On the mechanism of MgATP-dependent gating of CFTR Cl- channels

Structure and dynamics of NBD1 from CFTR characterized using crystallography and hydrogen/deuterium exchange mass spectrometry

The DeltaF508 mutation in nucleotide-binding domain 1 (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) is the predominant cause of cystic fibrosis. Previous biophysical studies on human F508 and DeltaF508 domains showed only local structural changes restricted to residues 509-511 and only minor differences in folding rate and stability. These results were remarkable because DeltaF508 was widely assumed to perturb domain folding based on the fact that it prevents trafficking of CFTR out of the endoplasmic reticulum. However, the previously reported crystal structures did not come from matched F508 and DeltaF508 constructs, and the DeltaF508 structure contained additional mutations that were required to obtain sufficient protein solubility. In this article, we present additional biophysical studies of NBD1 designed to address these ambiguities. Mass spectral measurements of backbone amide (1)H/(2)H exchange rates in matched F508 and DeltaF508 constructs reveal that DeltaF508 increases backbone dynamics at residues 509-511 and the adjacent protein segments but not elsewhere in NBD1. These measurements also confirm a high level of flexibility in the protein segments exhibiting variable conformations in the crystal structures. We additionally present crystal structures of a broader set of human NBD1 constructs, including one harboring the native F508 residue and others harboring the DeltaF508 mutation in the presence of fewer and different solubilizing mutations. The only consistent conformational difference is observed at residues 509-511. The side chain of residue V510 in this loop is mostly buried in all non-DeltaF508 structures but completely solvent exposed in all DeltaF508 structures. These results reinforce the importance of the perturbation DeltaF508 causes in the surface topography of NBD1 in a region likely to mediate contact with the transmembrane domains of CFTR. However, they also suggest that increased exposure of the 509-511 loop and increased dynamics in its vicinity could promote aggregation in vitro and aberrant intermolecular interactions that impede trafficking in vivo.

Relationship between nucleotide binding and ion channel gating in cystic fibrosis transmembrane conductance regulator

We have employed rate-equilibrium free energy relationship (REFER) analysis to characterize the dynamic events involved in the allosteric regulation of cystic fibrosis transmembrane conductance regulator (CFTR) function. A wide range of different hydrolysable and poorly hydrolysable nucleoside triphosphates were used to elucidate the role of ATP hydrolysis in CFTR function. The linearity of the REFER plots and Phi values near unity for all ligands tested implies that CFTR channel gating is a reversible thermally driven process with all structural reorganization in the binding site(s) completed prior to channel opening. This is consistent with the requirement for nucleotide binding for channel opening. However, the channel structural transition from the open to the closed state occurs independently of any events in the binding sites. Similar results were obtained on substitution of amino acids at coupling joints between both nucleotide binding domains (NBD) and cytoplasmic loops (CL) in opposite halves of the protein, indicating that any structural reorganization there also had occurred in the channel closed state. The fact that fractional Phi values were not observed in either of these distant sites suggests that there may not be a deterministic 'lever-arm' mechanism acting between nucleotide binding sites and the channel gate. These findings favour a stochastic coupling between binding and gating in which all structural transitions are thermally driven processes. We speculate that increase of channel open state probability is due to reduction of the number of the closed state configurations available after physical interaction between ligand bound NBDs and the channel.

Gating of the CFTR Cl- channel by ATP-driven nucleotide-binding domain dimerisation

The cystic fibrosis transmembrane conductance regulator (CFTR) plays a fundamental role in fluid and electrolyte transport across epithelial tissues. Based on its structure, function and regulation, CFTR is an ATP-binding cassette (ABC) transporter. These transporters are assembled from two membrane-spanning domains (MSDs) and two nucleotide-binding domains (NBDs). In the vast majority of ABC transporters, the NBDs form a common engine that utilises the energy of ATP hydrolysis to pump a wide spectrum of substrates through diverse transmembrane pathways formed by the MSDs. By contrast, in CFTR the MSDs form a pathway for passive anion flow that is gated by cycles of ATP binding and hydrolysis by the NBDs. Here, we consider how the interaction of ATP with two ATP-binding sites, formed by the NBDs, powers conformational changes in CFTR structure to gate the channel pore. We explore how conserved sequences from both NBDs form ATP-binding sites at the interface of an NBD dimer and highlight the distinct roles that each binding site plays during the gating cycle. Knowledge of how ATP gates the CFTR Cl- channel is critical for understanding CFTR's physiological role, its malfunction in disease and the mechanism of action of small molecules that modulate CFTR channel gating.

Review. ATP hydrolysis-driven gating in cystic fibrosis transmembrane conductance regulator

Proteins belonging to the ATP-binding cassette superfamily couple ATP binding and hydrolysis at conserved nucleotide-binding domains (NBDs) to diverse cellular functions. Most superfamily members are transporters, while cystic fibrosis transmembrane conductance regulator (CFTR), alone, is an ion channel. Despite this functional difference, recent results have suggested that CFTR shares a common molecular mechanism with other members. ATP binds to partial binding sites on the surface of the two NBDs, which then associate to form a NBD dimer, with complete composite catalytic sites now buried at the interface. ATP hydrolysis and gamma-phosphate dissociation, with the loss of molecular contacts linking the two sides of the composite site, trigger dimer dissociation. The conformational signals generated by NBD dimer formation and dissociation are transmitted to the transmembrane domains where, in transporters, they drive the cycle of conformational changes that translocate the substrate across the membrane; in CFTR, they result in opening and closing (gating) of the ion-permeation pathway.

The mechanism of ABC transporters: general lessons from structural and functional studies of an antigenic peptide transporter

The shuttling of substrates across a cellular membrane frequently requires a specialized ATP-binding cassette (ABC) transporter, which couples the energy of ATP binding and hydrolysis to substrate transport. Due to its importance in immunity, the ABC transporter associated with antigen processing (TAP) has been studied extensively and is an excellent model for other ABC transporters. The TAP protein pumps cytosolic peptides into the endoplasmic reticulum for loading onto class I major histocompatibility complex (MHC) for subsequent immune surveillance. Here, we outline a potential mechanism for the TAP protein with supporting evidence from bacterial transporter structures. The similarities and differences between TAP and other transporters support the notion that ABC transporters in general have adapted around a universal transport mechanism.

A mutation in CFTR modifies the effects of the adenylate kinase inhibitor Ap5A on channel gating

Mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. The CFTR anion channel is controlled by ATP binding and enzymatic activity at the two nucleotide-binding domains. CFTR exhibits two types of enzymatic activity: 1), ATPase activity in the presence of ATP and 2), adenylate kinase activity in the presence of ATP plus physiologic concentrations of AMP or ADP. Previous work showed that P(1),P(5)-di(adenosine-5')pentaphosphate (Ap(5)A), a specific adenylate kinases inhibitor, inhibited wild-type CFTR. In this study, we report that Ap(5)A increased activity of CFTR with an L1254A mutation. This mutation increased the EC50 for ATP by >10-fold and reduced channel activity by prolonging the closed state. Ap(5)A did not elicit current on its own nor did it alter ATP EC50 or maximal current. However, it changed the relationship between ATP concentration and current. At submaximal ATP concentrations, Ap(5)A stimulated current by stabilizing the channel open state. Whereas previous work indicated that adenylate kinase activity regulated channel opening, our data suggest that Ap(5)A binding may also influence channel closing. These results also suggest that a better understanding of the adenylate kinase activity of CFTR may be of value in developing new therapeutic strategies for cystic fibrosis.

CFTR function and prospects for therapy

Mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) epithelial anion channel cause cystic fibrosis (CF). The multidomain integral membrane glycoprotein, a member of the adenine nucleotide-binding cassette (ABC) transporter family, conserved in metazoan salt-transporting tissues, is required to control ion and fluid homeostasis on epithelial surfaces. This review considers different therapeutic strategies that have arisen from knowledge of CFTR structure and function as well as its biosynthetic processing, intracellular trafficking, and turnover.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

CLC-0 and CFTR: Chloride Channels Evolved From Transporters

CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl−channels play important roles in Cl−transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl−channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.

Prolonged treatment of cells with genistein modulates the expression and function of the cystic fibrosis transmembrane conductance regulator

BACKGROUND AND PURPOSE

Cystic fibrosis (CF) is caused by dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. In the search for new CF therapies, small molecules have been identified that rescue the defective channel gating of CF mutants (termed CFTR potentiators). Here, we investigate the long-term effects of genistein, the best-studied CFTR potentiator, on the expression and function of CFTR.

EXPERIMENTAL APPROACH

We pre-treated baby hamster kidney (BHK) cells expressing wild-type or F508del-CFTR (the most common CF mutant) with concentrations of genistein that potentiate (30 microM) or inhibit (100 microM) CFTR function for 2 or 24 h at 37 degrees C before examining CFTR maturation, expression and single-channel activity.

KEY RESULTS

Using the iodide efflux technique, we found that genistein pre-treatment failed to restore function to F508del-CFTR, but altered that of wild-type CFTR. Pre-treatment of cells with genistein for 2 h had little effect on CFTR processing, whereas pre-treatment for 24 h either augmented (30 microM genistein) or impaired (100 microM genistein) CFTR maturation. Using immunocytochemistry, we found that all genistein pre-treatments increased the localization of CFTR protein to the cell surface. However, following the incubation of cells with genistein (100 microM) for 2 h, individual CFTR Cl(-) channels exhibited characteristics of channel block upon channel activation.

CONCLUSIONS AND IMPLICATIONS

Genistein pre-treatment alters the maturation, cell surface expression and single-channel function of CFTR in ways distinct from its acute effects. Thus, CFTR potentiators have the potential to influence CFTR by mechanisms distinct from their effects on channel gating.

Conformational changes in a pore-lining helix coupled to cystic fibrosis transmembrane conductance regulator channel gating

Cystic fibrosis transmembrane conductance regulator (CFTR), the protein dysfunctional in cystic fibrosis, is unique among ATP-binding cassette transporters in that it functions as an ion channel. In CFTR, ATP binding opens the channel, and its subsequent hydrolysis causes channel closure. We studied the conformational changes in the pore-lining sixth transmembrane segment upon ATP binding by measuring state-dependent changes in accessibility of substituted cysteines to methanethiosulfonate reagents. Modification rates of three residues (resides 331, 333, and 335) near the extracellular side were 10-1000-fold slower in the open state than in the closed state. Introduction of a charged residue by chemical modification at two of these positions (resides 331 and 333) affected CFTR single-channel gating. In contrast, modifications of pore-lining residues 334 and 338 were not state-dependent. Our results suggest that ATP binding induces a modest conformational change in the sixth transmembrane segment, and this conformational change is coupled to the gating mechanism that regulates ion conduction. These results may establish a structural basis of gating involving the dynamic rearrangement of transmembrane domains necessary for vectorial transport of substrates in ATP-binding cassette transporters.

The hydroxyl group of S685 in Walker A motif and the carboxyl group of D792 in Walker B motif of NBD1 play a crucial role for multidrug resistance protein folding and function

Structural analysis of MRP1-NBD1 revealed that the Walker A S685 forms hydrogen-bond with the Walker B D792 and interacts with magnesium and the beta-phosphate of the bound ATP. We have found that substitution of the D792 with leucine resulted in misfolding of the protein. In this report we tested whether substitution of the S685 with residues that prevent formation of this hydrogen-bond would also cause misfolding. Indeed, substitution of the S685 with residues potentially preventing formation of this hydrogen-bond resulted in misfolding of the protein. In addition, some substitutions that might form hydrogen-bond with D792 also yielded immature protein. All these mutants are temperature-sensitive variants. However, these complex-glycosylated mature mutants prepared from the cells grown at 27 degrees C still significantly affect ATP binding and ATP-dependent solute transport. In contrast, substitution of the S685 with threonine yielded complex-glycosylated mature protein that is more active than the wild-type MRP1, indicating that the interaction between the hydroxyl group of 685 residue and the carboxyl group of D792 plays a crucial role for the protein folding and the interactions of the hydroxyl group at 685 with magnesium and the beta-phosphate of the bound ATP play an important role for ATP-binding and ATP-dependent solute transport.

Characterization of a 7,8-benzoflavone double effect on CFTR Cl(-) channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of adenosine triphosphate (ATP)-binding cassette (ABC) transporter ATPases. This protein forms a Cl(-) channel with a complex regulation; gene mutations cause cystic fibrosis disease. We investigated the interaction between the protein and the flavone UCCF-029 using the patch-clamp technique in the excised inside-out configuration in order to study the molecular mechanism of action for this potentiator on completely phosphorylated channel (25 U/ml protein kinase A) and a relatively low level of ATP (0.3 mM: ). Low concentrations of UCCF-029 (<50 nM: ) increase the open probability (p (o)), favoring the channel transition to an activated state, while high UCCF-029 (>50 nM: ) levels determine inhibition of the CFTR by a reduction of the total open time. Our data suggest that this drug can potentiate CFTR by binding to a specific site on the nucleotide binding domain, promoting dimer formation. The response of CFTR to variable concentrations of ATP is not modified by application of the potentiator UCCF-029 at either low, activatory, concentration or high, inhibitory, levels. Hence, we conclude that the potentiator may not interfere with binding of ATP but probably acts at an independent site in the protein, interacting directly with CFTR to modulate channel activity.

Cystic Fibrosis: Lessons from the Sweat Gland

Lessons from the sweat gland on cystic fibrosis (CF) began long before modern medicine became a science. In European folklore, the curse that “a child that taste salty when kissed will soon die” (Alonso y de los Ruyzes de Fonteca J. Diez Previlegios para Mugeres Prenadas. Henares, Spain, 1606) has been taken by many as a direct reference to cystic fibrosis [Busch R. Acta Univ Carol Med (Praha) 36: 13–15, 1990]. The high salt concentration in sweat from patients with CF is now accepted as almost pathognomonic with this fatal genetic disease, but the earliest descriptions of cystic fibrosis as a disease entity did not mention sweat or sweat glands (Andersen DH. Am J Dis Child 56: 344–399, 1938; Andersen DH, Hodges RG. Am J Dis Child 72: 62–80, 1946). Nonetheless, defective sweating soon became an inseparable, and major, component of the constellation of symptoms that diagnose “cystic fibrosis” (Davis PB. Am J Respir Crit Care Med 173: 475–482, 2006). The sweat gland has played a foremost role in diagnosing, defining pathophysiology, debunking misconceptions, and increasing our understanding of the effects of the disease on organs, tissues, cells, and molecules. The sweat gland has taught us much.

Restoration of W1282X CFTR activity by enhanced expression

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Premature termination codons represent a common minority of CFTR mutations, and are caused by base pair substitutions that produce abnormal stop codons in the coding sequence. Select aminoglycosides induce "translational readthrough" of premature stop codons and have been shown to restore full-length functional protein in a number of preclinical and clinical settings. We studied two well-described premature termination codons found in the distal open reading frame of CFTR, W1282X and R1162X, expressed in polarizing and nonpolarizing cells. Our findings indicate that W1282X CFTR-expressing cells demonstrate significantly greater CFTR activity when overexpressed compared with R1162X CFTR cells, even when truncated protein is the predominant form. In addition, our results show that the combination of stimulated expression and stop codon suppression produces additive effects on CFTR-mediated ion transport. These findings provide evidence that W1282X CFTR exhibits membrane localization and retained chloride channel function after enhanced expression, and suggest that patients harboring this mutation may be more susceptible to CFTR rescue.

G551D and G1349D, two CF-associated mutations in the signature sequences of CFTR, exhibit distinct gating defects

Mutations in the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR) result in cystic fibrosis (CF). CFTR is a chloride channel that is regulated by phosphorylation and gated by ATP binding and hydrolysis at its nucleotide binding domains (NBDs). G551D-CFTR, the third most common CF-associated mutation, has been characterized as having a lower open probability (Po) than wild-type (WT) channels. Patients carrying the G551D mutation present a severe clinical phenotype. On the other hand, G1349D, also a mutant with gating dysfunction, is associated with a milder clinical phenotype. Residues G551 and G1349 are located at equivalent positions in the highly conserved signature sequence of each NBD. The physiological importance of these residues lies in the fact that the signature sequence of one NBD and the Walker A and B motifs from the other NBD form the ATP-binding pocket (ABP1 and ABP2, named after the location of the Walker A motif) once the two NBDs dimerize. Our studies show distinct gating characteristics for these mutants. The G551D mutation completely eliminates the ability of ATP to increase the channel activity, and the observed activity is approximately 100-fold smaller than WT-CFTR. G551D-CFTR does not respond to ADP, AMP-PNP, or changes in [Mg(2+)]. The low activity of G551D-CFTR likely represents the rare ATP-independent gating events seen with WT channels long after the removal of ATP. G1349D-CFTR maintains ATP dependence, albeit with a Po approximately 10-fold lower than WT. Interestingly, compared to WT results, the ATP dose-response relationship of G1349D-CFTR is less steep and shows a higher apparent affinity for ATP. G1349D data could be well described by a gating model that predicts that binding of ATP at ABP1 hinders channel opening. Thus, our data provide a quantitative explanation at the single-channel level for different phenotypes presented by patients carrying these two mutations. In addition, these results support the idea that CFTR's two ABPs play distinct functional roles in gating.

Functional Analysis of Mutations in the Putative Binding Site for Cystic Fibrosis Transmembrane Conductance Regulator Potentiators

An increasing number of compounds able to potentiate the activity of mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have been identified by high throughput screening or by individual search of derivatives of known active compounds. Several lines of evidence suggest that most CFTR potentiators act through the same mechanism, probably by binding to the nucleotide binding domains to promote the activity of the protein and then, with lower affinity, to an inhibitory site. With the aim of identifying the activating binding site, we recently modeled the nucleotide binding domain dimer and predicted a common binding site for potentiators in its interface. To validate this model experimentally, we mutated some of the residues involved in the putative binding site, i.e. Arg(553), Ala(554), and Val(1293). The activity of CFTR potentiators was measured as apical membrane currents on polarized cells stably expressing wild type or mutated proteins. CFTR activity was elicited by application of a membrane-permeable cAMP analogue followed by increasing concentrations of potentiators. We found that all three mutants responded to cAMP, although the affinity of R553Q was higher than that of wild type CFTR. In R553Q and V1293G mutants, the dissociation constant of potentiators for the activating site was increased, whereas the dissociation constant for the inhibitory site was reduced. Our results show that the mutated residues are part of the activating binding site for potentiators, as suggested by the molecular model. In addition, these results suggest that the activating and inhibitory sites are not independent of each other.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

Thermodynamics of CFTR channel gating: a spreading conformational change initiates an irreversible gating cycle

CFTR is the only ABC (ATP-binding cassette) ATPase known to be an ion channel. Studies of CFTR channel function, feasible with single-molecule resolution, therefore provide a unique glimpse of ABC transporter mechanism. CFTR channel opening and closing (after regulatory-domain phosphorylation) follows an irreversible cycle, driven by ATP binding/hydrolysis at the nucleotide-binding domains (NBD1, NBD2). Recent work suggests that formation of an NBD1/NBD2 dimer drives channel opening, and disruption of the dimer after ATP hydrolysis drives closure, but how NBD events are translated into gate movements is unclear. To elucidate conformational properties of channels on their way to opening or closing, we performed non-equilibrium thermodynamic analysis. Human CFTR channel currents were recorded at temperatures from 15 to 35°C in inside-out patches excised from Xenopus oocytes. Activation enthalpies(ΔH^‡) were determined from Eyring plots. ΔH^‡ was 117 ± 6 and 69 ± 4 kJ/mol, respectively, for opening and closure of partially phosphorylated, and 96 ± 6 and 73 ± 5 kJ/mol for opening and closure of highly phosphorylated wild-type (WT) channels. ΔH^‡ for reversal of the channel opening step, estimated from closure of ATP hydrolysis–deficient NBD2 mutant K1250R and K1250A channels, and from unlocking of WT channels locked open with ATP+AMPPNP, was 43 ± 2, 39 ± 4, and 37 ± 6 kJ/mol, respectively. Calculated upper estimates of activation free energies yielded minimum estimates of activation entropies (ΔS^‡), allowing reconstruction of the thermodynamic profile of gating, which was qualitatively similar for partially and highly phosphorylated CFTR. ΔS^‡ appears large for opening but small for normal closure. The large ΔH^‡ and ΔS^‡ (TΔS^‡ ≥ 41 kJ/mol) for opening suggest that the transition state is a strained channel molecule in which the NBDs have already dimerized, while the pore is still closed. The small ΔS^‡ for normal closure is appropriate for cleavage of a single bond (ATP's beta-gamma phosphate bond), and suggests that this transition state does not require large-scale protein motion and hence precedes rehydration (disruption) of the dimer interface.

In vivo phosphorylation of CFTR promotes formation of a nucleotide-binding domain heterodimer

The human ATP-binding cassette (ABC) protein CFTR (cystic fibrosis transmembrane conductance regulator) is a chloride channel, whose dysfunction causes cystic fibrosis. To gain structural insight into the dynamic interaction between CFTR's nucleotide-binding domains (NBDs) proposed to underlie channel gating, we introduced target cysteines into the NBDs, expressed the channels in Xenopus oocytes, and used in vivo sulfhydryl-specific crosslinking to directly examine the cysteines' proximity. We tested five cysteine pairs, each comprising one introduced cysteine in the NH(2)-terminal NBD1 and another in the COOH-terminal NBD2. Identification of crosslinked product was facilitated by co-expression of NH(2)-terminal and COOH-terminal CFTR half channels each containing one NBD. The COOH-terminal half channel lacked all native cysteines. None of CFTR's 18 native cysteines was found essential for wild type-like, phosphorylation- and ATP-dependent, channel gating. The observed crosslinks demonstrate that NBD1 and NBD2 interact in a head-to-tail configuration analogous to that in homodimeric crystal structures of nucleotide-bound prokaryotic NBDs. CFTR phosphorylation by PKA strongly promoted both crosslinking and opening of the split channels, firmly linking head-to-tail NBD1-NBD2 association to channel opening.

CFTR (ABCC7) is a hydrolyzable-ligand-gated channel

As the product of the gene mutated in cystic fibrosis, the most common genetic disease of Caucasians, CFTR is an atypical ABC protein. From an evolutionary perspective, it is apparently a relatively young member of the ABC family, present only in metazoans where it plays a critical role in epithelial salt and fluid homeostasis. Functionally, the membrane translocation process it mediates, the passive bidirectional diffusion of small inorganic anions, is simpler than the vectorial transport of larger more complex substrates ("allocrites") by most ABC transporters. However, the control of the permeation pathway which cannot go unchecked is necessarily more stringent than in the case of the transporters. There is tight regulation by the phosphorylation/dephosphorylation of the unique CFTR R domain superimposed on the basic ABC regulation mode of ATP binding and hydrolysis at the dual nucleotide binding sites. As with other ABCC subfamily members, only the second of these sites is hydrolytic in CFTR. The phosphorylation and ATP binding/hydrolysis events do not strongly influence each other; rather, R domain phosphorylation appears to enable transduction of the nucleotide binding allosteric signal to the responding channel gate. ATP hydrolysis is not required for either the opening or closing gating transitions but efficiently clears the ligand-binding site enabling a new gating cycle to be initiated.

The two ATP binding sites of cystic fibrosis transmembrane conductance regulator (CFTR) play distinct roles in gating kinetics and energetics

Cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ABC (ATP binding cassette) transporter family, is a chloride channel whose activity is controlled by protein kinase–dependent phosphorylation. Opening and closing (gating) of the phosphorylated CFTR is coupled to ATP binding and hydrolysis at CFTR's two nucleotide binding domains (NBD1 and NBD2). Recent studies present evidence that the open channel conformation reflects a head-to-tail dimerization of CFTR's two NBDs as seen in the NBDs of other ABC transporters (Vergani et al., 2005). Whether these two ATP binding sites play an equivalent role in the dynamics of NBD dimerization, and thus in gating CFTR channels, remains unsettled. Based on the crystal structures of NBDs, sequence alignment, and homology modeling, we have identified two critical aromatic amino acids (W401 in NBD1 and Y1219 in NBD2) that coordinate the adenine ring of the bound ATP. Conversion of the W401 residue to glycine (W401G) has little effect on the sensitivity of the opening rate to [ATP], but the same mutation at the Y1219 residue dramatically lowers the apparent affinity for ATP by >50-fold, suggesting distinct roles of these two ATP binding sites in channel opening. The W401G mutation, however, shortens the open time constant. Energetic analysis of our data suggests that the free energy of ATP binding at NBD1, but not at NBD2, contributes significantly to the energetics of the open state. This kinetic and energetic asymmetry of CFTR's two NBDs suggests an asymmetric motion of the NBDs during channel gating. Opening of the channel is initiated by ATP binding at the NBD2 site, whereas separation of the NBD dimer at the NBD1 site constitutes the rate-limiting step in channel closing.

Structure and function of ABC transporters: the ATP switch provides flexible control

ATP-binding cassette (ABC) transporters are ubiquitous integral membrane proteins that facilitate the transbilayer movement of ligands. They comprise, minimally, two transmembrane domains, which impart ligand specificity, and two nucleotide-binding domains (NBDs), which power the transport cycle. Almost 25 years of biochemistry is reviewed in light of the recent structure analyses resulting in the ATP-switch model for function in which the NBDs switch between a dimeric conformation, closed around two molecules of ATP, and a nucleotide-free, dimeric 'open' conformation. The flexibility of this switching mechanism has evolved to provide different kinetic control for different transporters and has also been co-opted to diverse functions other than transmembrane transport.

Control of the CFTR channel's gates

Unique among ABC (ATP-binding cassette) protein family members, CFTR (cystic fibrosis transmembrane conductance regulator), also termed ABCC7, encoded by the gene mutated in cystic fibrosis patients, functions as an ion channel. Opening and closing of its anion-selective pore are linked to ATP binding and hydrolysis at CFTR's two NBDs (nucleotide-binding domains), NBD1 and NBD2. Isolated NBDs of prokaryotic ABC proteins form homodimers upon binding ATP, but separate after hydrolysis of the ATP. By combining mutagenesis with single-channel recording and nucleotide photolabelling on intact CFTR molecules, we relate opening and closing of the channel gates to ATP-mediated events in the NBDs. In particular, we demonstrate that two CFTR residues, predicted to lie on opposite sides of its anticipated NBD1-NBD2 heterodimer interface, are energetically coupled when the channels open but are independent of each other in closed channels. This directly links ATP-driven tight dimerization of CFTR's cytoplasmic NBDs to opening of the ion channel in the transmembrane domains. Evolutionary conservation of the energetically coupled residues in a manner that preserves their ability to form a hydrogen bond argues that this molecular mechanism, involving dynamic restructuring of the NBD dimer interface, is shared by all members of the ABC protein superfamily.

The ABC protein turned chloride channel whose failure causes cystic fibrosis

CFTR chloride channels are encoded by the gene mutated in patients with cystic fibrosis. These channels belong to the superfamily of ABC transporter ATPases. ATP-driven conformational changes, which in other ABC proteins fuel uphill substrate transport across cellular membranes, in CFTR open and close a gate to allow transmembrane flow of anions down their electrochemical gradient. New structural and biochemical information from prokaryotic ABC proteins and functional information from CFTR channels has led to a unifying mechanism explaining those ATP-driven conformational changes.

Insight in eukaryotic ABC transporter function by mutation analysis

With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

The block of CFTR by scorpion venom is state-dependent

Cystic fibrosis transmembrane conductance regulator (CFTR) adenosine triphosphate-dependent chloride channels are expressed in epithelial cells and are associated with a number of genetic disorders, including cystic fibrosis. Venom of the scorpion Leirus quinquestriatus hebraeus reversibly inhibits CFTR when applied to its cytoplasmic surface. To examine the state-dependence of inhibition we recorded wild-type and mutant CFTR channel currents using inside-out membrane patches from Xenopus oocytes. Application of either venom or diphenylamine-2-carboxylate to channels that were either activated (open) or resting (closed) indicate primarily closed state-dependent inhibition of CFTR by venom, whereas diphenylamine-2-carboxylate showed no state-dependence of block. Efficacy of venom-mediated macroscopic current inhibition was inversely related to channel activity. Analysis of single-channel and macropatch data indicated that venom could either inhibit channel opening, if it binds during an interburst closed state or in the absence of cytosolic adenosine triphosphate, or introduce new intraburst closed states, if it binds during an open event. The on-rate of venom binding for intraburst block could be modulated by changing CFTR activity with vanadate or adenylyl-imidodiphosphate, or by introducing the Walker A mutation K1250A. These findings represent the first description of state-dependent inhibition of CFTR and suggest that the active toxin could be used as a tool to study the conformational changes that occur during CFTR gating.

Differential sensitivity of the cystic fibrosis (CF)-associated mutants G551D and G1349D to potentiators of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel

The genetic disease cystic fibrosis (CF) is caused by loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. Two CF mutants, G551D and G1349D, affect equivalent residues in the highly conserved LSGGQ motifs that are essential components of the ATP-binding sites of CFTR. Both mutants severely disrupt CFTR channel gating by decreasing mean burst duration (MBD) and prolonging greatly the interburst interval (IBI). To identify small molecules that rescue the gating defects of G551D- and G1349D-CFTR and understand better how these agents work, we used the patch clamp technique to study the effects on G551D- and G1349D-CFTR of phloxine B, pyrophosphate (PP(i)), and 2'-deoxy ATP (2'-dATP), three agents that strongly enhance CFTR channel gating. Phloxine B (5 microm) potentiated robustly G551D-CFTR Cl- channels by altering both MBD and IBI. In contrast, phloxine B (5 microm) decreased the IBI of G1349D-CFTR, but this effect was insufficient to rescue G1349D-CFTR channel gating. PP(i) (5 mm) potentiated weakly G551D-CFTR and was without effect on the G1349D-CFTR Cl- channel. However, by altering both MBD and IBI, albeit with different efficacies, 2'-dATP (1 mm) potentiated both G551D- and G1349D-CFTR Cl- channels. Using the ATP-driven nucleotide-binding domain dimerization model of CFTR channel gating, we suggest that phloxine B, PP(i) and 2'-dATP alter channel gating by distinct mechanisms. We conclude that G551D- and G1349D-CFTR have distinct pharmacological profiles and speculate that drug therapy for CF is likely to be mutation-specific.

Control of the CFTR channel's gates

Unique among ABC (ATP-binding cassette) protein family members, CFTR (cystic fibrosis transmembrane conductance regulator), also termed ABCC7, encoded by the gene mutated in cystic fibrosis patients, functions as an ion channel. Opening and closing of its anion-selective pore are linked to ATP binding and hydrolysis at CFTR's two NBDs (nucleotide-binding domains), NBD1 and NBD2. Isolated NBDs of prokaryotic ABC proteins form homodimers upon binding ATP, but separate after hydrolysis of the ATP. By combining mutagenesis with single-channel recording and nucleotide photolabelling on intact CFTR molecules, we relate opening and closing of the channel gates to ATP-mediated events in the NBDs. In particular, we demonstrate that two CFTR residues, predicted to lie on opposite sides of its anticipated NBD1–NBD2 heterodimer interface, are energetically coupled when the channels open but are independent of each other in closed channels. This directly links ATP-driven tight dimerization of CFTR's cytoplasmic NBDs to opening of the ion channel in the transmembrane domains. Evolutionary conservation of the energetically coupled residues in a manner that preserves their ability to form a hydrogen bond argues that this molecular mechanism, involving dynamic restructuring of the NBD dimer interface, is shared by all members of the ABC protein superfamily.

State-dependent chemical reactivity of an engineered cysteine reveals conformational changes in the outer vestibule of the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels are gated by binding and hydrolysis of ATP at the nucleotide-binding domains (NBDs). We used covalent modification of CFTR channels bearing a cysteine engineered at position 334 to investigate changes in pore conformation that might accompany channel gating. In single R334C-CFTR channels studied in excised patches, modification by [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET+), which increases conductance, occurred only during channel closed states. This suggests that the rate of reaction of the cysteine was greater in closed channels than in open channels. R334C-CFTR channels in outside-out macropatches activated by ATP alone were modified with first order kinetics upon rapid exposure to MTSET+. Modification was much slower when channels were locked open by the addition of nonhydrolyzable nucleotide or when the R334C mutation was coupled to a second mutation, K1250A, which greatly decreases channel closing rate. In contrast, modification was faster in R334C/K464A-CFTR channels, which exhibit prolonged interburst closed states. These data indicate that the reactivity of the engineered cysteine in R334C-CFTR is state-dependent, providing evidence of changes in pore conformation coupled to ATP binding and hydrolysis at the NBDs. The data also show that maneuvers that lock open R334C-CFTR do so by locking channels into the prominent s2 subconductance state, suggesting that the most stable conducting state of the pore reflects the fully occupied, prehydrolytic state of the NBDs.

High affinity ATP/ADP analogues as new tools for studying CFTR gating

Previous studies using non-hydrolysable ATP analogues and hydrolysis-deficient cystic fibrosis transmembrane conductance regulator (CFTR) mutants have indicated that ATP hydrolysis precedes channel closing. Our recent data suggest that ATP binding is also important in modulating the closing rate. This latter hypothesis predicts that ATP analogues with higher binding affinities should stabilize the open state more than ATP. Here we explore the possibility of using N6-modified ATP/ADP analogues as high-affinity ligands for CFTR gating, since these analogues have been shown to be more potent than native ATP/ADP in other ATP-binding proteins. Among the three N6-modified ATP analogues tested, N6-(2-phenylethyl)-ATP (P-ATP) was the most potent, with a K(1/2) of 1.6 +/- 0.4 microm (>50-fold more potent than ATP). The maximal open probability (P(o)) in the presence of P-ATP was approximately 30% higher than that of ATP, indicating that P-ATP also has a higher efficacy than ATP. Single-channel kinetic analysis showed that as [P-ATP] was increased, the opening rate increased, whereas the closing rate decreased. The fact that these two kinetic parameters have different sensitivities to changes of [P-ATP] suggests an involvement of two different ATP-binding sites, a high-affinity site modulating channel closing and a low affinity site controlling channel opening. The effect of P-ATP on the stability of open states was more evident when ATP hydrolysis was abolished, either by mutating the nucleotide-binding domain 2 (NBD2) Walker B glutamate (i.e. E1371) or by using the non-hydrolysable ATP analogue AMP-PNP. Similar strategies to develop nucleotide analogues with a modified adenine ring could be valuable for future studies of CFTR gating.

A quantitative description of the activation and inhibition of CFTR by potentiators: Genistein

The CFTR, encoded by the gene mutated in cystic fibrosis (CF) patients, is responsible for cAMP dependent chloride transport in epithelia. Substances that activate CFTR have been suggested as possible CF therapy. Most substances investigated so far exert a dual effect on the CFTR: low concentrations stimulate CFTR, whereas higher concentrations inhibit CFTR. Besides, the CFTR phosphorylation level determines the apparent affinity of the drug. We have studied the properties of genistein, the well known CFTR potentiator, by measuring apical membrane current on epithelia formed by cells stably transfected with CFTR and stimulated with different concentrations of CPTcAMP. We propose a quantitative model to describe the activatory and inhibitory effect of genistein, accounting also for the cAMP dependent activation.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

CFTR: more than just a chloride channel

This review examines the cystic fibrosis transmembrane conductance regulator (CFTR) protein. After summarizing the ion channels regulated by CFTR, the review focuses on the functions of CFTR that do not relate directly to a disease mechanism based on a channelopathy. The key concept is that newly synthesized CFTR has to enter lipid vesicles which bud from the endoplasmic reticulum. This is abnormally low in DeltaF508 CFTR. Normal wild type vesicular CFTR enters a recycling pool of lipid vesicles which transiently dock with the apical membrane only for CFTR to be retrieved shortly after into a sub-apical recycling compartment. This retrieval is abnormally fast in DeltaF508 CFTR. The review discusses the relationship between this process and the difficult topic of fat metabolism and then explores the possible links between abnormal fatty acid turnover and inflammatory cascades that are abnormal in cystic fibrosis. Finally the review concentrates on the emerging functions of a protein kinase (AMP-activated kinase) which is bound near the C terminus of the CFTR protein whose functions could intergrate some of the abnormalities in lipid metabolism that result from mislocalization of CFTR in clinical disease.

CFTR gating II: Effects of nucleotide binding on the stability of open states

Previously, we demonstrated that ADP inhibits cystic fibrosis transmembrane conductance regulator (CFTR) opening by competing with ATP for a binding site presumably in the COOH-terminal nucleotide binding domain (NBD2). We also found that the open time of the channel is shortened in the presence of ADP. To further study this effect of ADP on the open state, we have used two CFTR mutants (D1370N and E1371S); both have longer open times because of impaired ATP hydrolysis at NBD2. Single-channel kinetic analysis of ΔR/D1370N-CFTR shows unequivocally that the open time of this mutant channel is decreased by ADP. ΔR/E1371S-CFTR channels can be locked open by millimolar ATP with a time constant of ∼100 s, estimated from current relaxation upon nucleotide removal. ADP induces a shorter locked-open state, suggesting that binding of ADP at a second site decreases the locked-open time. To test the functional consequence of the occupancy of this second nucleotide binding site, we changed the [ATP] and performed similar relaxation analysis for E1371S-CFTR channels. Two locked-open time constants can be discerned and the relative distribution of each component is altered by changing [ATP] so that increasing [ATP] shifts the relative distribution to the longer locked-open state. Single-channel kinetic analysis for ΔR/E1371S-CFTR confirms an [ATP]-dependent shift of the distribution of two locked-open time constants. These results support the idea that occupancy of a second ATP binding site stabilizes the locked-open state. This binding site likely resides in the NH₂-terminal nucleotide binding domain (NBD1) because introducing the K464A mutation, which decreases ATP binding affinity at NBD1, into E1371S-CFTR shortens the relaxation time constant. These results suggest that the binding energy of nucleotide at NBD1 contributes to the overall energetics of the open channel conformation.

CFTR gating I: Characterization of the ATP-dependent gating of a phosphorylation-independent CFTR channel (DeltaR-CFTR)

The CFTR chloride channel is activated by phosphorylation of serine residues in the regulatory (R) domain and then gated by ATP binding and hydrolysis at the nucleotide binding domains (NBDs). Studies of the ATP-dependent gating process in excised inside-out patches are very often hampered by channel rundown partly caused by membrane-associated phosphatases. Since the severed ΔR-CFTR, whose R domain is completely removed, can bypass the phosphorylation-dependent regulation, this mutant channel might be a useful tool to explore the gating mechanisms of CFTR. To this end, we investigated the regulation and gating of the ΔR-CFTR expressed in Chinese hamster ovary cells. In the cell-attached mode, basal ΔR-CFTR currents were always obtained in the absence of cAMP agonists. Application of cAMP agonists or PMA, a PKC activator, failed to affect the activity, indicating that the activity of ΔR-CFTR channels is indeed phosphorylation independent. Consistent with this conclusion, in excised inside-out patches, application of the catalytic subunit of PKA did not affect ATP-induced currents. Similarities of ATP-dependent gating between wild type and ΔR-CFTR make this phosphorylation-independent mutant a useful system to explore more extensively the gating mechanisms of CFTR. Using the ΔR-CFTR construct, we studied the inhibitory effect of ADP on CFTR gating. The Ki for ADP increases as the [ATP] is increased, suggesting a competitive mechanism of inhibition. Single channel kinetic analysis reveals a new closed state in the presence of ADP, consistent with a kinetic mechanism by which ADP binds at the same site as ATP for channel opening. Moreover, we found that the open time of the channel is shortened by as much as 54% in the presence of ADP. This unexpected result suggests another ADP binding site that modulates channel closing.

CFTR channel opening by ATP-driven tight dimerization of its nucleotide-binding domains

ABC (ATP-binding cassette) proteins constitute a large family of membrane proteins that actively transport a broad range of substrates. Cystic fibrosis transmembrane conductance regulator (CFTR), the protein dysfunctional in cystic fibrosis, is unique among ABC proteins in that its transmembrane domains comprise an ion channel. Opening and closing of the pore have been linked to ATP binding and hydrolysis at CFTR's two nucleotide-binding domains, NBD1 and NBD2 (see, for example, refs 1, 2). Isolated NBDs of prokaryotic ABC proteins dimerize upon binding ATP, and hydrolysis of the ATP causes dimer dissociation. Here, using single-channel recording methods on intact CFTR molecules, we directly follow opening and closing of the channel gates, and relate these occurrences to ATP-mediated events in the NBDs. We find that energetic coupling between two CFTR residues, expected to lie on opposite sides of its predicted NBD1-NBD2 dimer interface, changes in concert with channel gating status. The two monitored side chains are independent of each other in closed channels but become coupled as the channels open. The results directly link ATP-driven tight dimerization of CFTR's cytoplasmic nucleotide-binding domains to opening of the ion channel in the transmembrane domains. This establishes a molecular mechanism, involving dynamic restructuring of the NBD dimer interface, that is probably common to all members of the ABC protein superfamily.

Preferential phosphorylation of R-domain Serine 768 dampens activation of CFTR channels by PKA

CFTR (cystic fibrosis transmembrane conductance regulator), the protein whose dysfunction causes cystic fibrosis, is a chloride ion channel whose gating is controlled by interactions of MgATP with CFTR's two cytoplasmic nucleotide binding domains, but only after several serines in CFTR's regulatory (R) domain have been phosphorylated by cAMP-dependent protein kinase (PKA). Whereas eight R-domain serines have previously been shown to be phosphorylated in purified CFTR, it is not known how individual phosphoserines regulate channel gating, although two of them, at positions 737 and 768, have been suggested to be inhibitory. Here we show, using mass spectrometric analysis, that Ser 768 is the first site phosphorylated in purified R-domain protein, and that it and five other R-domain sites are already phosphorylated in resting Xenopus oocytes expressing wild-type (WT) human epithelial CFTR. The WT channels have lower activity than S768A channels (with Ser 768 mutated to Ala) in resting oocytes, confirming the inhibitory influence of phosphoserine 768. In excised patches exposed to a range of PKA concentrations, the open probability (P(o)) of mutant S768A channels exceeded that of WT CFTR channels at all [PKA], and the half-maximally activating [PKA] for WT channels was twice that for S768A channels. As the open burst duration of S768A CFTR channels was almost double that of WT channels, at both low (55 nM) and high (550 nM) [PKA], we conclude that the principal mechanism by which phosphoserine 768 inhibits WT CFTR is by hastening the termination of open channel bursts. The right-shifted P(o)-[PKA] curve of WT channels might explain their slower activation, compared with S768A channels, at low [PKA]. The finding that phosphorylation kinetics of WT or S768A R-domain peptides were similar provides no support for an alternative explanation, that early phosphorylation of Ser 768 in WT CFTR might also impair subsequent phosphorylation of stimulatory R-domain serines. The observed reduced sensitivity to activation by [PKA] imparted by Ser 768 might serve to ensure activation of WT CFTR by strong stimuli while dampening responses to weak signals.

Reversible silencing of CFTR chloride channels by glutathionylation

The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation- and ATP-dependent chloride channel that modulates salt and water transport across lung and gut epithelia. The relationship between CFTR and oxidized forms of glutathione is of potential interest because reactive glutathione species are produced in inflamed epithelia where they may be modulators or substrates of CFTR. Here we show that CFTR channel activity in excised membrane patches is markedly inhibited by several oxidized forms of glutathione (i.e., GSSG, GSNO, and glutathione treated with diamide, a strong thiol oxidizer). Three lines of evidence indicate that the likely mechanism for this inhibitory effect is glutathionylation of a CFTR cysteine (i.e., formation of a mixed disulfide with glutathione): (a) channels could be protected from inhibition by pretreating the patch with NEM (a thiol alkylating agent) or by lowering the bath pH; (b) inhibited channels could be rescued by reducing agents (e.g., DTT) or by purified glutaredoxins (Grxs; thiol disulfide oxidoreductases) including a mutant Grx that specifically reduces mixed disulfides between glutathione and cysteines within proteins; and (c) reversible glutathionylation of CFTR polypeptides in microsomes could be detected biochemically under the same conditions. At the single channel level, the primary effect of reactive glutathione species was to markedly inhibit the opening rates of individual CFTR channels. CFTR channel inhibition was not obviously dependent on phosphorylation state but was markedly slowed when channels were first “locked open” by a poorly hydrolyzable ATP analogue (AMP-PNP). Consistent with the latter finding, we show that the major site of inhibition is cys-1344, a poorly conserved cysteine that lies proximal to the signature sequence in the second nucleotide binding domain (NBD2) of human CFTR. This region is predicted to participate in ATP-dependent channel opening and to be occluded in the nucleotide-bound state of the channel based on structural comparisons to related ATP binding cassette transporters. Our results demonstrate that human CFTR channels are reversibly inhibited by reactive glutathione species, and support an important role of the region proximal to the NBD2 signature sequence in ATP-dependent channel opening.

Normal gating of CFTR requires ATP binding to both nucleotide-binding domains and hydrolysis at the second nucleotide-binding domain

ATP interacts with the two nucleotide-binding domains (NBDs) of CFTR to control gating. However, it is unclear whether gating involves ATP binding alone, or also involves hydrolysis at each NBD. We introduced phenylalanine residues into nonconserved positions of each NBD Walker A motif to sterically prevent ATP binding. These mutations blocked [alpha-(32)P]8-N(3)-ATP labeling of the mutated NBD and reduced channel opening rate without changing burst duration. Introducing cysteine residues at these positions and modifying with N-ethylmaleimide produced the same gating behavior. These results indicate that normal gating requires ATP binding to both NBDs, but ATP interaction with one NBD is sufficient to support some activity. We also studied mutations of the conserved Walker A lysine residues (K464A and K1250A) that prevent hydrolysis. By combining substitutions that block ATP binding with Walker A lysine mutations, we could differentiate the role of ATP binding vs. hydrolysis at each NBD. The K1250A mutation prolonged burst duration; however, blocking ATP binding prevented the long bursts. These data indicate that ATP binding to NBD2 allowed channel opening and that closing was delayed in the absence of hydrolysis. The corresponding NBD1 mutations showed relatively little effect of preventing ATP hydrolysis but a large inhibition of blocking ATP binding. These data suggest that ATP binding to NBD1 is required for normal activity but that hydrolysis has little effect. Our results suggest that both NBDs contribute to channel gating, NBD1 binds ATP but supports little hydrolysis, and ATP binding and hydrolysis at NBD2 are key for normal gating.

Functional roles of nonconserved structural segments in CFTR's NH2-terminal nucleotide binding domain

The cystic fibrosis transmembrane conductance regulator (CFTR), encoded by the gene mutated in cystic fibrosis patients, belongs to the family of ATP-binding cassette (ABC) proteins, but, unlike other members, functions as a chloride channel. CFTR is activated by protein kinase A (PKA)-mediated phosphorylation of multiple sites in its regulatory domain, and gated by binding and hydrolysis of ATP at its two nucleotide binding domains (NBD1, NBD2). The recent crystal structure of NBD1 from mouse CFTR (Lewis, H.A., S.G. Buchanan, S.K. Burley, K. Conners, M. Dickey, M. Dorwart, R. Fowler, X. Gao, W.B. Guggino, W.A. Hendrickson, et al. 2004. EMBO J. 23:282–293) identified two regions absent from structures of all other NBDs determined so far, a “regulatory insertion” (residues 404–435) and a “regulatory extension” (residues 639–670), both positioned to impede formation of the putative NBD1–NBD2 dimer anticipated to occur during channel gating; as both segments appeared highly mobile and both contained consensus PKA sites (serine 422, and serines 660 and 670, respectively), it was suggested that their phosphorylation-linked conformational changes might underlie CFTR channel regulation. To test that suggestion, we coexpressed in Xenopus oocytes CFTR residues 1–414 with residues 433–1480, or residues 1–633 with 668–1480, to yield split CFTR channels (called 414+433 and 633+668) that lack most of the insertion, or extension, respectively. In excised patches, regulation of the resulting CFTR channels by PKA and by ATP was largely normal. Both 414+433 channels and 633+668 channels, as well as 633(S422A)+668 channels (lacking both the extension and the sole PKA consensus site in the insertion), were all shut during exposure to MgATP before addition of PKA, but activated like wild type (WT) upon phosphorylation; this indicates that inhibitory regulation of nonphosphorylated WT channels depends upon neither segment. Detailed kinetic analysis of 414+433 channels revealed intact ATP dependence of single-channel gating kinetics, but slightly shortened open bursts and faster closing from the locked-open state (elicited by ATP plus pyrophosphate or ATP plus AMPPNP). In contrast, 633+668 channel function was indistinguishable from WT at both macroscopic and microscopic levels. We conclude that neither nonconserved segment is an essential element of PKA- or nucleotide-dependent regulation.

Inventory and Comparative Analysis of Rice and Arabidopsis ATP-binding Cassette (ABC) Systems

ATP-binding cassette (ABC) proteins constitute a large superfamily found in all kingdoms of living organisms. The recent completion of two draft sequences of the rice (Oryza sativa) genome allowed us to analyze and classify its ABC proteins and to compare to those in Arabidopsis thaliana. We identified a similar number of ABC proteins in rice and Arabidopsis (121 versus 120), despite the rice genome being more than three times the size of Arabidopsis. Both Arabidopsis and rice have representative members in all seven major subfamilies of ABC ATPases (A to G) commonly found in eukaryotes. This comparative analysis allowed the detection of 29 potential orthologous sequences in Arabidopsis and rice. However, plant share with prokaryotes a specific set of ABC systems that is not detected in animals. These ABC systems might be inherited from the cyanobacterial ancestor of chloroplasts. The present work provides the first complete inventory of rice ABC proteins and an updated inventory of those proteins in Arabidopsis.

How do ABC transporters drive transport?

Members of the ATP-binding cassette (ABC) superfamily are integral membrane proteins that hydrolyze ATP to drive transport. In the last two decades these proteins have been extensively characterized on a genetic and biochemical level, and in recent years high-resolution crystal structures of several nucleotide-binding domains and full-length transporters have extended our knowledge. Here we discuss the possible mechanisms of transport that have been derived from these crystal structures and the extensive available biochemical data.

The ATP switch model for ABC transporters

ABC transporters mediate active translocation of a diverse range of molecules across all cell membranes. They comprise two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Recent biochemical, structural and genetic studies have led to the ATP-switch model in which ATP binding and ATP hydrolysis, respectively, induce formation and dissociation of an NBD dimer. This provides an exquisitely regulated switch that induces conformational changes in the TMDs to mediate membrane transport.

Nucleotide exchange from the high-affinity ATP-binding site in SecA is the rate-limiting step in the ATPase cycle of the soluble enzyme and occurs through a specialized conformational state

We have characterized the kinetic and thermodynamic consequences of adenine nucleotide interaction with the low-affinity and high-affinity nucleotide-binding sites in free SecA. ATP binds to the hydrolytically active high-affinity site approximately 3-fold more slowly than ADP when SecA is in its conformational ground state, suggesting that ATP binding probably occurs when the enzyme is in another conformational state during the productive ATPase/transport cycle. The steady-state ATP hydrolysis rate is equivalent to the rate of ADP release from the high-affinity site under a number of conditions, indicating that this process is the rate-limiting step in the ATPase cycle of the free enzyme. Because efficient protein translocation requires at least a 100-fold acceleration in the ATPase rate, the rate-limiting process of ADP release from the high-affinity site is likely to play a controlling role in the conformational reaction cycle of SecA. This release process involves a large enthalpy of activation, suggesting that it involves a protein conformational change, and two observations indicate that this conformational change is different from the well-characterized endothermic conformational transition believed to gate the binding of SecA to SecYEG. First, nucleotide binding to the low-affinity site strongly inhibits the endothermic transition but does not reduce the rate of ADP release. Second, removal of Mg(2+) from an allosteric binding site on SecA does not perturb the endothermic transition but produces a 10-fold acceleration in the rate of ADP release. These divergent effects suggest that a specialized conformational transition mediates the rate-limiting ADP-release process in SecA. Finally, ADP, 2'-O-(N-methylanthraniloyl)-adenosine-5'-diphosphate (MANT-ADP), and adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S) bind with similar affinities to the high-affinity site and also to the low-affinity site as inferred from their consistent effects in inhibiting the endothermic transition. In contrast, adenosine 5'-(beta,gamma-imino)triphosphate (AMPPNP) shows 100-fold weaker affinity than ADP for the high-affinity site and no detectable interaction with the low-affinity site at concentrations up to 1 mM, suggesting that this nonhydrolyzable analogue may not be a faithful mimic of ATP in its interactions with SecA.

ATP increases the affinity between MutS ATPase domains. Implications for ATP hydrolysis and conformational changes

MutS is the key protein of the Escherichia coli DNA mismatch repair system. It recognizes mispaired and unpaired bases and has intrinsic ATPase activity. ATP binding after mismatch recognition by MutS serves as a switch that enables MutL binding and the subsequent initiation of mismatch repair. However, the mechanism of this switch is poorly understood. We have investigated the effects of ATP binding on the MutS structure. Crystallographic studies of ATP-soaked crystals of MutS show a trapped intermediate, with ATP in the nucleotide-binding site. Local rearrangements of several residues around the nucleotide-binding site suggest a movement of the two ATPase domains of the MutS dimer toward each other. Analytical ultracentrifugation experiments confirm such a rearrangement, showing increased affinity between the ATPase domains upon ATP binding and decreased affinity in the presence of ADP. Mutations of specific residues in the nucleotide-binding domain reduce the dimer affinity of the ATPase domains. In addition, ATP-induced release of DNA is strongly reduced in these mutants, suggesting that the two activities are coupled. Hence, it seems plausible that modulation of the affinity between ATPase domains is the driving force for conformational changes in the MutS dimer. These changes are driven by distinct amino acids in the nucleotide-binding site and form the basis for long-range interactions between the ATPase domains and DNA-binding domains and subsequent binding of MutL and initiation of mismatch repair.

A heteromeric complex of the two nucleotide binding domains of cystic fibrosis transmembrane conductance regulator (CFTR) mediates ATPase activity

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a member of the ABC superfamily of transporter proteins. Recently, crystal structures of intact, prokaryotic members of this family have been described. These structures suggested that ATP binding and hydrolysis occurs at two sites formed at the interface between their nucleotide binding domains (NBDs). In contrast to the prokaryotic family members, the NBDs of CFTR are asymmetric (both structurally and functionally), and previous to the present studies, it was not clear whether both NBDs are required for ATP hydrolysis. In order to assess the relative roles of the two NBDs of human CFTR, we purified and reconstituted NBD1 and NBD2, separately and together. We found that NBD1 and NBD2 by themselves exhibited relatively low ATPase activity. Co-assembly of NBD1 and NBD2 exhibited a 2-3-fold enhancement in catalytic activity relative to the isolated domains and this increase reflected enhanced ATP turnover (V(max)). These data provide the first direct evidence that heterodimerization of the NBD1 and NBD2 domains of CFTR is required to generate optimal catalytic activity.

The cystic fibrosis mutation G1349D within the signature motif LSHGH of NBD2 abolishes the activation of CFTR chloride channels by genistein

Cystic fibrosis (CF) is a common lethal genetic disease caused by autosomal recessive mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel that belongs to the ATP-Binding Cassette (ABC) family of transporters. The class III CF mutations G551D and G1349D are located within the "signature" sequence LSGGQ and LSHGH of NBD1 and NBD2, respectively. We have constructed by site-directed mutagenesis vectors encoding green fluorescent protein (GFP)-tagged wild-type (wt) CFTR or CFTR containing delF508, G551D, G1349D and G551D/G1349D to study their pharmacology after transient expression in COS-7 cells. We show that IBMX and the benzo[c]quinolizinium derivative MPB-91 stimulates the activity of G1349D-, G551D- and G551D/G1349D-CFTR only in the presence of cAMP-promoting agents like forskolin or cpt-cAMP. Similar half-maximal effective concentrations (EC(50)) of MPB-91 (22-36microM) have been determined for wt-, G551D-, G1349D- and G551D/G1349D-CFTR. The isoflavone genistein stimulates wild-type (wt)- and delF508-CFTR channel activity in a non-Michaelis-Menten manner. By contrast, the response of G1349D- and G551D-CFTR to genistein is dramatically altered. First, genistein is not able to stimulate G1349D- and G551D/G1349D-CFTR. Second, genistein stimulates G551D-CFTR without any inhibition at high concentration. We conclude from these results that whereas G551 in NBD1 is an important molecular site for inhibition of CFTR by genistein, the symmetrical G1349 in NBD2 is also one major site but for the activation of CFTR by genistein. Because both mutations alter specifically the mechanism of CFTR channel activation by genistein, we believe that the signature sequences of CFTR act as molecular switches that upon interaction with genistein turn on and off the channel.

Purification and crystallization of the cystic fibrosis transmembrane conductance regulator (CFTR)

The cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane protein that is mutated in patients suffering from cystic fibrosis. Here we report the purification and first crystallization of wild-type human CFTR. Functional characterization of the material showed it to be highly active. Electron crystallography of negatively stained two-dimensional crystals of CFTR has revealed the overall architecture of this channel for two different conformational states. These show a strong structural homology to two conformational states of another eukaryotic ATP-binding cassette transporter, P-glycoprotein. In contrast to P-glycoprotein, however, both conformational states can be observed in the presence of a nucleotide, which may be related to the role of CFTR as an ion channel rather than a transporter. The hypothesis that the two conformations could represent the "open" and "closed" states of the channel is considered.