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

Different fluorescent labels report distinct components of spHCN channel voltage sensor movement

We used voltage clamp fluorometry to probe the movement of the S4 helix in the voltage-sensing domain of the sea urchin HCN channel (spHCN) expressed in Xenopus oocytes. We obtained markedly different fluorescence responses with either ALEXA-488 or MTS-TAMRA covalently linked to N-terminal Cys332 of the S4 helix. With hyperpolarizing steps, ALEXA-488 fluorescence increased rapidly, consistent with it reporting the initial inward movement of S4, as previously described. In contrast, MTS-TAMRA fluorescence increased more slowly and its early phase correlated with that of channel opening. Additionally, a slow fluorescence component that tracked the development of the mode shift, or channel hysteresis, could be resolved with both labels. We quantitated this component as an increased deactivation tail current delay with concomitantly longer activation periods and found it to depend strongly on the presence of K+ ions in the pore. Using collisional quenching experiments and structural predictions, we established that ALEXA-488 was more exposed to solvent than MTS-TAMRA. We propose that components of S4 movement during channel activation can be kinetically resolved using different fluorescent probes to reveal distinct biophysical properties. Our findings underscore the need to apply caution when interpreting voltage clamp fluorometry data and demonstrate the potential utility of different labels to interrogate distinct biophysical properties of voltage-gated membrane proteins.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

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.

Structural models of the NaPi-II sodium-phosphate cotransporters

Progress towards understanding the molecular mechanisms of phosphate homeostasis through sodium-dependent transmembrane uptake has long been stymied by the absence of structural information about the NaPi-II sodium-phosphate transporters. For many other coupled transporters, even those unrelated to NaPi-II, internal repeated elements have been revealed as a key feature that is inherent to their function. Here, we review recent structure prediction studies for NaPi-II transporters. Attempts to identify structural templates for NaPi-II transporters have leveraged the structural repeat perspective to uncover an otherwise obscured relationship with the dicarboxylate-sodium symporters (DASS). This revelation allowed the prediction of three-dimensional structural models of human NaPi-IIa and flounder NaPi-IIb, whose folds were evaluated by comparison with available biochemical data outlining the transmembrane topology and solvent accessibility of various regions of the protein. Using these structural models, binding sites for sodium and phosphate were proposed. The predicted sites were tested and refined based on detailed electrophysiological and biochemical studies and were validated by comparison with subsequently reported structures of transporters belonging to the AbgT family. Comparison with the DASS transporter VcINDY suggested a conformational mechanism involving a large, two-domain structural change, known as an elevator-like mechanism. These structural models provide a foundation for further studies into substrate binding, conformational change, kinetics, and energetics of sodium-phosphate transport. We discuss future opportunities, as well as the challenges that remain.

Cation Interactions and Membrane Potential Induce Conformational Changes in NaPi-IIb

Voltage-dependence of Na(+)-coupled phosphate cotransporters of the SLC34 family arises from displacement of charges intrinsic to the protein and the binding/release of one Na(+) ion in response to changes in the transmembrane electric field. Candidate coordination residues for the cation at the Na1 site were previously predicted by structural modeling using the x-ray structure of dicarboxylate transporter VcINDY as template and confirmed by functional studies. Mutations at Na1 resulted in altered steady-state and presteady-state characteristics that should be mirrored in the conformational changes induced by membrane potential changes. To test this hypothesis by functional analysis, double mutants of the flounder SLC34A2 protein were constructed that contain one of the Na1-site perturbing mutations together with a substituted cysteine for fluorophore labeling, as expressed in Xenopus oocytes. The locations of the mutations were mapped onto a homology model of the flounder protein. The effects of the mutagenesis were characterized by steady-state, presteady-state, and fluorometric assays. Changes in fluorescence intensity (ΔF) in response to membrane potential steps were resolved at three previously identified positions. These fluorescence data corroborated the altered presteady-state kinetics upon perturbation of Na1, and furthermore indicated concomitant changes in the microenvironment of the respective fluorophores, as evidenced by changes in the voltage dependence and time course of ΔF. Moreover, iodide quenching experiments indicated that the aqueous nature of the fluorophore microenvironment depended on the membrane potential. These findings provide compelling evidence that membrane potential and cation interactions induce significant large-scale structural rearrangements of the protein.

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.

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.

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.

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.

Conformational changes associated with proton-dependent gating of ASIC1a

Acid-sensing ion channels are proton-gated Na(+) channels expressed predominantly in neurons. How channel structure translates an environmental stimulus into changes in pore permeability remains largely undefined. The pore of ASIC1 is defined by residues in the second transmembrane domain (TM2), although a segment of the outer vestibule is formed by residues of TM1. We used the voltage clamp fluorometry technique to define the role of the region preceding TM2 (pre-TM2) in activation and desensitization of mouse ASIC1a. Oocytes expressing E425C channels labeled with Alexa Fluor 488 C5-maleimide showed a change in the emission of the fluorescent probe in response to extracellular acidification. The time course of the change in fluorescence correlated with activation but not desensitization of E425C channels. The fluorescence emission did not change following extracellular acidification in oocytes carrying an inactivating mutation (W287G/E425C), although these channels were labeled and expressed at the plasma membrane. Our data indicate that pore opening occurs in conjunction with a conformational rearrangement of the pre-TM2. We observed a change in the emission of the fluorescent probe when labeled E425C channels transition from the desensitized to the resting state. The substituted-cysteine-accessibility method was used to determine whether the pre-TM2 has different conformations in the resting and desensitized states. State-dependent changes in accessibility to 2-[(trimethylammonium)ethyl]methanethiosulfonate bromide modification were observed in oocytes expressing K421C, K422C, Y424C, and E425C channels. Our results suggest that the pre-TM2 of ASIC1a undergoes dynamic conformational rearrangements during proton-dependent gating.

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.

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.

Ligand-specific conformational changes in the alpha1 glycine receptor ligand-binding domain

Understanding the activation mechanism of Cys loop ion channel receptors is key to understanding their physiological and pharmacological properties under normal and pathological conditions. The ligand-binding domains of these receptors comprise inner and outer beta-sheets and structural studies indicate that channel opening is accompanied by conformational rearrangements in both beta-sheets. In an attempt to resolve ligand-dependent movements in the ligand-binding domain, we employed voltage-clamp fluorometry on alpha1 glycine receptors to compare changes mediated by the agonist, glycine, and by the antagonist, strychnine. Voltage-clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. In the inner beta-sheet, we labeled residues in loop 2 and in binding domain loops D and E. At each position, strychnine and glycine induced distinct maximal fluorescence responses. The pre-M1 domain responded similarly; at each of four labeled positions glycine produced a strong fluorescence signal, whereas strychnine did not. This suggests that glycine induces conformational changes in the inner beta-sheet and pre-M1 domain that may be important for activation, desensitization, or both. In contrast, most labeled residues in loops C and F yielded fluorescence changes identical in magnitude for glycine and strychnine. A notable exception was H201C in loop C. This labeled residue responded differently to glycine and strychnine, thus underlining the importance of loop C in ligand discrimination. These results provide an important step toward mapping the domains crucial for ligand discrimination in the ligand-binding domain of glycine receptors and possibly other Cys loop receptors.

Illuminating the structure and function of Cys-loop receptors

Cys-loop receptors are an important class of ligand-gated ion channels. They mediate fast synaptic neurotransmission, are implicated in various 'channelopathies' and are important pharmacological targets. Recent progress in X-ray crystallography and electron microscopy has provided a considerable insight into the structure of Cys-loop receptors. However, data from these experiments only provide 'snapshots' of the proteins under investigation. They cannot provide information about the various conformations the protein adopts during transition from the closed to the open and desensitized states. Voltage-clamp fluorometry helps overcome this problem by simultaneously monitoring movements at the channel gate (through changes in current) and conformational rearrangements in a domain of interest (through changes in fluorescence) in real time. Thus, the technique can provide information on both transitional and steady state conformations and serves as a real time correlate of the channel structure and its function. Voltage-clamp fluorometry experiments on Cys-loop receptors have yielded a large body of data concerning the mechanisms by which agonists, antagonists and modulators act on these receptors. They have shed new light on the conformational mobility of both the ligand-binding and the transmembrane domain of Cys-loop receptors.

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.

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.

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.

Topology of the Type IIa Na+/Pi Cotransporter

The type IIa Na(+)/P(i) cotransporter (NaPi-IIa) plays a key role in the reabsorption of inorganic phosphate (P(i)) in the renal proximal tubule. The rat NaPi-IIa isoform is a protein of 637 residues for which different algorithms predict 8-12 transmembrane domains (TMDs). Epitope tagging experiments demonstrated that both the N and the C termini of NaPi-IIa are located intracellularly. Site-directed mutagenesis revealed two N-glycosylation sites in a large putative extracellular loop. Results from structure-function studies suggested the assembly of two similar opposed regions that possibly constitute part of the substrate translocation pathway for one phosphate ion together with three sodium ions. Apart from these topological aspects, other structural features of NaPi-IIa are not known. In this study, we have addressed the topology of NaPi-IIa using in vitro transcription/translation of HK-M0 and HK-M1 fusion vectors designed to test membrane insertion properties of cDNA sequences encoding putative NaPi-IIa TMDs. Based on the results of in vitro transcription/translation analyses, we propose a model of NaPi-IIa comprising 12 TMDs, with both N and C termini orientated intracellularly and a large hydrophilic extracellular loop between the fifth and sixth TMDs. The proposed model is in good agreement with the prediction of the NaPi-IIa structure obtained by the hidden Markov algorithm HMMTOP.