Membrane Carriers

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CHAPTER 9 Membrane Carriers

Carriers are integral membrane proteins that use electrochemical gradients to move select chemical substrates across lipid bilayers (Fig. 9-1). Transport by well-characterized carriers depends on a conformational change to move each substrate. Typically, the carriers work step by step, more like enzymes than channels. Channels simply provide a selective pore for transport, and they generally transport at much higher rates (see Chapter 10). Common substrates for carriers are ions and small soluble organic molecules, but in some cases, substrates are lipid soluble.

Figure 9-1 primary and secondary transport reactions. An ATP-driven pump produces a gradient of an ion, such as Na⁺, across a membrane. The triangles represent gradients of Na⁺ (purple), glucose (green), sugar (blue), and Ca²⁺ (light blue) across the membrane. This ion gradient drives secondary transport reactions mediated by carriers. Uniporters allow an ion or other solute to move across the membrane down its concentration gradient. Symporters and antiporters couple transport of an ion (Na⁺ in this example) down its concentration gradient with the transport of a solute (glucose or Ca²⁺ in these examples) up its concentration gradient. Antiporters carry out these reactions in succession, picking up Na⁺ outside, reorienting, dissociating Na⁺ inside, picking up Ca²⁺ inside, reorienting, and releasing Ca²⁺ outside.

Like pumps and channels, carriers are found in all membranes, wherever cells need to exchange molecules for metabolism or extrude wastes. Carriers are also known as facilitators or porters.

Carriers that transport a single substrate across a membrane down its concentra-tion gradient are called uniporters. Remarkably, many carriers also transport substrates up concentration gradients, provided that their passage through the carrier and across the membrane is coupled to the transport of another substrate down its electrochemical gradient. Glucose provides good examples of both downhill and uphill movement through different carriers. The GLUT1 uniporter allows glucose to move down its concentration gradient from plasma into red blood cells. On the other hand, the SGLT1 carrier uses a gradient of Na⁺ established by the Na⁺K⁺−ATPase pump to move glucose up its concentration gradient into intestinal cells. It is called a symporter, since glucose and Na⁺ move in the same direction. Another class of carriers called antiporters move a substrate in the opposite direction to the ion gradient driving the reaction. All carrier-mediated reactions are reversible, so substrates can move in either direction across the membrane, depending on the polarity of the driving forces.

When a carrier uses an ion gradient to provide the energy to transport a substrate, it is said to catalyze a secondary reaction. In this sense, pumps catalyze primary transport reactions, using energy from ATP hydrolysis, electron transport, or absorption of light to create ion gradients (see Chapter 8). Coupling an ion gradient created by pumps to drive transport by a carrier is called a chemiosmotic cycle (see Fig. 11-1).

Diversity of Carrier Proteins

Biological experimentation and exploration of genomes have revealed more than a hundred families of carriers, many of which can be grouped into superfamilies. The major facilitator superfamily (MFS) is the focus of this chapter, since it includes about one third of all known carrier proteins, including many of the best-characterized carriers. Thousands of MFS genes in all branches of the phylogenetic tree are likely to have arisen from a common ancestor. Two thirds of known carriers have other origins and structures but have converged on mechanical solutions for solute transport similar to MFS carriers. No one knows how many structurally distinct groups exist in nature. Box 9-1 illustrates a small selection of carrier families with different evolutionary origins and structures.

BOX 9-1 Crystal Structures of Diverse Carrier Proteins

Four crystal structures (Fig. 9-2) illustrate the diversity of carrier proteins. They differ in evolutionary origins and structures, but all function as carriers. They converged toward common mechanisms implemented by different structures. Conformational changes are believed to contribute to transport in all cases, so their mechanisms will be better understood when structures of additional conformations of each protein are available.

• MFS carriers consist of single polypeptides that form 10 to 14 (usually 12) transmembrane helices (Fig. 9-2B). Substrates bind in a pocket among these helices. A conformational change exposes this binding site on either side of the membrane so that substrates can bind and dissociate. These carriers are active as monomers.

• Mitochondrial adenine nucleotide carriers transport ATP and ADP across the membranes of mitochondria and chloroplasts. Their genes were formed by a threefold duplication and divergence of a sequence that encodes a pair of transmembrane helices. These six transmembrane helices form a cup exposing a nucleotide binding site near the middle of the membrane bilayer. These carriers are believed to operate in pairs to enable cooperative binding of ATP and ADP on opposite sides of the inner mitochondrial membrane. Once bound, the nucleotides are transported down their concentration gradients.

• Multidrug transporters help bacteria to live in hostile environments by extruding a wide variety of toxic hydrophobic chemicals. Substrates include bile salts, dyes, detergents, and lipid-soluble antibiotics. When overexpressed these carriers can make bacteria resistant to antibiotics. The protein is a homotrimer of huge subunits, each with 12 transmembrane helices. Large domains above the membrane help to span the periplasmic space. Substrates that are soluble in the membrane diffuse into a hydrophobic binding site in the center of the carrier and are transported out of the cell by a mechanism that depends on a proton gradient across the plasma membrane.

• Glutamate transporters remove the excitatory neurotransmitter glutamate from the synaptic cleft after a nerve impulse and transport glutamate into bacteria. The protein is a trimer of identical subunits, each composed of eight complex transmembrane helices. Each subunit has two α-helical hairpin loops that partially cross the bilayer and bind a glutamate. Each glutamate transported across the membrane is accompanied by three Na⁺ and one H⁺ moving in the same direction and one k⁺ moving in the opposite direction. Transport may involve transient fluctuations that open and close a gate near the bound glutamate, but the details are not known.

Structure of MFS Carrier Proteins

Crystal structures of two MFS carrier proteins from Escherichia coli (Fig. 9-3A-B) confirmed much of what had been learned about their organization from less direct methods. GlpT is a glycerol-3-phosphate–phosphate antiporter. LacY, the lactose permease, is a lactose-proton symporter. Both proteins consist of 12 transmembrane α-helices. The sequences and structures of the two halves of each protein are homologous, so it is believed that the original gene was created by duplication of an ancestral gene, which coded for a six-helix protein that formed functional dimers. As the MFS gene family grew during evolution, the ancient gene duplication and fusion process had two advantages. First, it allowed the two halves of each gene to diversify separately to increase specificity for a wide variety of substrates. Second, a single polypeptide simplifies assembly of a functional carrier, as two half-sized subunits do not have to find each other. If the two halves of a 12-helix MFS carrier are expressed in the same cell, they can assemble functional carriers, but less efficiently than the intact protein.

Figure 9-3 structures and transport reactions of two mfs carrier proteins from e. coli. A, LacY, a proton-lactose symporter. The red space-filling model is bound lactose. (PDB file: 1PV7.)B, GlpT, a glycerol-3-phosphate (G₃P)–phosphate antiporter. Bot-tom, Transport reactions carried out by postulated reorientation of transmembrane helices. Only 6 of the 12 transmembrane helices of each carrier are shown.

(References: Abramson J, Smirnova I, Kasho V, et al: Structure and mechanism of the lactose permease of E. coli. Science 301:610–615, 2003; Huang Y, Lemieux MJ, Song J, et al: Structure and mechanism of the glycerol-3-phosphate transporter from E. coli. Science 301:616–620, 2003.)

Both carriers bind substrates in the center of a cluster of transmembrane helices. LacY achieves specificity for lactose by providing geometrically favorable hydrogen bonds and hydrophobic interactions for the substrate. GlpT has a pair of conserved arginines near the middle of the bilayer that are required to bind phosphate.

MFS carriers are believed to alternate between two conformations: one with the substrate-binding site(s) open to the cytoplasmic side of the membrane, as in the crystals, and another with the substrate-binding site(s) exposed on the opposite side of the membrane. The architecture of both proteins is compatible with the proposed conformational change, but structures of the proteins in the alternate conformation must be understood before the mechanism can be established. When open to the periplasmic side of the membrane, GlpT binds glycerol-3-phosphate preferentially, since its affinity is higher than that of phosphate. Bound substrates may facilitate interconversion of the two conformations. When exposed to the cytoplasm, glycerol-3-phosphate dissociates and is replaced by phosphate, which is present at a higher concentration in the cytoplasm. Transport of lactose by LacY is coupled to transport of a proton. A glutamic acid is a likely candidate for proton binding, but it is not yet clear why the conformation of the protein that is open on the periplasmic side of the membrane favors binding of lactose plus a proton.

Carrier Physiology and Mechanisms

Investigators have identified about 500 different reactions that are attributable to secondary transporters and have characterized about a dozen carriers well enough to understand their mechanisms. These model systems (Table 9-1) provide a framework to divide carriers into three broad classes (Fig. 9-4) based on mechanism:

• Uniporters transport a single substrate that moves alone down its concentration gradient. This reaction is also called facilitated diffusion—facilitated in the sense that the carrier provides a low-resistance pathway across a poorly permeable lipid bilayer. GLUT carriers for glucose are an example of a uniporter found in mammalian tissues.

• Antiporters exchange substrates in opposite directions across a membrane. The driving ion moves in one direction, the driven substance in the other. The mitochondrial ANC exchanger for ADP and ATP is an antiporter (Fig. 9-2A).

• Symporters allow two or more substrates to move together in the same direction across a membrane. The driving ion and the driven substance move together across the membrane. This is also known as cotransport. Examples are the E. coli LacY protein (Fig. 9-3A) and the mammalian Na⁺−coupled glucose transporters.

Table 9-1 EXAMPLES OF CARRIER PROTEINS

Figure 9-4 carrier hypothesis with kinetic intermediates of three classes of carrier. C_o has the substrate-binding site oriented toward the outside of the cell. C_i has the substrate-binding site oriented inside. S, S₁, S₂, and Na are substrates. The arrows indicate transitions among the intermediates. Transport occurs when a substrate binds on one side of the membrane and is released on the opposite side after the carrier changes conformation.

Figure 9-2 structures of membrane carrier proteins. Ribbon diagrams illustrate their diversity. A, Bovine mitochondrial ADP/ATP transporter. (PDB file: 1OKC.)B, E. coli Lac Y. (PDB file: 1PV7.) C, E. coli AcrB multidrug transporter. The three identical subunits are shown in different colors. (PDB file: 1OY8.) D, Pyrococcus horikoshii glutamate transporter. The three identical subunits are shown in different colors. (PDB file: 1XFH.) E, Views of the exterior of the cell or mitochondrion with the presumed transport pathway shaded tan.

(References: Pebay-Peyroula E, Dahout-Gonzalez C, Kahn R, et al: Structure of mitochondrial ADP/ATP carrier in complex with carboxyatrachtyloside. Nature 426:39–44, 2003; Abramson J, Smirnova I, Kasho V, et al: Structure and mechanism of the lactose permease of E. coli. Science 301:610–615, 2003; Muakami S, Nakashima R, Yamashita E, Yamaguchi A: Crystal structure of bacterial multidrug efflux transporter AcrB. Nature 419:587–593, 2002; Yernool D, Boudker O, Jin Y, Gouaux E: Structure of a glutamate transporter homologue from Pyrococcus horikoshii. Nature 431:811–818, 2004.)

Dividing carriers into these three classes should not obscure the important point that these proteins are remarkably similar. In fact, relatively simple mutations can convert a carrier from one class to another.

A few carriers are more complicated than is indicated by this classification. For example, neurotransmitter carriers catalyze both antiporter and symporter reactions, with Na⁺ and Cl⁻ going in one direction and a neurotransmitter in the opposite direction. This example also makes the important general point that the stoichiometry of antiporter and symporter reactions need not be one-to-one. Table 9-1 provides other examples.

All three classes of MFS carriers use similar mechanisms to transfer bound substrates across membranes. This follows naturally from having a common evolutionary ancestor and similar architectures. They work like enzymes, binding substrates on one side of membranes, undergoing a conformational change that reorients this binding site, and releasing substrate on the opposite side of the membrane. Substrate concentrations on the two sides determine the direction of net transfer across the membrane. Whether a carrier is a uniporter, antiporter, or symporter depends simply on the number of substrate-binding sites and the rate and equilibrium constants for the various species to reorient across the membrane. The actual rate of transfer depends on the concentrations of substrates. A limited number of specific inhibitors (Table 9-2) have been useful in establishing physiological functions of carriers.

Table 9-2 TOOLS FOR STUDYING CARRIERS

Agent	Target
Furosemide^*	Na⁺/K⁺/2 Cl⁻ symporter
Amiloride^*	Na⁺/H⁺ antiporter
SITZ, DITZ	HCO₃⁻ /Cl⁻ antiporter
Cytochalasin B	GLUT isoforms
Phloretin	GLUT isoforms
Phlorizin	SGLT isoforms

* Used clinically as a drug.

Uniporters

The prefix “uni-” indicates that a single substrate moves across the membrane along its own electrochemical gradient. Nonelectrolytes, such as glucose, use uniporters simply to move across membranes down a chemical gradient. Movement of charged substrates is influenced by the membrane potential and by pH gradients in the case of weak acids or bases.

Classic experiments with GLUT1 in the plasma membrane of red blood cells led to the carrier concept in the 1950s. Human red blood cells are convenient to use because they express high concentrations of GLUT carriers and because blood banks can provide large quantities of cells. The time course of radioactive glucose accumulation (Fig. 9-5A) shows that transport is stereospecific for δ−glucose and that net transport stops when the concentrations of δ−glucose are equal inside and out. Slow equilibration of l-glucose across the membrane probably represents passive diffusion across the lipid bilayer, as this rate can be predicted from the solubility of glucose in membrane lipids. This experiment showed that something in the membrane causes an acceleration in the rate of glucose entry, giving rise to the concept of facilitated diffusion.

Figure 9-5 experiments on transport of radioactive glucose into red blood cells establishing the existence of membrane carriers. A, Time course of the uptake of δ− and l-glucose.B, Rate of uptake of δ− and l-glucose as a function of extracellular concentration. Uptake of l-glucose is by diffusion across the lipid bilayer. C, Rate of uptake of δ−glucose corrected for diffusion as a function of extracellular concentration. The curve is similar to the dependence of an enzyme on substrate concentration, yielding the maximum rate (V_max) at high substrate concentration and the apparent affinity of the carrier (K_m) for the substrate at half the maximal rate.

The dependence of the initial rate of δ−glucose entry on its concentration (Fig. 9-5B) provided evidence for a specific, saturable, carrier molecule in the membrane. Because the rate of δ−glucose entry includes both diffusion across the bilayer and movement through a carrier, the l-glucose rate must be used to correct for the rate of diffusion. Once this has been done, the rate of facilitated δ−glucose entry has a hyperbolic dependence on the concentration of δ−glucose (Fig. 9-5C). This concentration dependence is just like a bimolecular binding reaction (see Fig. 4-2) or the rate of a simple enzyme mechanism that depends on the rate of substrate binding to the enzyme. Thus, the substrate concentration at the half-maximal velocity provides an estimate of the affinity of the carrier for the substrate. At high substrate concentrations, the substrate-binding sites on the carrier are saturated, and the rate plateaus at a maximal velocity owing to rate-limiting conformational changes. These enzyme-like properties, along with the ability to stop facilitated transport with protein inhibitors, suggest that carriers are proteins with specific binding sites for their substrates.

The carrier hypothesis is now understood in terms of carrier proteins embedded in the lipid bilayer; these proteins bind substrate and undergo readily reversible first-order transitions between at least two different conformations (Fig. 9-3). One conformation exposes a substrate-binding site on one side of the membrane. Another conformation exposes a binding site on the other side. Thus, a single substrate molecule can bind on one side of the membrane and be released on the other, thus moving across the membrane. (The carrier does not physically diffuse across the membrane, as was formerly believed.)

Net transport requires a concentration gradient, as the conformational change that reorients substrate-binding sites is reversible, and substrate can move either way. The rates of substrate movement depend on the rates of formation of substrate-carrier complex on the two sides of the membrane. The rates of these second-order reactions depend directly on the substrate concentrations, so the binding sites of the carrier are more fully occupied on the uphill side, and the net movement of substrate is therefore in the downhill direction. When substrate concentrations are the same on the two sides, exchange continues without net movement because the carrier is equally saturated on both sides of the membrane.

This simple carrier model clarified a large body of confusing information and clearly distinguished car-riers from channels for the first time. The original car-rier model for the GLUT1 uniporter also led directly to kinetic schemes for antiporters and symporters (Fig. 9-4).

Carrier mechanisms involve a series of reversible reac-tions, including rate-limiting conformational changes in the carrier that move substrates across the membrane. Carriers generally translocate substrates at rates of 10⁻¹ to 10³ s⁻¹, similar to the rates of enzyme reac-tions, whereas channels transfer ions at rates of 10⁶ to 10⁹ s⁻¹ during the brief times that they open (see Chap-ter 10).

See Figure 11-2 for a depiction of epithelial cells using glucose symporters and uniporters to take up glucose from the intestine after a meal. See Figure 28-6 for an illustration of brown fat cells using uncoupling protein, a proton uniporter in mitochondria, to generate heat when animals arise from hibernation and mammalian infants are born.

Antiporters

Antiporters translocate two different substrates and use a concentration gradient of one substrate to drive another substrate up its concentration gradient. Like uniporters, these carriers undergo reversible, conformational changes that expose substrate-binding sites on one or the other side of a membrane. Two modifications of the uniporter mechanism provide a model for antiporters (Fig. 9-4). First, two substrates, S₁ and S₂, compete for binding antiporters. Second, a substrate-free carrier cannot undergo the conformational changes required to change the orientation of its binding sites. These differences make transport of two substrates dependent on each other in an obligate fashion.

For example, the heart 3Na⁺/Ca²⁺ exchanger binds either Na⁺ or Ca²⁺ and uses the large Na⁺ gradient across the plasma membrane to drive the transport of Ca²⁺ out of the cytoplasm up a concentration gradient (see Fig. 11-13). On the outer surface of the cell, the carrier binds three Na⁺. After the conformational change that reorients the binding site, the three Na⁺ dissociate inside and one Ca²⁺ binds. Reorientation of the binding site carries this Ca²⁺ to the outer surface of the cell, where it dissociates. (In addition to the substrate concentrations, the membrane potential must also be taken into account, as this exchange is not electrically neutral, and the potential may affect the binding of one or both substrates to the carrier.)

Antiporters generally exchange like substrates: cations for cations, anions for anions, sugars for sugars, and so on. The Na⁺/H⁺ antiporter of kidney, gut, and most other cells allows cells to manipulate their internal pH. Band 3 antiporter of red blood cells exchanges Cl⁻ for HCO₃⁻. Carbon dioxide produced in tissues by oxidative reactions diffuses into red blood cells, where a cytoplasmic enzyme—carbonic anhydrase—transforms carbon dioxide into HCO₃⁻. The antiporter provides a way for the HCO₃⁻ to return to the plasma, where it is carried to the lungs as the bicarbonate anion. UhpT antiporter (uptake of hexose phosphate transporter) allows E. coli to scavenge glucose 6-phosphate from the medium in exchange for inorganic phosphate. Antiporters in mitochondria, consisting of dimers (each with six helices like the presumed ancestor of MFS carriers), exchange cytoplasmic ADP for ATP synthesized by these organelles.

Symporters

The prefix “sym-” indicates that two substrates are transported in the same direction. A simple extension of the uniporter mechanism provides a model for symporters (Fig. 9-4). Like uniporters, the carrier has two conformations with substrate-binding sites open to either side of the membrane. The binding site may be free or occupied by a single substrate, like a uniporter, but in addition, two substrates can bind together. A second difference is that transmembrane reorientation of substrate-binding sites is much more favorable for free carrier and carrier with two bound substrates than for carrier with only one bound substrate. This feature of symporters minimizes leaks of one substrate across the membrane.

E. coli LacY symporter uses a proton gradient across the plasma membrane to drive accumulation of lactose (Fig. 9-3A). The proton gradient is created by the respiratory chain under aerobic conditions or by the F-type ATPase pump under anaerobic conditions (see Fig. 8-5). Protons move down their concentration gradient as lactose moves up its concentration gradient into the cell. Mutations in LacY can cause internal leaks that uncouple sugar transport from proton movements. Vertebrate SGLT1 symporter carries out a comparable reaction for intestinal epithelial cells using the Na⁺ gradient to take up glucose from the lumen of the gut (see Fig. 11-2).

Two key experiments (Fig. 9-6) established the symporter concept. The first demonstrated that extracellular Na⁺ was required for intestinal cells with the SGLT1 transporter to accumulate δ−glucose against a concentration gradient. This experiment left open the possibility that Na⁺ simply activates the carrier in some way without being used directly to drive glucose accumulation. Second, an experiment with LacY demonstrated that sugar and cation move across the membrane together. Not only was a proton gradient required for sugar transport but also a high concentration of another sugar (a nonmetabolizable lactose analog) in the medium could drive H⁺ into the cell along with the sugar. Additional experiments confirmed that the stoichiometry of the reaction was one lactose transported in for every H⁺ transported in. (Because this transport reaction is not electrically neutral, the membrane potential is a factor, and another pathway must be available to balance the charge—e.g., by carrying k⁺ in the opposite direction.) These experiments established that both substrates move together across the membrane with a fixed stoichiometry and that a concentration gradient of either can drive the other. Parallel experiments on plasma membrane vesicles isolated from vertebrate kidney cells showed that Na⁺ moving inward down its electrochemical gradient can drag glucose in with it. When the Na⁺ concentration is equal on the two sides, the carrier facilitates the movement of glucose across the membrane but not its net accumulation.