The transport of potassium through lipid bilayer membranes by the neutral carriers valinomycin and monactin

Summary Stationary conductance experiments on neutral and negatively charged bilayer membranes in the presence of valinomycin or monactin agree with a recently proposed carrier transport model, which is common to both carrier types. This model assumes an interface reaction between a cation from the aqueous solution and a carrier molecule from the membrane phase to establish charge transport across the interface. The transport across the membrane interior is described by some kind of Eyring model. The discussion of the current-voltage characteristic, the dependence of membrane conductance on the carrier and K⁺ concentrations, and the comparison with appropriate experiments allow correlation of the different rate constants of the transport model. The results show that the rate constants partly depend on the surface charge of the membranes. This dependency can be described by introducing the Gouy-Chapman theory for charged surfaces into the transport model.It was found that the carrier molecules could be added either to the aqueous phase or to the membrane-forming solution. The quantitative treatment of this phenomenon gives an evaluation of the partition coefficient of the carrier molecules between the membrane bulk phase and water.     

Engineering the lac permease for purification and crystallization

The lactose permease is being used as a model system for the rational redesign of a membrane protein with the goal of increasing the likelihood of crystallization. Various modifications to the protein have been added for the purposes of purification, stability, and potential for crystallization. The addition of six consecutive histidines at the C-terminus of the protein allows for the rapid purification by nickel-chelate chromatography, and the insertion of an entire protein domain into one of the inner cytoplasmic loops of the permease gives the resulting protein a larger hydrophilic surface area. The increase in polar surface area makes the fusion protein easier to handle and more likely to crystallize. In particular, the introduction of cytochromeb562 ofE. coli into the central hydrophilic domain of the lac permease results in a fusion protein with the transport activity of the permease and the visible absorbance spectrum of the cytochrome. The red permease is very easy to monitor through the steps of expression, purification, concentration, and crystallization.     

Na+-dependent sugar transport in a cultured epithelial cell line from pig kidney

Summary A Na⁺-dependent hexose transport system with similar characteristics to that observed in the kidney is retained in a cultured epithelial cell line from pig kidney (LLC-PK₁). The active transport of methyl-d-glucoside ( MGP), a nonmetabolizable sugar, which shares the glucose-galactose transport system in kidney cells is mediated through a Na⁺-dependent, substrate-saturable process. The kinetic analysis of the effect of Na⁺ on the uptake of MGP indicated that the Na⁺-sugar cotransport system is an affinity type system in which the binding of either sugar or Na⁺ to carrier increases the affinity for the other ligand without affecting theV _max. The sequence of selectivity for different sugars studied by the inhibition produced in the uptake of MGP is very similar to that reported in rat kidney, rabbit kidney cortex slices, and rabbit renal brush border membrane vesicles. Phlorizin, even at very low concentration, almost completely inhibits MGP uptake. Conversely, phloretin at the same low concentration stimulated the sugar accumulation by inhibition of efflux, probably at the level of the basolateral membrane. Sulfhydryl group inhibitors also blocked the MGP uptake, suggesting that these groups were required for normal functioning of the sugar carrier system. This sugar transport system is an important functional marker to study the molecular events associated with the development of polarization in epithelial cells.     

Regulation of methyl beta-galactoside permease activity in pts and crr mutants of Salmonella typhimurium

Summary We have studied the regulation of the synthesis and activity of a major galactose transport system, that of methyl -galactoside (MglP), in mutants of Salmonella typhimurium. Two classes of mutation that result in a (partially) defective phosphoenolpyruvate: sugar phosphotransferase system (PTS) interfere with MglP synthesis. pts mutations, which eliminate the general proteins of the PTS Enzyme I and/or HPr and crr mutations, which result in a defective glucose-specific factor III^Gle of the PTS, lead to a low MglP activity, as measured by methyl -galactoside transport. In both ptsH,I, and crr mutants the amount of galactose binding protein, one of the components of MglP, is only 5%–20% of that in wild-type cells, as measured with a specific antibody. We conclude that synthesis of MglP is inhibited in pts and crr mutants. Once the transport system is synthesized, its transport activity is not sensitive to PTS sugars (i.e., no inducer exclusion occurs). The defect in pts and crr mutants with respect to MglP synthesis can be relieved in two ways: by externally added cyclic adenosine 3, 5-monophosphate (cAMP) or by a mutation in the cAMP binding protein. The conclusion that MglP synthesis is dependent on cAMP is supported by the finding that its synthesis is also defective in mutants that lack adenylate cyclase. pts and crr mutations do not affect growth of S. typhimurium on galactose, however, since the synthesis and activity of the other major galactose transport system, the galactose permease (GalP), is not sensitive to these mutations. If the galactose permease is eliminated by mutation, growth of pts and crr mutants on low concentrations of galactose becomes very slow due to inhibited MglP synthesis. Residual growth observed at high galactose concentrations is the result of yet another transport system with low affinity for galactose.     

Molecular organization of the Escherichia coli gab cluster: nucleotide sequence of the structural genes gabD and gabP and expression of the GABA permease gene

We have determined the nucleotide sequences of two structural genes of the Escherichia coli gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway: gabD, coding for succinic semialdehyde dehydrogenase (SSDH, EC 1.2.1.16) and gabP, coding for the 4-aminobutyrate (GABA) transport carrier (GABA permease). We have previously reported the nucleotide sequence of the third structural gene of the cluster, gabT, coding for glutamate: succinic semialdehyde transaminase (EC 2.6.1.19). All three gab genes are transribed unidirectionally and their orientation within the cluster is 5-gabD-gabT-gabP-3. gabT and gabP are separated by an intergenic region of 234-bp, which contains three repetetive extragenic palindromic (REP) sequences. The gabD gene consists of 1,449 nucleotides specifying a protein of 482 amino acids with a molecular mass of 51.7 kDa. The protein shows significant homologies to the NAD⁺-dependent aldehyde dehydrogenase (EC 1.2.1.3) from Aspergillus nidulans and several mammals, and to the tumor associated NADP⁺-dependent aldehyde dehydrogenase (EC 1.2.1.4) from rat. The permease gene gabP comprises 1,401 nucleotides coding a highly hydrophobic protein of 466 amino acids with a molecular mass of 51.1 kDa. The GABA permease shows features typical for an integral membrane protein and is highly homologous to the aromatic acid carrier from E. coli, the proline, arginine and histidine permeases from Saccharomyces cerevisiae and the proline transport protein from A. nidulans. Uptake of GABA was increased ca. 5-fold in transformants of E. coli containing gabP plasmids. Strong overexpression of the gabP gene under control of the isopropyl-2-d-thiogalactoside (IPTG) inducible tac promoter, however, resulted in a severe growth inhibition of the transformed strains. The GABA carrier was characterized using moderately overexpressing transformants. The K _m of GABA uptake was found to be 11.8 M and the V_max 0.33 nmol/min · mg cells. Uptake of GABA was stimulated by ammonium sulfate and abolished by 2,4-dinitrophenol. Aspartate competed with GABA for uptake.     

Protein kinase C activates the renal apical membrane Na+/H+ exchanger

Summary Studies were performed on purified brush-border membranes from the kidney of the rabbit to examine the relation between protein kinase C and the Na⁺/H⁺ exchanger in these membranes. The brush-border membranes were transiently opened by exposure to hypotonic media and the membrane proteins phosphorylated by exposure to ATP and phorbol esters or partially purified protein kinase C. The membranes were resealed and the intravesicular space acidified by incubation in a sodium-free isotonic solution (pH 5.5). The rate of uptake of 1mm ²²Na⁺ (pH 7.5), with and without amiloride (1mm), was assayed and the proton gradient-stimulated, amiloride-inhibitable component of²²Na⁺ taken as a measure of the activity of the Na⁺/H⁺ exchanger. 12-0-tetradecanoyl phorbol-13-acetate (TPA) increased the amiloride-sensitive component of²²Na⁺ uptake TPA did not affect the amiloride-insensitive component of²²Na⁺ uptake or the equilibrium concentration of sodium. TPA also did not affect the rate of dissipation of the proton gradient in the absence of sodium or the rate of sodium-dependent or-independent uptake ofd-glucose. Other active phorbol esters stimulated the rate of Na⁺/H⁺ exchange, but phorbol esters of the 4 configuration did not. Incubation of the opened membranes in partially purified protein kinase C increased the rate of proton gradient-stimulated, amiloride-inhibitable sodium uptake. The stimulatory effect of TPA and protein kinase C was not additive. In the absence of ATP, neither TPA nor protein kinase C affected Na⁺/H⁺ exchange transport. To determine the membrane-bound protein substrates, parallel experiments were conducted with -[³²P] ATP in the phosphorylating solutions. The reaction was stopped by SDS and the phosphoproteins resolved by PAGE and autoradiography. TPA stimulation of protein kinase C resulted in phosphorylation of approximately 13 membrane-bound proteins ranging in apparent molecule from 15,000 to 140,000 daltons. These studies indicate that activation of endogenous renal brush-border protein kinase C by phorbol esters or exposure of these membranes to exogenous protein kinase C increases the rate of proton gradient-stimulated, amiloride-inhibitable sodium transport. Protein kinase C activation also results in phosphorylation of a finite number of membrane-bound proteins.     

Electrogenic properties of the cloned Na+/glucose cotransporter: II. A transport model under nonrapid equilibrium conditions

Summary The results of the accompanying electrophysiological study of the cloned Na⁺/glucose cotransporter from small intestine (Parent, L., Supplisson, S., Loo, D.D.F., Wright, E.M. (1992) J. Membrane Biol. 125:49–62) were evaluated in terms of a kinetic model. The steady-state and presteady-state cotransporter properties are described by a 6-state ordered kinetic model (mirror symmetry) with a Na⁺:MDG stoichiometry of 2. Carrier translocation in the membrane as well as Na⁺ and sugar binding and dissociation are treated as a function of their individual rate constants. Empty carrier translocation and Na⁺ binding/ dissociation are the only steps considered to be voltage dependent. Currents were associated with the translocation of the negatively charged carrier in the membrane. Negative membrane potential facilitates sugar transport. One numerical solution was found for the 14 rate constants that account quantitatively for our experiment observations: i.e., (i) sigmoidal shape of the sugar-specific current-voltage curves (absence of outward currents and inward current saturation at high negative potentials), (ii) Na⁺ and voltage dependence of K _0.5 ^sugar and i _max ^sugar , (iii) sugar and voltage dependence of K _0.5 ^Na and i _max ^Na , (iv) presteady-state currents and their dependence on external Na⁺, MDG and membrane potential, and (v) and carrier Na⁺ leak current. We conclude that the main voltage effect is on carrier translocation. Na⁺ ions that migrate from the extracellular medium to their binding sites sense 25 to 35% of the transmembrane voltage, whereas charges associated with the carrier translocation experiences 60 to 75% of the membrane electrical field. Internal Na⁺ ion binding is not voltage dependent. In our nonrapid equilibrium model, the rate-limiting step for sugar transport is a function of the membrane potential, [Na]₀ and [MDG]₀. At 0 mV and at saturating [Na]₀ and [MDG]₀, the rate-limiting step for sugar transport is the empty carrier translocation (5 sec^–1). As the membrane potential is made more negative, the empty carrier translocation gets faster and the internal Na⁺ dissociation becomes increasingly rate limiting. However, as [Na]₀ is decreased to less than 10 mm, the rate-limiting step is the external Na⁺ ions binding in the 0 to –150 mV potential range. At 0 mV, the external Na⁺ dissociation constant KNa is 80 mm and decreases to 24 mm at –150 mV. The external sugar dissociation constant KNaS is estimated to be 200 m and voltage independent. Finally, the internal leak pathway (CNa₂ translocation) is insignificant. While we cannot rule out a more complex kinetic model, the electrical properties of the cloned Na⁺/glucose cotransporter are found to be adequately described by this 6-state kinetic model.We are grateful to Drs. A. Berteloot, S. Ciani, and J.-Y. Lapointe for stimulating discussions and thank our colleagues for comments. L.P. was recipient of a post-doctoral fellowship from the Medical Research Council of Canada. This work was supported by a grant from the U.S. Public Health Service DK 19567.     

High-affinity sodium-dependent uptake of ascorbic acid by rat osteoblasts

Summary Ascorbic acid is essential for the formation of bone by osteoblasts, but the mechanism by which osteoblasts transport ascorbate has not been investigated previously. We examined the uptake ofl-[¹⁴C]ascorbate by a rat osteoblast-like cell line (ROS 17/2.8) and by primary cultures of rat calvaria cells. In both systems, cells accumulatedl-[¹⁴C]ascorbate during incubations of 1–30 min at 37°C. Unlike propionic acid, which diffuses across membranes in protonated form, ascorbic acid did not markedly alter cytosolic pH. Initial ascorbate uptake rate saturated with increasing substrate concentration, reflecting a high-affinity interaction that could be described by Michaelis-Menten kinetics (apparentK _m =30±2 m andV _max=1460±140 nmol ascorbate/g protein/min in ROS 17/2.8 cells incubated with 138mm extracellular Na⁺). Consistent with a stereoselective carrier-mediated mechanism, unlabeledl-ascorbate was a more potent inhibitor (IC₅₀=30±5 m) ofl-[¹⁴C]ascorbate transport than wasd-isoascorbate (IC₅₀=380±55 m). Uptake was dependent on both temperature and Na⁺, since it was inhibited by cooling to 4°C and by substitution of K⁺, Li⁺ or N-methyl-d-glucamine for extracellular Na⁺. Decreasing the external Na⁺ concentration lowered both the affinity of the transporter for ascorbate and the apparent maximum velocity of transport. We conclude that osteoblasts possess a stereoselective, high-affinity, Na⁺-dependent transport system for ascorbate. This system may play a role in the regulation of bone formation.     

A protonmotive force as the source of energy for galactoside transport in energy depletedEscherichia Coli

Summary Uracil transport inSaccharomyces cerevisiae is mediated by a specific permease which does not recognize other pyrimidines such as uridine, cytosine, thymine, 2-hydroxypyrimidine or 5-amino-uracil; hypoxanthine and 6-amino-uracil slightly inhibit the uptake of uracil in a strain lacking cytosine permease activity. Wild type cells concentrate extracellular uracil before its transformation into UMP and subsequent incorporation into nucleic acids. A strain lacking UMP pyrophosphorylase and uridine ribohydrolase (strainfur 1–8 rh, in which the endogenous production as well as the utilization of uracil are lacking) is able to concentrate¹⁴C-2 uracil from the medium. At the same time no other¹⁴C-2 labelled compound could be detected in this strain, thus suggesting that the uptake of uracil in yeast occurs by active transport which is not coupled to the UMP pyrophosphorylase. The optimal pH of uracil uptake in standard growth conditions was 4.3. It was deduced from experiments performed on strainfur 1–8 rh with³H-5 and¹⁴C-2 uracil that the intracellular pool of uracil is recycled once the steady-state has been reached. First order kinetics with similar rate constants were observed for uracil efflux in strainfur 1–8 rh (k min^–1=0.75±0.08) as well as in the strain lacking uracil permease,fur 1–8 rh fur 4–6 (k min^–1=0.60±0.08). The intracellular pool of¹⁴C-2 uracil can be chased in strainfur 1–8 rh by addition of³H uracil without inducing a large initial acceleration of the exit rate (the rate constant remained at 0.60). 2-4-dinitrophenol inhibits the uptake of uracil but also reduces the efflux of uracil in strainfur 1–8 rh fur 4–6. These data and the comparison with cytosine transport in the same organism support the hypothesis that, whereas uracil uptake is a permease mediated active transport, the efflux of uracil does not involve the uracil uptake permease. A coefficient of permeability of 7.4×10^–7 cm sec^–1 was calculated for uracil.     

The mechanistic nature of the membrane potential dependence of sodium-sugar cotransport in small intestine

Summary Methods are described which demonstrate the use of unidirectional influx of¹⁴C-tetraphenylphosphonium (¹⁴C-TPP⁺) into isolated intestinal epithelial cells as a quantitative sensor of the magnitude of membrane potentials created by experimentally imposed ion gradients. Using this technique the quantitative relationship between membrane potential () and Na⁺-dependent sugar influx was determined for these cells at various Na⁺ and -methylglucoside (-MG) concentrations. The results show a high degree of dependence for the transport Michaelis constant but a maximum velocity for transport which is independent of . No transinhibition by intracellular sugar (40mm) can be detected. Sugar influx in the absence of Na⁺ is insensitive to 1.3mm phlorizin and independent of . The mechanistic implications of these results were evaluated using the quality of fit between calculated and experimentally observed kinetic constants for rate equations derived from several transport models. The analysis shows that for models in which translocation is the potential-dependent step the free carrier cannot be neutral. If it is anionic, the transporter must be functionally asymmetric. A model in which Na⁺ binding is the potential-dependent step (Na⁺ well concept) also provides an appropriate kinetic fit to the experimental data, and must be considered as a possible mechanistic basis for function of the system.     

Characterization of a potassium carrier in rabbit reticulocyte cell membrane

Summary In this study we present evidence that high ouabain-resistant Rb influx, carried out by the rabbit reticulocyte membrane, is composed of carrier-mediated Rb influx and passive diffusion across the cell membrane. To meet this end, an assay was developed by which the two ouabain-resistant Rb influxes could be measured separately.Whereas theK _m for Rb of the carrier (12.5mm) did not change by increasing the pH, theVm was markedly reduced. At the optimal pH (6.0–6.5) theVm was 6–8 mmol h^–1 liter^–1 and fell to zero at pH 8.0. This may indicate a possible role of H⁺ ions in this transport mechanism.The carrier is inhibited by furosemide and ethacrynic acid, similarly to pump II in the erythrocyte and kidney. In addition, its activity is dependent upon the ionic content of the medium. The K(Rb) carrier appeared not to be involved in an active transport since depletion of ATP had no effect on the carrier activity. The carrier activity was also measured in rabbit erythrocytes and was found to be 10 times lower than that of rabbit reticulocytes. TheK _m for Rb, optimal pH, and high sensitivity to furosemide and ethacrynic acid of the erythrocyte and the reticulocyte carrier are similar.Our study suggests that maturation of reticulocytes to erythrocytes is accompanied by a loss or inactivation of most of a K (or Rb) carrier very active in the reticulocyte cell.     

Incorporation of [214C]malonyl CoA into fatty acids by a cell-free extract of Catharanthus roseus suspension culture cells

A cell-free extract containing the enzymes for de-novo synthesis, elongation and desaturation of fatty acids was prepared from cultured cells of Catharanthus roseus G. Don. ¹⁴C-Fatty acids synthesized by the extract from [2-¹⁴C]malonyl CoA substrate were palmitic (16:0), stearic (18:0) and oleic (18:1). Dialyzed extract was active and stable at room temperature and at 4° C, but was inactivated on boiling. There was an absolute requirement for NADPH for incorporation of [2-¹⁴C]malonyl CoA into total fatty acids. Escherichia coli acyl carrier protein stimulated total fatty-acid synthesis without affecting the relative ratio of individual fatty acids. Total fatty-acid synthesis at a rate of 45 nmol·mg^-1 protein·h^-1 occurred at a substrate level of 73 M malonyl CoA, cofactor levels of 500 M NADPH, 30 g·ml^-1 E. coli ACP, and 1.0 mg·ml^-1 extract protein. Total fatty acid synthesis was also sensitive to cerulenin and CoA levels. Variations in the relative abundance of individual ¹⁴C-fatty acids were regulated by concentrations of [¹⁴C]malonyl CoA. NADPH and ferredoxin, as well as by pH, temperature and length of incubation. Fatty-acid synthetase enzymes responsible for [¹⁴C]palmitic acid were rapidly saturated at a low substrate level (0.3 M malonyl CoA). Increasing the level of [2-¹⁴C]malonyl CoA permitted further synthesis of [¹⁴C]stearate and [¹⁴C]oleate. Desaturation of [¹⁴C]stearate to [¹⁴C]oleate was stimulated by increasing the levels of NADPH and ferredoxin. The desaturase and elongase enzymes were sensitive to acidic pH. The desaturase was also unstable at 41° C, although fatty acid synthetase and elongase were unaffected by this temperature.Abbreviation ACP Acyl carrier protein     

Role of substrate binding forces in exchange-only transport systems: II. Implications for the mechanism of the anion exchanger of red cells

Summary The transition-state theory of exchange-only membrane transport is applied to experimental results in the literature on the anion exchanger of red cells. Two central features of the system are in accord with the theory: (i) forming the transition state in translocation involves a carrier conformational change; (ii) substrate specificity is expressed in transport rates rather than affinities. The expression of specificity is consistent with other evidence for a conformational intermediate (not the transition state) formed in the translocation of all substrates. The theory, in conjunction with concepts derived from the chemistry of macrocyclic ion inclusion complexes, prescribes certain essential properties in the transport site. Separate substites are required for the preferred substrates. Cl^– and HCO ₃ ^– , to account for tight binding in the transition state (K _diss1m). Further, the following mechanism is suggested. A substrate anion initially forms a loose surface complex at one subsite, but in the transition state the subsites converge to form an inclusion complex in which the binding forces are greatly increased through a chelation effect. The conformational change at the substrate site, which is driven by the mounting forces of binding, sets in train a wider conformational change that converts the carrier from an immobile to a mobile form. Though simple, this composite-site mechanism explains many unsual features of the system. It accounts for substrate inhibition, partially noncompetitive inhibition of one substrate by another, and tunneling, which is net transport under conditions where exchange should prevail, according to other models. All three types of behavior result from the formation of a ternary complex in which substrate anions are bound at both subsites. The mechanism also accounts for the enormous range of substrate structures accepted by the system, for the complex inhibition by the organic sulfate NAP-taurine, and for the involvement of several cationic side chains and two different protein domains in the transport site.     

Amino-acid transport into cultured tobacco cells

Arginine transport in suspension-cultured cells of Nicotiana tabacum L. cv. Wisconsin-38 was investigated. Cells that were preincubated in the presence of Ca²⁺ for 6 h prior to transport exhibited stimulated transport rates. After the preincubation treatment, initial rates of uptake were constant for at least 45 min. Arginine accumulated in the cells against a concentration gradient; this accumulation was not the result of exchange diffusion. Arginine uptake over a concentration range of 2.5 M to 1 mM was characterized by simple Michaelis-Menten kinetics with a K_m of 0.1 mM and a V_max of 9,000 nmol g^-1 fresh weight h^-1. Transport was inhibited by several compounds including carbonylcyanide-m-chlorophenylhydrazone, 2,4-dinitrophenol, N,N-dicyclohexylcarbodiimide, and N-ethylmaleimide. Inhibition by these compounds was not the result of increased efflux resulting from membrane damage. A variety of amino acids and analogs, with the exception of D-arginine, inhibited transport, indicating that arginine transport was mediated by a general L-aminoacid permease. Competition experiments indicated that arginine and lysine exhibited cross-competition for transport, with K_i values similar to respective K_m values. Arginine transport and low-affinity lysine transport are probably mediated by the same system in these cells.Abbreviations BTP Bis Tris Propane - CCCP Carbonylcyanide-m-chlorophenylhydrazone - DCCD N,N-dicyclohexylcarbodiimide - DNP 2,4-dinitrophenol - DTT Dithiothreitol - NEM N-ethylmaleimide - MES 2(N-morpholino)ethanesulfonic acid - TCA trichloroacetic acid This paper is the third in a series on amino-acid transport into cultured tobacco cells. For parts I and II, see Harrington and Henke (1981) and Harrington et al. (1981)     

Ferrioxamine transport mutants and the identification of the ferrioxamine receptor protein (FoxA) inErwinia herbicola (Enterobacter agglomerans)

Summary Iron deprivation ofErwinia herbicola (Enterobacter agglomerans) induces the biosynthesis of six high-M _r outer-membrane proteins and large amounts of ferrioxamine E. Mutagenesis withN-methyl-N-nitro-N-nitrosoguanidine and selection with ferrimycin A yielded mutants ofE. herbicola K4 (wild type), defective in the expression of a 76-kDa outer-membrane protein, as determined by SDS/polyacrylamide gel electrophoresis. While in bioassays wild-type cells showed growth promotion in the presence of ferrioxamines (B, D₁, D₂, E, G), enterobactin, citrate, ferrichrome and coprogen, these mutants failed to respond to ferrioxamines. Moreover, experiments with⁵⁵Fe-labelled siderophores confirmed that iron transport mediated by ferrioxamine E and B in the mutants was completely inhibited, whereas iron transport by other hydroxamate siderophores, such as ferrichrome and coprogen was unaffected. The results are evidence that the 76-kDa protein in the outer membrane represents the receptor protein (FoxA) for ferrioxamines inE. herbicola.     

Mutation of Asparagine 76 in the Center of Glutamine Transporter SNAT3 Modulates Substrate-induced Conductances and Na+ Binding

The glutamine transporter SLC38A3 (SNAT3) plays an important role in the release of glutamine from brain astrocytes and the uptake of glutamine into hepatocytes. It is related to the vesicular GABA (γ-aminobutyric acid) transporter and the SLC36 family of proton-amino acid cotransporters. The transporter carries out electroneutral Na⁺-glutamine cotransport-H⁺ antiport. In addition, substrate-induced uncoupled cation currents are observed. Mutation of asparagine 76 to glutamine or histidine in predicted transmembrane helix 1 abolished all substrate-induced currents. Mutation of asparagine 76 to aspartate rendered the transporter Na⁺-independent and resulted in a gain of a large substrate-induced chloride conductance in the absence of Na⁺. Thus, a single residue is critical for coupled and uncoupled ion flows in the glutamine transporter SNAT3. Homology modeling of SNAT3 along the structure of the related benzyl-hydantoin permease from Microbacterium liquefaciens reveals that Asn-76 is likely to be located in the center of the membrane close to the translocation pore and forms part of the predicted Na⁺ -binding site.The amino acid and auxin permease superfamily comprises a wide variety of transport proteins. In mammals, three distinct solute carrier families (SLC) belong to this superfamily, namely SLC32, SLC36, and SLC38 (1). Despite belonging to the same superfamily, the three solute carrier families have different transport mechanisms. The SLC32 family has only one member, the vesicular inhibitory amino acid transporter, which supposedly carries out a H⁺-GABA (γ-aminobutyric acid) antiport (2). The SLC36 family comprises four members, two of which have been characterized in more detail. These are the proton amino acid cotransporters 1 and 2 (PAT1 and 2) that carry out glycine and proline uptake in kidney and intestine and are mutated in iminoglycinuria (3, 4). The SLC38 family is comprised of 11 members, 5 of which have been characterized in more detail (5). Two different transport mechanisms are found within this family, namely the Na⁺-amino acid cotransporters SNAT1, SNAT2, and SNAT4 and the Na⁺-amino acid cotransporters-H⁺-antiporters SNAT3 and SNAT5. Transporters of the superfamily play a key role in inhibitory and excitatory neurotransmission, metabolite absorption, and liver metabolism. Despite their important roles in mammalian physiology, relatively little is known about the structure and function of these transporters.The activity of ion-coupled membrane transporters is frequently associated with currents which de- or hyperpolarize the cell membrane. These currents may be due to electrogenic transport stoichiometry and/or to a non-stoichiometric ion conductance (6). Transport-associated ion conductances have been identified in a number of transporters but have been particularly well studied in several Na⁺-coupled neurotransmitter transporters (7–11). Transport-associated conductances have also been observed in electroneutral transporters that do not carry out net charge movement (8, 12–15). The glutamine transporter SNAT3, for instance, has a transport mechanism in which glutamine uptake is coupled to the cotransport of 1Na⁺ and the antiport of 1H⁺ and, hence, is unaffected by changes of the membrane potential (13, 16). Despite the electroneutral transport mechanism, substrate uptake is accompanied by inward currents, which are carried by cations below pH 7 and by protons at alkaline pH. In addition, a substrate-independent cation conductance and a Na⁺/H⁺ exchange activity has been observed (17). Non-stoichiometric currents can be mediated by the same ions that are involved in the coupled transport process, such as in the case of SNAT3, but may also be carried by different ions. Stoichiometric glutamate transport, for instance, involves Na⁺, H⁺, and K⁺ ions, whereas the glutamate transport-associated conductance is carried by chloride (18).A crucial question concerning transporter-associated ion conductances is whether the conducting pore coincides with the translocation pathway of the substrate and whether both use the same critical residues. In the case of the glutamate transporters, evidence has been presented suggesting that different residues are critical for the anion conductance than for substrate transport (19, 20) but that they all line the same pathway (21). Here we show that asparagine 76 of SNAT3 is critical for substrate-induced ion conductance and affects binding of the cosubstrate Na⁺. In addition we show that this residue is likely to be localized in the translocation pore in the center of the membrane.     

Delineation of sodium-stimulated amino acid transport pathways in rabbit kidney brush border vesicles

Summary We have confirmed previous demonstrations of sodium gradient-stimulated transport ofl-alanine, phenylalanine, proline, and -alanine, and in addition demonstrated transport of N-methylamino-isobutyric acid (MeAIB) and lysine in isolated rabbit kidney brush border vesicles. In order to probe the multiplicity of transport pathways available to each of these¹⁴C-amino acids, we measured the ability of test amino acids to inhibit tracer uptake. To obtain a rough estimate of nonspecific effects, e.g., dissipation of the transmembrane sodium electrochemical potential gradient, we measured the ability ofd-glucose to inhibit tracer uptake.l-alanine and phenylalanine were completely mutually inhibitory. Roughly 75% of the¹⁴C-l-alanine uptake could be inhibited by proline and -alanine, while lysine and MeAIB were no more effective thand-glucose. Roughly 50% of the¹⁴C-phenylalanine uptake could be inhibited by proline and -alanine; lysine was as effective as proline and -alanine, and the effects of pairs of these amino acids at 50mm each were not cumulative. MeAIB was no more effective thand-glucose. We conclude that three pathways mediate the uptake of neutral,l, -amino acids. One system is inaccessible to lysine, proline, and -alanine. The second system carries a major fraction of thel-alanine flux; it is sensitive to proline and -alanine, but not to lysine. The third system carries half the¹⁴C-phenylalanine flux, and it is sensitive to proline, lysine, and -alanine. Since the neutral,l, -amino acid fluxes are insensitive to MeAIB, we conclude that they are not mediated by the classicalA system, and since all of thel-alanine flux is inhibited by phenylalanine, we conclude that it is not mediated by the classicalASC system.l-alanine and phenylalanine completely inhibit uptake of lysine. MeAIB is no more effective thand-glucose in inhibiting lysine uptake, while proline and -alanine appear to inhibit a component of the lysine flux. We conclude that the¹⁴C-lysine fluxes are mediated by two systems, one, shared with phenylalanine, which is inhibited by proline, -alanine, andl-alanine, and one which is inhibited byl-alanine and phenylalanine but inaccessible to proline, -alanine, and MeAIB. Fluxes of¹⁴C-proline and¹⁴C-MeAIB are completely inhibited byl-alanine, phenylalanine, proline, and MeAIB, but they are insensitive to lysine. Proline and MeAIB, as well as alanine and phenylalanine, but not lysine, inhibit¹⁴C--alanine uptake. However, -alanine inhibits only 38% of the¹⁴C-proline uptake and 57% of the MeAIB uptake. We conclude that two systems mediate uptake of proline and MeAIB, and that one of these systems also transports -alanine.     

Design principles for active transport systems

We have previously described simple models for active transport and have derived steady state equations for the unidirectional flux of substrate in terms of a minimal set of kinetic parameters. Here we consider how to maximize the pumping rate of a carrier-enzyme through the optimal utilization of the ATP hydrolysis reaction. The equations for net flux contain rate constants and dissociation constants and these determine the maximum velocities and affinities measured in transport kinetic analysis. It is assumed that the rate constants can evolve to the diffusion limited rate of substrate binding as has apparently occurred in the enzyme triosephosphate isomerase (Knowles & Albery, 1977). The dissociation constants of the rate limiting intermediates fit the affinities for substrates on different sides of the membrane and are dependent on the basic free energy levels (Hill, 1976) of the carrier substrate system. From our analysis it is clear that there are three ways to design a system with optimal affinities and that the choice is linked to the sequence of substrate binding. It is possible to use free energy differences of isomerization (Boyer, 1975) or ligand-ligand interactions (Weber, 1975) both of which have been described previously, but which are incorporated here into a unified treatment. A third possibility is to couple the binding step of a transported ligand to the progress of a chemical reaction as might occur, for example, if Na⁺ must be bound before the carrier can be phosphorylated. In this way the free energy of hydrolysis can be used not only to drive the overall pumping reaction, but also to fix differentially the affinity for substrate on either side of the membrane, as required for rapid pumping.     

Sodium-alanine cotransport in oocytes ofXenopus laevis: Correlation of alanine and sodium fluxes with potential and current changes

Summary The sodium-dependentl-alanine transport across the plasma membrane of oocytes ofXenopus laevis was studied by means of [¹⁴C]-l-alanine,²²Na⁺ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 21 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 21. The sodium-l-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess ofd-alanine overl-alanine caused about 60% inhibition.     

Repression and inactivation by glucose of the maltose transport system of Candida utilis

Summary The maltose utilization system of Candida utilis was affected by glucose through two different mechanisms: catabolite repression and inactivation. Maltose permease was under the control of both, whereas -glucosidase was only repressed.In glucose-maltose continuous culture, both sugars were consumed simultaneously at glucose steady-state concentrations in the fermentor below 100 mg/l, corresponding to dilution rates lower than 0.4 h^-1. At higher dilution rates, and consequently higher glucose concentrations, repression increased steeply, being complete when glucose concentration reached 170 mg/l.Glucose induced inactivation of maltose permease, in maltose-growing and resting cells, by decreasing V _max, without changing maltose affinity for its transport system. The inactivation process apparently required the entrance of the inactivator into the cell and its subsequent phosphorylation because: 1) The specific inactivation rate showed a dependence on glucose similar to that of glucose transport and 2) only rapidly phosphorylated glucose analogues could mimic the inactivation effect.