well with those of the corresponding mutants of the yeast transporter. Residues exhibiting high affinity for glucose were medium-sized, non-aromatic, uncharged and irrelevant to hydrogen-bond capability, suggesting an important role of van der Waals interaction. Sensitivity to phloretin, a specific inhibitor for the presumed exofacial glucose binding site, was decreased in two mutants, whereas that to cytochalasin B, a specific inhibitor for the presumed endofacial glucose binding site, was unchanged in the mutants. These results suggest that Ile(287) is a key residue for maintaining high glucose affinity in GLUT1 as revealed in Hxt2 and is located at or near the exofacial glucose binding site.

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.

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.

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.

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

A model has been proposed for the exofacial configuration of the Glut1 glucose transporter in which eight transmembrane domains form an inner helical bundle stabilized by four outer helices. The role of transmembrane segment 12, predicted to be an outer helix in this hypothetical model, was examined by cysteine-scanning mutagenesis and the substituted cysteine accessibility method using the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). A previously characterized functional cysteine-less Glut1 molecule was used to produce 21 Glut1 point mutants by changing each residue along helix 12 to a cysteine residue. These mutants were then expressed in Xenopus oocytes, and their protein levels, functional activities, and sensitivities to pCMBS were determined. Strikingly, in contrast to all nine other predicted Glut1 transmembrane helices that have been previously examined by this method, none of the 21 helix 12 single-cysteine mutants exhibited significant inhibition of specific transport activity. Also unlike most other Glut1 transmembrane domains in which solvent-accessible residues lie along a single face of the helix, mutations in five consecutive residues predicted to lie close to the exofacial face of the membrane resulted in sensitivity to pCMBS-induced transport inhibition. These results suggest that helix 12 plays a passive stabilizing role in the structure of Glut1 and is not directly involved in the transport mechanism. Additionally, the pCMBS data indicate that the predicted exoplasmic end of helix 12 is completely exposed to the external solvent when the transporter is in its exofacial configuration.

Cysteine-scanning Mutagenesis and Substituted Cysteine Accessibility Analysis of Transmembrane Segment 4 of the Glut1 Glucose Transporter

A low resolution model has been proposed for the exofacial conformation of the Glut1 glucose transporter in which eight transmembrane segments form an inner helical bundle stabilized by four outer helices. The role of transmembrane segment 4, predicted to be an inner helix in this structural model, was investigated by cysteine-scanning mutagenesis in conjunction with the substituted cysteine accessibility method using the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). A functional, cysteine-less, parental Glut1 molecule was used to produce 21 Glut1 point mutants by individually changing each residue along transmembrane helix 4 to a cysteine. The single cysteine mutants were then expressed in Xenopus oocytes, and their expression levels, transport activities, and sensitivities to pCMBS were determined. In striking contrast to all of the other seven predicted inner helices, none of the 21 helix 4 single-cysteine mutants was demonstrably inhibited by pCMBS. However, cysteine substitution within helix 4 resulted in an unusually high number of severely transport-defective mutants. The low absolute transport activities of two of these mutants (G130C and G134C) were due to their extremely low levels of expression, presumably a result of structural instability and consequent degradation in oocytes, suggesting that these two residues play an important role in maintaining the native structure of Glut1. The other two transport-defective mutants (Y143C and E146C) exhibited low specific transport activities, implying that these two residues play an important role in the transport cycle. Based on these data, we conclude that the exoplasmic end of helix 4 lies outside the inner helical bundle in the exofacial configuration of Glut1.

Identification of a hydrophobic residue as a key determinant of fructose transport by the facilitative hexose transporter SLC2A7 (GLUT7)

Until recently, the only facilitated hexose transporter GLUT proteins (SLC2A) known to transport fructose were GLUTs 2 and 5. However, the recently cloned GLUT7 can also transport fructose as well as glucose. Comparison of sequence alignments indicated that GLUTs 2, 5, and 7 all had an isoleucine residue at position "314" (GLUT7), whereas the non-fructose-transporting isoforms, GLUTs 1, 3, and 4, had a valine at this position. Mutation of Ile-314 to a valine in GLUT7 resulted in a loss of fructose transport, whereas glucose transport remained completely unaffected. Similar results were obtained with GLUTs 2 and 5. Energy minimization modeling of GLUT7 indicated that Ile-314 projects from transmembrane domain 7 (TM7) into the lumen of the aqueous pore, where it could form a hydrophobic interaction with tryptophan 89 from TM2. A valine residue at 314 appeared to produce a narrowing of the vestibule when compared with the isoleucine. It is proposed that this hydrophobic interaction across the pore forms a selectivity filter restricting the access of some hexoses to the substrate binding site(s) within the aqueous channel. The presence of a selectivity filter in the extracellular vestibule of GLUT proteins would allow for subtle changes in substrate specificity without changing the kinetic parameters of the protein.

Predicting the three-dimensional structure of the human facilitative glucose transporter glut1 by a novel evolutionary homology strategy: insights on the molecular mechanism of substrate migration, and binding sites for glucose and inhibitory molecules

The glucose transporters (GLUT/SLC2A) are members of the major facilitator superfamily. Here, we generated a three-dimensional model for Glut1 using a two-step strategy: 1), GlpT structure as an initial homology template and 2), evolutionary homology using glucose-6-phosphate translocase as a template. The resulting structure (PDB No. 1SUK) exhibits a water-filled passageway communicating the extracellular and intracellular domains, with a funnel-like exofacial vestibule (infundibulum), followed by a 15 A-long x 8 A-wide channel, and a horn-shaped endofacial vestibule. Most residues which, by mutagenesis, are crucial for transport delimit the channel, and putative sugar recognition motifs (QLS, QLG) border both ends of the channel. On the outside of the structure there are two positively charged cavities (one exofacial, one endofacial) delimited by ATP-binding Walker motifs, and an exofacial large side cavity of yet unknown function. Docking sites were found for the glucose substrate and its inhibitors: glucose, forskolin, and phloretin at the exofacial infundibulum; forskolin, and phloretin at an endofacial site next to the channel opening; and cytochalasin B at a positively charged endofacial pocket 3 A away from the channel. Thus, 1SUK accounts for practically all biochemical and mutagenesis evidence, and provides clues for the transport process.

Transmembrane segment 3 of the Glut1 glucose transporter is an outer helix

A model has been proposed for the structure of the Glut1 glucose transporter based on the results of mutagenesis studies and homology modeling in which eight transmembrane segments form an inner helical bundle surrounded by four outer helices. The role of transmembrane segment 3 in this structural model was investigated using cysteine-scanning mutagenesis in conjunction with the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). Twenty-one Glut1 mutants were created from a fully functional, cysteine-less, parental Glut1 molecule by successively changing each residue along transmembrane helix 3 to a cysteine. The single cysteine mutants were then expressed in Xenopus oocytes, and their expression levels, transport activities, and sensitivities to pCMBS were determined. Cysteine substitution at methionine 96 abolished transport activity, whereas substitutions at the other positions resulted in either modest reductions or no significant effect on transport activity. In striking contrast to all other helices that have been examined to date, only one of the 21 helix 3 single-cysteine mutants was inhibited by pCMBS, suggesting that only a small portion of this helix is exposed to the external solvent. This result is consistent with predictions based on our current structural model, in which helix 3 is one of four outer helices that surround the inner helical bundle that comprises the aqueous substrate-binding cavity. An updated two-dimensional model for the orientation of the 12 transmembrane helices and the conformation of the exofacial glucose-binding site of Glut1 is presented that is consistent with existing experimental data.

Functional characterization of an insulin-responsive glucose transporter (GLUT4) from fish adipose tissue

Glucose transport across the plasma membrane is mediated by a family of glucose transporter proteins (GLUTs), several of which have been identified in mammalian, avian, and, more recently, in fish species. Here, we report on the cloning of a salmon GLUT from adipose tissue with a high sequence homology to mammalian GLUT4 that has been named okGLUT4. Kinetic analysis of glucose transport following expression in Xenopus laevis oocytes demonstrated a 7.6 +/- 1.4 mM K(m) for 2-deoxyglucose (2-DG) transport measured under zero-trans conditions and 14.4 +/- 1.5 mM by equilibrium exchange of 3-O-methylglucose. Transport of 2-DG by okGLUT4-injected oocytes was stereospecific and was competed by D-glucose, D-mannose, and, to a lesser extent, D-galactose and D-fructose. In addition, 2-DG uptake was inhibited by cytochalasin B and ethylidene glucose. Moreover, insulin stimulated glucose uptake in Xenopus oocytes expressing okGLUT4 and in isolated trout adipocytes, which contain the native form of okGLUT4. Despite differences in protein motifs important for insulin-stimulated translocation of mammalian GLUT4, okGLUT4 was able to translocate to the plasma membrane from intracellular localization sites in response to insulin when expressed in 3T3-L1 adipocytes. These data demonstrate that okGLUT4 is a structural and functional fish homolog of mammalian GLUT4 but with a lower affinity for glucose, which could in part explain the lower ability of fish to clear a glucose load.

Relative Proximity and Orientation of Helices 4 and 8 of the GLUT1 Glucose Transporter

A structure has been proposed for glucose transporter-1 (GLUT1) based upon homology modeling that is consistent with the results of numerous mutagenesis studies (Mueckler, M., and Makepeace, C. (2004) J. Biol. Chem. 279, 10494-10499). To further test and refine this model, the relative orientation and proximity of transmembrane helices 4 and 8 were analyzed by chemical crosslinking of di-cysteine mutants created in a reporter GLUT1 construct. All six native cysteine residues of GLUT1 were changed to either glycine or serine residues by site-directed mutagenesis, resulting in a functional Glut1 construct with Cys mutated to Gly/Ser (C-less). The GLUT1 reporter molecule was engineered from C-less GLUT1 by creating a unique cleavage site for factor Xa protease within the central cytoplasmic loop and by eliminating the site of N-linked glycosylation. Fourteen functional di-cysteine mutants were then created from the C-less reporter construct, each mutant containing a single cysteine residue in helix 4 and one cysteine residue in helix 8. These mutants were expressed in Xenopus oocytes, and the sensitivity of each mutant to intramolecular crosslinking by two homo-bifunctional, thiol-specific crosslinking reagents, bismaleimidehexane and 1,4-phenylenedimaleimide, was ascertained by protease cleavage followed by immunoblot analysis. Four pairs of cysteine residues, Cys(148)/Cys(328), Cys(145)/Cys(328), Cys(148)/Cys(325), and Cys(145)/Cys(325), were observed to be in close enough proximity to be susceptible to crosslinking by one or both reagents. All five of the cysteine residues susceptible to crosslinking are predicted to lie on the same face of helix 4 or 8 and to reside close to the cytoplasmic face of the membrane. These data indicate that the cytoplasmic ends of helices 4 and 8 lie within 6-16 A of one another and that the two helices twist or tilt such that they are further than 16 A apart toward the center and the exoplasmic side of the membrane. An updated model for the clustering of the transmembrane helices of GLUT1 is presented based on these data.

Analysis of transmembrane segment 8 of the GLUT1 glucose transporter by cysteine-scanning mutagenesis and substituted cysteine accessibility

The GLUT1 glucose transporter has been proposed to form an aqueous substrate translocation pathway via the clustering of several amphipathic transmembrane helices (Mueckler, M., Caruso, C., Baldwin, S. A., Panico, M., Blench, I., Morris, H. R., Allard, W. J., Lienhard, G. E., and Lodish, H. F. (1985) Science 229, 941-945). The possible role of transmembrane helix 8 in the formation of this permeation pathway was investigated using cysteine-scanning mutagenesis and the membrane-impermeant sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). Twenty-one GLUT1 mutants were created from a fully functional cysteine-less parental GLUT1 molecule by successively changing each residue along transmembrane segment 8 to a cysteine. The mutant proteins were then expressed in Xenopus oocytes, and their membrane concentrations, 2-deoxyglucose uptake activities, and sensitivities to pCMBS were determined. Four positions within helix 8, alanine 309, threonine 310, serine 313, and glycine 314, were accessible to pCMBS as judged by the inhibition of transport activity. All four of these residues are clustered along one face of a putative alpha-helix. These results suggest that transmembrane segment 8 of GLUT1 forms part of the sugar permeation pathway. Updated two-dimensional models for the orientation of the 12 transmembrane helices and the conformation of the exofacial glucose binding site of GLUT1 are proposed that are consistent with existing experimental data and homology modeling based on the crystal structures of two bacterial membrane transporters.

Functional studies of threonine 310 mutations in Glut1: T310I is pathogenic, causing Glut1 deficiency

We have previously reported on a patient with the Glut1 deficiency syndrome (Online Mendelian Inheritance in Man number 606777) carrying a heterozygous T310I missense mutation in the GLUT1 gene (Klepper, J., Wang, D., Fischbarg, J., Vera, J. C., Jarjour, I. T., O'Driscoll, K. R., and De Vivo, D. C. (1999) Neurochem. Res. 24, 587-594). To investigate the molecular basis for the associated functional deficit, we constructed T310A, T310S, and T310I human GLUT1 mutants for expression in Xenopus laevis oocytes via cRNA injection. For all mutants, glucose transport was decreased, and osmotic water permeability (Pf) was increased. Km values for 3-O-methylglucose (3-OMG) uptake under zero-trans influx and equilibrium exchange influx conditions were, respectively, 13 +/- 1 and 68 +/- 5 mm for wild-type Glut1, 5 +/- 1 and 25 +/- 6 mm for T310A, 6 +/- 3 and 30 +/- 6 mm for T310I, and 5 +/- 1 and 48 +/- 5 mm for T310S. Compared with wild-type Glut1, we determined the following. (a). Zero-trans and equilibrium exchange influx values of 3-OMG were significantly decreased, respectively, 15 and 5% in T310A, 8 and 3% in T310I, and 40 and 34% in T310S mutants. (b). Zero-trans efflux of 3-OMG and dehydroascorbic acid uptake were significantly decreased in mutants. (c). The relative Pf values for T310A, T310I, and T310S were increased 3-, 4.8-, and 3.5-fold compared with wild-type values. We found a very high negative correlation between the rate of glucose uptake and Pf (-0.93), and between hydropathy and uptake (-0.92), a moderate correlation between hydropathy and Pf (0.73), and a minimal correlation between uptake, Pf, and molecular weight. These findings are consistent with a central role for hydropathy rather than size at position 310 of this mutation.

Functional consequences of an in vivo mutation in exon 10 of the human GLUT1 gene

The functional consequences of an in vivo heterozygous insertion mutation in the human facilitated glucose transporter isoform 1 (GLUT1) gene were investigated. The resulting frameshift in exon 10 changed the primary structure of the C-terminus from 42 in native GLUT1 to 61 amino acid residues in the mutant. Kinetic studies on a patient's erythrocytes were substantiated by expressing the mutant cDNA in Xenopus laevis oocytes. K(m) and V(max) values were clearly decreased explaining pathogenicity. Targeting to the plasma membrane was comparable between mutant and wild-type GLUT1. Transport inhibition by cytochalasin B was more effective in the mutant than in the wild-type transporter. The substrate specificity of GLUT1 remained unchanged.

Characterization of a cysteine-less human reduced folate carrier: localization of a substrate-binding domain by cysteine-scanning mutagenesis and cysteine accessibility methods

The human reduced folate carrier (hRFC) mediates the transport of reduced folates and classical anti-folates into mammalian cells. Whereas the functionally important domains in hRFC are poorly characterized, previous studies with anti-folate-resistant cells suggest critical roles for transmembrane domain (TMD) 1 and residues (Gly44, Glu45, Ser46 and Ile48) in or flanking this region. An hRFC mutant devoid of cysteine residues was prepared by deleting the C-terminal 56 amino acids, including four cysteine residues, and mutagenizing the remaining cysteine residues to serine residues. A fully functional cysteine-less hRFC protein was expressed in transport-impaired MtxRIIOuaR2-4 Chinese-hamster ovary cells. To explore the role of residues in or flanking TMD1 in transport, all 24 amino acids from Trp25 to Ile48 of hRFC were mutated individually to cysteine residues, and the mutant hRFCs were transfected into MtxRIIOuaR2-4 cells. All of the 24 cysteine mutants were expressed and, with the exception of R42C (Arg42-->Cys), were capable of mediating methotrexate uptake above the low level in MtxRIIOuaR2-4 cells. We found that by treating the transfected cells with the small, water-soluble, thiol-reactive anionic reagent, sodium (2-sulphonatoethyl) methanethiosulphonate, methotrexate transport by several of the cysteine-substituted hRFC mutants was significantly inhibited, including Q40C, G44C, E45C and I48C. Sodium (2-sulphonatoethyl) methanethiosulphonate transport inhibition of the Q40C, G44C and I48C mutants was protected by leucovorin [(6R, S)-5-formyltetrahydrofolate], indicating that these residues lie at or near a substrate-binding site. Using surface-labelling reagents [N-biotinylaminoethyl methanethiosulphonate and 3-(N-maleimidylpropionyl)biocytin, combined with 4-acetamido-4'-maleimidylstilbene-2,2'-disulphonic acid] with cysteine mutants from positions 37-48, the extracellular TMD1 boundary was found to lie between residues 39 and 40, and amino acids 44-46 and 48 were localized to the TMD1 exofacial loop. Collectively, our results imply that amino acids 40, 44, 48 and, possibly, 42 serve important roles in hRFC transport, albeit not as structural components of the putative transmembrane channel for folate substrates.

Changes in glucose transport and water permeability resulting from the T310I pathogenic mutation in Glut1 are consistent with two transport channels per monomer

We studied glucose and water passage across wild type (WT) glucose transporter Glut1 and its T310I pathogenic mutant, expressing them in Xenopus laevis oocytes. We found that the T310I mutation produced a 8-fold decrease in glucose transport (zero-trans influx, 13 +/- 2% compared with WT), accompanied by a 2.8-fold increase in the osmotic water permeability (P(f) 280 +/- 40% compared with WT), and no change in the diffusional water permeability (P(d)). The dependence of glucose and water transports on the amounts of mutant cRNA injected was identical exponential buildups (k = 19.7 ng), suggesting that they depend similarly on the quaternary structure. The E(a) values for P(f) were 16 +/- 0.4 (WT) and 11 +/- 1 kcal mol(-1) (T310I). We report for the first time that 10 mm d-glucose and l-glucose inhibit P(f) by approximately 45% in the WT but not in the T310I mutant. In addition, 10 mm maltose reduces P(f) (15-20%) in both cases. However, 5 mm l-glucose increased the P(f) of T310I, consistent with a cooperative effect. These experimental observations and an analysis of our three-dimensional model strongly suggest the presence of two channels per Glut1 monomer, one of which can be blocked by the mutation T310I.

Analysis of transmembrane segment 10 of the Glut1 glucose transporter by cysteine-scanning mutagenesis and substituted cysteine accessibility

The Glut1 glucose transporter has been proposed to form an aqueous sugar translocation pathway through the lipid bilayer via the clustering of several transmembrane helices (Mueckler, M., Caruso, C., Baldwin, S. A., Panico, M., Blench, I., Morris, H. R., Allard, W. J., Lienhard, G. E., and Lodish, H. F. (1985) Science 229, 941-945). The participation of transmembrane helix 10 in the formation of this putative aqueous tunnel was tested using cysteine-scanning mutagenesis in conjunction with the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). A series of 21 mutants was created from a fully functional, cysteine-less, parental Glut1 molecule by changing each residue within putative transmembrane segment 10 to cysteine. Each mutant was then expressed in Xenopus oocytes, and its plasma membrane content, 2-deoxyglucose uptake activity, and sensitivity to pCMBS were measured. Helix 10 exhibited a highly distinctive reaction profile to scanning mutagenesis whereby cysteine substitution at residues within the cytoplasmic N-terminal half of the helix tended to increase specific transport activity, whereas substitution at residues within the exoplasmic C-terminal half of the helix tended to decrease specific transport activity. Four residues within helix 10 were clearly accessible to pCMBS as judged by inhibition or stimulation of transport activity. All four of these residues were clustered along one face of a putative alpha-helix. These results combined with previously published data suggest that transmembrane segment 10 of Glut1 forms part of the sugar permeation pathway. Two-dimensional models for the conformation of the 12 transmembrane helices and the exofacial glucose-binding site of Glut1 are proposed that are consistent with existing experimental data.

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.

The large cytoplasmic loop of the glucose transporter GLUT1 is an essential structural element for function

Alanine scanning mutagenesis and the introduction of deletions and insertions were used to address the role of the large cytoplasmic loop in 2-deoxy-D-glucose (2-DOG) uptake by GLUT1 expressed in Xenopus oocytes. Alanine scanning mutagenesis of 29 amino acid residues that are identical or homologous in GLUT1 to GLUT4 demonstrated that the transport activities of only a few variants were affected. Progressive truncation of the loop by six deletions leaving intact 59 (delta236-241), 49 (delta231-246), 39 (delta226-251), 28 (delta221-257), 18 (delta216-262), or 10 (delta213-267) amino acid residues resulted in a progressive decrease in 2-DOG uptake. Compared with wild-type GLUT1 the uptake rates varied between 33% for the delta236-241 mutant and 4% for the delta213-267 mutant. Insertional mutagenesis using hexaalanine or hexaglycine to fill in the deletion 236D-241L restored 2-DOG uptake to 73% of wild-type GLUT1 in the case of hexaalanine, whereas hexaglycine insertion was without effect. Confocal laser microscopy demonstrated that a deletion of six amino acid residues did not influence the expression level in the plasma membrane (delta236-241 mutant), whereas the plasma membrane fluorescence of the delta213-267 mutant was comparable with that of water-injected Xenopus oocytes. Computer-aided secondary structure prediction of the loop suggested that it consists of a long alpha-helix bundle interrupted or kinked by the highly conserved glycine-233.

Repacking of the transmembrane domains of P-glycoprotein during the transport ATPase cycle

P-glycoprotein (P-gp) is an ABC (ATP-binding cassette) transporter, which hydrolyses ATP and extrudes cytotoxic drugs from mammalian cells. P-gp consists of two transmembrane domains (TMDs) that span the membrane multiple times, and two cytoplasmic nucleotide-binding domains (NBDs). We have determined projection structures of P-gp trapped at different steps of the transport cycle and correlated these structures with function. In the absence of nucleotide, an approximately 10 A resolution structure was determined by electron cryo-microscopy of two-dimensional crystals. The TMDs form a chamber within the membrane that appears to be open to the extracellular milieu, and may also be accessible from the lipid phase at the interfaces between the two TMDs. Nucleotide binding causes a repacking of the TMDs and reduction in drug binding affinity. Thus, ATP binding, not hydrolysis, drives the major conformational change associated with solute translocation. A third distinct conformation of the protein was observed in the post-hydrolytic transition state prior to release of ADP/P(i). Biochemical data suggest that these rearrangements may involve rotation of transmembrane alpha-helices. A mechanism for transport is suggested.

Sequence and functional analysis of GLUT10: a glucose transporter in the Type 2 diabetes-linked region of chromosome 20q12-13.1

We have carried out a detailed sequence and functional analysis of a novel human facilitative glucose transporter, designated GLUT10, located in the Type 2 diabetes-linked region of human chromosome 20q12-13.1. The GLUT10 gene is located between D20S888 and D20S891 and is encoded by 5 exons spanning 26.8 kb of genomic DNA. The human GLUT10 cDNA encodes a 541 amino acid protein that shares between 31 and 35% amino acid identity with human GLUT1-8. The predicted amino acid sequence of GLUT10 is nearly identical in length to the recently described GLUT9 homologue, but is longer than other known members of the GLUT family. In addition, we have cloned the mouse cDNA homolog of GLUT10 that encodes a 537 amino acid protein that shares 77.3% identity with human GLUT10. The amino acid sequence probably has 12 predicted transmembrane domains and shares characteristics of other mammalian glucose transporters. Human and mouse GLUT10 retain several sequence motifs characteristic of mammalian glucose transporters including VP497ETKG in the cytoplasmic C-terminus, G73R[K,R] between TMD2 and TMD3 (PROSITE PS00216), VD92RAGRR between TMD8 and TMD9 (PROSITE PS00216), Q242QLTG in TMD7, and tryptophan residues W430 (TMD10) and W454 (TMD11), that correspond to trytophan residues previously implicated in GLUT1 cytochalasin B binding and hexose transport. Neither human nor mouse GLUT10 retains the full P[E,D,N]SPR motif after Loop6 but instead is replaced with P186AG[T,A]. A PROSITE search also shows that GLUT10 has lost the SUGAR TRANSPORT 2 pattern (PS00217), a result of the substitution G113S in TMD4, while all other known human GLUTs retain the glycine and the pattern match. The significance of this substitution is unknown. Sites for N-linked glycosylation are predicted at N334ATG between TMD8 and TMD9 and N526STG in the cytoplasmic C-terminus. Northern hybridization analysis identified a single 4.4-kb transcript for GLUT10 in human heart, lung, brain, liver, skeletal muscle, pancreas, placenta, and kidney. By RT-PCR analysis, GLUT10 mRNA was also detected in fetal brain and liver. When expressed in Xenopus oocytes, human GLUT10 exhibited 2-deoxy-D-glucose transport with an apparent Km of approximately 0.3 mM. D-Glucose and D-galactose competed with 2-deoxy-D-glucose and transport was inhibited by phloretin. The gene localization and functional properties suggest a role for GLUT10 in glucose metabolism and Type 2 diabetes.

Functional consequences of the autosomal dominant G272A mutation in the human GLUT1 gene

The first autosomal dominant missense mutation (G272A) reported within the human GLUT1 gene and shared by three affected family members was investigated in respect to functional consequences. Substitution of glycine-91 by site-directed mutagenesis with either aspartate or alanine resulted in a significant decrease in transport activity of GLUT1 expressed in Xenopus oocytes. Expression of mutant transporters was confirmed by immunoblot, 2-deoxy-glucose uptake and confocal laser microscopy. The data agree with 3-O-methyl-glucose uptake into patient erythrocytes and indicate that the loss of glycine rather than a hydrophilic side chain (Gly91Asp) defines the functional consequences of this mutation.

The presence of an aqueous cavity in the proton-pumping pathway of the pyridine nucleotide transhydrogenase of Escherichia coli is suggested by the reaction of the enzyme with sulfhydryl inhibitors

The pyridine nucleotide transhydrogenase of Escherichia coli carries out transmembrane proton translocation coupled to transfer of a hydride ion equivalent between NAD(+) and NADP(+). The membrane domain (domain II) of the enzyme is composed of 13 transmembrane helices. Previous studies (N. A. Glavas et al., Biochemistry 34, 7694-7702, 1995) have suggested that betaHis91 in transmembrane helix 9 is involved in the translocation pathway of protons across the membrane. In this study we have replaced amino acid residues on the same face of helix 9 as betaHis91 by single cysteine residues. We then examined the effect of the sulfhydryl inhibitors N-ethylmaleimide (NEM) and p-chloromercuriphenylsulfonate (pCMPS) on enzyme activity and, in the case of [(14)C]NEM, as an enzyme label. The pattern of enzyme inhibition and labelling is consistent with the presence of an aqueous cavity through domain II from the cytosolic surface to the region of betaHis91. Residue betaAsn222 in helix 13, which appears also to be involved in the proton pathway across domain II, may interface with this aqueous cavity. A further series of mutants of betaGlu124 on helix 10 confirms the proposal (P. D. Bragg and C. Hou, Arch. Biochem. Biophys. 363, 182-190, 1999) that this residue is involved in passive permeation of protons across domain II.