Dual roles of the sixth transmembrane segment of the CFTR chloride channel in gating and permeation

Vx-770 potentiates CFTR function by promoting decoupling between the gating cycle and ATP hydrolysis cycle

Vx-770 (Ivacaftor), a Food and Drug Administration (FDA)-approved drug for clinical application to patients with cystic fibrosis (CF), shifts the paradigm from conventional symptomatic treatments to therapeutics directly tackling the root of the disease: functional defects of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel caused by pathogenic mutations. The underlying mechanism for the action of Vx-770 remains elusive partly because this compound not only increases the activity of wild-type (WT) channels whose gating is primarily controlled by ATP binding/hydrolysis, but also improves the function of G551D-CFTR, a disease-associated mutation that abolishes CFTR's responsiveness to ATP. Here we provide a unified theory to account for this dual effect of Vx-770. We found that Vx-770 enhances spontaneous, ATP-independent activity of WT-CFTR to a similar magnitude as its effects on G551D channels, a result essentially explaining Vx-770's effect on G551D-CFTR. Furthermore, Vx-770 increases the open time of WT-CFTR in an [ATP]-dependent manner. This distinct kinetic effect is accountable with a newly proposed CFTR gating model depicting an [ATP]-dependent "reentry" mechanism that allows CFTR shuffling among different open states by undergoing multiple rounds of ATP hydrolysis. We further examined the effect of Vx-770 on R352C-CFTR, a unique mutant that allows direct observation of hydrolysis-triggered gating events. Our data corroborate that Vx-770 increases the open time of WT-CFTR by stabilizing a posthydrolytic open state and thereby fosters decoupling between the gating cycle and ATP hydrolysis cycle. The current study also suggests that this unique mechanism of drug action can be further exploited to develop strategies that enhance the function of CFTR.

The CFTR ion channel: gating, regulation, and anion permeation

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated anion channel with two remarkable distinctions. First, it is the only ATP-binding cassette (ABC) transporter that is known to be an ion channel--almost all others function as transport ATPases. Second, CFTR is the only ligand-gated channel that consumes its ligand (ATP) during the gating cycle--a consequence of its enzymatic activity as an ABC transporter. We discuss these special properties of CFTR in the context of its evolutionary history as an ABC transporter. Other topics include the mechanisms by which CFTR gating is regulated by phosphorylation of its unique regulatory domain and our current view of the CFTR permeation pathway (or pore). Understanding these basic operating principles of the CFTR channel is central to defining the mechanisms of action of prospective cystic fibrosis drugs and to the development of new, rational treatment strategies.

The power stroke driven by ATP binding in CFTR as studied by molecular dynamics simulations

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel belonging to the ATP binding cassette (ABC) protein superfamily. Currently, it remains unclear how ATP binding causes the opening of the channel gate at the molecular level. To clarify this mechanism, we first constructed an atomic model of the inward-facing CFTR using the X-ray structures of other ABC proteins. Molecular dynamics (MD) simulations were then performed to explore the structure and dynamics of the inward-facing CFTR in a membrane environment. In the MgATP-bound state, two nucleotide-binding domains (NBDs) formed a head-to-tail type of dimer, in which the ATP molecules were sandwiched between the Walker A and signature motifs. Alternatively, one of the final MD structures in the apo state was similar to that of a "closed-apo" conformation found in the X-ray analysis of ATP-free MsbA. Principal component analysis for the MD trajectory indicated that NBD dimerization causes significant structural and dynamical changes in the transmembrane domains (TMDs), which is likely indicative of the formation of a chloride ion access path. This study suggests that the free energy gain from ATP binding acts as a driving force not only for NBD dimerization but also for NBD-TMD concerted motions.

Tuning of CFTR chloride channel function by location of positive charges within the pore

High unitary Cl(-) conductance in the cystic fibrosis transmembrane conductance regulator Cl(-) channel requires a functionally unique, positively charged lysine residue (K95) in the inner vestibule of the channel pore. Here we used a mutagenic approach to investigate the ability of other sites in the pore to host this important positive charge. The loss of conductance observed in the K95Q mutation was >50% rescued by substituting a lysine for each of five different pore-lining amino acids, suggesting that the exact location of the fixed positive charge is not crucial to support high conductance. Moving the positive charge also restored open-channel blocker interactions that are lost in K95Q. Introducing a second positive charge in addition to that at K95 did not increase conductance at any site, but did result in a striking increase in the strength of block by divalent Pt(NO(2))(4)(2-) ions. Based on the site dependence of these effects, we propose that although the exact location of the positive charge is not crucial for normal pore properties, transplanting this charge to other sites results in a diminution of its effectiveness that appears to depend on its location along the axis of the pore.

Nonintegral stoichiometry in CFTR gating revealed by a pore-lining mutation

Cystic fibrosis transmembrane conductance regulator (CFTR) is a unique member of the ATP-binding cassette (ABC) protein superfamily. Unlike most other ABC proteins that function as active transporters, CFTR is an ATP-gated chloride channel. The opening of CFTR's gate is associated with ATP-induced dimerization of its two nucleotide-binding domains (NBD1 and NBD2), whereas gate closure is facilitated by ATP hydrolysis-triggered partial separation of the NBDs. This generally held theme of CFTR gating-a strict coupling between the ATP hydrolysis cycle and the gating cycle-is put to the test by our recent finding of a short-lived, post-hydrolytic state that can bind ATP and reenter the ATP-induced original open state. We accidentally found a mutant CFTR channel that exhibits two distinct open conductance states, the smaller O1 state and the larger O2 state. In the presence of ATP, the transition between the two states follows a preferred O1→O2 order, a telltale sign of a violation of microscopic reversibility, hence demanding an external energy input likely from ATP hydrolysis, as such preferred gating transition was abolished in a hydrolysis-deficient mutant. Interestingly, we also observed a considerable amount of opening events that contain more than one O1→O2 transition, indicating that more than one ATP molecule may be hydrolyzed within an opening burst. We thus conclude a nonintegral stoichiometry between the gating cycle and ATP consumption. Our results lead to a six-state gating model conforming to the classical allosteric mechanism: both NBDs and transmembrane domains hold a certain degree of autonomy, whereas the conformational change in one domain will facilitate the conformational change in the other domain.

Locating a plausible binding site for an open-channel blocker, GlyH-101, in the pore of the cystic fibrosis transmembrane conductance regulator

High-throughput screening has led to the identification of small-molecule blockers of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, but the structural basis of blocker binding remains to be defined. We developed molecular models of the CFTR channel on the basis of homology to the bacterial transporter Sav1866, which could permit blocker binding to be analyzed in silico. The models accurately predicted the existence of a narrow region in the pore that is a likely candidate for the binding site of an open-channel pore blocker such as N-(2-naphthalenyl)-[(3,5-dibromo-2,4-dihydroxyphenyl)methylene]glycine hydrazide (GlyH-101), which is thought to act by entering the channel from the extracellular side. As a more-stringent test of predictions of the CFTR pore model, we applied induced-fit, virtual, ligand-docking techniques to identify potential binding sites for GlyH-101 within the CFTR pore. The highest-scoring docked position was near two pore-lining residues, Phe337 and Thr338, and the rates of reactions of anionic, thiol-directed reagents with cysteines substituted at these positions were slowed in the presence of the blocker, consistent with the predicted repulsive effect of the net negative charge on GlyH-101. When a bulky phenylalanine that forms part of the predicted binding pocket (Phe342) was replaced with alanine, the apparent affinity of the blocker was increased ∼200-fold. A molecular mechanics-generalized Born/surface area analysis of GlyH-101 binding predicted that substitution of Phe342 with alanine would substantially increase blocker affinity, primarily because of decreased intramolecular strain within the blocker-protein complex. This study suggests that GlyH-101 blocks the CFTR channel by binding within the pore bottleneck.

Relative movements of transmembrane regions at the outer mouth of the cystic fibrosis transmembrane conductance regulator channel pore during channel gating

Multiple transmembrane (TM) segments line the pore of the cystic fibrosis transmembrane conductance regulator Cl(-) channel; however, the relative alignment of these TMs and their relative movements during channel gating are unknown. To gain three-dimensional structural information on the outer pore, we have used patch clamp recording to study the proximity of pairs of cysteine side chains introduced into TMs 6 and 11, using both disulfide cross-linking and Cd(2+) coordination. Following channel activation, disulfide bonds could apparently be formed between three cysteine pairs (of 15 studied): R334C/T1122C, R334C/G1127C, and T338C/S1118C. To examine the state dependence of cross-linking, we combined these cysteine mutations with a nucleotide-binding domain mutation (E1371Q) that stabilizes the channel open state. Investigation of the effects of the E1371Q mutation on disulfide bond formation and Cd(2+) coordination suggests that although R334C/T1122C and T338C/S1118C are closer together in the channel open state, R334C/G1127C are close together and can form disulfide bonds only when the channel is closed. These results provide important new information on the three-dimensional structure of the outer mouth of the cystic fibrosis transmembrane conductance regulator channel pore: TMs 6 and 11 are close enough together to form disulfide bonds in both open and closed channels. Moreover, the altered relative locations of residues in open and in closed channels that we infer allow us to propose that channel opening and closing may be associated with a relative translational movement of TMs 6 and 11, with TM6 moving "down" (toward the cytoplasm) during channel opening.

Role of the juxtamembrane region of cytoplasmic loop 3 in the gating and conductance of the cystic fibrosis transmembrane conductance regulator chloride channel

Opening and closing of the cystic fibrosis transmembrane conductance regulator chloride channel are controlled by interactions of ATP with its cytoplasmic nucleotide binding domains (NBDs). The NBDs are connected to the transmembrane pore via four cytoplasmic loops. These loops have been suggested to play roles both in channel gating and in forming a cytoplasmic extension of the channel pore. To investigate the structure and function of one of these cytoplasmic loops, we have used patch clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced into loop 3. We find that methanethiosulfonate (MTS) reagents modify cysteines introduced at 14 of 16 sites studied in the juxtamembrane region of loop 3, in all cases leading to inhibition of channel function. In most cases, both the functional effects of modification and the rate of modification were similar for negatively and positively charged MTS reagents. Single-channel recordings indicated that, at all sites, inhibition was the result of an MTS reagent-induced decrease in channel open probability; in no case was the Cl(-) conductance of open channels altered by modification. These results indicate that loop 3 is readily accessible to the cytoplasm and support the involvement of this region in the control of channel gating. However, our results do not support the hypothesis that this region is close enough to the Cl(-) permeation pathway to exert any influence on permeating Cl(-) ions. We propose that either the cytoplasmic pore is very wide or cytoplasmic Cl(-) ions use other routes to access the transmembrane pore.

Identification of a novel post-hydrolytic state in CFTR gating

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters, ubiquitous proteins found in all kingdoms of life, catalyze substrates translocation across biological membranes using the free energy of ATP hydrolysis. Cystic fibrosis transmembrane conductance regulator (CFTR) is a unique member of this superfamily in that it functions as an ATP-gated chloride channel. Despite difference in function, recent studies suggest that the CFTR chloride channel and the exporter members of the ABC protein family may share an evolutionary origin. Although ABC exporters harness the free energy of ATP hydrolysis to fuel a transport cycle, for CFTR, ATP-induced dimerization of its nucleotide-binding domains (NBDs) and subsequent hydrolysis-triggered dimer separation are proposed to be coupled, respectively, to the opening and closing of the gate in its transmembrane domains. In this study, by using nonhydrolyzable ATP analogues, such as pyrophosphate or adenylyl-imidodiphosphate as baits, we captured a short-lived state (state X), which distinguishes itself from the previously identified long-lived C2 closed state by its fast response to these nonhydrolyzable ligands. As state X is caught during the decay phase of channel closing upon washout of the ligand ATP but before the channel sojourns to the C2 closed state, it likely emerges after the bound ATP in the catalysis-competent site has been hydrolyzed and the hydrolytic products have been released. Thus, this newly identified post-hydrolytic state may share a similar conformation of NBDs as the C2 closed state (i.e., a partially separated NBD and a vacated ATP-binding pocket). The significance of this novel state in understanding the structural basis of CFTR gating is discussed.

Cystic fibrosis transmembrane conductance regulator: a molecular model defines the architecture of the anion conduction path and locates a "bottleneck" in the pore

We developed molecular models for the cystic fibrosis transmembrane conductance regulator chloride channel based on the prokaryotic ABC transporter, Sav1866. Here we analyze predicted pore geometry and side-chain orientations for TM3, TM6, TM9, and TM12, with particular attention being paid to the location of the rate-limiting barrier for anion conduction. Side-chain orientations assayed by cysteine scanning were found to be from 77 to 90% in accord with model predictions. The predicted geometry of the anion conduction path was defined by a space-filling model of the pore and confirmed by visualizing the distribution of water molecules from a molecular dynamics simulation. The pore shape is that of an asymmetric hourglass, comprising a shallow outward-facing vestibule that tapers rapidly toward a narrow "bottleneck" linking the outer vestibule to a large inner cavity extending toward the cytoplasmic extent of the lipid bilayer. The junction between the outer vestibule and the bottleneck features an outward-facing rim marked by T338 in TM6 and I1131 in TM12, consistent with the observation that cysteines at both of these locations reacted with both channel-permeant and channel-impermeant, thiol-directed reagents. Conversely, cysteines substituted for S341 in TM6 or T1134 in TM12, predicted by the model to lie below the rim of the bottleneck, were found to react exclusively with channel-permeant reagents applied from the extracellular side. The predicted dimensions of the bottleneck are consistent with the demonstrated permeation of Cl(-), pseudohalide anions, water, and urea.

Structural basis for the channel function of a degraded ABC transporter, CFTR (ABCC7)

Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily, but little is known about how this ion channel that harbors an uninterrupted ion permeation pathway evolves from a transporter that works by alternately exposing its substrate conduit to the two sides of the membrane. Here, we assessed reactivity of intracellularly applied thiol-specific probes with cysteine residues substituted into the 12th transmembrane segment (TM12) of CFTR. Our experimental data showing high reaction rates of substituted cysteines toward the probes, strong blocker protection of cysteines against reaction, and reaction-induced alterations in channel conductance support the idea that TM12 of CFTR contributes to the lining of the ion permeation pathway. Together with previous work, these findings raise the possibility that pore-lining elements of CFTR involve structural components resembling those that form the substrate translocation pathway of ABC transporters. In addition, comparison of reaction rates in the open and closed states of the CFTR channel leads us to propose that upon channel opening, the wide cytoplasmic vestibule tightens and the pore-lining TM12 rotates along its helical axis. This simple model for gating conformational changes in the inner pore domain of CFTR argues that the gating transition of CFTR and the transport cycle of ABC proteins share analogous conformational changes. Collectively, our data corroborate the popular hypothesis that degradation of the cytoplasmic-side gate turned an ABC transporter into the CFTR channel.

Alternating access to the transmembrane domain of the ATP-binding cassette protein cystic fibrosis transmembrane conductance regulator (ABCC7)

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a member of the ATP-binding cassette (ABC) protein family, most members of which act as active transporters. Actively transporting ABC proteins are thought to alternate between "outwardly facing" and "inwardly facing" conformations of the transmembrane substrate pathway. In CFTR, it is assumed that the outwardly facing conformation corresponds to the channel open state, based on homology with other ABC proteins. We have used patch clamp recording to quantify the rate of access of cysteine-reactive probes to cysteines introduced into two different transmembrane regions of CFTR from both the intracellular and extracellular solutions. Two probes, the large [2-sulfonatoethyl]methanethiosulfonate (MTSES) molecule and permeant Au(CN)(2)(-) ions, were applied to either side of the membrane to modify cysteines substituted for Leu-102 (first transmembrane region) and Thr-338 (sixth transmembrane region). Channel opening and closing were altered by mutations in the nucleotide binding domains of the channel. We find that, for both MTSES and Au(CN)(2)(-), access to these two cysteines from the cytoplasmic side is faster in open channels, whereas access to these same sites from the extracellular side is faster in closed channels. These results are consistent with alternating access to the transmembrane regions, however with the open state facing inwardly and the closed state facing outwardly. Our findings therefore prompt revision of current CFTR structural and mechanistic models, as well as having broader implications for transport mechanisms in all ABC proteins. Our results also suggest possible locations of both functional and dysfunctional ("vestigial") gates within the CFTR permeation pathway.

Conformational change opening the CFTR chloride channel pore coupled to ATP-dependent gating

Opening and closing of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel are controlled by ATP binding and hydrolysis by its nucleotide binding domains (NBDs). This is presumed to control opening of a single "gate" within the permeation pathway, however, the location of such a gate has not been described. We used patch clamp recording to monitor access of cytosolic cysteine reactive reagents to cysteines introduced into different transmembrane (TM) regions in a cysteine-less form of CFTR. The rate of modification of Q98C (TM1) and I344C (TM6) by both [2-sulfonatoethyl] methanethiosulfonate (MTSES) and permeant Au(CN)(2)(-) ions was reduced when ATP concentration was reduced from 1mM to 10μM, and modification by MTSES was accelerated when 2mM pyrophosphate was applied to prevent channel closure. Modification of K95C (TM1) and V345C (TM6) was not affected by these manoeuvres. We also manipulated gating by introducing the mutations K464A (in NBD1) and E1371Q (in NBD2). The rate of modification of Q98C and I344C by both MTSES and Au(CN)(2)(-) was decreased by K464A and increased by E1371Q, whereas modification of K95C and V345C was not affected. These results suggest that access from the cytoplasm to K95 and V345 is similar in open and closed channels. In contrast, modifying ATP-dependent channel gating alters access to Q98 and I344, located further into the pore. We propose that ATP-dependent gating of CFTR is associated with the opening and closing of a gate within the permeation pathway at the level of these pore-lining amino acids.

Differential contribution of TM6 and TM12 to the pore of CFTR identified by three sulfonylurea-based blockers

Previous studies suggested that four transmembrane domains 5, 6, 11, 12 make the greatest contribution to forming the pore of the CFTR chloride channel. We used excised, inside-out patches from oocytes expressing CFTR with alanine-scanning mutagenesis in amino acids in TM6 and TM12 to probe CFTR pore structure with four blockers: glibenclamide (Glyb), glipizide (Glip), tolbutamide (Tolb), and Meglitinide. Glyb and Glip blocked wildtype (WT)-CFTR in a voltage-, time-, and concentration-dependent manner. At V (M) = -120 mV with symmetrical 150 mM Cl(-) solution, fractional block of WT-CFTR by 50 μM Glyb and 200 μM Glip was 0.64 ± 0.03 (n = 7) and 0.48 ± 0.02 (n = 7), respectively. The major effects on block by Glyb and Glip were found with mutations at F337, S341, I344, M348, and V350 of TM6. Under similar conditions, fractional block of WT-CFTR by 300 μM Tolb was 0.40 ± 0.04. Unlike Glyb, Glip, and Meglitinide, block by Tolb lacked time-dependence (n = 7). We then tested the effects of alanine mutations in TM12 on block by Glyb and Glip; the major effects were found at N1138, T1142, V1147, N1148, S1149, S1150, I1151, and D1152. From these experiments, we infer that amino acids F337, S341, I344, M348, and V350 of TM6 face the pore when the channel is in the open state, while the amino acids of TM12 make less important contributions to pore function. These data also suggest that the region between F337 and S341 forms the narrow part of the CFTR pore.

Cystic fibrosis transmembrane conductance regulator: temperature-dependent cysteine reactivity suggests different stable conformers of the conduction pathway

Cysteine scanning has been widely used to identify pore-lining residues in mammalian ion channels, including the cystic fibrosis transmembrane conductance regulator (CFTR). These studies, however, have been typically conducted at room temperature rather than human body temperature. Reports of substantial effects of temperature on gating and anion conduction in CFTR channels as well as an unexpected pattern of cysteine reactivity in the sixth transmembrane segment (TM6) prompted us to investigate the effect of temperature on the reactivity of cysteines engineered into TM6 of CFTR. We compared reaction rates at temperatures ranging from 22 to 37 °C for cysteines placed on either side of an apparent size-selective accessibility barrier previously defined by comparing reactivity toward channel-permeant and channel-impermeant, thiol-directed reagents. The results indicate that the reactivity of cysteines at three positions extracellular to the position of the accessibility barrier, 334, 336, and 337, is highly temperature-dependent. At 37 °C, cysteines at these positions were highly reactive toward MTSES(-), whereas at 22 °C, the reaction rates were 2-6-fold slower to undetectable. An activation energy of 157 kJ/mol for the reaction at position 337 is consistent with the hypothesis that, at physiological temperature, the extracellular portion of the CFTR pore can adopt conformations that differ significantly from those that can be accessed at room temperature. However, the position of the accessibility barrier defined empirically by applying channel-permeant and channel-impermeant reagents to the extracellular aspect of the pore is not altered. The results illuminate previous scanning results and indicate that the assay temperature is a critical variable in studies designed to use chemical modification to test structural models for the CFTR anion conduction pathway.

Functional arrangement of the 12th transmembrane region in the CFTR chloride channel pore based on functional investigation of a cysteine-less CFTR variant

The membrane-spanning part of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel comprises 12 transmembrane (TM) α-helices, arranged into two pseudo-symmetrical groups of six. While TM6 in the N-terminal TMs is known to line the pore and to make an important contribution to channel properties, much less is known about its C-terminal counterpart, TM12. We have used patch clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of TM12 in a cysteine-less variant of CFTR. We find that methanethiosulfonate (MTS) reagents irreversibly modify cysteines substituted for TM12 residues N1138, M1140, S1141, T1142, Q1144, W1145, V1147, N1148, and S1149 when applied to the cytoplasmic side of open channels. Cysteines sensitive to internal MTS reagents were not modified by extracellular [2-(trimethylammonium)ethyl] MTS, consistent with MTS reagent impermeability. Both S1141C and T1142C could be modified by intracellular [2-sulfonatoethyl] MTS prior to channel activation; however, N1138C and M1140C, located deeper into the pore from its cytoplasmic end, were modified only after channel activation. Comparison of these results with previous work on CFTR-TM6 allows us to develop a model of the relative positions, functional contributions, and alignment of these two important TMs lining the CFTR pore. We also propose a mechanism by which these seemingly structurally symmetrical TMs make asymmetric contributions to the functional properties of the channel pore.

Functional differences in pore properties between wild-type and cysteine-less forms of the CFTR chloride channel

Studies of the structure and function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel have been advanced by the development of functional channel variants in which all 18 endogenous cysteine residues have been mutated ("cys-less" CFTR). However, cys-less CFTR has a slightly higher single-channel conductance than wild-type CFTR, raising questions as to the suitability of cys-less as a model of the wild-type CFTR pore. We used site-directed mutagenesis and patch-clamp recording to investigate the origin of this conductance difference and to determine the extent of functional differences between wild-type and cys-less CFTR channel permeation properties. Our results suggest that the conductance difference is the result of a single substitution, of C343: the point mutant C343S has a conductance similar to cys-less, whereas the reverse mutation, S343C in a cys-less background, restores wild-type conductance levels. Other cysteine substitutions (C128S, C225S, C376S, C866S) were without effect. Substitution of other residues for C343 suggested that conductance is dependent on amino acid side chain volume at this position. A range of other functional pore properties, including interactions with channel blockers (Au[CN] (2) (-) , 5-nitro-2-[3-phenylpropylamino]benzoic acid, suramin) and anion permeability, were not significantly different between wild-type and cys-less CFTR. Our results suggest that functional differences between these two CFTR constructs are of limited scale and scope and result from a small change in side chain volume at position 343. These results therefore support the use of cys-less as a model of the CFTR pore region.

Alignment of transmembrane regions in the cystic fibrosis transmembrane conductance regulator chloride channel pore

Different transmembrane (TM) α helices are known to line the pore of the cystic fibrosis TM conductance regulator (CFTR) Cl⁻ channel. However, the relative alignment of these TMs in the three-dimensional structure of the pore is not known. We have used patch-clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of the pore-lining first TM (TM1) of a cysteine-less variant of CFTR. We find that methanethiosulfonate (MTS) reagents irreversibly modify cysteines substituted for TM1 residues K95, Q98, P99, and L102 when applied to the cytoplasmic side of open channels. Residues closer to the intracellular end of TM1 (Y84–T94) were not apparently modified by MTS reagents, suggesting that this part of TM1 does not line the pore. None of the internal MTS reagent-reactive cysteines was modified by extracellular [2-(trimethylammonium)ethyl] MTS. Only K95C, closest to the putative intracellular end of TM1, was apparently modified by intracellular [2-sulfonatoethyl] MTS before channel activation. Comparison of these results with recent work on CFTR-TM6 suggests a relative alignment of these two important TMs along the axis of the pore. This alignment was tested experimentally by formation of disulfide bridges between pairs of cysteines introduced into these two TMs. Currents carried by the double mutants K95C/I344C and Q98C/I344C, but not by the corresponding single-site mutants, were inhibited by the oxidizing agent copper(II)-o-phenanthroline. This inhibition was irreversible on washing but could be reversed by the reducing agent dithiothreitol, suggesting disulfide bond formation between the introduced cysteine side chains. These results allow us to develop a model of the relative positions, functional contributions, and alignment of two important TMs lining the CFTR pore. Such functional information is necessary to understand and interpret the three-dimensional structure of the pore.

On the mechanism of CFTR inhibition by a thiazolidinone derivative

The effects of a thiazolidinone derivative, 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (or CFTRinh-172), on cystic fibrosis transmembrane conductance regulator (CFTR) gating were studied in excised inside-out membrane patches from Chinese hamster ovary cells transiently expressing wild-type and mutant CFTR. We found that the application of CFTRinh-172 results in an increase of the mean closed time and a decrease of the mean open time of the channel. A hyperbolic relationship between the closing rate and [CFTRinh-172] suggests that CFTRinh-172 does not act as a simple pore blocker. Interestingly, the potency of inhibition increases as the open time of the channel is increased with an IC50 in the low nanomolar range for CFTR channels locked in an open state for tens of seconds. Our studies also provide evidence that CFTRinh-172 can bind to both the open state and the closed state. However, at least one additional step, presumably reflecting inhibitor-induced conformational changes, is required to shut down the conductance after the binding of the inhibitor to the channel. Using the hydrolysis-deficient mutant E1371S as a tool as the closing rate of this mutant is dramatically decreased, we found that CFTRinh-172-dependent inhibition of CFTR channel gating, in two aspects, mimics the inactivation of voltage-dependent cation channels. First, similar to the recovery from inactivation in voltage-gated channels, once CFTR is inhibited by CFTRinh-172, reopening of the channel can be seen upon removal of the inhibitor in the absence of adenosine triphosphate (ATP). Second, ATP induced a biphasic current response on inhibitor-bound closed channels as if the ATP-opened channels "inactivate" despite a continuous presence of ATP. A simplified six-state kinetic scheme can well describe our data, at least qualitatively. Several possible structural mechanisms for the effects of CFTRinh-172 will be discussed.