The voltage dependence of a cloned mammalian renal type II Na+/Pi cotransporter (NaPi-2)

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

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.

Characterization of Transport Activity of SLC11 Transporters in Xenopus laevis Oocytes by Fluorophore Quenching

Membrane proteins are involved in different physiological functions and are the target of pharmaceutical and abuse drugs. Xenopus laevis oocytes provide a powerful heterologous expression system for functional studies of these proteins. Typical experiments investigate transport using electrophysiology and radiolabeled uptake. A two-electrode voltage clamp is suitable only for electrogenic proteins, and uptake measurements require the existence of radiolabeled substrates and adequate laboratory facilities.Recently, Dictyostelium discoideum Nramp1 and NrampB were characterized using multidisciplinary approaches. NrampB showed no measurable electrogenic activity, and it was investigated in Xenopus oocytes by acquiring confocal images of the quenching of injected fluorophore calcein.This method is adequate to measure the variation in emitted fluorescence, and thus transporter activity indirectly, but requires long experimental procedures to collect statistically consistent data. Considering that optimal expression of heterologous proteins lasts for 48-72 h, a slow acquiring process requires the use of more than one batch of oocytes to complete the experiments. Here, a novel approach to measure substrate uptake is reported. Upon injection of a fluorophore, oocytes were incubated with the substrate and the transport activity measured, evaluating fluorescence quenching in a microplate reader. The technique permits the testing of tens of oocytes in different experimental conditions simultaneously, and thus the collection of significant statistical data for each batch, saving time and animals.The method was tested with different metal transporters (SLC11), DMT1, DdNramp1, and DdNrampB, and verified with the peptide transporter PepT1 (SLC15). Comparison with traditional methods (uptake, two-electrode voltage clamp) and with quenching images acquired by fluorescence microscopy confirmed its efficacy.

Potent Inhibition of Biphasic Tubular Reabsorption of Lithium by Acetazolamide and Foscarnet in Rats

Lithium is mainly excreted into urine, and a large fraction of lithium filtered through glomeruli is reabsorbed in the proximal tubule. However, the mechanisms responsible for lithium reabsorption remain unclear. We previously reported that the reabsorption of lithium was biphasic in rats, and that foscarnet inhibited lithium reabsorption with a high affinity type. We herein evaluated the effects of acetazolamide and foscarnet on the renal excretion of lithium in rats treated with lithium chloride at 2 doses. In rats intravenously injected with a bolus of 25 mg/kg lithium chloride, acetazolamide facilitated the urinary excretion of lithium, and increased the fractional excretion of lithium from 0.446 to 0.953, near the theoretically maximum value. At a dose of 2.5 mg/kg lithium chloride, the fractional excretion of lithium was 0.241 in control rats, 0.420 in rats administered acetazolamide, and 0.976 in rats administered acetazolamide and foscarnet. These results showed the potent inhibition of lithium reabsorption by acetazolamide and foscarnet in rats. And, it was exhibited that the effects of acetazolamide on lithium reabsorption differed with the dosages of lithium administered.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Distinct functional properties of two electrogenic isoforms of the SLC34 Na-Pi cotransporter

Inorganic phosphate (Pi ) is crucial for proper cellular function in all organisms. In mammals, type II Na-Pi cotransporters encoded by members of the Slc34 gene family play major roles in the maintenance of Pi homeostasis. However, the molecular mechanisms regulating Na-Pi cotransporter activity within the plasma membrane are largely unknown. In the present study, we used two approaches to examine the effect of changing plasma membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 ) levels on the activities of two electrogenic Na-Pi cotransporters, NaPi-IIa and NaPi-IIb. To deplete plasma membrane PI(4,5)P2 in Xenopus oocytes, we utilized Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), which dephosphorylates PI(4,5)P2 to phosphatidylinositol 4-phosphate (PI(4)P). Upon activation of Ci-VSP, NaPi-IIb currents were significantly decreased, whereas NaPi-IIa currents were unaffected. We also used the rapamycin-inducible Pseudojanin (PJ) system to deplete both PI(4,5)P2 and PI(4)P from the plasma membrane of cultured Neuro 2a cells. Depletion of PI(4,5)P2 and PI(4)P using PJ significantly reduced NaPi-IIb activity, but NaPi-IIa activity was unaffected, which excluded the possibility that NaPi-IIa is equally sensitive to PI(4,5)P2 and PI(4)P. These results indicate that NaPi-IIb activity is regulated by PI(4,5)P2 , whereas NaPi-IIa is not sensitive to either PI(4,5)P2 or PI(4)P. In addition, patch clamp recording of NaPi-IIa and NaPi-IIb currents in cultured mammalian cells enabled kinetic analysis with higher temporal resolution, revealing their distinct kinetic properties.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

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.

Family resemblances: A common fold for some dimeric ion-coupled secondary transporters

The structures of two bacterial antiporters that act as multidrug resistance efflux pumps, MtrF and YdaH, resemble each other and that of the sodium-coupled succinate symporter VcINDY.

Membrane transporter proteins catalyze the passage of a broad range of solutes across cell membranes, allowing the uptake and efflux of crucial compounds. Because of the difficulty of expressing, purifying, and crystallizing integral membrane proteins, relatively few transporter structures have been elucidated to date. Although every membrane transporter has unique characteristics, structural and mechanistic similarities between evolutionarily diverse transporters have been identified. Here, we compare two recently reported structures of membrane proteins that act as antimicrobial efflux pumps, namely MtrF from Neisseria gonorrhoeae and YdaH from Alcanivorax borkumensis, both with each other and with the previously published structure of a sodium-dependent dicarboxylate transporter from Vibrio cholerae, VcINDY. MtrF and YdaH belong to the p-aminobenzoyl-glutamate transporter (AbgT) family and have been reported as having architectures distinct from those of all other families of transporters. However, our comparative analysis reveals a similar structural arrangement in all three proteins, with highly conserved secondary structure elements. Despite their differences in biological function, the overall “design principle” of MtrF and YdaH appears to be almost identical to that of VcINDY, with a dimeric quaternary structure, helical hairpins, and clear boundaries between the transport and scaffold domains. This observation demonstrates once more that the same secondary transporter architecture can be exploited for multiple distinct transport modes, including cotransport and antiport. Based on our comparisons, we detected conserved motifs in the substrate-binding region and predict specific residues likely to be involved in cation or substrate binding. These findings should prove useful for the future characterization of the transport mechanisms of these families of secondary active transporters.

Identification of the first sodium binding site of the phosphate cotransporter NaPi-IIa (SLC34A1)

Transporters of the SLC34 family (NaPi-IIa,b,c) catalyze uptake of inorganic phosphate (Pi) in renal and intestinal epithelia. The transport cycle requires three Na(+) ions and one divalent Pi to bind before a conformational change enables translocation, intracellular release of the substrates, and reorientation of the empty carrier. The electrogenic interaction of the first Na(+) ion with NaPi-IIa/b at a postulated Na1 site is accompanied by charge displacement, and Na1 occupancy subsequently facilitates binding of a second Na(+) ion at Na2. The voltage dependence of cotransport and presteady-state charge displacements (in the absence of a complete transport cycle) are directly related to the molecular architecture of the Na1 site. The fact that Li(+) ions substitute for Na(+) at Na1, but not at the other sites (Na2 and Na3), provides an additional tool for investigating Na1 site-specific events. We recently proposed a three-dimensional model of human SLC34a1 (NaPi-IIa) including the binding sites Na2, Na3, and Pi based on the crystal structure of the dicarboxylate transporter VcINDY. Here, we propose nine residues in transmembrane helices (TM2, TM3, and TM5) that potentially contribute to Na1. To verify their roles experimentally, we made single alanine substitutions in the human NaPi-IIa isoform and investigated the kinetic properties of the mutants by voltage clamp and (32)P uptake. Substitutions at five positions in TM2 and one in TM5 resulted in relatively small changes in the substrate apparent affinities, yet at several of these positions, we observed significant hyperpolarizing shifts in the voltage dependence. Importantly, the ability of Li(+) ions to substitute for Na(+) ions was increased compared with the wild-type. Based on these findings, we adjusted the regions containing Na1 and Na3, resulting in a refined NaPi-IIa model in which five positions (T200, Q206, D209, N227, and S447) contribute directly to cation coordination at Na1.

The sodium/multivitamin transporter: a multipotent system with therapeutic implications

The Na(+)/multivitamin transporter (SMVT) is a member of the solute:sodium symporter family that catalyzes the Na(+)-dependent uptake of the structurally diverse water-soluble vitamins pantothenic acid (vitamin B5) and biotin (vitamin H), α-lipoic acid-a vitamin-like substance with strong antioxidant properties-and iodide. The organic substrates of SMVT play central roles in the cellular metabolism and are, therefore, essential for normal human health and development. For example, biotin deficiency leads to growth retardation, dermatological disorders, and neurological disorders. Animal studies have shown that biotin deficiency during pregnancy is directly correlated to embryonic growth retardation, congenital malformation, and death of the embryo. This chapter focuses on the structural and functional features of the human isoform of SMVT (hSMVT); the discovery of which was greatly facilitated by the cloning and expression of hSMVT in tractable expression systems. Special emphasis will be given to mechanistic implications of the transport process of hSMVT that will inform our understanding of the molecular determinants of hSMVT-mediated transport in dynamic context to alleviate the development and optimization of hSMVT as a multipotent platform for drug delivery.

Structural fold and binding sites of the human Na⁺-phosphate cotransporter NaPi-II

Phosphate plays essential biological roles and its plasma level in humans requires tight control to avoid bone loss (insufficiency) or vascular calcification (excess). Intestinal absorption and renal reabsorption of phosphate are mediated by members of the SLC34 family of sodium-coupled transporters (NaPi-IIa,b,c) whose membrane expression is regulated by various hormones, circulating proteins, and phosphate itself. Consequently, NaPi-II proteins are also potentially important pharmaceutical targets for controlling phosphate levels. Their crucial role in Pi homeostasis is underscored by pathologies resulting from naturally occurring SLC34 mutations and SLC34 knockout animals. SLC34 isoforms have been extensively studied with respect to transport mechanism and structure-function relationships; however, the three-dimensional structure is unknown. All SLC34 transporters share a duplicated motif comprising a glutamine followed by a stretch of threonine or serine residues, suggesting the presence of structural repeats as found in other transporter families. Nevertheless, standard bioinformatic approaches fail to clearly identify a suitable template for molecular modeling. Here, we used hydrophobicity profiles and hidden Markov models to define a structural repeat common to all SLC34 isoforms. Similar approaches identify a relationship with the core regions in a crystal structure of Vibrio cholerae Na(+)-dicarboxylate transporter VcINDY, from which we generated a homology model of human NaPi-IIa. The aforementioned SLC34 motifs in each repeat localize to the center of the model, and were predicted to form Na(+) and Pi coordination sites. Functional relevance of key amino acids was confirmed by biochemical and electrophysiological analysis of expressed, mutated transporters. Moreover, the validity of the predicted architecture is corroborated by extensive published structure-function studies. The model provides key information for elucidating the transport mechanism and predicts candidate substrate binding sites.

The SLC34 family of sodium-dependent phosphate transporters

The SLC34 family of sodium-driven phosphate cotransporters comprises three members: NaPi-IIa (SLC34A1), NaPi-IIb (SLC34A2), and NaPi-IIc (SLC34A3). These transporters mediate the translocation of divalent inorganic phosphate (HPO4 (2-)) together with two (NaPi-IIc) or three sodium ions (NaPi-IIa and NaPi-IIb), respectively. Consequently, phosphate transport by NaPi-IIa and NaPi-IIb is electrogenic. NaPi-IIa and NaPi-IIc are predominantly expressed in the brush border membrane of the proximal tubule, whereas NaPi-IIb is found in many more organs including the small intestine, lung, liver, and testis. The abundance and activity of these transporters are mostly regulated by changes in their expression at the cell surface and are determined by interactions with proteins involved in scaffolding, trafficking, or intracellular signaling. All three transporters are highly regulated by factors including dietary phosphate status, hormones like parathyroid hormone, 1,25-OH2 vitamin D3 or FGF23, electrolyte, and acid-base status. The physiological relevance of the three members of the SLC34 family is underlined by rare Mendelian disorders causing phosphaturia, hypophosphatemia, or ectopic organ calcifications.

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.

Conferring electrogenicity to the electroneutral phosphate cotransporter NaPi-IIc (SLC34A3) reveals an internal cation release step

The SLC34 family of Na(+)-dependent inorganic phosphate cotransporters comprises two electrogenic isoforms (NaPi-IIa, NaPi-IIb) and an electroneutral isoform (NaPi-IIc). Both fulfill essential physiological roles in mammalian phosphate homeostasis. By substitution of three conserved amino acids, found in all electrogenic isoforms, at corresponding sites in NaPi-IIc, electrogenicity was re-established and the Na(+)/P i stoichiometry increased from 2:1 to 3:1. However, this engineered electrogenic construct (AAD-IIc) had a reduced apparent P i affinity and different presteady-state kinetics from the wild-type NaPi-IIa/b. We investigated AAD-IIc using electrophysiology and voltage clamp fluorometry to elucidate the compromised behavior. The activation energy for cotransport was threefold higher than for NaPi-IIc and 1.5-fold higher than for NaPi-IIa and the temperature dependence of presteady-state charge displacements suggested that the large activation energy was associated with the empty carrier reorientation. AAD-IIc shows a weak interaction of external Na(+) ions with the electric field, and thus retains the electroneutral cooperative interaction of two Na(+) ions preceding external P i binding of NaPi-IIc. Most of the presteady-state charge movement was accounted for by the empty carrier (in the absence of external P i ), and the cytosolic release of one Na(+) ion (in the presence of P i ). Simulations using a kinetic model recapitulated the presteady-state and steady-state behavior and allowed identification of two critical partial reactions: the final release of Na(+) to the cytosol and external P i binding. Fluorometric recordings from AAD-IIc mutants with Cys substituted at functionally important sites established that AAD-IIc undergoes substrate- and voltage-dependent conformational changes that correlated qualitatively with its presteady-state kinetics.

Using lithium to probe sequential cation interactions with GAT1

Li(+) interacts with the Na(+)/Cl(-)-dependent GABA transporter, GAT1, under two conditions: in the absence of Na(+) it induces a voltage-dependent leak current; in the presence of Na(+) and GABA, Li(+) stimulates GABA-induced steady-state currents. The amino acids directly involved in the interaction with the Na(+) and Li(+) ions at the so-called "Na2" binding site have been identified, but how Li(+) affects the kinetics of GABA cotransport has not been fully explored. We expressed GAT1 in Xenopus oocytes and applied the two-electrode voltage clamp and (22)Na uptake assays to determine coupling ratios and steady-state and presteady-state kinetics under experimental conditions in which extracellular Na(+) was partially substituted by Li(+). Three novel findings are: 1) Li(+) reduced the coupling ratio between Na(+) and net charge translocated during GABA cotransport; 2) Li(+) increased the apparent Na(+) affinity without changing its voltage dependence; 3) Li(+) altered the voltage dependence of presteady-state relaxations in the absence of GABA. We propose an ordered binding scheme for cotransport in which either a Na(+) or Li(+) ion can bind at the putative first cation binding site (Na2). This is followed by the cooperative binding of the second Na(+) ion at the second cation binding site (Na1) and then binding of GABA. With Li(+) bound to Na2, the second Na(+) ion binds more readily GAT1, and despite a lower apparent GABA affinity, the translocation rate of the fully loaded carrier is not reduced. Numerical simulations using a nonrapid equilibrium model fully recapitulated our experimental findings.

Mechanism of proton/substrate coupling in the heptahelical lysosomal transporter cystinosin

Secondary active transporters use electrochemical gradients provided by primary ion pumps to translocate metabolites or drugs "uphill" across membranes. Here we report the ion-coupling mechanism of cystinosin, an unusual eukaryotic, proton-driven transporter distantly related to the proton pump bacteriorhodopsin. In humans, cystinosin exports the proteolysis-derived dimeric amino acid cystine from lysosomes and is impaired in cystinosis. Using voltage-dependence analysis of steady-state and transient currents elicited by cystine and neutralization-scanning mutagenesis of conserved protonatable residues, we show that cystine binding is coupled to protonation of a clinically relevant aspartate buried in the membrane. Deuterium isotope substitution experiments are consistent with an access of this aspartate from the lysosomal lumen through a deep proton channel. This aspartate lies in one of the two PQ-loop motifs shared by cystinosin with a set of eukaryotic membrane proteins of unknown function and is conserved in about half of them, thus suggesting that other PQ-loop proteins may translocate protons.

Phosphate transport kinetics and structure-function relationships of SLC34 and SLC20 proteins

Transport of inorganic phosphate (P(i)) is mediated by proteins belonging to two solute carrier families (SLC20 and SLC34). Members of both families transport P(i) using the electrochemical gradient for Na(+). The role of the SLC34 members as essential players in mammalian P(i) homeostasis is well established, whereas that of SLC20 proteins is less well defined. The SLC34 family comprises the following three isoforms that preferentially cotransport divalent P(i) and are expressed in epithelial tissue: the renal NaPi-IIa and NaPi-IIc are responsible for reabsorbing P(i) in the proximal tubule, whereas NaPi-IIb is more ubiquitously expressed, including the small intestine, where it mediates dietary P(i) absorption. The SLC20 family comprises two members (PiT-1, PiT-2) that preferentially cotransport monovalent P(i) and are expressed in epithelial as well as nonepithelial tissue. The transport kinetics of members of both families have been characterized in detail using heterologous expression in Xenopus oocytes. For the electrogenic NaPi-IIa/b, and PiT-1,-2, conventional electrophysiological techniques together with radiotracer methods have been applied, as well as time-resolved fluorometric measurements that allow new insights into local conformational changes of the protein during the cotransport cycle. For the electroneutral NaPi-IIc, conventional tracer uptake and fluorometry have been used to elucidate its transport properties. The 3-D structures of these proteins remain unresolved and structure-function studies have so far concentrated on defining the topology and identifying sites of functional importance.

Lithium interactions with Na+-coupled inorganic phosphate cotransporters: insights into the mechanism of sequential cation binding

Type IIa/b Na(+)-coupled inorganic phosphate cotransporters (NaPi-IIa/b) are considered to be exclusively Na(+) dependent. Here we show that Li(+) can substitute for Na(+) as a driving cation. We expressed NaPi-IIa/b in Xenopus laevis oocytes and performed two-electrode voltage-clamp electrophysiology and uptake assays to investigate the effect of external Li(+) on their kinetics. Replacement of 50% external Na(+) with Li(+) reduced the maximum transport rate and the rate-limiting plateau of the P(i)-induced current began at less hyperpolarizing potentials. Simultaneous electrophysiology and (22)Na uptake on single oocytes revealed that Li(+) ions can substitute for at least one of the three Na(+) ions necessary for cotransport. Presteady-state assays indicated that Li(+) ions alone interact with the empty carrier; however, the total charge displaced was 70% of that with Na(+) alone, or when 50% of the Na(+) was replaced by Li(+). If Na(+) and Li(+) were both present, the midpoint potential of the steady-state charge distribution was shifted towards depolarizing potentials. The charge movement in the presence of Li(+) alone reflected the interaction of one Li(+) ion, in contrast to 2 Na(+) ions when only Na was present. We propose an ordered binding scheme for cotransport in which Li(+) competes with Na(+) to occupy the putative first cation interaction site, followed by the cooperative binding of one Na(+) ion, one divalent P(i) anion, and a third Na(+) ion to complete the carrier loading. With Li(+) bound, the kinetics of subsequent partial reactions were significantly altered. Kinetic simulations of this scheme support our experimental data.

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.

Phosphate transporters in renal, gastrointestinal, and other tissues

Inorganic phosphate (Pi) is essential for all living organisms. Bound to organic molecules, Pi fulfills structural, metabolic, and signaling tasks. Therefore, cell growth and maintenance depends on efficient transport of Pi across cellular membranes into the intracellular space. Uptake of Pi requires energy because the substrate is transported against its electrochemical gradient. Till recently, 2 major families of physiologically relevant Pi-specific transporters have been identified: the solute carrier families Slc34 and Slc20. Interestingly, phylogenetic links can be detected between prokaryotic and eukaryotic transporters in both families. Because less complex model organisms are often instrumental in establishing paradigms for protein function in human beings, a brief assessment of Slc34 and Slc20 phylogeny is of interest.

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.

Anionic leak currents through the Na+/monocarboxylate cotransporter SMCT1

SMCT1 is a Na-coupled cotransporter of short chain monocarboxylates, which is expressed in the apical membrane of diverse epithelia such as colon, renal cortex, and thyroid. We previously reported that SMCT1 cotransport was reduced by extracellular Cl− replacement with cyclamate− and that the protein exhibited an ostensible anionic leak current. In this paper, we have revisited the interaction between small monovalent anions and SMCT cotransport and leak currents. We found that the apparent Cl− dependence of cotransport was due to inhibition of this protein by the replacement anion cyclamate, whereas several other replacement anions function as substrates for SMCT1; a suitable replacement anion (MES−) was identified. The observed outward leak currents represented anionic influx and favored larger anions (NO3−>I−>Br−>Cl−); currents in excess of 1 μA (at +50 mV) could be observed and exhibited a quasilinear relationship with anion concentrations up to 100 mM. Application of 25 mM bicarbonate did not produce measurable leak currents. The leak current displayed outward rectification, which disappeared when external Na+ was replaced by N-methyl-d-glucamine+. More precisely, external Na+ blocked the leak current in both directions, but its Ki value rose rapidly when membrane potential became positive. Thus SMCT1 possesses a anionic leak current that becomes significant whenever external Na+ concentration is reduced. The presence of this leak current may represent a second function for SMCT1 in addition to cotransporting short chain fatty acids, and future experiments will determine whether this function serves a physiological role in tissues where SMCT1 is expressed.

Different effects of arsenate and phosphonoformate on Pitransport adaptation in opossum kidney cells

The main nonhormonal mechanism for controlling inorganic phosphate (Pi) homeostasis is renal adaptation of the proximal tubular Pitransport rate to changes in dietary phosphate content. Opossum kidney (OK) cell line is an in vitro renal model that maintains the ability of renal adaptation to the extracellular Piconcentration. We have studied how two competitive inhibitors of Pitransport, arsenate [As(V)] and phosphonoformate (PFA), affect adaptation to low and high Piconcentrations. OK cells show very high affinity for As(V) (inhibitory constant, Ki0.12 mM) when compared with the rat kidney. As(V) very efficiently reversed the adaptation of OK cells to low Pi(0.1 mM), whereas PFA induced adaptation similar to 0.1 mM Pi. Adaptation with 2 mM Pior As(V) was characterized by decreases in the maximal velociy ( Vmax) of Pitransport and an abundance of the NaPi-IIa Pitransporter in the plasma membrane, shown by the protein biotinylation. Conversely, PFA and 0.1 mM Piincreased the Vmaxand transporter abundance. Changes in the Vmaxwere limited to a 50% variation, which was not paralleled by changes in the concentration of Pior of the inhibitor. OK cells are very sensitive to As(V), but the effects are reversible and noncytotoxic. These effects can be interpreted as As(V) being transported into the cell, thereby mimicking a high Piconcentration. PFA blocks the uptake of Pibut is not transported, and it therefore simulates a low Piconcentration inside the cell. To conclude, a mathematical definition of the adaptation process is reported, thereby explaining the limited changes in Pitransport Vmax.

Substrate interactions of the electroneutral Na+-coupled inorganic phosphate cotransporter (NaPi-IIc)

The SLC34 solute carrier family comprises the electrogenic NaPi-IIa/b and the electroneutral NaPi-IIc, which display Na(+) : P(i) cotransport stoichiometries of 3 : 1 and 2 : 1, respectively. We previously proposed that NaPi-IIc lacks one of the three Na(+) interaction sites hypothesised for the electrogenic isoforms, but, unlike NaPi-IIa/b, its substrate binding order is undetermined. By expressing NaPi-IIc in Xenopus oocytes, isotope influx and efflux assays gave results consistent with Na(+) being the first and last substrate to bind. To further investigate substrate interactions, we applied a fluorometry-based technique that uses site-specific labelling with a fluorophore to characterize substrate-induced conformational changes. A novel Cys was introduced in the third extracellular loop of NaPi-IIc that could be labelled with a reporter fluorophore (MTS-TAMRA). Although labelling resulted in suppression of cotransport as previously reported for the electrogenic isoforms, changes in fluorescence were induced by changes in extracellular Na(+) concentration in the absence of P(i) and by changes in extracellular P(i) concentration in presence of Na(+). These data, combined with (32)P uptake data, also support a binding scheme in which Na(+) is the first substrate to interact. Moreover, the apparent P(i) affinity from fluorometry agreed with that from (32)P uptake, confirming the applicability of the fluorometric technique for kinetic studies of electroneutral carriers. Analysis of the fluorescence data showed that like the electrogenic NaPi-IIb, 2 Na(+) ions interact cooperatively with NaPi-IIc before P(i) binding, which implies that only one of these is translocated. This result provides compelling evidence that SLC34 proteins share common motifs for substrate interaction and that cotransport and substrate binding stoichiometries are not necessarily equivalent.

Measuring ion transport activities in Xenopus oocytes using the ion-trap technique

The ion-trap technique is an experimental approach allowing measurement of changes in ionic concentrations within a restricted space (the trap) comprised of a large-diameter ion-selective electrode apposed to a voltage-clamped Xenopus laevis oocyte. The technique is demonstrated with oocytes expressing the Na(+)/glucose cotransporter (SGLT1) using Na(+)- and H(+)-selective electrodes and with the electroneutral H(+)/monocarboxylate transporter (MCT1). In SGLT1-expressing oocytes, bath substrate diffused into the trap within 20 s, stimulating Na(+)/glucose influx, which generated a measurable decrease in the trap Na(+) concentration ([Na(+)](T)) by 0.080 +/- 0.009 mM. Membrane hyperpolarization produced a further decrease in [Na(+)](T), which was proportional to the increased cotransport current. In a Na(+)-free, weakly buffered solution (pH 5.5), H(+) drives glucose transport through SGLT1, and this was monitored with a H(+)-selective electrode. Proton movements can also be clearly detected on adding lactate to an oocyte expressing MCT1 (pH 6.5). For SGLT1, time-dependent changes in [Na(+)](T) or [H(+)](T) were also detected during a membrane potential pulse (150 ms) in the presence of substrate. In the absence of substrate, hyperpolarization triggered rapid reorientation of SGLT1 cation binding sites, accompanied by cation capture from the trap. The resulting change in [Na(+)](T) or [H(+)](T) is proportional to the pre-steady-state charge movement. The ion-trap technique can thus be used to measure steady-state and pre-steady-state transport activities and provides new opportunities for studying electrogenic and electroneutral ion transport mechanisms.

Phosphate transporters: a tale of two solute carrier families

Phosphate is an essential component of life and must be actively transported into cells against its electrochemical gradient. In vertebrates, two unrelated families of Na+ -dependent P(i) transporters carry out this task. Remarkably, the two families transport different P(i) species: whereas type II Na+/P(i) cotransporters (SCL34) prefer divalent HPO(4)(2-), type III Na(+)/P(i) cotransporters (SLC20) transport monovalent H2PO(4)(-). The SCL34 family comprises both electrogenic and electroneutral members that are expressed in various epithelia and other polarized cells. Through regulated activity in apical membranes of the gut and kidney, they maintain body P(i) homeostasis, and in salivary and mammary glands, liver, and testes they play a role in modulating the P(i) content of luminal fluids. The two SLC20 family members PiT-1 and PiT-2 are electrogenic and ubiquitously expressed and may serve a housekeeping role for cell P(i) homeostasis; however, also more specific roles are emerging for these transporters in, for example, bone mineralization. In this review, we focus on recent advances in the characterization of the transport kinetics, structure-function relationships, and physiological implications of having two distinct Na+/P(i) cotransporter families.

Deciphering PiT transport kinetics and substrate specificity using electrophysiology and flux measurements

Members of the SLC20 family or type III Na(+) -coupled P(i) cotransporters (PiT-1, PiT-2) are ubiquitously expressed in mammalian tissue and are thought to perform a housekeeping function for intracellular P(i) homeostasis. Previous studies have shown that PiT-1 and PiT-2 mediate electrogenic P(i) cotransport when expressed in Xenopus oocytes, but only limited kinetic characterizations were made. To address this shortcoming, we performed a detailed analysis of SLC20 transport function. Three SLC20 clones (Xenopus PiT-1, human PiT-1, and human PiT-2) were expressed in Xenopus oocytes. Each clone gave robust Na(+)-dependent (32)P(i) uptake, but only Xenopus PiT-1 showed sufficient activity for complete kinetic characterization by using two-electrode voltage clamp and radionuclide uptake. Transport activity was also documented with Li(+) substituted for Na(+). The dependence of the P(i)-induced current on P(i) concentration was Michaelian, and the dependence on Na(+) concentration indicated weak cooperativity. The dependence on external pH was unique: the apparent P(i) affinity constant showed a minimum in the pH range 6.2-6.8 of approximately 0.05 mM and increased to approximately 0.2 mM at pH 5.0 and pH 8.0. Xenopus PiT-1 stoichiometry was determined by dual (22)Na-(32)P(i) uptake and suggested a 2:1 Na(+):P(i) stoichiometry. A correlation of (32)P(i) uptake and net charge movement indicated one charge translocation per P(i). Changes in oocyte surface pH were consistent with transport of monovalent P(i). On the basis of the kinetics of substrate interdependence, we propose an ordered binding scheme of Na(+):H(2)PO(4)(-):Na(+). Significantly, in contrast to type II Na(+)-P(i) cotransporters, the transport inhibitor phosphonoformic acid did not inhibit PiT-1 or PiT-2 activity.

Temperature dependence of steady-state and presteady-state kinetics of a type IIb Na+/P i cotransporter

The temperature dependence of the transport kinetics of flounder Na(+)-coupled inorganic phosphate (P(i)) cotransporters (NaPi-IIb) expressed in Xenopus oocytes was investigated using radiotracer and electrophysiological assays. (32)P(i) uptake was strongly temperature-dependent and decreased by approximately 80% at a temperature change from 25 degrees C to 5 degrees C. The corresponding activation energy (E (a)) was approximately 14 kcal mol(-1) for the cotransport mode. The temperature dependence of the cotransport and leak modes was determined from electrogenic responses to 1 mM P(i) and phosphonoformic acid (PFA), respectively, under voltage clamp. The magnitude of the P(i)- and PFA-induced changes in holding current decreased with temperature. E (a) at -100 mV for the cotransport and leak modes was approximately 16 kcal mol(-1) and approximately 11 kcal mol(-1), respectively, which suggested that the leak is mediated by a carrier, rather than a channel, mechanism. Moreover, E (a) for cotransport was voltage-independent, suggesting that a major conformational change in the transport cycle is electroneutral. To identify partial reactions that confer temperature dependence, we acquired presteady-state currents at different temperatures with 0 mM P(i) over a range of external Na(+). The relaxation time constants increased, and the peak time constant shifted toward more positive potentials with decreasing temperature. Likewise, there was a depolarizing shift of the charge distribution, whereas the total available charge and apparent valency predicted from single Boltzmann fits were temperature-independent. These effects were explained by an increased temperature sensitivity of the Na(+)-debinding rate compared with the other voltage-dependent rate constants.

Structural and functional basis of amino acid specificity in the invertebrate cotransporter KAAT1

The substrate specificity of KAAT1, a Na+- and K+-dependent neutral amino acid cotransporter cloned from the larva of the invertebrate Manduca sexta and belonging to the SLC6A gene family has been investigated using electrophysiological and radiotracer methods. The specificity of KAAT1 was compared to that of CAATCH1, a strictly related transporter with different amino acid selectivity. Competition experiments between different substrates indicate that both transporters bind leucine more strongly than threonine and proline, the difference between KAAT1 and CAATCH1 residing in the incapacity of the latter to complete the transport cycle in presence of leucine. The behaviour of CAATCH1 is mimicked by the S308T mutant form of KAAT1, constructed on the basis of the atomic structure of a leucine-transporting bacterial member of the family, which indicates the participation of this residue in the leucine-binding site. The reverse mutation T308S in CAATCH1 conferred to this transporter the ability to transport leucine in presence of K+. These results may be interpreted by a kinetic scheme in which, in presence of Na+, the leucine-bound state of the transporter is relatively stable, while in presence of K+ and at negative potentials the progression of the leucine-bound form along the cycle is favoured. In this context serine 308 appears to be important in allowing the change to the inward-facing conformation of the transporter following substrate binding, rather than in determining the binding specificity.

Electrogenic kinetics of a mammalian intestinal type IIb Na(+)/P(i) cotransporter

The kinetics of a type IIb Na(+)-coupled inorganic phosphate (Pi) cotransporter (NaPi-IIb) cloned from mouse small intestine were studied using the two-electrode voltage clamp applied to Xenopus oocytes. In the steady state, mouse NaPi-IIb showed a curvilinear I-V relationship, with rate-limiting behavior only for depolarizing potentials. The Pi dose dependence was Michaelian, with an apparent affinity constant for Pi (Km(pi)) of 10 +/- 1 microM: at -60 mV. Unlike for rat NaPi-IIa, (Km(pi)) increased with membrane hyperpolarization, as reported for human NaPi-IIa, flounder NaPi-IIb and zebrafish NaPi-IIb2. The apparent affinity constant for Na(+) (Km(na)) was 23 +/- 1 mM: at -60 mV, and the Na(+) activation was cooperative with a Hill coefficient of approximately 2. Pre-steady-state currents were documented in the absence of Pi and showed a strong dependence on external Na(+). The hyperpolarizing shift of the charge distribution midpoint potential was 65 mV/log[Na]. Approximately half the moveable charge was attributable to the empty carrier. A comparison of the voltage dependence of steady-state Pi-induced current and pre-steady-state charge movement indicated that for -120 mV <or= V <or= 0 mV the voltage dependence of the empty carrier was the main determinant of the curvilinear steady-state cotransport characteristic. External protons partially inhibited NaPi-IIb steady-state activity, independent of the titration of mono- and divalent Pi, and immobilized pre-steady-state charge movements associated with the first Na(+) binding step.

Proximal tubular handling of phosphate: A molecular perspective

Members of the SLC34 gene family of solute carriers encode for three Na+-dependent phosphate (P i) cotransporter proteins, two of which (NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3) control renal reabsorption of P i in the proximal tubule of mammals, whereas NaPi-IIb/SCLC34A2 mediates P i transport in organs other than the kidney. The P i transport mechanism has been extensively studied in heterologous expression systems and structure-function studies have begun to reveal the intricacies of the transport cycle at the molecular level using techniques such as cysteine scanning mutagenesis, and voltage clamp fluorometry. Moreover, sequence differences between the three types of cotransporters have been exploited to obtain information about the molecular determinants of hormonal sensitivity and electrogenicity. Renal handling of P i is regulated by hormonal and non-hormonal factors. Changes in urinary excretion of P i are almost invariably mirrored by changes in the apical expression of NaPi-IIa and NaPi-IIc in proximal tubules. Therefore, understanding the mechanisms that control the apical expression of NaPi-IIa and NaPi-IIc as well as their functional properties is critical to understanding how an organism achieves P i homeostasis.

Effect of substrate on the pre-steady-state kinetics of the Na(+)/glucose cotransporter

When measuring Na(+)/glucose cotransporter (SGLT1) activity in Xenopus oocytes with the two-electrode voltage-clamp technique, pre-steady-state currents dissipate completely in the presence of saturating alpha-methyl-glucose (alphaMG, a nonhydrolyzable glucose analog) concentrations. In sharp contrast, two SGLT1 mutants (C255A and C511A) that lack a recently identified disulfide bridge express the pre-steady-state currents in the presence of alphaMG. The dose-dependent effects of alphaMG on pre-steady-state currents were studied for wild-type (wt) SGLT1 and for the two mutants. Increases in alphaMG concentration reduced the total transferred charge (partially for the mutants, totally for wt SGLT1), shifted the transferred charge versus membrane potential (Q-V) curve toward positive potentials, and significantly modified the time constants of the pre-steady-state currents. A five-state kinetic model is proposed to quantitatively explain the effect of alphaMG on pre-steady-state currents. This analysis reveals that the reorientation of free transporter is the slowest step for wt SGLT1 either in the presence or in the absence of alphaMG. In contrast, the conformational change of the fully loaded mutant transporters constitutes their rate-limiting step in the presence of substrate and explains the persistence of pre-steady-state currents in this situation.

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.

State-dependent conformations of the translocation pathway in the tyrosine transporter Tyt1, a novel neurotransmitter:sodium symporter from Fusobacterium nucleatum

The gene of a novel prokaryotic member (Tyt1) of the neurotransmitter:sodium symporter (NSS) family has been cloned from Fusobacterium nucleatum. In contrast to eukaryotic and some prokaryotic NSSs, which contain 12 transmembrane domains (TMs), Tyt1 contains only 11 TMs, a characteristic shared by approximately 70% of prokaryotic NSS homologues. Nonetheless upon heterologous expression in an engineered Escherichia coli host, Tyt1 catalyzes robust Na+-dependent, highly selective l-tyrosine transport. Genetic engineering of Tyt1 variants devoid of cysteines or with individually retained endogenous cysteines at positions 18 or 238, at the cytoplasmic ends of TM1 and TM6, respectively, preserved normal transport activity. Whereas cysteine-less Tyt1 was resistant to the inhibitory effect of sulfhydryl-alkylating reagents, N-ethylmaleimide inhibited transport by Tyt1 variants containing either one or both of the endogenous cysteines, and this inhibition was altered by the substrates sodium and tyrosine, consistent with substrate-induced dynamics in the transport pathway. Our findings support a binding model of Tyt1 function in which an ordered sequence of substrate-induced structural changes reflects distinct conformational states of the transporter. This work identifies Tyt1 as the first functional bacterial NSS member putatively consisting of only 11 TMs and shows that Tyt1 is a suitable model for the study of NSS dynamics with relevance to structure/function relationships of human NSSs, including the dopamine, norepinephrine, serotonin, and gamma-aminobutyric acid transporters.

Functionally important residues in the predicted 3(rd) transmembrane domain of the type IIa sodium-phosphate co-transporter (NaPi-IIa)

The type IIa Na(+)/P(i), cotransporter (NaPi-IIa) mediates electrogenic transport of three Na(+) and one divalent P(i) ion (and one net positive charge) across the cell membrane. Sequence comparison of electrogenic NaPi-IIa and IIb isoforms with the electroneutral NaPi-IIc isoform pointed to the third transmembrane domain (TMD-3) as a possibly significant determinant of substrate binding. To elucidate the role of TMD-3 in the topology and mechanism underlying NaPi-IIa function we subjected it to cysteine scanning mutagenesis. The constructs were expressed in Xenopus oocytes and P(i) transport kinetics were assayed by electrophysiology and radiotracer uptake. Cys substitution resulted in only marginally altered kinetics of P(i) transport in those mutants providing sufficient current for analysis. Only one site, at the extracellular end of TMD-3, appeared to be accessible to methanethiosulfonate reagents. However, additional mutations carried out at D224 (replaced by E, G or N) and N227 (replaced by D or Q) resulted in markedly altered voltage and substrate dependencies of the P(i)-dependent currents. Replacing Asp-224 (highly conserved in electrogenic a and b isoforms) with Gly (the residue found in the electroneutral c isoform) resulted in a mutant that mediated electroneutral Na(+)-dependent P(i) transport. Since electrogenic NaPi-II transports 3 Na(+)/transport cycle, whereas electroneutral NaPi-IIc only transports 2, we speculate that this loss of electrogenicity might result from the loss of one of the three Na(+) binding sites in NaPi-IIa.

Renouncing electroneutrality is not free of charge: switching on electrogenicity in a Na+-coupled phosphate cotransporter

Renal type IIa Na+-coupled inorganic phosphate (Pi) cotransporters (NaPi-IIa) mediate divalent Pi transport in an electrogenic manner, whereas the renal type IIc isoform (NaPi-IIc) is electroneutral, yet it shows high sequence identity with NaPi-IIa. Dual uptake (32Pi/22Na) assays confirmed that NaPi-IIc displayed Na+-coupled Pi cotransport with a 2:1 (Na+:Pi) stoichiometry compared with 3:1 established for NaPi-IIa. This finding suggested that the electrogenicity of NaPi-IIa arises from the interaction of an additional Na+ ion compared with NaPi-IIc. To identify the molecular elements responsible for the functional difference between isoforms, we used chimera and amino acid replacement approaches. Transport activity of chimeras constructed with NaPi-IIa and NaPi-IIc indicated that residues within the first six transmembrane domains were essential for the electrogenicity of NaPi-IIa. Sequence comparison between electrogenic and electroneutral isoforms revealed differences in the charge and polarity of residues clustered in three areas, one of which included part of the predicted third transmembrane domain. Here, substitution of three residues with their NaPi-IIa equivalents in NaPi-IIc (S189A, S191A, and G195D) resulted in a transporter that displayed a 1:1 charge/Pi coupling, a 3:1 Na+:Pi stoichiometry, and transient currents that resembled pre-steady-state relaxations. The mutant's weaker voltage dependency and 10-fold lower apparent Pi affinity compared with NaPi-IIa indicated that other residues important for the NaPi-IIa kinetic fingerprint exist. Our findings demonstrate that, through a minimal number of side chain substitutions, we can effect a switch from electroneutral to electrogenic cotransporter function, concomitant with the appearance of a cosubstrate interaction site.

Electrogenic glutamate transporters in the CNS: molecular mechanism, pre-steady-state kinetics, and their impact on synaptic signaling

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The spatiotemporal profile of the glutamate concentration in the synapse is critical for excitatory synaptic signalling. The control of this spatiotemporal concentration profile requires the presence of large numbers of synaptically localized glutamate transporters that remove pre-synaptically released glutamate by uptake into neurons and adjacent glia cells. These glutamate transporters are electrogenic and utilize energy stored in the transmembrane potential and the Na+/K+-ion concentration gradients to accumulate glutamate in the cell. This review focuses on the kinetic and electrogenic properties of glutamate transporters, as well as on the molecular mechanism of transport. Recent results are discussed that demonstrate the multistep nature of the transporter reaction cycle. Results from pre-steady-state kinetic experiments suggest that at least four of the individual transporter reaction steps are electrogenic, including reactions associated with the glutamate-dependent transporter halfcycle. Furthermore, the kinetic similarities and differences between some of the glutamate transporter subtypes and splice variants are discussed. A molecular mechanism of glutamate transport is presented that accounts for most of the available kinetic data. Finally, we discuss how synaptic glutamate transporters impact on glutamate receptor activity and how transporters may shape excitatory synaptic transmission.

Functional characterization of high-affinity Na(+)/dicarboxylate cotransporter found in Xenopus laevis kidney and heart

The SLC13 gene family includes sodium-coupled transporters for citric acid cycle intermediates and sulfate. The present study describes the sequence and functional characterization of a SLC13 family member from Xenopus laevis, the high-affinity Na(+)/dicarboxylate cotransporter xNaDC-3. The cDNA sequence of xNaDC-3 codes for a protein of 602 amino acids that is approximately 70% identical to the sequences of mammalian NaDC-3 orthologs. The message for xNaDC-3 is found in the kidney, liver, intestine, and heart. The xNaDC-3 has a high affinity for substrate, including a K(m) for succinate of 4 muM, and it is inhibited by the NaDC-3 test substrates 2,3-dimethylsuccinate and adipate. The transport of succinate by xNaDC-3 is dependent on sodium, with sigmoidal activation kinetics, and lithium can partially substitute for sodium. As with other members of the family, xNaDC-3 is electrogenic and exhibits inward substrate-dependent currents in the presence of sodium. However, other electrophysiological properties of xNaDC-3 are unique and involve large leak currents, possibly mediated by anions, that are activated by binding of sodium or lithium to a single site.

Structure-function relations of the first and fourth extracellular linkers of the type IIa Na+/Pi cotransporter: II. Substrate interaction and voltage dependency of two functionally important sites

Functionally important sites in the predicted first and fourth extracellular linkers of the type IIa Na⁺/P_i cotransporter (NaPi-IIa) were identified by cysteine scanning mutagenesis (Ehnes et al., 2004). Cysteine substitution or modification with impermeant and permeant methanethiosulfonate (MTS) reagents at certain sites resulted in changes to the steady-state voltage dependency of the cotransport mode (1 mM P_i, 100 mM Na⁺ at pH 7.4) of the mutants. At Gly-134 (ECL-1) and Met-533 (ECL-4), complementary behavior of the voltage dependency was documented with respect to the effect of cys-substitution and modification. G134C had a weak voltage dependency that became even stronger than that of the wild type (WT) after MTS incubation. M533C showed a WT-like voltage dependency that became markedly weaker after MTS incubation. To elucidate the underlying mechanism, the steady-state and presteady-state kinetics of these mutants were studied in detail. The apparent affinity constants for P_i and Na⁺ did not show large changes after MTS exposure. However, the dependency on external protons was changed in a complementary manner for each mutant. This suggested that cys substitution at Gly-134 or modification of Cys-533 had induced similar conformational changes to alter the proton modulation of transport kinetics. The changes in steady-state voltage dependency correlated with changes in the kinetics of presteady-state charge movements determined in the absence of P_i, which suggested that voltage-dependent transitions in the transport cycle were altered. The steady-state and presteady-state behavior was simulated using an eight-state kinetic model in which the transition rate constants of the empty carrier and translocation of the fully loaded carrier were found to be critical determinants of the transport kinetics. The simulations predict that cys substitution at Gly-134 or cys modification of Cys-533 alters the preferred orientation of the empty carrier from an inward to outward-facing conformation for hyperpolarizing voltages.

Computational modelling of H+-coupled peptide transport via human PEPT1

H+-coupled peptide transporter 1 (PEPT1) mediates the transport of small peptides and peptide-like drugs in a pH- and voltage-dependent manner. Here, we investigated the transport mechanisms of PEPT1 for neutral and charged substrates by experimental studies and computational simulation. Uptake studies revealed that the Michaelis-Menten constant (Km) of glycylsarcosine (Gly-Sar), a neutral substrate, decreased with a fall in pH from 7.4 to 5.5, but at pH 5.0, the Km increased again. In contrast, the Km value of an anionic substrate, ceftibuten, declined steadily with decreasing pH. Based on these findings and information from the literature, we hypothesized the transport mechanisms in which (1) H+ binds to not only the H+-binding site, but also the substrate-binding site; and (2) H+ at the substrate-binding site inhibits the interaction of neutral and cationic substrates, but is necessary for that of anionic substrates. To validate these hypotheses, a computational model was constructed and various properties of substrate transport by PEPT1 were simulated. Our model reproduced the voltage dependence, hyperbolic saturation and bell-shaped pH-profile of Gly-Sar transport. Moreover, the various transport properties of negatively and positively charged substrates were also reconstructed. These findings indicated that the inferred mechanisms are able to sufficiently interpret the transport of both neutral and charged substrates by PEPT1.

Structure-function relations of the first and fourth predicted extracellular linkers of the type IIa Na+/Pi cotransporter: I. Cysteine scanning mutagenesis

The putative first intracellular and third extracellular linkers are known to play important roles in defining the transport properties of the type IIa Na⁺-coupled phosphate cotransporter (Kohler, K., I.C. Forster, G. Stange, J. Biber, and H. Murer. 2002b. J. Gen. Physiol. 120:693–705). To investigate whether other stretches that link predicted transmembrane domains are also involved, the substituted cysteine accessibility method (SCAM) was applied to sites in the predicted first and fourth extracellular linkers (ECL-1 and ECL-4). Mutants based on the wild-type (WT) backbone, with substituted novel cysteines, were expressed in Xenopus oocytes, and their function was assayed by isotope uptake and electrophysiology. Functionally important sites were identified in both linkers by exposing cells to membrane permeant and impermeant methanethiosulfonate (MTS) reagents. The cysteine modification reaction rates for sites in ECL-1 were faster than those in ECL-4, which suggested that the latter were less accessible from the extracellular medium. Generally, a finite cotransport activity remained at the end of the modification reaction. The change in activity was due to altered voltage-dependent kinetics of the P_i-dependent current. For example, cys substitution at Gly-134 in ECL-1 resulted in rate-limiting, voltage-independent cotransport activity for V ≤ −80 mV, whereas the WT exhibited a linear voltage dependency. After cys modification, this mutant displayed a supralinear voltage dependency in the same voltage range. The opposite behavior was documented for cys substitution at Met-533 in ECL-4. Modification of cysteines at two other sites in ECL-1 (Ile-136 and Phe-137) also resulted in supralinear voltage dependencies for hyperpolarizing potentials. Taken together, these findings suggest that ECL-1 and ECL-4 may not directly form part of the transport pathway, but specific sites in these linkers can interact directly or indirectly with parts of NaPi-IIa that undergo voltage-dependent conformational changes and thereby influence the voltage dependency of cotransport.

Substrate interactions in the human type IIa sodium-phosphate cotransporter (NaPi-IIa)

We have characterized the kinetics of substrate transport in the renal type IIa human sodium-phosphate cotransporter (NaPi-IIa). The transporter was expressed in Xenopus laevis oocytes, and steady-state and pre-steady-state currents and substrate uptakes were characterized by voltage-clamp and isotope flux. First, by measuring simultaneous uptake of a substrate (32Pi, 22Na) and charge in voltage-clamped oocytes, we established that the human NaPi-IIa isoform operates with a Na:Pi:charge stoichiometry of 3:1:1 and that the preferred transported Pi species is HPO4(2-). We then probed the complex interrelationship of substrates, pH, and voltage in the NaPi-IIa transport cycle by analyzing both steady-state and pre-steady-state currents. Steady-state current measurements show that the apparent HPO4(2-) affinity is voltage dependent and that this voltage dependency is abrogated by lowering the pH or the Na+ concentration. In contrast, the voltage dependency of the apparent Na+ affinity increased when pH was lowered. Pre-steady-state current analysis shows that Na+ ions bind first and influence the preferred orientation of the transporter in the absence of Pi. Pre-steady-state charge movement was partially suppressed by complete removal of Na+ from the bath, by reducing extracellular pH (both in the presence and absence of Na+), or by adding Pi (in the presence of 100 mM Na). None of these conditions suppressed charge movement completely. The results allowed us to modify previous models for the transport cycle of NaPi-II transporters by including voltage dependency of HPO4(2-) binding and proton modulation of the first Na+ binding step.

Aromatic amino acid transporter AAT-9 of Caenorhabditis elegans localizes to neurons and muscle cells

The Caenorhabditis elegans genome encodes nine homologues of mammalian glycoprotein-associated amino acid transporters. Two of these C. elegans proteins (AAT-1 and AAT-3) have been shown to function as catalytic subunits (light chains) of heteromeric amino acid transporters. These proteins need to associate with a glycoprotein heavy chain subunit (ATG-2) to reach the cell surface in a manner similar to that of their mammalian homologues. AAT-1 and AAT-3 contain a cysteine residue in the second putative extracellular loop through which a disulfide bridge can form with a heavy chain. In contrast, six C. elegans members of this family (AAT-4 to AAT-9) lack such a cysteine residue. We show here that one of these transporter proteins, AAT-9, reaches the cell surface in Xenopus oocytes without an exogenous heavy chain and that it functions as an exchanger of aromatic amino acids. Two-electrode voltage clamp experiments demonstrate that AAT-9 displays a substrate-activated conductance. Immunofluorescence shows that it is expressed close to the pharyngeal bulbs within C. elegans neurons. The selective expression of an aat-9 promoter-green fluorescent protein construct in several neurons of this region and in wall muscle cells around the mouth supports and extends these localization data. Taken together, the results show that AAT-9 is expressed in excitable cells of the nematode head and pharynx in which it may provide a pathway for aromatic amino acid transport.