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

Allicin Facilitates Airway Surface Liquid Hydration by Activation of CFTR

Airway epithelium plays critical roles in regulating airway surface liquid (ASL), the alteration of which causes mucus stasis symptoms. Allicin is a compound released from garlic and harbors the capacity of lung-protection. However, the potential regulatory effects of allicin on airway epithelium remain elusive. This study aimed to investigate the effects of allicin on ion transport across airway epithelium and evaluate its potential as an expectorant. Application of allicin induced Cl⁻ secretion across airway epithelium in a concentration-dependent manner. Blockade of cystic fibrosis transmembrane conductance regulator (CFTR) or inhibition of adenylate cyclase-cAMP signaling pathway attenuated allicin-induced Cl⁻ secretion in airway epithelial cells. The in vivo study showed that inhaled allicin significantly increased the ASL secretion in mice. These results suggest that allicin induces Cl⁻ and fluid secretion across airway epithelium via activation of CFTR, which might provide therapeutic strategies for the treatment of chronic pulmonary diseases associated with ASL dehydration.

Contribution of the eighth transmembrane segment to the function of the CFTR chloride channel pore

Our molecular understanding of the cystic fibrosis transmembrane conductance regulator (CFTR)-the chloride channel that is mutated in cystic fibrosis-has been greatly enhanced by a number of recent atomic-level structures of the protein in different conformations. One surprising aspect of these structures was the finding that the eighth of CFTR's 12 membrane-spanning segments (TM8) appeared close to the channel pore. Although functional evidence supports a role for other TMs in forming the pore, such a role for TM8 has not previously been reported. Here, we use patch-clamp recording to investigate the functional role of TM8. Using substituted cysteine accessibility mutagenesis, we find that three amino acid side-chains in TM8 (Y913, Y914, and Y917) are exposed to the extracellular, but not the intracellular, solution. Cysteine cross-linking experiments suggest that Y914 and Y917 are in close proximity to L102 (TM1) and F337 (TM6), respectively, suggesting that TM8 contributes to the narrow selectivity filter region of the pore. Different amino acid substitutions suggest that Y914, and to a lesser extent Y917, play important roles in controlling anion flux through the open channel. Furthermore, substitutions that reduce side-chain volume at Y917 severely affect channel gating, resulting in a channel with an extremely unstable open state. Our results suggest that pore-lining TM8 is among the most important TMs controlling the permeation phenotype of the CFTR channel, and also that movement of TM8 may be critically involved in channel gating.

Cytoplasmic pathway followed by chloride ions to enter the CFTR channel pore

Most ATP-binding cassette (ABC) proteins function as ATP-dependent membrane pumps. One exception is the cystic fibrosis transmembrane conductance regulator (CFTR), an ABC protein that functions as a Cl(-) ion channel. As such, the CFTR protein must form a continuous pathway for the movement of Cl(-) ions from the cytoplasm to the extracellular solution when in its open channel state. Extensive functional investigations have characterized most parts of this Cl(-) permeation pathway. However, one region remains unexplored-the pathway connecting the cytoplasm to the membrane-spanning pore. We used patch clamp recording and extensive substituted cysteine accessibility mutagenesis to identify amino acid side-chains in cytoplasmic regions of CFTR that lie close to the pathway taken by Cl(-) ions as they pass from the cytoplasm through this pathway. Our results suggest that Cl(-) ions enter the permeation pathway via a single lateral tunnel formed by the cytoplasmic parts of the protein, and then follow a fairly direct central pathway towards the membrane-spanning parts of the protein. However, this pathway is not lined continuously by any particular part of the protein; instead, the contributions of different cytoplasmic regions of the protein appear to change as the permeation pathway approaches the membrane, which appears to reflect the ways in which different cytoplasmic regions of the protein are oriented towards its central axis. Our results allow us to define for the first time the complete Cl(-) permeation pathway in CFTR, from the cytoplasm to the extracellular solution.

Metal bridges to probe membrane ion channel structure and function

Ion channels are integral membrane proteins that undergo important conformational changes as they open and close to control transmembrane flux of different ions. The molecular underpinnings of these dynamic conformational rearrangements are difficult to ascertain using current structural methods. Several functional approaches have been used to understand two- and three-dimensional dynamic structures of ion channels, based on the reactivity of the cysteine side-chain. Two-dimensional structural rearrangements, such as changes in the accessibility of different parts of the channel protein to the bulk solution on either side of the membrane, are used to define movements within the permeation pathway, such as those that open and close ion channel gates. Three-dimensional rearrangements – in which two different parts of the channel protein change their proximity during conformational changes – are probed by cross-linking or bridging together two cysteine side-chains. Particularly useful in this regard are so-called metal bridges formed when two or more cysteine side-chains form a high-affinity binding site for metal ions such as Cd2+ or Zn2+. This review describes the use of these different techniques for the study of ion channel dynamic structure and function, including a comprehensive review of the different kinds of conformational rearrangements that have been studied in different channel types via the identification of intra-molecular metal bridges. Factors that influence the affinities and conformational sensitivities of these metal bridges, as well as the kinds of structural inferences that can be drawn from these studies, are also discussed.

Functional Architecture of the Cytoplasmic Entrance to the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel Pore

As an ion channel, the cystic fibrosis transmembrane conductance regulator must form a continuous pathway for the movement of Cl(-) and other anions between the cytoplasm and the extracellular solution. Both the structure and the function of the membrane-spanning part of this pathway are well defined. In contrast, the structure of the pathway that connects the cytoplasm to the membrane-spanning regions is unknown, and functional roles for different parts of the protein forming this pathway have not been described. We used patch clamp recording and substituted cysteine accessibility mutagenesis to identify positively charged amino acid side chains that attract cytoplasmic Cl(-) ions to the inner mouth of the pore. Our results indicate that the side chains of Lys-190, Arg-248, Arg-303, Lys-370, Lys-1041, and Arg-1048, located in different intracellular loops of the protein, play important roles in the electrostatic attraction of Cl(-) ions. Mutation and covalent modification of these residues have charge-dependent effects on the rate of Cl(-) permeation, demonstrating their functional role in maximization of Cl(-) flux. Other nearby positively charged side chains were not involved in electrostatic interactions with Cl(-). The location of these Cl(-)-attractive residues suggests that cytoplasmic Cl(-) ions enter the pore via a lateral portal located between the cytoplasmic extensions to the fourth and sixth transmembrane helices; a secondary, functionally less relevant portal might exist between the extensions to the 10th and 12th transmembrane helices. These results define the cytoplasmic mouth of the pore and show how it attracts Cl(-) ions from the cytoplasm.

Metal bridges illuminate transmembrane domain movements during gating of the cystic fibrosis transmembrane conductance regulator chloride channel

Opening and closing of the cystic fibrosis transmembrane conductance regulator are controlled by ATP binding and hydrolysis by the cytoplasmic nucleotide-binding domains. Different conformational changes in the channel pore have been described during channel opening and closing; however, the relative importance of these changes to the process of gating the pore is not known. We have used patch clamp recording to identify high affinity Cd(2+) bridges formed between pairs of pore-lining cysteine residues introduced into different transmembrane α-helices (TMs). Seven Cd(2+) bridges were identified forming between cysteines in TMs 6 and 12. Interestingly, each of these Cd(2+) bridges apparently formed only in closed channels, and their formation stabilized the closed state. In contrast, a single Cd(2+) bridge identified between cysteines in TMs 1 and 12 stabilized the channel open state. Analysis of the pattern of Cd(2+) bridge formation in different channel states suggests that lateral separation and convergence of different TMs, rather than relative rotation or translation of different TMs, is the key conformational change that causes the channel pore to open and close.

Interaction between 2 extracellular loops influences the activity of the cystic fibrosis transmembrane conductance regulator chloride channel

Activity of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is thought to be controlled by cytoplasmic factors. However, recent evidence has shown that overall channel activity is also influenced by extracellular anions that interact directly with the extracellular loops (ECLs) of the CFTR protein. Very little is known about the structure of the ECLs or how substances interacting with these ECLs might affect CFTR function. We used patch-clamp recording to investigate the accessibility of cysteine-reactive reagents to cysteines introduced throughout ECL1 and 2 key sites in ECL4. Furthermore, interactions between ECL1 and ECL4 were investigated by the formation of disulfide crosslinks between cysteines introduced into these 2 regions. Crosslinks could be formed between R899C (in ECL4) and a number of sites in ECL1 in a manner that was dependent on channel activity, suggesting that the relative orientation of these 2 loops changes on activation. Formation of these crosslinks inhibited channel function, suggesting that relative movement of these ECLs is important to normal channel function. Implications of these findings for the effects of mutations in the ECLs that are associated with cystic fibrosis and interactions with extracellular substances that influence channel activity are discussed.

Directed evolution of P-glycoprotein cysteines reveals site-specific, non-conservative substitutions that preserve multidrug resistance

Pgp (P-glycoprotein) is a prototype ABC (ATP-binding-cassette) transporter involved in multidrug resistance of cancer. We used directed evolution to replace six cytoplasmic Cys (cysteine) residues in Pgp with all 20 standard amino acids and selected for active mutants. From a pool of 75000 transformants for each block of three Cys, we identified multiple mutants that preserved drug resistance and yeast mating activity. The most frequent substitutions were glycine and serine for Cys427 (24 and 20%, respectively) and Cys1070 (37 and 25%) of the Walker A motifs in the NBDs (nucleotide-binding domains), Cys1223 in NBD2 (25 and 8%) and Cys638 in the linker region (24 and 16%), whereas close-by Cys669 tolerated glycine (16%) and alanine (14%), but not serine (absent). Cys1121 in NBD2 showed a clear preference for positively charged arginine (38%) suggesting a salt bridge with Glu269 in the ICL2 (intracellular loop 2) may stabilize domain interactions. In contrast, three Cys residues in transmembrane α-helices could be successfully replaced by alanine. The resulting CL (Cys-less) Pgp was fully active in yeast cells, and purified proteins displayed drug-stimulated ATPase activities indistinguishable from WT (wild-type) Pgp. Overall, directed evolution identified site-specific, non-conservative Cys substitutions that allowed building of a robust CL Pgp, an invaluable new tool for future functional and structural studies, and that may guide the construction of other CL proteins where alanine and serine have proven unsuccessful.

Delineating the extracellular water-accessible surface of the proton-coupled folate transporter

The proton-coupled folate transporter (PCFT) was recently identified as the major uptake route for dietary folates in humans. The three-dimensional structure of PCFT and its detailed interplay with function remain to be determined. We screened the water-accessible extracellular surface of HsPCFT using the substituted-cysteine accessibility method, to investigate the boundaries between the water-accessible surface and inaccessible buried protein segments. Single-cysteines, engineered individually at 40 positions in a functional cysteine-less HsPCFT background construct, were probed for plasma-membrane expression in Xenopus oocytes with a bilayer-impermeant primary-amine-reactive biotinylating agent (sulfosuccinimidyl 6-(biotinamido) hexanoate), and additionally for water-accessibility of the respective engineered cysteine with the sulfhydryl-selective biotinylating agent 2-((biotinoyl)amino)ethyl methanethiosulfonate. The ratio between Cys-selective over amine-selective labeling was further used to evaluate three-dimensional models of HsPCFT generated by homology / threading modeling. The closest homologues of HsPCFT with a known experimentally-determined three-dimensional structure are all members of one of the largest membrane protein super-families, the major facilitator superfamily (MFS). The low sequence identity - 14% or less – between HsPCFT and these templates necessitates experiment-based evaluation and model refinement of homology / threading models. With the present set of single-cysteine accessibilities, the models based on GlpT and PepT_St are most promising for further refinement.

Two salt bridges differentially contribute to the maintenance of cystic fibrosis transmembrane conductance regulator (CFTR) channel function

Previous studies have identified two salt bridges in human CFTR chloride ion channels, Arg(352)-Asp(993) and Arg(347)-Asp(924), that are required for normal channel function. In the present study, we determined how the two salt bridges cooperate to maintain the open pore architecture of CFTR. Our data suggest that Arg(347) not only interacts with Asp(924) but also interacts with Asp(993). The tripartite interaction Arg(347)-Asp(924)-Asp(993) mainly contributes to maintaining a stable s2 open subconductance state. The Arg(352)-Asp(993) salt bridge, in contrast, is involved in stabilizing both the s2 and full (f) open conductance states, with the main contribution being to the f state. The s1 subconductance state does not require either salt bridge. In confirmation of the role of Arg(352) and Asp(993), channels bearing cysteines at these sites could be latched into a full open state using the bifunctional cross-linker 1,2-ethanediyl bismethanethiosulfonate, but only when applied in the open state. Channels remained latched open even after washout of ATP. The results suggest that these interacting residues contribute differently to stabilizing the open pore in different phases of the gating cycle.

Substitution of transmembrane domain Cys residues alters pH(o)-sensitive anion transport by AE2/SLC4A2 anion exchanger

AE2/SLC4A2 is the most widely expressed of the Na(+)-independent SLC4 Cl(-)/HCO3 (-) exchangers and is essential for postnatal survival, but its structure remains unknown. We have generated and expressed a mouse AE2 construct devoid of transmembrane domain cysteine (Cys) residues, mAE2Cys-less, to enhance the utility of Cys-substitution mutagenesis for structural and structure-function studies of mAE2. mAE2Cys-less expressed in Xenopus oocytes exhibited partial reduction of stilbene disulfonate-sensitive anion exchange activity. This activity was independent of the mAE2 N-terminal cytosolic domain and was accompanied by near-normal surface expression, without change in K 1/2 for extracellular Cl(-). mAE2Cys-less exhibited wildtype activation of anion exchange by hypertonicity and by NH4Cl, and wildtype inhibition of anion exchange by acidic intracellular pH (pHi) in the absence of NH4 (+). However, inhibition of anion exchange by extracellular pH (pHo) exhibited an alkaline shifted pHo(50) value of at least 0.6-0.7 pH units. Although SO4 (2-) transport by mAE2Cys-less resembled wildtype mAE2 in its stimulation by acidic pHo, the absence of transmembrane domain Cys residues abrogated activation of oxalate transport by acidic pHo. The contrasting enhancement of SO4 (2-) transport by alkaline pHo in the mAE1 anion translocation pathway mutant E699Q (Am J Physiol Cell Physiol 295: C302) was phenocopied by the corresponding mutant E1007Q in both AE2 and AE2Cys-less. However, the absence of transmembrane domain Cys residues exacerbated the reduced basal anion transport function exhibited by this and other missense substitutions at AE2 residue E1007. AE2Cys-less will be a valuable experimental tool for structure-function studies of the SLC4 gene family, but its utility for studies of AE2 regulation by extracellular pH must be evaluated in the context of its alkaline-shifted pHo sensitivity, resembling that of AE2 gastric parietal cell variant AE2c1.

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.

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.

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.