Expression of substrate specificity in facilitated transport systems

Estimation of glucose uptake by ovarian follicular cells

In vitro maturation (IVM) of mammalian oocytes provides an alternative to traditional in vitro fertilization techniques for clinical treatment of infertility or animal breeding. IVM involves the collection of oocytes from the ovary prior to ovulation, with maturation occurring in a laboratory environment. The success of IVM is highly sensitive to the in vitro nutrient environment. The nurse cells surrounding the oocyte, known as cumulus cells, regulate this environment and removal of these cells reduces the ability of the oocyte to develop following insemination. Determining the nature of the interaction between the oocyte and cumulus cells, collectively called the cumulus-oocyte complex (COC), is a difficult task experimentally. Here we use a combination of experimental and mathematical techniques to investigate glucose transport within bovine COCs and find quantitative estimates of the glucose uptake rates of the oocyte and cumulus cells. Surprisingly, our modeling shows the rate of uptake of glucose by the oocyte to increase and then decrease with concentration, a result that needs further experimental investigation but which supports the expectation that high and low glucose concentrations are detrimental to oocyte development. The methodology described is suitable for use across species and for investigating the transport of other important nutrients within the COC.

The structural basis of secondary active transport mechanisms

Secondary active transporters couple the free energy of the electrochemical potential of one solute to the transmembrane movement of another. As a basic mechanistic explanation for their transport function the model of alternating access was put forward more than 40 years ago, and has been supported by numerous kinetic, biochemical and biophysical studies. According to this model, the transporter exposes its substrate binding site(s) to one side of the membrane or the other during transport catalysis, requiring a substantial conformational change of the carrier protein. In the light of recent structural data for a number of secondary transport proteins, we analyze the model of alternating access in more detail, and correlate it with specific structural and chemical properties of the transporters, such as their assignment to different functional states in the catalytic cycle of the respective transporter, the definition of substrate binding sites, the type of movement of the central part of the carrier harboring the substrate binding site, as well as the impact of symmetry on fold-specific conformational changes. Besides mediating the transmembrane movement of solutes, the mechanism of secondary carriers inherently involves a mechanistic coupling of substrate flux to the electrochemical potential of co-substrate ions or solutes. Mainly because of limitations in resolution of available transporter structures, this important aspect of secondary transport cannot yet be substantiated by structural data to the same extent as the conformational change aspect. We summarize the concepts of coupling in secondary transport and discuss them in the context of the available evidence for ion binding to specific sites and the impact of the ions on the conformational state of the carrier protein, which together lead to mechanistic models for coupling.

A General Channel Model Accounts for Channel, Carrier, Countertransport and Cotransport Kinetics

In this work we propose a unifying model of mediated membrane transport, based upon the idea that the integral membrane proteins involved in these processes operate via complex channel mechanisms. In the first part, we briefly review literature about the structural aspects of membrane transporters. We conclude that there is a substantial amount of evidence suggesting that most membrane proteins performing transport are embodied with channel-like structures that may constitute the translocation paths. This includes cases where the phenomenological transport kinetics do not correspond to the classical channel behavior. In the second part of this article we introduce the general channel model of mediated transport and employ it to derive specific examples, like simple one- or two-ligand channels, water-ligand channels, simple carriers, co- and counter-transport systems and more complex water-ligand carriers. We show that, for the most part, these particular cases can be obtained by the application of the techniques of diagram reduction to the full model. The necessary conditions for diagram reduction reflect physical properties of the protein and its surroundings.

Erythrocyte membrane transport

Erythrocytes are endowed with functional entities that support either cellular functions or the systemic delivery of O2 from lung to tissue and removal of CO2 from tissue to lung. The latter depend largely on the blood's circulatory capacity. They are associated, respectively, with cytosolic haemoglobin and the major membrane polypeptide band 3 (anion exchanger 1, AE1). As a membrane transporter, AE1 mediates Cl-/HCO3- exchange, thus enhancing the blood capacity for carrying CO2 and for acid-base homeostasis. By interacting with lipids and proteins, the multifunctional AE1 tethers the membrane cytoskeleton multiprotein complex to the membrane and confers upon erythrocytes both mechanical and viscoelastic properties. Those in turn allow cells to withstand the shear forces of circulation and squeeze through capillaries. Most other major membrane transporters are apparently essential for maintaining a stable erythrocyte cell shape and flexibility via a functional membrane cytoskeleton. These include the membrane transporters of glucose, nucleoside and purine for fueling the Na/K and Ca pumps via ATP production, and of amino acid and oxidized glutathione transport for maintaining the cell redox status. All membrane transporters detected in mature erythrocytes are synthesized early in erythrocyte differentiation. Their contribution to erythrocyte and to systemic physiology is presently being re-assessed by targeted gene disruption and replacement. For example, organisms with reduced or disrupted AE1 gene expression showed major erythrocyte instabilities and defective anion exchange capacity and acidosis, but remain alive.

Transport of phospholipid synthesis precursors and lipid trafficking into malaria-infected erythrocytes

Phospholipid biosynthesis in Plasmodium is of crucial importance considering the high degree of membrane biogenesis. In the de novo phosphatidylcholine pathway, the major plasmodial phospholipid, choline, first enters infected erythrocytes by a transport-mediated process, whose main kinetic characteristics are the same as in normal cells except for a considerable increase in Vm. The kinetic and functional characterizations of the choline carrier (affinity, specificity, stereoselectivity, asymmetric cyclic model, ionic dependence, limiting step in carrier translocation) have now been done, although there is no information concerning its nature and structure, despite the fact that it is likely an outstanding pharmacological target. Other unanswered questions concern the mechanisms for choline entry into the parasite. The intense lipid trafficking between the intracellular parasite and the host cell membrane also indicates that Plasmodium controls its own lipid composition as well as that of its host cell. Organelles that house the machinery for lipid synthesis, and mechanisms for trafficking and sorting, have not yet been described because of the lack of appropriate tools, but they could address fundamental questions in the contemporary cell biology of this parasite.

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

Antimalarial Activity of 77 Phospholipid Polar Head Analogs: Close Correlation Between Inhibition of Phospholipid Metabolism and In Vitro Plasmodium Falciparum Growth

Seventy-seven potential analogs of phospholipid polar heads, choline and ethanolamine, were evaluated in vitro as inhibitors ofPlasmodium falciparum growth. Their IC50 ranged from 10−3 to 10−7 mol/L. Ten compounds showed similar antimalarial activity when tested against three different parasite strains (2 chloroquine-sensitive strains and 1 chloroquine-resistant strain). Compounds showing marked antimalarial activity were assayed for their effects on phospholipid metabolism. The most active compounds (IC50 of 1 to 0.03 μmol/L) were inhibitors of de novo phosphatidylcholine (PC) biosynthesis from choline. For a series of 50 compounds, there was a close correlation between impairment of phospholipid biosynthesis and inhibition of in vitro malaria parasite growth. High choline concentrations caused a marked specific shift in the curves for PC biosynthesis inhibition. Concentrations inhibiting 50% PC metabolism from choline were in close agreement with the Ki of these compounds for the choline transporter inPlasmodium knowlesi-infected erythrocytes. By contrast, measurement of the effects of 12 of these compounds on rapidly dividing lymphoblastoid cells showed a total absence of correlation between parasite growth inhibition and human lymphoblastoid cell growth inhibition. Specific antimalarial effects of choline or ethanolamine analogs are thus likely mediated by their alteration of phospholipid metabolism. This indicates that de novo PC biosynthesis from choline is a very realistic target for new malaria chemotherapy, even against pharmacoresistant strains.

Kinetic and energetic characterization of solute flux through the reconstituted aspartate/glutamate carrier from beef heart mitochondria after modification with mercurials

The functional switch from specific, coupled antiport to unspecific unidirectional transport (efflux) of the reconstituted aspartate/glutamate carrier from mitochondria after chemical modification with mersalylic acid was investigated in kinetic and energetic terms. The rate of mercurial-induced efflux was determined for a number of solutes which differ from the physiological substrate aspartate in structure, size and charge, namely oxoglutarate, sulfate, glucose, lysine and arginine. These values were compared to the rates of efflux as well as antiport of aspartate. Measurement of the temperature dependence of all rates led to evaluation of the activation energy of the different substrates. The activation energy was similar for all substrates and for both transport modes, whereas the efflux rates could be ordered in the following sequence: anions > uncharged solutes > cations. When extrapolating to Vmax conditions, the resulting turnover numbers for uniport substrates become similar and exceed the turnover numbers for aspartate and glutamate antiport. Trans-inhibition of efflux was only observed in the case of externally added aspartate or glutamate and only for internal anionic substrates (at the cis side), thus indicating that after efflux induction the specificity of the external binding site is fully and that of the internal site is partially retained. The consequence of these results for understanding the transport function of the aspartate/glutamate carrier in the slippage mode (uniport) is discussed in energetic and kinetic terms.

Interpreting the effects of specific protein modification on antiport coupling mechanisms: the case of the aspartate/glutamate exchanger

Reaction of two cysteine residues in the aspartate/glutamate carrier of mitochondria is reported to abolish exchange but to actuate a passive one-way exit of extremely low substrate affinity and specificity, but with the same activation energy as antiport (Dierks, T., Salentin, A. and Krämer, R. (1990) Biochim. Biophys. Acta 1028, 281-288). This behaviour, reminiscent of a channel, becomes understandable when the required control over carrier mobility by the substrate is allowed for. Whether the transport mechanism involves a substrate site alternately exposed on opposite sides of the membrane or sites simultaneously exposed on both sides, and whether the substrate acts by converting an immobile carrier conformation to an inherently mobile intermediate or by stabilizing the transition state in carrier movement, the same fundamental relationship emerges: the ratio of coupled to uncoupled rates (antiport relative to net flux) is limited by the ratio of substrate dissociation constants in successive carrier conformations, one immobile, the other mobile; the increment in the binding energy in the two forms must therefore be large. Shifts in the equilibrium between these conformations and shifts in their relative affinities for the substrate can account for the properties of the modified transport system, which, it is concluded, functions as a carrier, not a channel.

Transport properties of single-file pores with two conformational states

Complex facilitative membrane transporters of specific ligands may operate via inner channels subject to conformational transitions. To describe some properties of these systems, we introduce here a kinetic model of coupled transport of two species, L and w, through a two-conformational pore. The basic assumptions of the model are: a) single-file of, at most, n molecules inside the channel; b) each pore state is open to one of the compartments only; c) there is at most only one vacancy per pore; d) inside the channel, a molecule of L occupies the same positions as a molecule of w; and e) there is at most only one molecule of L per pore. We develop a general representation of the kinetic diagram of the model that is formally similar to the one used to describe one-vacancy transport through a one-conformational single-file pore. In many cases of biological importance, L could be a hydrophilic (ionic or nonionic) ligand and w could be water. The model also finds application to describe solute (w) transport under saturation conditions. In this latter case, L would be another solute, or a tracer of w. We derive steady-state expressions for the fluxes of L and w, and for the permeability coefficients. The main results obtained from the analysis of the model are the following. 1) Under the condition of equilibrium of w, the expression derived for the flux of L is formally indistinguishable from the one obtainable from a standard four-state model of ligand transport mediated by a two-conformational transporter. 2) When L is a tracer of w, we can derive an expression for the ratio between the main isotope and tracer permeability coefficients (Pw/Pd). We find that the near-equilibrium permeability ratio satisfies (n - 1) < or = (Pw/Pd)eq < or = n, a result previously derived for the one-conformational, single-file pore for the case that n > or = 2. 3) The kinetic model studied here represents a generalization of the carrier concept. In fact, for the case that n = 1 (corresponding to the classical single-occupancy carrier), the near-equilibrium permeability ratio satisfies 0 < or = (Pw/Pd)eq < or = 1, which is characteristic of a carrier performing exchange-diffusion.

Interpreting the effects of site-directed mutagenesis on active transport systems

Single amino acid substitutions in the lactose permease of Escherichia coli are known to elicit behaviour, such as the transformation of an active into a passive system, not explained by current co-transport models. The behaviour, it is shown, can be explained by an expanded reaction scheme that takes account of the required alternation of the carrier, in the course of the coupled reaction, between mobile and immobile conformations or between conformations that bind either only one substrate or both substrates. The extended model links such behaviour to altered conformational equilibria or binding regions. Thus, mutations that affect the equilibrium between a mobile one-site conformation of the free carrier and an immobile conformation having sites for both substrates allow passive transport of the second substrate in an ordered mechanism, and mutations in a secondary substrate binding region that affects this conformational change allow passive transport of the first substrate. Mutations in regions interacting with a substrate in the transition state in carrier movement, as well as in the initial binding sites, can also be distinguished. The analysis applies to both primary and secondary active transport.

Coupling mechanisms in active transport

In primary and secondary active transport, the mobility and specificity of the carrier are controlled, over the course of the transport reaction, in accordance with a set of 'rules'. The rules are shown to depend on two mechanisms: a substrate--either the driving substrate (a transported ion or ATP) or the driven substrate--may shift a conformational equilibrium or accelerate a rate-limiting conformational change. From an analysis of coupling mechanisms the following conclusions emerge. (i) The ratio of coupled to uncoupled flux, which should be large, cannot be greater than the ratio of substrate dissociation constants in an initial complex and a conformationally altered state. A minimum value for the increased binding force can be estimated from steady-state constants. (ii) In an ordered mechanism, slippage is expected at high concentrations of the substrate adding to the carrier second, while slippage of the first substrate should remain low. (iii) Slippage in coupled transport is minimized if the driven substrate is last on in loading the carrier and last off in unloading, while the reverse order makes the affinity high in loading and low in unloading, as required for efficient transfer from one compartment to another; hence the preferred mechanism may depend on prevailing physiological conditions. (iv) A coupled transport system can be transformed into a facilitated system for one substrate or both if the control of carrier mobility is undermined through modification of the carrier.

Inhibitors of choline transport in alveolar type II epithelial cells

Isolated alveolar type II epithelial cells (granular pneumocytes) from rat lung accumulate free choline against a concentration gradient by an energy-dependent saturable transport process with apparent Km approximately 18 microM. In order to evaluate the structural requirements for choline transport by these cells, the inhibition of the initial rate of cellular uptake of [3H]choline (5 microM) by its analogue was measured. There was no significant inhibition of substrate uptake by analogues lacking an amino group while the presence of a quaternary nitrogen was most effective. N,N'-dimethylethanolamine (apparent Ki, 7 microM) and n-decylcholine (apparent Ki, 0.5 microM) were potent competitive inhibitors of choline transport. Substitution of the hydroxyl group in choline greatly diminished the inhibitory effect; fluorocholine, thiocholine, betaine, and betaine aldehyde showed little or no inhibition. This requirement for a hydroxyl group raises the possibility of hydrogen bonding of choline with the transport protein. The choline transport system in granular pneumocytes appears to differ from that in synaptosomes by the lower affinity of the carrier for substrate and for hemicholinium-3 and from that in erythrocytes by the role of the hydroxyl in the substrate molecule. The availability of inhibitory analogues for choline transport will facilitate isolation and study of the granular pneumocyte choline transport protein.

Anomeric preference of fluoroglucose exchange across human red-cell membranes. 19F-n.m.r. studies

The rates of exchange across the human red-cell membrane of the alpha- and beta-anomers of the glucose derivative 3-fluoro-3-deoxy-D-glucose (3FG) were measured, under equilibrium-exchange conditions, using a 19F-n.m.r.-magnetization-exchange procedure. In experiments carried out over a range of 3FG concentrations (3.4-113 mM), the alpha-anomer was found to be transported with a smaller Km (greater apparent affinity) than the beta-anomer. In two experiments carried out at 34 and 37 degrees C the ratio (alpha/beta) of the Michaelis constants for exchange was 0.75 +/- 0.07 and 0.83 +/- 0.07 respectively and the Vmax for 3FG exchange was 28 +/- 3 and 33 +/- 3 mmol.s-1.litre of cells-1 respectively. In several experiments carried out at a single 3FG concentration (17 mM) and at 37 degrees C, using red cells from four individuals, the rate of exchange of the alpha-anomer across the membrane was significantly higher than that of the beta-anomer. The weighted mean value of the above-mentioned ratio was 0.79 +/- 0.07 for the four donors.