Conformational changes couple Na+ and glucose transport

Flux coupling, not specificity, shapes the transport and phylogeny of SLC6 glycine transporters

ATB[Formula: see text] (SLC6A14) is a member of the amino acid transporter branch of the SLC6 family along with GlyT1 (SLC6A9) and GlyT2 (SLC6A5), two glycine-specific transporters coupled to 2:1 and 3:1 Na[Formula: see text]:Cl[Formula: see text], respectively. In contrast, ATB[Formula: see text] exhibits broad substrate specificity for all neutral and cationic amino acids, and its ionic coupling remains unsettled. Using the reversal potential slope method, we demonstrate a 3:1:1 Na[Formula: see text]:Cl[Formula: see text]:Gly stoichiometry for ATB[Formula: see text] that is consistent with its 2.1 e/Gly charge coupling. Like GlyT2, ATB[Formula: see text] behaves as a unidirectional transporter with virtually no glycine efflux at negative potentials after uptake, except by heteroexchange as remarkably shown by leucine activation of NMDARs in Xenopus oocytes coexpressing both membrane proteins. Analysis and computational modeling of the charge movement of ATB[Formula: see text] reveal a higher affinity for sodium in the absence of substrate than GlyT2 and a gating mechanism that locks Na[Formula: see text] into the apo-transporter at depolarized potentials. A 3:1 Na[Formula: see text]:Cl[Formula: see text] stoichiometry justifies the concentrative transport properties of ATB[Formula: see text] and explains its trophic role in tumor growth, while rationalizing its phylogenetic proximity to GlyT2 despite their extreme divergence in specificity.

Integrated AlphaFold2 and DEER investigation of the conformational dynamics of a pH-dependent APC antiporter

The Amino Acid-Polyamine-Organocation (APC) transporter GadC contributes to the survival of pathogenic bacteria under extreme acid stress by exchanging extracellular glutamate for intracellular γ-aminobutyric acid (GABA). Its structure, determined in an inward-facing conformation at alkaline pH, consists of the canonical LeuT-fold with a conserved five-helix inverted repeat, thereby resembling functionally divergent transporters such as the serotonin transporter SERT and the glucose-sodium symporter SGLT1. However, despite this structural similarity, it is unclear if the conformational dynamics of antiporters such as GadC follow the blueprint of these or other LeuT-fold transporters. Here, we used double electron-electron resonance (DEER) spectroscopy to monitor the conformational dynamics of GadC in lipid bilayers in response to acidification and substrate binding. To guide experimental design and facilitate the interpretation of the DEER data, we generated an ensemble of structural models in multiple conformations using a recently introduced modification of AlphaFold2 . Our experimental results reveal acid-induced conformational changes that dislodge the Cterminus from the permeation pathway coupled with rearrangement of helices that enables isomerization between inward- and outward-facing states. The substrate glutamate, but not GABA, modulates the dynamics of an extracellular thin gate without shifting the equilibrium between inward- and outward-facing conformations. In addition to introducing an integrated methodology for probing transporter conformational dynamics, the congruence of the DEER data with patterns of structural rearrangements deduced from ensembles of AlphaFold2 models illuminates the conformational cycle of GadC underpinning transport and exposes yet another example of the divergence between the dynamics of different families in the LeuT-fold.

Principles of Alternating Access in LeuT-fold Transporters: Commonalities and Divergences

Found in all domains of life, transporters belonging to the LeuT-fold class mediate the import and exchange of hydrophilic and charged compounds such as amino acids, metals, and sugar molecules. Nearly two decades of investigations on the eponymous bacterial transporter LeuT have yielded a library of high-resolution snapshots of its conformational cycle linked by solution-state experimental data obtained from multiple techniques. In parallel, its topology has been observed in symporters and antiporters characterized by a spectrum of substrate specificities and coupled to gradients of distinct ions. Here we review and compare mechanistic models of transport for LeuT, its well-studied homologs, as well as functionally distant members of the fold, emphasizing the commonalities and divergences in alternating access and the corresponding energy landscapes. Our integrated summary illustrates how fold conservation, a hallmark of the LeuT fold, coincides with divergent choreographies of alternating access that nevertheless capitalize on recurrent structural motifs. In addition, it highlights the knowledge gap that hinders the leveraging of the current body of research into detailed mechanisms of transport for this important class of membrane proteins.

Functional characterization of Atlantic salmon (Salmo salar L.) PepT2 transporters

Abstract

The high‐affinity/low‐capacity system Slc15a2 (PepT2) is responsible for the reuptake of di/tripeptides from the renal proximal tubule, but it also operates in many other tissues and organs. Information regarding PepT2 in teleost fish is limited and, to date, functional data are available from the zebrafish (Danio rerio) only. Here, we report the identification of two slc15a2 genes in the Atlantic salmon (Salmo salar) genome, namely slc15a2a and slc15a2b. The two encoded PepT2 proteins share 87% identity and resemble both structurally and functionally the canonical vertebrate PepT2 system. The mRNA tissue distribution analyses reveal a widespread distribution of slc15a2a transcripts, being more abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and the distal part of the gastrointestinal tract. The function of the two transporters was investigated by heterologous expression in Xenopus laevis oocytes and two‐electrode voltage‐clamp recordings of transport and presteady‐state currents. Both PepT2a and PepT2b in the presence of Gly‐Gln elicit pH‐dependent and Na⁺ independent inward currents. The biophysical and kinetic analysis of the recorded currents defined the transport properties, confirming that the two Atlantic salmon PepT2 proteins behave as high‐affinity/low‐capacity transporters. The recent structures and the previous kinetic schemes of rat and human PepT2 qualitatively account for the characteristics of the two Atlantic salmon proteins. This study is the first to report on the functional expression of two PepT2‐type transporters that operate in the same vertebrate organism as a result of (a) gene duplication process(es).

Key points

Two slc15a2‐type genes, slc15a2a and slc15a2b coding for PepT2‐type peptide transporters were found in the Atlantic salmon.

slc15a2a transcripts, widely distributed in the fish tissues, are abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and distal gastrointestinal tract.

Amino acids involved in vertebrate Slc15 transport function are conserved in PepT2a and PepT2b proteins.

Detailed kinetic analysis indicates that both PepT2a and PepT2b operate as high‐affinity transporters.

The kinetic schemes and structures proposed for the mammalian models of PepT2 are suitable to explain the function of the two Atlantic salmon transporters.

SSM-based electrophysiology, a label-free real-time method reveals sugar binding & transport events in SGLT1

Here, we present a solid-supported membrane (SSM)-based electrophysiological approach to study sugar binding and Na⁺/glucose cotransport by SGLT1 in membrane vesicles. SSM-based electrophysiology delivers a cumulative real-time current readout from numerous SGLT1 proteins simultaneously using a gold-coated sensor chip. In contrast to conventional techniques, which mainly operate with voltage steps, currents are triggered by sugar or sodium addition. Sugar concentration jumps in the presence of sodium lead to transport currents between 5 and 10 nA. Remarkably, in the absence of sodium (i.e. no transport), we observed fast pre-steady-state (PSS) currents with time constants between 3 and 10 ms. These PSS currents mainly originate from sugar binding. Sodium binding does not induce PSS currents. Due to high time resolution, PSS currents were distinguished from transport and eventually correlated with conformational transitions within the sugar translocation pathway. In addition, we analyzed the impact of driving forces on transport and binding currents, showing that membrane voltage and sodium concentration gradients lead to an increased transport rate without affecting sugar binding kinetics. We also compared Na⁺/sugar efflux with physiologically relevant influx and found similar transport rates, but lower affinity in efflux mode. SSM-based electrophysiology is a powerful technique, which overcomes bottlenecks for transport measurements observed in other techniques such as the requirement of labels or the lack of real-time data. Rapid solution exchange enables the observation of substrate-induced electrogenic events like conformational transitions, opening novel perspectives for in-depth functional studies of SGLT1 and other transporters.

Characterization of SGLT1-mediated glucose transport in Caco-2 cell monolayers, and absence of its regulation by sugar or epinephrine

Caco-2 cells are increasingly used to study the absorption of drugs and nutrients, including D-glucose, an important nutrient that mainly gets absorbed from the intestine by the sodium/glucose cotransporter 1 (SGLT1). However, disadvantages of Caco-2 cells for such studies have been reported, e.g., D-glucose cannot elicit translocation of the intracellular pool of SGLT1 to the apical membrane, the origin of the cells affects glucose uptake, and Caco-2 cells exhibit heterogeneity. This study aimed to characterize SGLT1-mediated glucose transport across Caco-2 cell monolayers. We found that at lower glucose concentrations (5 mM) SGLT1 contributes more to total glucose transport than at higher (10 mM) glucose concentrations, suggesting contributions by another transporter at higher glucose concentrations. This contrasts with the in vivo situation, where SGLT1 dominant glucose transporter at all glucose concentrations. We also tested whether known regulators like sugars or catecholamines can stimulate glucose transport across Caco-2 cell monolayers. Neither epinephrine nor 2-deoxy-D-glucose could stimulate glucose transport. Moreover, the epinephrine could not induce accumulation of cyclic adenosine monophosphate (cAMP) in Caco-2 cells, indicating the absence of a functional β₂-adrenoceptor in Caco-2 cells, which could explain the lack of epinephrine effect on glucose transport. Also, Caco-2 cells may lack some kinases required for increased SGLT1 transport. Overall, SGLT1-mediated glucose transport and its regulation in Caco-2 cells differ from that in vivo, and caution is advised when extrapolating glucose transport results obtained with this model to the in vivo situation.

Plant glucose transporter structure and function

The carbohydrate D-glucose is the main source of energy in living organisms. In contrast to animals, as well as most fungi, bacteria, and archaea, plants are capable to synthesize a surplus of sugars characterizing them as autothrophic organisms. Thus, plants are de facto the source of all food on earth, either directly or indirectly via feed to livestock. Glucose is stored as polymeric glucan, in animals as glycogen and in plants as starch. Despite serving a general source for metabolic energy and energy storage, glucose is the main building block for cellulose synthesis and represents the metabolic starting point of carboxylate- and amino acid synthesis. Finally yet importantly, glucose functions as signalling molecule conveying the plant metabolic status for adjustment of growth, development, and survival. Therefore, cell-to-cell and long-distance transport of photoassimilates/sugars throughout the plant body require the fine-tuned activity of sugar transporters facilitating the transport across membranes. The functional plant counterparts of the animal sodium/glucose transporters (SGLTs) are represented by the proton-coupled sugar transport proteins (STPs) of the plant monosaccharide transporter(-like) family (MST). In the framework of this special issue on “Glucose Transporters in Health and Disease,” this review gives an overview of the function and structure of plant STPs in comparison to the respective knowledge obtained with the animal Na⁺-coupled glucose transporters (SGLTs).

Unique structural features in an Nramp metal transporter impart substrate-specific proton cotransport and a kinetic bias to favor import

Natural resistance-associated macrophage protein (Nramp) transporters enable uptake of essential transition metal micronutrients in numerous biological contexts. These proteins are believed to function as secondary transporters that harness the electrochemical energy of proton gradients by "coupling" proton and metal transport. Here we use the Deinococcus radiodurans (Dra) Nramp homologue, for which we have determined crystal structures in multiple conformations, to investigate mechanistic details of metal and proton transport. We untangle the proton-metal coupling behavior of DraNramp into two distinct phenomena: ΔpH stimulation of metal transport rates and metal stimulation of proton transport. Surprisingly, metal type influences substrate stoichiometry, leading to manganese-proton cotransport but cadmium uniport, while proton uniport also occurs. Additionally, a physiological negative membrane potential is required for high-affinity metal uptake. To begin to understand how Nramp's structure imparts these properties, we target a conserved salt-bridge network that forms a proton-transport pathway from the metal-binding site to the cytosol. Mutations to this network diminish voltage and ΔpH dependence of metal transport rates, alter substrate selectivity, perturb or eliminate metal-stimulated proton transport, and erode the directional bias favoring outward-to-inward metal transport under physiological-like conditions. Thus, this unique salt-bridge network may help Nramp-family transporters maximize metal uptake and reduce deleterious back-transport of acquired metals. We provide a new mechanistic model for Nramp proton-metal cotransport and propose that functional advantages may arise from deviations from the traditional model of symport.

The contribution of voltage clamp fluorometry to the understanding of channel and transporter mechanisms

Cowgill and Chanda discuss the importance of voltage clamp fluorometry to the functional interpretation of ion channel and transporter structures.

Key advances in single particle cryo-EM methods in the past decade have ushered in a resolution revolution in modern biology. The structures of many ion channels and transporters that were previously recalcitrant to crystallography have now been solved. Yet, despite having atomistic models of many complexes, some in multiple conformations, it has been challenging to glean mechanistic insight from these structures. To some extent this reflects our inability to unambiguously assign a given structure to a particular physiological state. One approach that may allow us to bridge this gap between structure and function is voltage clamp fluorometry (VCF). Using this technique, dynamic conformational changes can be measured while simultaneously monitoring the functional state of the channel or transporter. Many of the important papers that have used VCF to probe the gating mechanisms of channels and transporters have been published in the Journal of General Physiology. In this review, we provide an overview of the development of VCF and discuss some of the key problems that have been addressed using this approach. We end with a brief discussion of the outlook for this technique in the era of high-resolution structures.

L amino acid transporter structure and molecular bases for the asymmetry of substrate interaction

L-amino acid transporters (LATs) play key roles in human physiology and are implicated in several human pathologies. LATs are asymmetric amino acid exchangers where the low apparent affinity cytoplasmic side controls the exchange of substrates with high apparent affinity on the extracellular side. Here, we report the crystal structures of an LAT, the bacterial alanine-serine-cysteine exchanger (BasC), in a non-occluded inward-facing conformation in both apo and substrate-bound states. We crystallized BasC in complex with a nanobody, which blocks the transporter from the intracellular side, thus unveiling the sidedness of the substrate interaction of BasC. Two conserved residues in human LATs, Tyr 236 and Lys 154, are located in equivalent positions to the Na1 and Na2 sites of sodium-dependent APC superfamily transporters. Functional studies and molecular dynamics (MD) calculations reveal that these residues are key for the asymmetric substrate interaction of BasC and in the homologous human transporter Asc-1.

L-Amino acid Transporters (LATs) are asymmetric amino acid exchangers. Here the authors determine the crystal structure of a prokaryotic LAT, the alanine-serine-cysteine exchanger (BasC) and identify key residues for asymmetric substrate interaction in both BasC and the homologous human transporter Asc-1 through functional studies.

The molecular mechanism of SLC34 proteins: insights from two decades of transport assays and structure-function studies

The expression cloning some 25 years ago of the first member of SLC34 solute carrier family, the renal sodium-coupled inorganic phosphate cotransporter (NaPi-IIa) from rat and human tissue, heralded a new era of research into renal phosphate handling by focussing on the carrier proteins that mediate phosphate transport. The cloning of NaPi-IIa was followed by that of the intestinal NaPi-IIb and renal NaPi-IIc isoforms. These three proteins constitute the main secondary-active Na⁺-driven pathways for apical entry of inorganic phosphate (P_i) across renal and intestinal epithelial, as well as other epithelial-like organs. The key role these proteins play in mammalian P_i homeostasis was revealed in the intervening decades by numerous in vitro and animal studies, including the development of knockout animals for each gene and the detection of naturally occurring mutations that can lead to P_i-handling dysfunction in humans. In addition to characterising their physiological regulation, research has also focused on understanding the underlying transport mechanism and identifying structure-function relationships. Over the past two decades, this research effort has used real-time electrophysiological and fluorometric assays together with novel computational biology strategies to develop a detailed, but still incomplete, understanding of the transport mechanism of SLC34 proteins at the molecular level. This review will focus on how our present understanding of their molecular mechanism has evolved in this period by highlighting the key experimental findings.

Active site voltage clamp fluorometry of the sodium glucose cotransporter hSGLT1

Site-directed fluorometry was used to understand conformational changes of the Na⁺/glucose symporter. SGLT1 functions by a mechanism where the substrate-binding site alternates between the two faces of the membrane, but little is known about the underlying conformational changes. Rhodamines were covalently inserted into the substrate cavity, and changes of fluorescence were measured in real time with the opening and closing of the outer gate as SGLT1 was driven between inward and outward conformations using voltage jumps. Structural modeling indicated that the quenching with gating opening was due to an increased solvation of rhodamine and an increase in polar residues lining the wall of the cavity. This experimental approach will lead to a better understanding of the mechanism of membrane transport.

In the human sodium glucose cotransporter (hSGLT1) cycle, the protein undergoes conformational changes where the sugar-binding site alternatively faces the external and internal surfaces. Functional site-directed fluorometry was used to probe the conformational changes at the sugar-binding site. Residues (Y290, T287, H83, and N78) were mutated to cysteines. The mutants were expressed in Xenopus laevis oocytes and tagged with environmentally sensitive fluorescent rhodamines [e.g., tetramethylrhodamine (TMR)-thiols]. The fluorescence intensity was recorded as the mutants were driven into different conformations using voltage jumps. Sugar binding and transport by the fluorophore-tagged mutants were blocked, but Na⁺ binding and the voltage-dependent conformational transitions were unaffected. Structural models indicated that external Na⁺ binding opened a large aqueous vestibule (600 ų) leading to the sugar-binding site. The fluorescence of TMR covalently linked to Y290C, T287C, and H83C decreased as the mutant proteins were driven from the inward to the outward open Na⁺-bound conformation. The time courses of fluorescence changes (milliseconds) were close to the SGLT1 capacitive charge movements. The quench in rhodamine fluorescence indicated that the environment of the chromophores became more polar with opening of the external gates as the protein transitioned from the inward to outward facing state. Structural analyses showed an increase in polar side chains and a decrease in hydrophobic side chains lining the vestibule, and this was reflected in solvation of the chromophore. The results demonstrate the opening and closing of external gates in real time, with the accompanying changes of polarity of the sugar vestibule.

Functional Site-Directed Fluorometry

Initially developed in the mid-1990s to examine the conformational changes of the canonical Shaker voltage-gated potassium channel, functional site-directed fluorometry has since been expanded to numerous other voltage-gated and ligand-gated ion channels as well as transporters, pumps, and other integral membrane proteins. The power of functional site-directed fluorometry, also known as voltage-clamp fluorometry, lies in its ability to provide information on the conformational changes in a protein in response to changes in its environment with high temporal resolution while simultaneously monitoring the function of that protein. Over time, applications of site-directed fluorometry have expanded to examine the interactions of ion channels with modulators ranging from membrane potential to ligands to accessory protein subunits to lipids. In the future, the range of questions answerable by functional site-directed fluorometry and its interpretive power should continue to improve, making it an even more powerful technique for dissecting the conformational dynamics of ion channels and other membrane proteins.

The role of transporter ectodomains in drug recognition and binding: phlorizin and the sodium–glucose cotransporter

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

Correlating charge movements with local conformational changes of a Na(+)-coupled cotransporter

To gain insight into the steady-state and dynamic characteristics of structural rearrangements of an electrogenic secondary-active cotransporter during its transport cycle, two measures of conformational change (pre-steady-state current relaxations and intensity of fluorescence emitted from reporter fluorophores) were investigated as a function of membrane potential and external substrate. Cysteines were substituted at three believed-new sites in the type IIb Na(+)-coupled inorganic phosphate cotransporter (SLC34A2 flounder isoform) that were predicted to be involved in conformational changes. Labeling at one site resulted in substantial suppression of transport activity, whereas for the other sites, function remained comparable to the wild-type. For these mutants, the properties of the pre-steady-state charge relaxations were similar for each, whereas fluorescence intensity changes differed significantly. Fluorescence changes could be accounted for by simulations using a five-state model with a unique set of apparent fluorescence intensities assigned to each state according to the site of labeling. Fluorescence reported from one site was associated with inward and outward conformations, whereas for the other sites, including four previously indentified sites, emissions were associated principally with one or the other orientation of the transporter. The same membrane potential change induced complementary changes in fluorescence at some sites, which suggested that the microenvironments of the respective fluorophores experience concomitant changes in polarity. In response to step changes in voltage, the pre-steady-state current relaxation and the time course of change in fluorescence intensity were described by single exponentials. For one mutant the time constants matched well with and without external Na(+), providing direct evidence that this label reports conformational changes accompanying intrinsic charge movement and cation interactions.

Functional identification and characterization of sodium binding sites in Na symporters

Sodium cotransporters from several different gene families belong to the leucine transporter (LeuT) structural family. Although the identification of Na(+) in binding sites is beyond the resolution of the structures, two Na(+) binding sites (Na1 and Na2) have been proposed in LeuT. Na2 is conserved in the LeuT family but Na1 is not. A biophysical method has been used to measure sodium dissociation constants (Kd) of wild-type and mutant human sodium glucose cotransport (hSGLT1) proteins to identify the Na(+) binding sites in hSGLT1. The Na1 site is formed by residues in the sugar binding pocket, and their mutation influences sodium binding to Na1 but not to Na2. For the canonical Na2 site formed by two -OH side chains, S392 and S393, and three backbone carbonyls, mutation of S392 to cysteine increased the sodium Kd by sixfold. This was accompanied by a dramatic reduction in the apparent sugar and phlorizin affinities. We suggest that mutation of S392 in the Na2 site produces a structural rearrangement of the sugar binding pocket to disrupt both the binding of the second Na(+) and the binding of sugar. In contrast, the S393 mutations produce no significant changes in sodium, sugar, and phlorizin affinities. We conclude that the Na2 site is conserved in hSGLT1, the side chain of S392 and the backbone carbonyl of S393 are important in the first Na(+) binding, and that Na(+) binding to Na2 promotes binding to Na1 and also sugar binding.

Conformational changes represent the rate-limiting step in the transport cycle of maize sucrose transporter1

Proton-driven Suc transporters allow phloem cells of higher plants to accumulate Suc to more than 1 M, which is up to ~1000-fold higher than in the surrounding extracellular space. The carrier protein can accomplish this task only because proton and Suc transport are tightly coupled. This study provides insights into this coupling by resolving the first step in the transport cycle of the Suc transporter SUT1 from maize (Zea mays). Voltage clamp fluorometry measurements combining electrophysiological techniques with fluorescence-based methods enable the visualization of conformational changes of SUT1 expressed in Xenopus laevis oocytes. Using the Suc derivate sucralose, binding of which hinders conformational changes of SUT1, the association of protons to the carrier could be dissected from transport-associated movements of the protein. These combined approaches enabled us to resolve the binding of protons to the carrier and its interrelationship with the alternating movement of the protein. The data indicate that the rate-limiting step of the reaction cycle is determined by the accessibility of the proton binding site. This, in turn, is determined by the conformational change of the SUT1 protein, alternately exposing the binding pockets to the inward and to the outward face of the membrane.

Simulated annealing reveals the kinetic activity of SGLT1, a member of the LeuT structural family

The Na⁺/glucose cotransporter (SGLT1) is the archetype of membrane proteins that use the electrochemical Na⁺ gradient to drive uphill transport of a substrate. The crystal structure recently obtained for vSGLT strongly suggests that SGLT1 adopts the inverted repeat fold of the LeuT structural family for which several crystal structures are now available. What is largely missing is an accurate view of the rates at which SGLT1 transits between its different conformational states. In the present study, we used simulated annealing to analyze a large set of steady-state and pre–steady-state currents measured for human SGLT1 at different membrane potentials, and in the presence of different Na⁺ and α-methyl-d-glucose (αMG) concentrations. The simplest kinetic model that could accurately reproduce the time course of the measured currents (down to the 2 ms time range) is a seven-state model (C₁ to C₇) where the binding of the two Na⁺ ions (C₄→C₅) is highly cooperative. In the forward direction (Na⁺/glucose influx), the model is characterized by two slow, electroneutral conformational changes (59 and 100 s⁻¹) which represent reorientation of the free and of the fully loaded carrier between inside-facing and outside-facing conformations. From the inward-facing (C₁) to the outward-facing Na-bound configuration (C₅), 1.3 negative elementary charges are moved outward. Although extracellular glucose binding (C₅→C₆) is electroneutral, the next step (C₆→C₇) carries 0.7 positive charges inside the cell. Alignment of the seven-state model with a generalized model suggested by the structural data of the LeuT fold family suggests that electrogenic steps are associated with the movement of the so-called thin gates on each side of the substrate binding site. To our knowledge, this is the first model that can quantitatively describe the behavior of SGLT1 down to the 2 ms time domain. The model is highly symmetrical and in good agreement with the structural information obtained from the LeuT structural family.

Functional regulation of sugar assimilation by N-glycan-specific interaction of pancreatic α-amylase with glycoproteins of duodenal brush border membrane

Porcine pancreatic α-amylase (PPA) binds to N-linked glycans of glycoproteins (Matsushita, H., Takenaka, M., and Ogawa, H. (2002) J. Biol Chem., 277, 4680-4686). Immunostaining revealed that PPA is located at the brush-border membrane (BBM) of enterocytes in the duodenum and that the binding is inhibited by mannan but not galactan, indicating that PPA binds carbohydrate-specifically to BBM. The ligands for PPA in BBM were identified as glycoprotein N-glycans that are significantly involved in the assimilation of glucose, including sucrase-isomaltase (SI) and Na(+)/Glc cotransporter 1 (SGLT1). Binding of SI and SGLT1 in BBM to PPA was dose-dependent and inhibited by mannan. Using BBM vesicles, we found functional changes in PPA and its ligands in BBM due to the N-glycan-specific interaction. The starch-degrading activity of PPA and maltose-degrading activity of SI were enhanced to 240 and 175%, respectively, while Glc uptake by SGLT1 was markedly inhibited by PPA at high but physiologically possible concentrations, and the binding was attenuated by the addition of mannose-specific lectins, especially from Galanthus nivalis. Additionally, recombinant human pancreatic α-amylases expressed in yeast and purified by single-step affinity chromatography exhibited the same carbohydrate binding specificity as PPA in binding assays with sugar-biotinyl polymer probes. The results indicate that mammalian pancreatic α-amylases share a common carbohydrate binding activity and specifically bind to the intestinal BBM. Interaction with N-glycans in the BBM activated PPA and SI to produce much Glc on the one hand and to inhibit Glc absorption by enterocytes via SGLT1 in order to prevent a rapid increase in blood sugar on the other.

Mouse SGLT3a generates proton-activated currents but does not transport sugar

Sodium-glucose cotransporters (SGLTs) are secondary active transporters belonging to the SLC5 gene family. SGLT1, a well-characterized member of this family, electrogenically transports glucose and galactose. Human SGLT3 (hSGLT3), despite sharing a high amino acid identity with human SGLT1 (hSGLT1), does not transport sugar, although functions as a sugar sensor. In contrast to humans, two different genes in mice and rats code for two different SGLT3 proteins, SGLT3a and SGLT3b. We previously cloned and characterized mouse SGLT3b (mSGLT3b) and showed that, while it does transport sugar like SGLT1, it likely functions as a physiological sugar sensor like hSGLT3. In this study, we cloned mouse SGLT3a (mSGLT3a) and characterized it by expressing it in Xenopus laevis oocytes and performing electrophysiology and sugar transport assays. mSGLT3a did not transport sugar, and sugars did not induce currents at pH 7.4, though acidic pH induced inward currents that increased in the presence of sugar. Moreover, mutation of residue 457 from glutamate to glutamine resulted in a Na+-dependent transport of sugar that was inhibited by phlorizin. To corroborate our results in oocytes, we expressed and characterized mSGLT3a in mammalian cells and confirmed our findings. In addition, we cloned, expressed, and characterized rat SGLT3a in oocytes and found characteristics similar to mSGLT3a. In summary, acidic pH induces currents in mSGLT3a, and sugar-induced currents are increased at acidic pH, but wild-type SGLT3a does not transport sugar.

Bridging the gap between structure and kinetics of human SGLT1

The Na(+)-glucose cotransporter hSGLT1 is a member of a class of membrane proteins that harness Na(+) electrochemical gradients to drive uphill solute transport. Although hSGLT1 belongs to one gene family (SLC5), recent structural studies of bacterial Na(+) cotransporters have shown that Na(+) transporters in different gene families have the same structural fold. We have constructed homology models of hSGLT1 in two conformations, the inward-facing occluded (based on vSGLT) and the outward open conformations (based on Mhp1), mutated in turn each of the conserved gates and ligand binding residues, expressed the SGLT1 mutants in Xenopus oocytes, and determined the functional consequences using biophysical and biochemical assays. The results establish that mutating the ligand binding residues produces profound changes in the ligand affinity (the half-saturation concentration, K(0.5)); e.g., mutating sugar binding residues increases the glucose K(0.5) by up to three orders of magnitude. Mutation of the external gate residues increases the Na(+) to sugar transport stoichiometry, demonstrating that these residues are critical for efficient cotransport. The changes in phlorizin inhibition constant (K(i)) are proportional to the changes in sugar K(0.5), except in the case of F101C, where phlorizin K(i) increases by orders of magnitude without a change in glucose K(0.5). We conclude that glucose and phlorizin occupy the same binding site and that F101 is involved in binding to the phloretin group of the inhibitor. Substituted-cysteine accessibility methods show that the cysteine residues at the position of the gates and sugar binding site are largely accessible only to external hydrophilic methanethiosulfonate reagents in the presence of external Na(+), demonstrating that the external sugar (and phlorizin) binding vestibule is opened by the presence of external Na(+) and closes after the binding of sugar and phlorizin. Overall, the present results provide a bridge between kinetics and structural studies of cotransporters.

Optical sensors for monitoring dynamic changes of intracellular metabolite levels in mammalian cells

Knowledge of the in vivo levels, distribution and flux of ions and metabolites is crucial to our understanding of physiology in both healthy and diseased states. The quantitative analysis of the dynamics of ions and metabolites with subcellular resolution in vivo poses a major challenge for the analysis of metabolic processes. Genetically encoded Förster resonance energy transfer (FRET) sensors can be used for real-time in vivo detection of metabolites. FRET sensor proteins, for example, for glucose, can be targeted genetically to any cellular compartment, or even to subdomains (e.g., a membrane surface), by adding signal sequences or fusing the sensors to specific proteins. The sensors can be used for analyses in individual mammalian cells in culture, in tissue slices and in intact organisms. Applications include gene discovery, high-throughput drug screens or systematic analysis of regulatory networks affecting uptake, efflux and metabolism. Quantitative analyses obtained with the help of FRET sensors for glucose or other ions and metabolites provide valuable data for modeling of flux. Here we provide a detailed protocol for monitoring glucose levels in the cytosol of mammalian cell cultures through the use of FRET glucose sensors; moreover, the protocol can be used for other ions and metabolites and for analyses in other organisms, as has been successfully demonstrated in bacteria, yeast and even intact plants. The whole procedure typically takes ∼4 d including seeding and transfection of mammalian cells; the FRET-based analysis of transfected cells takes ∼5 h.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Plant sucrose transporters from a biophysical point of view

The majority of higher plants use sucrose as their main mobile carbohydrate. Proton-driven sucrose transporters play a crucial role in cell-to-cell and long-distance distribution of sucrose throughout the plant. A very negative plant membrane potential and the ability of sucrose transporters to accumulate sucrose concentrations of more than 1 M indicate that plants evolved transporters with unique structural and functional features. The knowledge about the transport mechanism and structural/functional domains of these nano-machines is, however, still fragmentary. In this review, the current knowledge about the biophysical properties of plant sucrose transporters is summarized and discussed.

Determination of the Na(+)/glucose cotransporter (SGLT1) turnover rate using the ion-trap technique

The Na(+)/glucose cotransporter (SGLT1) is a membrane protein that couples the transport of two Na(+) ions and one glucose molecule using the so-called alternating access mechanism. According to this principle, each cotransporter molecule can adopt either of two main conformations: one with the binding sites accessible to the extracellular solution and one with the binding sites facing the intracellular solution. The turnover rate (TOR) is the number of complete cycles that each protein performs per second. Determination of the TOR has important consequences for investigation of the cotransport mechanism, as none of the rate constants involved in mediating transport in a given direction (conformational changes and binding and unbinding reactions) can be slower than the TOR measured under the same conditions. In addition, the TOR can be used to estimate the number of cotransporter molecules involved in generating a given ensemble activity. In this study, we obtain an independent estimation of the TOR for human SGLT1 expressed in Xenopus laevis oocytes applying the ion-trap technique. This approach detects the quantity of ions released in or taken up from the restricted space existing between the oocyte plasma membrane and the tip of a large ion-selective electrode. Taking advantage of the fact that hSGLT1 in the absence of Na(+) can cotransport glucose with protons, we used a pH electrode to determine a TOR of 8.00 ± 1.3 s⁻¹ in the presence of 35 mM α-methyl-glucose at -150 mV (pH 5.5). For the same group of oocytes, a TOR of 13.3 ± 2.4 s⁻¹ was estimated under near-V(max) conditions, i.e., in the presence of 90 mM Na(+) and 5 mM α-methyl-glucose. Under these circumstances, the average cotransport current was -1.08 ± 0.61 μA (n = 14), and this activity was generated by an average of 3.6 ± 0.7 × 10¹¹ cotransporter molecules/oocyte.

Sugar binding residue affects apparent Na+ affinity and transport stoichiometry in mouse sodium/glucose cotransporter type 3B

SGLT1 is a sodium/glucose cotransporter that moves two Na(+) ions with each glucose molecule per cycle. SGLT3 proteins belong to the same family and are described as glucose sensors rather than glucose transporters. Thus, human SGLT3 (hSGLT3) does not transport sugar, but extracellular glucose depolarizes the cell in which it is expressed. Mouse SGLT3b (mSGLT3b), although it transports sugar, has low apparent sugar affinity and partially uncoupled stoichiometry compared with SGLT1, suggesting that mSGLT3b is also a sugar sensor. The crystal structure of the Vibrio parahaemolyticus SGLT showed that residue Gln(428) interacts directly with the sugar. The corresponding amino acid in mammalian proteins, 457, is conserved in all SGLT1 proteins as glutamine. In SGLT3 proteins, glutamate is the most common residue at this position, although it is a glycine in mSGLT3b and a serine in rat SGLT3b. To test the contribution of this residue to the function of SGLT3 proteins, we constructed SGLT3b mutants that recapitulate residue 457 in SGLT1 and hSGLT3, glutamine and glutamate, respectively. The presence of glutamine at residue 457 increased the apparent Na(+) and sugar affinities, whereas glutamate decreased the apparent Na(+) affinity. Moreover, glutamate transported more cations per sugar molecule than the wild type protein. We propose a model where cations are released intracellularly without the release of sugar from an intermediate state. This model explains the uncoupled charge:sugar transport phenotype observed in wild type and G457E-mSGLT3b compared with SGLT1 and the sugar-activated cation transport without sugar transport that occurs in hSGLT3.

Sucrose- and H-dependent charge movements associated with the gating of sucrose transporter ZmSUT1

Background

In contrast to man the majority of higher plants use sucrose as mobile carbohydrate. Accordingly proton-driven sucrose transporters are crucial for cell-to-cell and long-distance distribution within the plant body. Generally very negative plant membrane potentials and the ability to accumulate sucrose quantities of more than 1 M document that plants must have evolved transporters with unique structural and functional features.

Methodology/Principal Findings

To unravel the functional properties of one specific high capacity plasma membrane sucrose transporter in detail, we expressed the sucrose/H⁺ co-transporter from maize ZmSUT1 in Xenopus oocytes. Application of sucrose in an acidic pH environment elicited inward proton currents. Interestingly the sucrose-dependent H⁺ transport was associated with a decrease in membrane capacitance (C_m). In addition to sucrose C_m was modulated by the membrane potential and external protons. In order to explore the molecular mechanism underlying these C_m changes, presteady-state currents (I_pre) of ZmSUT1 transport were analyzed. Decay of I_pre could be best fitted by double exponentials. When plotted against the voltage the charge Q, associated to I_pre, was dependent on sucrose and protons. The mathematical derivative of the charge Q versus voltage was well in line with the observed C_m changes. Based on these parameters a turnover rate of 500 molecules sucrose/s was calculated. In contrast to gating currents of voltage dependent-potassium channels the analysis of ZmSUT1-derived presteady-state currents in the absence of sucrose (I = Q/τ) was sufficient to predict ZmSUT1 transport-associated currents.

Conclusions

Taken together our results indicate that in the absence of sucrose, ‘trapped’ protons move back and forth between an outer and an inner site within the transmembrane domains of ZmSUT1. This movement of protons in the electric field of the membrane gives rise to the presteady-state currents and in turn to C_m changes. Upon application of external sucrose, protons can pass the membrane turning presteady-state into transport currents.

The actual ionic nature of the leak current through the Na+/glucose cotransporter SGLT1

Expression of the Na(+)/glucose cotransporter SGLT1 in Xenopus oocytes is characterized by a phlorizin-sensitive leak current (in the absence of glucose) that was originally called a "Na(+) leak" and represents some 5-10% of the maximal Na(+)/glucose cotransport current. We analyzed the ionic nature of the leak current using a human SGLT1 mutant (C292A) displaying a threefold larger leak current while keeping a reversal potential (V(R)) of approximately -15 mV as observed for wt SGLT1. V(R) showed only a modest negative shift when extracellular Na(+) concentration ([Na(+)](o)) was lowered and it was completely insensitive to changes in extracellular Cl(-). When extracellular pH (pH(o)) was decreased from 7.5 to 6.5 and 5.5, V(R) shifted by +15 and +40 mV, respectively, indicating that protons may be the main charge carrier at low pH(o) but other ions must be involved at pH(o) 7.5. In the presence of 15 mM [Na(+)](o) (pH(o) = 7.5), addition of 75 mM of either Na(+), Li(+), Cs(+), or K(+) generated similar increases in the leak current amplitude. This observation, which was confirmed with wt SGLT1, indicates a separate pathway for the leak current with respect to the cotransport current. This means that, contrary to previous beliefs, the leak current cannot be accounted for by the translocation of the Na-loaded and glucose-free cotransporter. Using chemical modification and different SGLT1 mutants, a relationship was found between the cationic leak current and the passive water permeability suggesting that water and cations may share a common pathway through the cotransporter.

Elucidating conformational changes in the gamma-aminobutyric acid transporter-1

The GABA transporter-1 (GAT-1) has three current-generating modes: GABA-coupled current, Li+-induced leak current, and Na+-dependent transient currents. We earlier hypothesized that Li+ is able to substitute for the first Na+ in the transport cycle and thereby induce a distinct conformation in GAT-1 and that the onset of the Li+-induced leak current at membrane potentials more negative than -50 mV was due to a voltage-dependent conformational change of the Li+-bound transporter. In this study, we set out to verify this hypothesis and seek insight into the structural dynamics underlying the leak current, as well as the sodium-dependent transient currents, by applying voltage clamp fluorometry to tetramethylrhodamine 6-maleimide-labeled GAT-1 expressed in Xenopus laevis oocytes. MTSET accessibility studies demonstrated the presence of two distinct conformations of GAT-1 in the presence of Na+ or Li+. The voltage-dependent fluorescence intensity changes obtained in Li+ buffer correlated with the Li+-induced leak currents, i.e. both were highly voltage-dependent and only present at hyperpolarized potentials (<-50 mV). The transient currents correlated directly with the voltage-dependent fluorescence data obtained in sodium buffer and the associated conformational changes were distinct from those associated with the Li+-induced leak current. The inhibitor potency of SKF89976A of the Li+- versus Na+-bound transporter confirmed the cationic dependence of the conformational occupancy. Our observations suggest that the microdomain situated at the external end of transmembrane I is involved in different conformational changes taking place either during the binding and release of sodium or during the initiation of the Li+-induced leak current.

Effects on conformational states of the rabbit sodium/glucose cotransporter through modulation of polarity and charge at glutamine 457

The high affinity sodium/glucose cotransporter (SGLT1) couples transport of Na(+) and glucose. Previous studies established that mutant Q457C human SGLT1 retains full activity, and sugar translocation is abolished in mutant Q457R or in mutant Q457C after reaction with methanethiosulfonate derivatives, but Na(+) and sugar binding remain intact. To explore the mechanism by which modulation of Q457 abolishes transport, Q457C and Q457R of rabbit SGLT1 were studied using chemical modification and the two-electrode voltage-clamp technique. Compared to wild-type SGLT1, Q457C exhibits approximately 20-fold reduction in phloridzin affinity and preferential occupancy of an inward-facing state. Alkylation of Q457C by [(2-trimethylammonium) ethyl] methanethiosulphonate bromide, (MTSET), reverses these changes while blocking transport. Analysis of pre-steady-state currents in the absence of sugar yields three decay constants for each of Q457C, Q457C-MTSET and Q457R. Comparison of Q457C-MTSET and Q457R with Q457C and wild-type, reveals that inhibition of transport is accompanied by a decrease in magnitude and voltage-independence of the slow decay constant at negative potentials. But fast and medium decays remain unchanged. Computer simulation of transient currents suggests that introduction of positive charge at position 457 leads to a predominant outward rather than inward-facing conformational state. Taken together, the results suggest that glutamine 457, in addition to being involved in sugar binding, is a residue that is sensitive to conformational changes of the carrier.

The crystal structure of a sodium galactose transporter reveals mechanistic insights into Na+/sugar symport

Membrane transporters that use energy stored in sodium gradients to drive nutrients into cells constitute a major class of proteins. We report the crystal structure of a member of the solute sodium symporters (SSS), the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT). The approximately 3.0 angstrom structure contains 14 transmembrane (TM) helices in an inward-facing conformation with a core structure of inverted repeats of 5 TM helices (TM2 to TM6 and TM7 to TM11). Galactose is bound in the center of the core, occluded from the outside solutions by hydrophobic residues. Surprisingly, the architecture of the core is similar to that of the leucine transporter (LeuT) from a different gene family. Modeling the outward-facing conformation based on the LeuT structure, in conjunction with biophysical data, provides insight into structural rearrangements for active transport.

How drugs interact with transporters: SGLT1 as a model

Drugs are transported by cotransporters with widely different turnover rates. We have examined the underlying mechanism using, as a model system, glucose and indican (indoxyl-beta-D-glucopyranoside) transport by human Na+/glucose cotransporter (hSGLT1). Indican is transported by hSGLT1 at 10% of the rate for glucose but with a fivefold higher apparent affinity. We expressed wild-type hSGLT1 and mutant G507C in Xenopus oocytes and used electrical and optical methods to measure the kinetics of glucose (using nonmetabolized glucose analogue alpha-methyl-D-glucopyranoside, alphaMDG) and indican transport, alone and together. Indican behaved as a competitive inhibitor of alphaMDG transport. To examine protein conformations, we recorded SGLT1 capacitive currents (charge movements) and fluorescence changes in response to step jumps in membrane voltage, in the presence and absence of indican and/or alphaMDG. In the absence of sugar, voltage jumps elicited capacitive SGLT currents that decayed to steady state with time constants (tau) of 3-20 ms. These transient currents were abolished in saturating alphaMDG but only slightly reduced (10%) in saturating indican. SGLT1 G507C rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages. Maximal fluorescence increased approximately 150% in saturating indican but decreased approximately 50% in saturating alphaMDG. Modeling indicated that the rate-limiting step for indican transport is sugar translocation, whereas for alphaMDG it is dissociation of Na+ from the internal binding sites. The inhibitory effects of indican on alphaMDG transport are due to its higher affinity and a 100-fold lower translocation rate. Our results indicate that competition between substrates and drugs should be taken into consideration when targeting transporters as drug delivery systems.

Transmembrane IV of the high-affinity sodium-glucose cotransporter participates in sugar binding

Investigation of the structure/function relationships of the sodium-glucose transporter (SGLT1) is crucial to understanding the cotransporter mechanism. In the present study, we used cysteine-scanning mutagenesis and chemical modification by methanethiosulfonate (MTS) derivatives to test whether predicted transmembrane IV participates in sugar binding. Five charged and polar residues (K139, Q142, T156, K157, and D161) and two glucose/galactose malabsorption missense mutations (I147 and S159) were replaced with cysteine. Mutants I147C, T156C, and K157C exhibited sufficient expression to be studied in detail using the two-electrode voltage-clamp method in Xenopus laevis oocytes and COS-7 cells. I147C was similar in function to wild-type and was not studied further. Mutation of lysine-157 to cysteine (K157C) causes loss of phloridzin and alpha-methyl-D-glucopyranoside (alphaMG) binding. These functions are restored by chemical modification with positively charged (2-aminoethyl) methanethiosulfonate hydrobromide (MTSEA). Mutation of threonine-156 to cysteine (T156C) reduces the affinity of alphaMG and phloridzin for T156C by approximately 5-fold and approximately 20-fold, respectively. In addition, phloridzin protects cysteine-156 in T156C from alkylation by MTSEA. Therefore, the presence of a positive charge or a polar residue at 157 and 156, respectively, affects sugar binding and sugar-induced Na(+) currents.

How membrane proteins sense voltage

The ionic gradients across cell membranes generate a transmembrane voltage that regulates the function of numerous membrane proteins such as ion channels, transporters, pumps and enzymes. The mechanisms by which proteins sense voltage is diverse: ion channels have a conserved, positively charged transmembrane region that moves in response to changes in membrane potential, some G-protein coupled receptors possess a specific voltage-sensing motif and some membrane pumps and transporters use the ions that they transport across membranes to sense membrane voltage. Characterizing the general features of voltage sensors might lead to the discovery of further membrane proteins that are voltage regulated.

Site-directed mutagenesis investigation of coupling properties of metal ion transport by DCT1

DCT1 (NRAMP2, DMT1, slc11a2) is a member of the NRAMP family and functions as general metal ion transporter in mammals; defective DCT1 causes anemia. The driving force for metal ion transport is protonmotive force, where protons are transported in the same direction as metal ions. The stoichiometry between metal ion and proton varies under different conditions due to mechanistic proton slip. To better understand this phenomenon, we performed site-directed mutagenesis of DCT1 and analyzed the mutants by measurement of metal ion uptake activity and electrophysiology in Xenopus laevis oocytes. A single reciprocal mutation, I144F, between DCT1 and the homologous yeast transporter Smf1p located in putative transmembrane domain 2 abolished the metal ion transport activity of DCT1, significantly increased the slip currents, and generated sodium slip currents. A double mutation adding F227I in transmembrane domain 4 to I144F in transmembrane domain 2 restored the uptake activity of DCT1 and reduced the slip currents. These results demonstrate the importance of these regions in coupling of metal ions and protons as well as the possible proximity of I144 and F227 in the folded structure of DCT1.

Amino Acid Residues in Transmembrane Segment IX of the Na+/I– Symporter Play a Role in Its Na+ Dependence and Are Critical for Transport Activity

The Na+/I- symporter (NIS) is a key plasma membrane glycoprotein that mediates Na+-dependent active I- transport in the thyroid, lactating breast, and other tissues. The OH group of the side chain at position 354 in transmembrane segment (TMS) IX of NIS has been demonstrated to be essential for NIS function, as revealed by the study of the congenital I- transport defect-causing T354P NIS mutation. TMS IX has the most beta-OH group-containing amino acids (Ser and Thr) of any TMS in NIS. We have thoroughly characterized the functional significance of all Ser and Thr in TMS IX in NIS, as well as of other residues in TMS IX that are highly conserved in other transporters of the SLC5A protein family. Here we show that five beta-OH group-containing residues (Thr-351, Ser-353, Thr-354, Ser-356, and Thr-357) and Asn-360, all of which putatively face the same side of the helix in TMS IX, plus Asp-369, located in the membrane/cytosol interface, play key roles in NIS function and seem to be involved in Na+ binding/translocation.

Three surface subdomains form the vestibule of the Na+/glucose cotransporter SGLT1

A combination of biophysical and biochemical approaches was employed to probe the topology, arrangement, and function of the large surface subdomains of SGLT1 in living cells. Using atomic force microscopy on the single molecule level, Chinese hamster ovary cells overexpressing SGLT1 were probed with atomic force microscopy tips carrying antibodies against epitopes of different subdomains. Specific single molecule recognition events were observed with antibodies against loop 6-7, loop 8-9, and loop 13-14, demonstrating the extracellular orientation of these subdomains. The addition of D-glucose in Na+-containing medium decreased the binding probability of the loop 8-9 antibody, suggesting a transport-related conformational change in the region between amino acids 339 and 356. Transport studies with mutants C345A, C351A, C355A, or C361S supported a role for these amino acids in determining the affinity of SGLT1 for D-glucose. MTSET, [2-(trimethylammonium)ethyl] methanethiosulfonate and dithiothreitol inhibition patterns on alpha-methyl-glucoside uptake by COS-7 cells expressing C255A, C560A, or C608A suggested the presence of a disulfide bridge between Cys255 and Cys608. This assumption was corroborated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry showing mass differences in peptides derived from transporters biotinylated in the absence and presence of dithiothreitol. These results indicate that loop 6-7 and loop 13-14 are connected by a disulfide bridge. This bridge brings also loop 8-9 into close vicinity with the former subdomains to create a vestibule for sugar binding.

Transporters as Channels

This review investigates some key aspects of transport mechanisms and recent advances in our understanding of this ubiquitous cellular process. The prevailing model of cotransport is the alternating access model, which suggests that large conformational changes in the transporter protein accompany cotransport. This model rests on decades of research and has received substantial support because many transporter characteristics are explained using its premises. New experiments, however, have revealed the existence of channels in transporters, an idea that is in conflict with traditional models. The alternating access model is the subject of previous detailed reviews. Here we concentrate on the relatively recent data that document primarily the channel properties of transporters. In some cases, namely, the observation of single-transporter currents, the evidence is direct. In other cases the evidence--for example, from fluctuation analysis or transporter currents too large to be described as anything other than channel-like--is indirect. Although the existence of channels in transporters is not in doubt, we are far from understanding the significance of this property. In the online Supplemental Material , we review some pertinent aspects of ion channel theory and cotransport physiology to provide background for the channels and transporters presented here. We discuss the existence of channels in transporters, and we speculate on the biological significance of this newly unveiled property of transport proteins.

Conformational dynamics of hSGLT1 during Na+/glucose cotransport

This study examines the conformations of the Na⁺/glucose cotransporter (SGLT1) during sugar transport using charge and fluorescence measurements on the human SGLT1 mutant G507C expressed in Xenopus oocytes. The mutant exhibited similar steady-state and presteady-state kinetics as wild-type SGLT1, and labeling of Cys507 by tetramethylrhodamine-6-maleimide had no effect on kinetics. Our strategy was to record changes in charge and fluorescence in response to rapid jumps in membrane potential in the presence and absence of sugar or the competitive inhibitor phlorizin. In Na⁺ buffer, step jumps in membrane voltage elicited presteady-state currents (charge movements) that decay to the steady state with time constants τ_med (3–20 ms, medium) and τ_slow (15–70 ms, slow). Concurrently, SGLT1 rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages (ΔF). The charge vs. voltage (Q-V) and fluorescence vs. voltage (ΔF-V) relations (for medium and slow components) obeyed Boltzmann relations with similar parameters: zδ (apparent valence of voltage sensor) ≈ 1; and V_0.5 (midpoint voltage) between −15 and −40 mV. Sugar induced an inward current (Na⁺/glucose cotransport), and reduced maximal charge (Q_max) and fluorescence (ΔF_max) with half-maximal concentrations (K_0.5) of 1 mM. Increasing [αMDG]_o also shifted the V_0.5 for Q and ΔF to more positive values, with K_0.5's ≈ 1 mM. The major difference between Q and ΔF was that at saturating [αMDG]_o, the presteady-state current (and Q_max) was totally abolished, whereas ΔF_max was only reduced 50%. Phlorizin reduced both Q_max and ΔF_max (K_i ≈ 0.4 μM), with no changes in V_0.5's or relaxation time constants. Simulations using an eight-state kinetic model indicate that external sugar increases the occupancy probability of inward-facing conformations at the expense of outward-facing conformations. The simulations predict, and we have observed experimentally, that presteady-state currents are blocked by saturating sugar, but not the changes in fluorescence. Thus we have isolated an electroneutral conformational change that has not been previously described. This rate-limiting step at maximal inward Na⁺/sugar cotransport (saturating voltage and external Na⁺ and sugar concentrations) is the slow release of Na⁺ from the internal surface of SGLT1. The high affinity blocker phlorizin locks the cotransporter in an inactive conformation.

Voltage-clamp fluorometry in the local environment of the C255-C511 disulfide bridge of the Na+/glucose cotransporter

We recently identified a functionally important disulfide bridge between C255 and C511 of the human Na+/glucose cotransporter SGLT1. In this study, voltage-clamp fluorometry was used to characterize the fluorescence of four different dyes attached to C255 and C511 under various ionic and substrate/inhibitor conditions. State-dependent fluorescence changes (DeltaF) were observed when TMR5M or TMR6M dyes were attached to C255 and C511 or when Alexa488 was bound to C511. TMR5M-C511 was extremely sensitive to membrane potential (Vm) and to external Na+ and alphaMG (a nonmetabolizable glucose analog) concentrations. A progressive increase in alphaMG concentration drastically changed the maximal voltage-dependent DeltaF and produced a positive shift in the midpoint of the DeltaF-Vm curve. By determining specific fluorescence intensity for each state of the cotransporter, our steady-state fluorescence data could be reproduced using the rate constants previously proposed for a five-state kinetic model exclusively derived from electrophysiological measurements. Our results bring an independent support to the proposed kinetic model and show that the binding of alphaMG substrate significantly modifies the environment of C255 and C511.

Active sugar transport in health and disease

Secondary active glucose transport occurs by at least four members of the SLC5 gene family. This review considers the structure and function of two premier members, SGLT1 and SGLT2, and their role in intestinal glucose absorption and renal glucose reabsorption. Genetics disorders of SGLTs include Glucose-Galactose Malabsorption, and Familial Renal Glucosuria. SGLT1 plays a central role in Oral Rehydration Therapy used so effectively to treat secretory diarrhoea such as cholera. Increasing attention is being focused on SGLTs as drug targets for the therapy of diabetes.

Voltage clamp fluorometric measurements on a type II Na+-coupled Pi cotransporter: shedding light on substrate binding order

Voltage clamp fluorometry (VCF) combines conventional two-electrode voltage clamp with fluorescence measurements to detect protein conformational changes, as sensed by a fluorophore covalently attached to the protein. We have applied VCF to a type IIb Na⁺-coupled phosphate cotransporter (NaPi-IIb), in which a novel cysteine was introduced in the putative third extracellular loop and expressed in Xenopus oocytes. Labeling this cysteine (S448C) with methanethiosulfonate (MTS) reagents blocked cotransport function, however previous electrophysiological studies (Lambert G., I.C. Forster, G. Stange, J. Biber, and H. Murer. 1999. J. Gen. Physiol. 114:637–651) suggest that substrate interactions with the protein can still occur, thus permitting study of a limited subset of states. After labeling S448C with the fluorophore tetramethylrhodamine MTS, we detected voltage- and substrate-dependent changes in fluorescence (ΔF), which suggested that this site lies in an environment that is affected by conformational change in the protein. ΔF was substrate dependent (no ΔF was detectable in 0 mM Na⁺) and showed little correlation with presteady-state charge movements, indicating that the two signals provide insight into different underlying physical processes. Interpretation of ion substitution experiments indicated that the substrate binding order differs from our previous model (Forster, I., N. Hernando, J. Biber, and H. Murer. 1998. J. Gen. Physiol. 112:1–18). In the new model, two (rather than one) Na⁺ ions precede P_i binding, and only the second Na⁺ binding transition is voltage dependent. Moreover, we show that Li⁺, which does not drive cotransport, interacts with the first Na⁺ binding transition. The results were incorporated in a new model of the transport cycle of type II Na⁺/P_i cotransporters, the validity of which is supported by simulations that successfully predict the voltage and substrate dependency of the experimentally determined fluorescence changes.

Mapping conformational changes of a type IIb Na+/Pi cotransporter by voltage clamp fluorometry

The fluorescence of a fluorophore depends on its environment, and if attached to a protein it may report on conformational changes. We have combined two-electrode voltage clamp with simultaneous fluorescence measurements to detect conformational changes in a type IIb Na(+)/P(i) cotransporter expressed in Xenopus oocytes. Four novel Cys, labeled with a fluorescent probe, yielded voltage- and substrate-dependent changes in fluorescence (F). Neither Cys substitution nor labeling significantly altered the mutant electrogenic properties. Different F responses to voltage and substrate were recorded at the four sites. S155C, located in an intracellular re-entrant loop in the first half of the protein, and E451C, located in an extracellular re-entrant loop in the second half of the protein, both showed Na(+), Li(+), and P(i)-dependent F signals. S226C and Q319C, located at opposite ends of a large extracellular loop in the middle of the protein, mainly responded to changes in Na(+) and Li(+). Hyperpolarization increased F for S155C and S226C but decreased F for Q319C and E451C. The labeling and F response of S155C, confirmed that the intracellular loop containing Ser-155 is re-entrant as it is accessible from the extracellular milieu. The behavior of S155C and E451C indicates a strong involvement of the two re-entrant loops in conformational changes during the transport cycle. Moreover, the data for S226C and Q319C suggest that also the large extracellular loop is associated with transport function. Finally, the reciprocal voltage dependences of the S155C-E451C and S226C-Q319C pairs suggest reciprocal conformational changes during the transport cycle for their respective local environments.

Ligands on the string: single-molecule AFM studies on the interaction of antibodies and substrates with the Na+-glucose co-transporter SGLT1 in living cells

Atomic force microscopy (AFM) was used to probe topology, conformational changes and initial substratecarrier interactions of Na+-glucose co-transporter (SGLT1) in living cells on a single-molecule level. By scanning SGLT1-transfected Chinese hamster ovary (CHO) cells with AFM tips carrying an epitope-specific antibody directed against the extramembranous C-terminal loop 13, significant recognition events could be detected. Specificity was confirmed by the absence of events in nontransfected CHO cells and by the use of free antigen and free antibody superfusion. Thus, contrary to computer predictions on SGLT1 topology, loop 13 seems to be part of the extracellular surface of the transporter. Binding probability of the antibody decreased upon addition of phlorizin, a specific inhibitor of SGLT1, suggesting a considerable conformational change of loop 13 when the inhibitor occludes the sugar translocation pathway. Using an AFM tip carrying 1-thio-D-glucose, direct evidence could be obtained that in the presence of Na+ a sugarbinding site appears on the transporter surface. The binding site accepts the sugar residue of the glucoside phlorizin, free D-glucose, and D-galactose, but not free Lglucose and probably represents the first of several selectivity filters of the transporter. This work demonstrates the potential of AFM to study the presence and dynamics of plasma membrane transporters in intact cells on the single molecule level.

Molecular interactions between dipeptides, drugs and the human intestinal H+ -oligopeptide cotransporter hPEPT1

The human intestinal proton-coupled oligopeptide transporter hPEPT1 has been implicated in the absorption of pharmacologically active compounds. We have investigated the interactions between a comprehensive selection of drugs, and wild-type and variant hPEPT1s expressed in Xenopus oocytes, using radiotracer uptake and electrophysiological methods. The beta-lactam antibiotics ampicillin, amoxicillin, cephalexin and cefadroxil, the antineoplastics delta-aminolevulinic acid (delta-ALA) and bestatin, and the neuropeptide N-acetyl-Asp-Glu (NAAG), were transported, as judged by their ability to evoke inward currents. When the drugs were added in the presence of the typical substrate glycylsarcosine (Gly-Sar), the inward currents were equal or less than that induced by Gly-Sar alone. This suggests that the drugs are transported at a lower turnover rate than Gly-Sar, but may also point towards complex interactions between dipeptides, drugs and the transporter. Gly-Sar and the drugs also modified the kinetics of hPEPT1 presteady-state charge movement, by causing a reduction in maximum charge (Qmax) and a shift of the midpoint voltage (V0.5) to more negative potentials. Our results indicate that the substrate selectivity of hPEPT1 is: Gly-Sar > NAAG, delta-ALA, bestatin > cefadroxil, cephalexin > ampicillin, amoxicillin. Based on steady-state and presteady-state analysis of Gly-Sar and cefadroxil transport, we proposed an extension of the 6-state kinetic model for hPEPT1 function that globally accounts for the observed presteady-state and steady-state kinetics of neutral dipeptide and drug transport. Our model suggests that, under saturating conditions, the rate-limiting step of the hPEPT1 transport cycle is the reorientation of the empty carrier within the membrane. Variations in rates of drug cotransport are predicted to be due to differences in affinity and turnover rate. Oral availability of drugs may be reduced in the presence of physiological concentrations of dietary dipeptides in the gut, suggesting that oral delivery drugs should be taken on an empty stomach. The common hPEPT1 single-nucleotide polymorphisms Ser117Asn and Gly419Ala retained the essential kinetic and drug recognition characteristics of the wild type, suggesting that neither variant is likely to have a major impact on oral absorption of drugs.