Aspartate 55 in the Na+/proline permease of Escherichia coli is essential for Na+-coupled proline uptake

Prokaryotic Solute/Sodium Symporters: Versatile Functions and Mechanisms of a Transporter Family

The solute/sodium symporter family (SSS family; TC 2.A.21; SLC5) consists of integral membrane proteins that use an existing sodium gradient to drive the uphill transport of various solutes, such as sugars, amino acids, vitamins, or ions across the membrane. This large family has representatives in all three kingdoms of life. The human sodium/iodide symporter (NIS) and the sodium/glucose transporter (SGLT1) are involved in diseases such as iodide transport defect or glucose-galactose malabsorption. Moreover, the bacterial sodium/proline symporter PutP and the sodium/sialic acid symporter SiaT play important roles in bacteria–host interactions. This review focuses on the physiological significance and structural and functional features of prokaryotic members of the SSS family. Special emphasis will be given to the roles and properties of proteins containing an SSS family domain fused to domains typically found in bacterial sensor kinases.

Core Transmembrane Domain 6 Plays a Pivotal Role in the Transport Cycle of the Sodium/Proline Symporter PutP

Crystal structures of transporters with a LeuT-type structural fold assign core transmembrane domain 6 (TM6') a central role in substrate binding and translocation. Here, the function of TM6' in the sodium/proline symporter PutP, a member of the solute/sodium symporter family, was investigated. A complete scan of TM6' identified eight amino acids as particularly important for PutP function. Of these residues, Tyr-248, His-253, and Arg-257 impact sodium binding, whereas Arg-257 and Ala-260 may participate in interactions leading to closure of the inner gate. Furthermore, the previous suggestion of an involvement of Trp-244, Tyr-248, and Pro-252 in proline binding is further supported. In addition, substitution of Gly-245, Gly-247, and Gly-250 affects the amount of PutP in the membrane. A Cys accessibility analysis suggests an involvement of the inner half of TM6' in the formation of a hydrophilic pathway that is open to the inside in the absence of ligands and closed in the presence of sodium and proline. In conclusion, the results demonstrate that TM6' plays a central role in substrate binding and release on the inner side of the membrane also in PutP and extend the knowledge on functionally relevant amino acids in transporters with a LeuT-type structural fold.

Glu-311 in External Loop 4 of the Sodium/Proline Transporter PutP Is Crucial for External Gate Closure

The available structural information on LeuT and structurally related transporters suggests that external loop 4 (eL4) and the outer end of transmembrane domain (TM) 10' participate in the reversible occlusion of the outer pathway to the solute binding sites. Here, the functional significance of eL4 and the outer region of TM10' are explored using the sodium/proline symporter PutP as a model. Glu-311 at the tip of eL4, and various amino acids around the outer end of TM10' are identified as particularly crucial for function. Substitutions at these sites inhibit the transport cycle, and affect in part ligand binding. In addition, changes at selected sites induce a global structural alteration in the direction of an outward-open conformation. It is suggested that interactions between the tip of eL4 and the peptide backbone at the end of TM10' participate in coordinating conformational alterations underlying the alternating access mechanism of transport. Together with the structural information on LeuT-like transporters, the results further specify the idea that common design and functional principles are maintained across different transport families.

Biosynthesis of Proline

Proline was among the last biosynthetic precursors to have its biosynthetic pathway unraveled. This review recapitulates the findings on the biosynthesis and transport of proline. Glutamyl kinase (GK) catalyzes the ATP-dependent phosphorylation of L-glutamic acid. Purification of γ-GK from Escherichia coli was facilitated by the expression of the proB and proA genes from a high-copy-number plasmid and the development of a specific coupled assay based on the NADPH-dependent reduction of GP by γ-glutamyl phosphate reductase (GPR). GPR catalyzes the NADPH-dependent reduction of GP to GSA. Site directed mutagenesis was used to identify residues that constitute the active site of E. coli GK. This analysis indicated that there is an overlap between the binding sites for glutamate and the allosteric inhibitor proline, suggesting that proline competes with the binding of glutamate. The review also summarizes the genes involved in the metabolism of proline in E. coli and Salmonella. Among the completed genomic sequences of Enterobacteriaceae, genes specifying all three proline biosynthetic enzymes can be discerned in E. coli, Shigella, Salmonella enterica, Serratia marcescens, Erwinia carotovora, Yersinia, Photorhabdus luminescens, and Sodalis glossinidius strain morsitans. The intracellular proline concentration increases with increasing external osmolality in proline-overproducing mutants. This apparent osmotic regulation of proline accumulation in the overproducing strains may be the result of increased retention or recapture of proline, achieved by osmotic stimulation of the ProP or ProU proline transport systems. A number of proline analogs can be incorporated into proteins in vivo or in vitro.

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.

Identification of a second substrate-binding site in solute-sodium symporters

The structure of the sodium/galactose transporter (vSGLT), a solute-sodium symporter (SSS) from Vibrio parahaemolyticus, shares a common structural fold with LeuT of the neurotransmitter-sodium symporter family. Structural alignments between LeuT and vSGLT reveal that the crystallographically identified galactose-binding site in vSGLT is located in a more extracellular location relative to the central substrate-binding site (S1) in LeuT. Our computational analyses suggest the existence of an additional galactose-binding site in vSGLT that aligns to the S1 site of LeuT. Radiolabeled galactose saturation binding experiments indicate that, like LeuT, vSGLT can simultaneously bind two substrate molecules under equilibrium conditions. Mutating key residues in the individual substrate-binding sites reduced the molar substrate-to-protein binding stoichiometry to ~1. In addition, the related and more experimentally tractable SSS member PutP (the Na(+)/proline transporter) also exhibits a binding stoichiometry of 2. Targeting residues in the proposed sites with mutations results in the reduction of the binding stoichiometry and is accompanied by severely impaired translocation of proline. Our data suggest that substrate transport by SSS members requires both substrate-binding sites, thereby implying that SSSs and neurotransmitter-sodium symporters share common mechanistic elements in substrate transport.

Extracellular loop 4 of the proline transporter PutP controls the periplasmic entrance to ligand binding sites

The Na(+)/proline symporter (PutP), like several other Na(+)-coupled symporters, belongs to the so-called LeuT-fold structural family, which features ten core transmembrane domains (cTMs) connected by extra- and intracellular loops. The role of these loops has been discussed in context with the gating function in the alternating access model of secondary active transport processes. Here we report the complete spin-labeling site scan of extracellular loop 4 (eL4) in PutP that reveals the presence of two α-helical segments, eL4a and eL4b. Among the eL4 residues that are directly implicated in the functional dynamics of the transporter, Phe314 in eL4b anchors the loop by means of hydrophobic contacts to cTM1 close to the ligand binding sites. We propose that ligand-induced conformational changes at the binding sites are transmitted via the anchoring residue to eL4 and through eL4 further to adjacent cTMs, leading to closure of the extracellular gate.

The sodium/proline transporter PutP of Helicobacter pylori

Helicobacter pylori is cause of chronic gastritis, duodenal ulcer and gastric carcinoma in humans. L-proline is a preferred energy source of the microaerophilic bacterium. Previous analyses revealed that HpputP and HpputA, the genes that are predicted to play a central role in proline metabolism as they encode for the proline transporter and proline dehydrogenase, respectively, are essential for stomach colonization. Here, the molecular basis of proline transport in H. pylori by HpPutP was investigated experimentally for the first time. Measuring radiolabeled substrate transport in H. pylori and E. coli heterologously expressing HpputP as well as in proteoliposomes reconstituted with HpPutP, we demonstrate that the observed proline transport in H. pylori is mediated by HpPutP. HpPutP is specific and exhibits a high affinity for L-proline. Notably, L-proline transport is exclusively dependent on Na⁺ as coupling ion, i.e., Na⁺/L-proline symport, reminiscent to the properties of PutP of E. coli even though H. pylori lives in a more acidic environment. Homology model-based structural comparisons and substitution analyses identified amino acids crucial for function. HpPutP-catalyzed proline uptake was efficiently inhibited by the known proline analogs 3,4-dehydro-D,L-proline and L-azetidine-2-carboxylic acid.

The sodium/iodide symporter: state of the art of its molecular characterization

The sodium/iodide symporter (NIS or SLC5A5) is an intrinsic membrane protein implicated in iodide uptake into thyroid follicular cells. It plays a crucial role in iodine metabolism and thyroid regulation and its function is widely exploited in the diagnosis and treatment of benign and malignant thyroid diseases. A great effort is currently being made to develop a NIS-based gene therapy also allowing the radiotreatment of nonthyroidal tumors. NIS is also expressed in other tissues, such as salivary gland, stomach and mammary gland during lactation, where its physiological role remains unclear. The molecular identity of the thyroid iodide transporter was elucidated approximately fifteen years ago. It belongs to the superfamily of sodium/solute symporters, SSS (and to the human transporter family, SLC5), and is composed of 13 transmembrane helices and 643 amino acid residues in humans. Knowledge concerning NIS structure/function relationship has been obtained by taking advantage of the high resolution structure of one member of the SSS family, the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT), and from studies of gene mutations leading to congenital iodine transport defects (ITD). This review will summarize current knowledge regarding the molecular characterization of NIS.

Conserved tyrosine in the first transmembrane segment of solute:sodium symporters is involved in Na+-coupled substrate co-transport

Solute:sodium symporters (SSSs) transport vital molecules across the plasma membrane of all living organisms. vSGLT, the Na(+)/galactose transporter of Vibrio parahemeolyticus, is the only SSS for which high resolution structural information is available, revealing a LeuT-like fold and a Na(+)-binding site analogous to the Na2 site of LeuT. Whereas the core transmembrane segments (TMs) of SSSs share high structural similarity with other transporters of LeuT-like fold, TM1 does not correspond to any TM in those structural homologs and was only resolved for the backbone atoms in the initial vSGLT structure (Protein Data Bank code 3DH4). To assess the role of TM1 in Na(+)-coupled substrate symport by the SSSs, here we have studied the role of a conserved residue in TM1 by computational modeling in conjunction with radiotracer transport and binding studies. Based on our sequence alignment and much topological data for homologous PutP, the Na(+)/proline transporter, we have simulated a series of vSGLT models with shifted TM1 residue assignments. We show that in two converged vSGLT models that retained the original TM1 backbone conformation, a conserved residue, Tyr-19, is associated with the Na(+) binding interaction network. In silico and in vitro mutagenesis of homologous Tyr-14 in PutP revealed the involvement of this conserved residue in Na(+)-dependent substrate binding and transport. Thus, our combined computational and experimental data provide the first clues about the importance of a conserved residue in TM1, a unique TM in the proteins with LeuT-like fold, in the Na(+)-coupled symport mechanism of SSSs.

Role of Asp187 and Gln190 in the Na+/proline symporter (PutP) of Escherichia coli

Asp187 and Gln190 were predicted as conserved and closely located at the Na(+) binding site in a topology and homology model structure of Na(+)/proline symporter (PutP) of Escherichia coli. The replacement of Asp187 with Ala or Leu did not affect proline transport activity; whereas, change to Gln abolished the active transport. The binding affinity for Na(+) or proline of these mutants was similar to that of wild-type (WT) PutP. This result indicates Asp187 to be responsible for active transport of proline without affecting the binding. Replacement of Gln190 with Ala, Asn, Asp, Leu and Glu had no effect on transport or binding, suggesting that it may not have a role in the transport. However, in the negative D187Q mutant, a second mutation, of Gln190 to Glu or Leu, restored 46 or 7% of the transport activity of WT, respectively, while mutation to Ala, Asn or Asp had no effect. Thus, side chain at position 190 has a crucial role in suppressing the functional defect of the D187Q mutant. We conclude that Asp187 is responsible for transport activity instead of coupling-ion binding by constituting the translocation pathway of the ion and Gln190 provides a suppressing mutation site to regain PutP functional activity.

Homology model of the Na+/proline transporter PutP of Escherichia coli and its functional implications

Na(+)/solute symporters are essential membrane integrated proteins that couple the flow of Na(+) ions driven by electrochemical Na(+) gradients to the transport of solutes across biological membranes. Here, we used a combination of molecular modeling techniques and evolutionary conservation analysis to construct and validate a first model of the Na(+)/proline symporter PutP of Escherichia coli based on the crystal structure of the bacterial Na(+)/galactose symporter vSGLT. Ligand docking experiments were employed to gain information about residues involved in proline binding. The proposed model is consistent with the available experimental data and was further validated by amino acid substitutions and kinetic and protein chemical analyses. Combination of the results of molecular modeling and functional studies predicts the location and organization of the Na(+) and proline binding sites. Remarkably, as proposed computationally and discovered here experimentally, residues Y140, W244, and Y248 of transmembrane segments 4 and 7 are found to be particularly important for PutP function and suggested to participate in proline binding and/or gating.

Substrate-dependent proton antiport in neurotransmitter:sodium symporters

Neurotransmitter:sodium symporters (NSS), targets for psychostimulants and therapeutic drugs, play a critical role in neurotransmission. Whereas eukaryotic NSS exhibit Cl⁻-dependent transport, bacterial NSS feature Cl⁻-independent substrate transport. Recently we showed in LeuT and Tyt1 that mutation of an acidic side chain near one of the Na⁺-binding sites renders substrate binding and/or transport Cl⁻ dependent. We reasoned that the negative charge - provided either by Cl⁻ or by the transporter itself - is required for substrate translocation. Here we show that Tyt1 reconstituted in proteoliposomes is strictly dependent on the Na⁺ gradient and is stimulated by an inside negative membrane potential and by an inversely-oriented H⁺ gradient. Remarkably, Na⁺/substrate symport elicited H⁺ efflux, indicative of Na⁺/substrate symport-coupled H⁺ antiport. Mutations that render the transport phenotype Cl⁻-dependent essentially abolish the pH dependence. We propose unifying features of charge balance by all NSS members with similar mechanistic features but with different molecular solutions.

Conserved glutamate residues Glu-343 and Glu-519 provide mechanistic insights into cation/nucleoside cotransport by human concentrative nucleoside transporter hCNT3

Human concentrative nucleoside transporter 3 (hCNT3) utilizes electrochemical gradients of both Na(+) and H(+) to accumulate pyrimidine and purine nucleosides within cells. We have employed radioisotope flux and electrophysiological techniques in combination with site-directed mutagenesis and heterologous expression in Xenopus oocytes to identify two conserved pore-lining glutamate residues (Glu-343 and Glu-519) with essential roles in hCNT3 Na(+)/nucleoside and H(+)/nucleoside cotransport. Mutation of Glu-343 and Glu-519 to aspartate, glutamine, and cysteine severely compromised hCNT3 transport function, and changes included altered nucleoside and cation activation kinetics (all mutants), loss or impairment of H(+) dependence (all mutants), shift in Na(+):nucleoside stoichiometry from 2:1 to 1:1 (E519C), complete loss of catalytic activity (E519Q) and, similar to the corresponding mutant in Na(+)-specific hCNT1, uncoupled Na(+) currents (E343Q). Consistent with close-proximity integration of cation/solute-binding sites within a common cation/permeant translocation pore, mutation of Glu-343 and Glu-519 also altered hCNT3 nucleoside transport selectivity. Both residues were accessible to the external medium and inhibited by p-chloromercuribenzene sulfonate when converted to cysteine.

Function of transmembrane domain IX in the Na+/proline transporter PutP

Selected residues of transmembrane domain (TM) IX were previously shown to play key roles in ligand binding and transport in members of the Na(+)/solute symporter family. Using the Na(+)/proline transporter PutP as a model, a complete Cys scanning mutagenesis of TM IX (positions 324 to 351) was performed here to further investigate the functional significance of the domain. G328, S332, Q345, and L346 were newly identified as important for Na(+)-coupled proline uptake. Placement of Cys at one of these positions altered K(m(pro)) (S332C and L346C, 3- and 21-fold decreased, respectively; Q345C, 38-fold increased), K(0.5(Na+)) (S332C, 13-fold decreased; Q345C, 19-fold increased), and/or V(max) [G328C, S332C, Q345C, and L346C, 3-, 22-, 2-, and 8-fold decreased compared to PutP(wild type), respectively]. Membrane-permeant N-ethylmaleimide inhibited proline uptake into cells containing PutP with Cys at distinct positions in the middle (T341C) and cytoplasmic half of TM IX (C344, L347C, V348C, and S351C) and had little or no effect on all other single Cys PutP variants. The inhibition pattern was in agreement with the pattern of labeling with fluorescein-5-maleimide. In addition, Cys placed into the cytoplasmic half of TM IX (C344, L347C, V348C, and S351C) was protected from fluorescein-5-maleimide labeling by proline while Na(+) alone had no effect. Membrane-impermeant methanethiosulfonate ethyltrimethylammonium modified Cys in the middle (A337C and T341C) and periplasmic half (L331C) but not in the cytoplasmic half of TM IX in intact cells. Furthermore, Cys at the latter positions was partially protected by Na(+) but not by proline. Based on these results, a model is discussed according to which residues of TM IX participate in the formation of ligand-sensitive, hydrophilic cavities in the protein that may reconstitute part of the Na(+) and/or proline translocation pathway of PutP.

Electrostatic and potential cation-pi forces may guide the interaction of extracellular loop III with Na+ and bile acids for human apical Na+-dependent bile acid transporter

The hASBT (human apical Na(+)-dependent bile acid transporter) constitutes a key target of anti-hypercholesterolaemic therapies and pro-drug approaches; physiologically, hASBT actively reclaims bile acids along the terminal ileum via Na(+) co-transport. Previously, TM (transmembrane segment) 7 was identified as part of the putative substrate permeation pathway using SCAM (substitute cysteine accessibility mutagenesis). In the present study, SCAM was extended through EL3 (extracellular loop 3; residues Arg(254)-Val(286)) that leads into TM7 from the exofacial matrix. Activity of most EL3 mutants was significantly hampered upon cysteine substitution, whereas ten (out of 31) were functionally inactive (<10% activity). Since only E282C lacked plasma membrane expression, EL3 amino acids predominantly fulfill critical functional roles during transport. Oppositely charged membrane-impermeant MTS (methanethiosulfonate) reagents {MTSET [(2-trimethylammonium) ethyl MTS] and MTSES [(2-sulfonatoethyl) MTS]} produced mostly similar inhibition profiles wherein only middle and descending loop segments (residues Thr(267)-Val(286)) displayed significant MTS sensitivity. The presence of bile acid substrate significantly reduced the rates of MTS modification for all MTS-sensitive mutants, suggesting a functional association between EL3 residues and bile acids. Activity assessments at equilibrative [Na(+)] revealed numerous Na(+)-sensitive residues, possibly performing auxiliary functions during transport such as transduction of protein conformational changes during translocation. Integration of these data suggests ligand interaction points along EL3 via electrostatic interactions with Arg(256), Glu(261) and probably Glu(282) and a potential cation-pi interaction with Phe(278). We conclude that EL3 amino acids are essential for hASBT activity, probably as primary substrate interaction points using long-range electrostatic attractive forces.

Role of Ser-340 and Thr-341 in transmembrane domain IX of the Na+/proline transporter PutP of Escherichia coli in ligand binding and transport

The Na+/solute symporter family comprises more than 400 members of pro- and eukaryotic origin. Using the Na+/proline transporter PutP of Escherichia coli as a model, the role of two conserved residues, Ser-340 and Thr-341, is investigated to obtain insights into the mechanism of transport catalyzed by members of this family. Substitution of these amino acids alters the transport kinetics of cells and proteoliposomes containing the PutP variants significantly. In particular, the apparent affinities for Na+ and Li+ are reduced by 2 orders of magnitude or more. Also proline binding is affected, albeit to a lesser extent than ion binding. Thereby, the presence of a hydroxyl group at position 341 is essential for high affinity ligand binding. Furthermore, Cys placed at position 340 or 341 reacts with sulfhydryl reagents of different polarity, indicating accessibility from the water phase. In addition, Cys cross-linking suggests proximity of the residues to other amino acids previously shown to be crucial for ligand binding. For these reasons it is suggested that Ser-340 and Thr-341 are located in a ligand translocation pathway. Furthermore, it is proposed that the side chain of Thr-341 directly participates in Na+ binding.

Conformational flexibility of helix VI is essential for substrate permeation of the human apical sodium-dependent bile acid transporter

The present study characterizes the methanethiosulfonate (MTS) inhibition profiles of 26 consecutive cysteine-substituted mutants comprising transmembrane (TM) helix 6 of the human apical Na(+)-dependent bile acid transporter (SLC10A2). TM6 is linked exofacially to TM7 via extracellular loop 3. TM7 was identified previously as lining part of the substrate permeation path ( Mol Pharmacol 70: 1565, 2006 ). Most TM6 cysteine replacements were well tolerated, except for five residues with either severely hampered (I229C, G249C) or abolished (P234C, G237C, G241C) activity. Disruption of protein synthesis or folding and stability may account for lack of activity for mutant P234C. Subsequent Pro234 amino acid replacement reveals its participation in both structural and functional aspects of the transport cycle. Application of polar MTS reagents (1 mM) significantly inhibited the activity of six mutants (V235C, S239C, F242C, R246C, A248C, and Y253C), for which rates of modification were almost fully reversed (except Y253C) upon inclusion of bile acid substrates or removal of Na(+) from the MTS preincubation medium. Activity assessments at equilibrative [Na(+)] revealed numerous Na(+)-sensitive residues, suggesting their proximity in or around Na(+) interaction sites. In silico modeling reveals the intimate and potentially cooperative orientation of MTS-accessible TM6 residues toward functionally important TM7 amino acids, substantiating TM6 participation during the transport cycle. We conclude a functional requirement for helical flexibility imparted by Pro234, Gly237, and Gly241, probably forming a "conformational switch" requisite for substrate turnover; meanwhile, MTS-accessible residues, which line a helical face spatially distinct from this switch, may participate during substrate permeation.

Conserved Glutamate Residues Are Critically Involved in Na+/Nucleoside Cotransport by Human Concentrative Nucleoside Transporter 1 (hCNT1)

Human concentrative nucleoside transporter 1 (hCNT1), the first discovered of three human members of the SLC28 (CNT) protein family, is a Na+/nucleoside cotransporter with 650 amino acids. The potential functional roles of 10 conserved aspartate and glutamate residues in hCNT1 were investigated by site-directed mutagenesis and heterologous expression in Xenopus oocytes. Initially, each of the 10 residues was replaced by the corresponding neutral amino acid (asparagine or glutamine). Five of the resulting mutants showed unchanged Na+-dependent uridine transport activity (D172N, E338Q, E389Q, E413Q, and D565N) and were not investigated further. Three were retained in intracellular membranes (D482N, E498Q, and E532Q) and thus could not be assessed functionally. The remaining two (E308Q and E322Q) were present in normal quantities at cell surfaces but exhibited low intrinsic transport activities. Charge replacement with the alternate acidic amino acid enabled correct processing of D482E and E498D, but not of E532D, to cell surfaces and also yielded partially functional E308D and E322D. Relative to wild-type hCNT1, only D482E exhibited normal transport kinetics, whereas E308D, E308Q, E322D, E322Q, and E498D displayed increased K50(Na+) and/or Km(uridine) values and diminished Vmax(Na+) and Vmax(uridine) values. E322Q additionally exhibited uridine-gated uncoupled Na+ transport. Together, these findings demonstrate roles for Glu-308, Glu-322, and Glu-498 in Na+/nucleoside cotransport and suggest locations within a common cation/nucleoside translocation pore. Glu-322, the residue having the greatest influence on hCNT1 transport function, exhibited uridine-protected inhibition by p-chloromercuriphenyl sulfonate and 2-aminoethyl methanethiosulfonate when converted to cysteine.

The importance of company: Na+ and Cl- influence substrate interaction with SLC6 transporters and other proteins

SLC6 transporters, which include transporters for gamma-aminobutyric acid (GABA), norepinephrine, dopamine, serotonin, glycine, taurine, L-proline, creatine, betaine, and neutral cationic amino acids, require Na+ and Cl- for their function, and this review covers the interaction between transporters of this family with Na+ and Cl- from a structure-function standpoint. Because detailed structure-function information regarding ion interactions with SLC6 transporters is limited, we cover other proteins cotransporting Na+ or Cl- with substrate (SLClA2, PutP, SLC5A1, melB), or ion binding to proteins in general (rhodanese, ATPase, LacY, thermolysine, angiotensin-converting enzyme, halorhodopsin, CFTR). Residues can be involved in directly binding Na+ or Cl-, in coupling ion binding to conformational changes in transporter, in coupling Na+ or Cl- movement to transport, or in conferring ion selectivity. Coordination of ions can involve a number of residues, and portions of the substrate and coupling ion binding sites can be distal in space in the tertiary structure of the transporter, with other portions that are close in space thought to be crucial for the coupling process. The reactivity with methanethiosulfonate reagents of cysteines placed in strategic positions in the transporter provides a readout for conformational changes upon ion or substrate binding. More work is needed to establish the relationships between ion interactions and oligomerization of SLC6 transporters.

Secondary transport of amino acids in prokaryotes

Amino acid transport is a ubiquitous phenomenon and serves a variety of functions in prokaryotes, including supply of carbon and nitrogen for catabolic and anabolic processes, pH homeostasis, osmoprotection, virulence, detoxification, signal transduction and generation of electrochemical ion gradients. Many of the participating proteins have eukaryotic relatives and are successfully used as model systems for exploration of transporter structure and function. Distribution, physiological roles, functional properties, and structure-function relationships of prokaryotic alpha-amino acid transporters are discussed.

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.

Identification of functionally relevant residues of the rat ileal apical sodium-dependent bile acid cotransporter

The mechanisms underlying the transport of bile acids by apical sodium-dependent bile acid transporter (Asbt) are not well defined. To further identify the functionally relevant residues, thirteen conserved negatively (Asp and Glu) and positively (Lys and Arg) charged residues plus Cys-270 of rat Asbt were replaced with Ala or Gln by site-directed mutagenesis. Seven of the fourteen residues of rat Asbt were identified as functionally important by taurocholate transport studies, substrate inhibition assays, confocal microscopy, and electrophysiological methods. The results showed that Asp-122, Lys-191, Lys-225, Lys-256, Glu-261, and Lys-312,Lys-313 residues of rat Asbt are critical for transport function and may determine substrate specificity. Arg-64 may be located at a different binding site to assist in interaction with non-bile acid organic anions. For bile acid transport by Asbt, Na(+) ion movement is a voltage-dependent process that tightly companied with taurocholate movement. Asp-122 and Glu-261 play a critical role in the interaction of a Na(+) ion and ligand with Asbt. Cys-270 is not essential for the transport process. These studies provide new details about the amino acid residues of Asbt involved in binding and transport of bile acids and Na(+).

Charge translocation during cosubstrate binding in the Na+/proline transporter of E.coli

Charge translocation associated with the activity of the Na(+)/proline cotransporter PutP of Escherichia coli was analyzed for the first time. Using a rapid solution exchange technique combined with a solid-supported membrane (SSM), it was demonstrated that Na(+)and/or proline individually or together induce a displacement of charge. This was assigned to an electrogenic Na(+)and/or proline binding process at the cytoplasmic face of the enzyme with a rate constant of k>50s(-1) which preceeds the rate-limiting step. Based on the kinetic analysis of our electrical signals, the following characteristics are proposed for substrate binding in PutP. (1) Substrate binding is electrogenic not only for Na(+), but also for the uncharged cosubstrate proline. The charge displacement associated with the binding of both substrates is of comparable size and independent of the presence of the respective cosubstrate. (2) Both substrates can bind individually to the transporter. Under physiological conditions, an ordered binding mechanism prevails, while at sufficiently high concentrations, each substrate can bind in the absence of the other. (3) Both substrate binding sites interact cooperatively with each other by increasing the affinity and/or the speed of binding of the respective cosubstrate. (4) Proline binding proceeds in a two-step process: low affinity (approximately 1mM) electroneutral substrate binding followed by a nearly irreversible electrogenic conformational transition.

Transmembrane domain II of the Na+/proline transporter PutP of Escherichia coli forms part of a conformationally flexible, cytoplasmic exposed aqueous cavity within the membrane

The Na+/proline transporter PutP of Escherichia coli is a member of a large family of Na+/substrate symporters. Previous work on PutP suggests an involvement of the region ranging from Asp-55 to Gly-58 in binding of Na+ and/or proline (Pirch, T., Quick, M., Nietschke, M., Langkamp, M., Jung, H. (2002) J. Biol. Chem. 277, 8790-8796). In this study, a complete Cys scanning mutagenesis of transmembrane domain II (TM II) of PutP was performed to further elucidate the role of the TM in the transport process. Strong defects of PutP function were observed upon substitution of Ala-48, Ala-53, Trp-59, and Gly-63 by Cys in addition to the previously characterized residues Asp-55, Ser-57, and Gly-58. However, except for Asp-55 none of these residues proved essential for function. The activity of eight mutants was sensitive to N-ethylmaleimide inhibition with the sensitive positions clustering predominantly on a hydrophilic face in the cytoplasmic half of TM II. The same face was also highly accessible to the bulky sulfhydryl reagent fluorescein 5-maleimide in randomly oriented membrane vesicles, suggesting an unrestricted accessibility of the corresponding amino acid positions via an aqueous pathway. Na+ stimulated the reactivity of Cys toward fluorescein 5-maleimide at two positions while proline inhibited reaction of the sulfhydryl group at nine positions. Taken together, the results demonstrate that TM II of PutP is of particular functional importance. It is proposed that hydrophilic residues in the cytoplasmic half of TM II participate in the formation of an aqueous cavity in the membrane that allows Na+ and/or proline binding to residues located in the middle of the TM (e.g. Asp-55 and Ser-57). In addition, the data indicate that TM II participates in Na+- and proline-induced conformational alterations.

Transport of taurocholate by mutants of negatively charged amino acids, cysteines, and threonines of the rat liver sodium-dependent taurocholate cotransporting polypeptide Ntcp

The relevance of functional amino acids for taurocholate transport by the sodium-dependent taurocholate cotransporting polypeptide Ntcp was determined by site-directed mutagenesis. cRNA from 28 single-points mutants of the rat liver Ntcp clone was expressed in Xenopus laevis oocytes. Mutations were generated in five conserved negatively charged amino acids (aspartates and glutamates) which were present in nine members of the SBAT-family, in two nonconserved negatively charged amino acids, in all eight Ntcp-cysteines, and in two threonines from a protein kinase C consensus region of the Ntcp C-terminus. Functional amino acids were Asp115, Glu257, and Cys266, which were found to be essential for the maintenance of taurocholic acid transport. Asp115 is located in the large intracellular loop III, whereas Glu257 and Cys266 are located in the large extracellular loop VI. Four mutations of threonines from the C-terminus of the Ntcp by alanines or tyrosines showed no effects on sodium-dependent taurocholate transport. Introduction of the FLAG(R) motif into several transport negative point mutations demonstrated that all mutated proteins besides one were present within the cell membrane of the oocytes and provided proof that an insertion defect has not caused transport deficiency by these Ntcp mutants. The latter was observed only with the transport negative mutant Asp24Asn. In conclusion, loop amino acids are required for sodium-dependent substrate translocation by the Ntcp.

The sodium/substrate symporter family: structural and functional features

Members of the sodium/substrate symporter family (SSSF, TC 2.A.21) catalyze the uptake of a wide variety of solutes including sugars, proline, pantothenate, and iodide into cells of pro- and eukaryotic origin. Extensive analyses of the topology of different SSSF proteins suggest an arrangement of 13 transmembrane domains as a common topological motif. Regions involved in sodium and/or substrate binding were identified. Furthermore, protein chemical and spectroscopic studies reveal ligand-induced structural alterations which are consistent with close interactions between the sites of sodium and substrate binding, thereby supporting an ordered binding mechanism for transport.

Sites important for Na+ and substrate binding in the Na+/proline transporter of Escherichia coli, a member of the Na+/solute symporter family

To elucidate the functional importance of transmembrane domain II in the Na(+)/proline transporter (PutP) of Escherichia coli we analyzed the effect of replacing Ser-54 through Gly-58. Substitution of Asp-55 or Met-56 dramatically reduces the apparent affinity for Na(+) and Li(+) in a cation-dependent manner. Conversely, Cys in place of Gly-58 significantly reduces only the apparent proline affinity while substitution of Ser-57 results in a dramatic reduction of the apparent proline and cation affinities. Interestingly, upon increasing the proline concentration the apparent Na(+) affinity of Ser-57 replacement mutants converges toward the wild-type value, indicating a close cooperativity between cation and substrate site(s). This notion is supported by the fact that Na(+)-stimulated site-specific fluorescence labeling of a single Cys at position 57 is completely reversed by the addition of proline. Similar results are obtained upon labeling of a Cys at position 54 or 58. Taken together, these results indicate that Asp-55 and Met-56 are located at or close to the ion-binding site while Ser-54, Ser-57, and Gly-58 may be close to the proline translocation pathway. In addition, the data prod at an involvement of the latter residues in ligand-induced conformational dynamics that are crucial for cation-coupled transport.

Towards the molecular mechanism of Na(+)/solute symport in prokaryotes

The Na(+)/solute symporter family (SSF, TC No. 2.A.21) contains more than 40 members of pro- and eukaryotic origin. Besides their sequence similarity, the transporters share the capability to utilize the free energy stored in electrochemical Na(+) gradients for the accumulation of solutes. As part of catabolic pathways most of the transporters are most probably involved in the acquisition of nutrients. Some transporters play a role in osmoadaptation. With a high resolution structure still missing, a combination of genetic, protein chemical and spectroscopic methods has been used to gain new insights into the structure and molecular mechanism of action of the transport proteins. The studies suggest a common 13-helix motif for all members of the SSF according to which the N-terminus is located in the periplasm and the C-terminus is directed into the cytoplasm (except for proteins containing a N- or C-terminal extension). Furthermore, an amino acid substitution analysis of the Na(+)/proline transporter (PutP) of Escherichia coli, a member of the SSF, has identified regions of particular functional importance. For example, amino acids of TM II of PutP proved to be critical for high affinity binding of Na(+) and proline. In addition, it was shown that ligand binding induces widespread conformational alterations in the transport protein. Taken together, the studies substantiate the common idea that Na(+)/solute symport is the result of a series of ligand-induced structural changes.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Neutralization of a conserved amino acid residue in the human Na+/glucose transporter (hSGLT1) generates a glucose-gated H+ channel

The role of conserved Asp204 in the human high affinity Na+/glucose cotransporter (hSGLT1) was investigated by site-directed mutagenesis combined with functional assays exploiting the Xenopus oocyte expression system. Substitution of H+ for Na+ reduces the apparent affinity of hSGLT1 for glucose from 0.3 to 6 mm. The apparent affinity for H+ (7 microm) is about three orders of magnitude higher than for Na+ (6 mm). Cation/glucose cotransport exhibits a coupling ratio of 2 Na+ (or 2 H+):1. Pre-steady-state kinetics indicate that similar Na+ - or H+ -induced conformational changes are the basis for coupled transport. Replacing Asp204 with Glu increases the apparent affinity for H+ by >20-fold with little impact on the apparent Na+ affinity. This implies that the length of the carboxylate side chain is critical for cation selectivity. Neutralization of Asp204 (Asp --> Asn or Cys) reveals glucose-evoked H(+) currents that were one order of magnitude greater than Na(+) currents. These phlorizin-sensitive H+ currents reverse and are enhanced by internal acidification of oocytes. Together with a H(+) to sugar stoichiometry as high as 145:1, these results favor a glucose-gated H+ channel activity of the mutant. Our observations support the idea that cotransporters and channels share common features.

Topology of the Na+/proline transporter of Escherichia coli

Hydropathy profile analysis of the amino acid sequence of the Na+/proline transporter of Escherichia coli (PutP) suggests that the protein consists of 12 transmembrane domains (TMs) which are connected by hydrophilic loops (Nakao, T., Yamato, I., and Anraku, Y. (1987) Mol. Gen. Genet. 208, 70-75). We have tested this prediction by applying a gene fusion approach in combination with a Cys accessibility analysis and site-specific proteolysis. Characterization of a series of PutP-alkaline phosphatase (PhoA) and PutP-beta-galactosidase (LacZ) hybrid proteins yields a reciprocal activity pattern of the reporter proteins that is in agreement with the topology of TMs III to XII of the 12-helix model. Placement of the PutP-PhoA and PutP-LacZ junction sites closer to the N terminus does not yield conclusive results. As a prerequisite for further topology studies, a functional PutP molecule devoid of all five native Cys residues (Cys-free PutP) is generated. Subsequently, amino acids in Cys-free PutP are replaced individually with Cys, and the accessibility of the sulfhydryl groups is analyzed. Surprisingly, Cys residues placed close to the N terminus of PutP (Ile-3 --> Cys, Thr-5 --> Cys) or into putative TM II (Ser-71 --> Cys, Glu-75 --> Cys) are highly accessible to membrane permeant and impermeant thiol reagents in intact cells. In contrast, Cys at the C terminus (Ser-502 --> Cys) reacts only with the membrane permeant but not with the impermeant reagent in intact cells. These results contradict the 12-helix motif and indicate a periplasmic location of the N terminus whereas the C terminus faces the cytoplasm. In addition, a transporter with Cys in place of Leu-37 (putative periplasmic loop (pL2) shows the same accessibility pattern as the Cys at the C terminus. Furthermore, PutP which has been purified and reconstituted into proteoliposomes in an inside-out orientation, is readily cleaved by the endoproteinase AspN before Asp-33 (pL2), Asp-112 (putative cytoplasmic loop (cL3), Asp-262 (cL7), and Asp-356 (cL9). These results suggest a cytosolic location of Asp-33 and Leu-37, thereby implying the formation of an additional TM formed by amino acids of pL2. Based on these observations, a new secondary structure model is proposed according to which the protein consists of 13 TMs with the N terminus on the outside and the C terminus facing the cytoplasm. The 13-helix structure is discussed as a common topological motif for all members of the Na+/solute cotransporter family.

Topology and function of the Na+/proline transporter of Escherichia coli, a member of the Na+/solute cotransporter family

The Na+/proline transporter of Escherichia coli (PutP) is a member of the Na+/solute contransporter family (SCF) and catalyzes the uptake of proline by a Na+ dependent transport mechanism. Hydropathy profile analysis suggests that the protein consists of 12 transmembrane domains (TMs) that traverse the membrane in zigzag fashion connected by hydrophilic loops. However, analysis of a series of putP-phoA (PutP-alkaline phosphatase) and putP-lacZ (PutP-beta-galactosidase) fusions and site-directed labeling of the transporter indicate a 13-helix motif with the N-terminus on the outside and the C-terminus facing the cytoplasm. The findings are discussed with respect to a common topological motif for all members of the SCF. Furthermore, amino acid substitution analysis indicates that the N-terminal part of PutP is important for ion binding. Thus, Asp55 (putative TM II) is essential for transport and proposed to interact directly with Na+. The functional importance of TM II is further confirmed by the observation that replacement of Arg40, Ser50, Ala53, or Ser57 alters transport kinetics dramatically.