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

Structural mechanisms of CFTR function and dysfunction

Hwang et al. integrate new structural insights with prior functional studies to reveal the functional anatomy of CFTR chloride channels.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel plays a critical role in regulating transepithelial movement of water and electrolyte in exocrine tissues. Malfunction of the channel because of mutations of the cftr gene results in CF, the most prevalent lethal genetic disease among Caucasians. Recently, the publication of atomic structures of CFTR in two distinct conformations provides, for the first time, a clear overview of the protein. However, given the highly dynamic nature of the interactions among CFTR’s various domains, better understanding of the functional significance of these structures requires an integration of these new structural insights with previously established biochemical/biophysical studies, which is the goal of this review.

Architecture and functional properties of the CFTR channel pore

The main function of the cystic fibrosis transmembrane conductance regulator (CFTR) is as an ion channel for the movement of small anions across epithelial cell membranes. As an ion channel, CFTR must form a continuous pathway across the cell membrane-referred to as the channel pore-for the rapid electrodiffusional movement of ions. This review summarizes our current understanding of the architecture of the channel pore, as defined by electrophysiological analysis and molecular modeling studies. This includes consideration of the characteristic functional properties of the pore, definition of the overall shape of the entire extent of the pore, and discussion of how the molecular structure of distinct regions of the pore might control different facets of pore function. Comparisons are drawn with closely related proteins that are not ion channels, and also with structurally unrelated proteins with anion channel function. A simple model of pore function is also described.

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.

Regulatory Crosstalk by Protein Kinases on CFTR Trafficking and Activity

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a member of the ATP binding cassette (ABC) transporter superfamily that functions as a cAMP-activated chloride ion channel in fluid-transporting epithelia. There is abundant evidence that CFTR activity (i.e., channel opening and closing) is regulated by protein kinases and phosphatases via phosphorylation and dephosphorylation. Here, we review recent evidence for the role of protein kinases in regulation of CFTR delivery to and retention in the plasma membrane. We review this information in a broader context of regulation of other transporters by protein kinases because the overall functional output of transporters involves the integrated control of both their number at the plasma membrane and their specific activity. While many details of the regulation of intracellular distribution of CFTR and other transporters remain to be elucidated, we hope that this review will motivate research providing new insights into how protein kinases control membrane transport to impact health and disease.

Structural Changes Fundamental to Gating of the Cystic Fibrosis Transmembrane Conductance Regulator Anion Channel Pore

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial cell anion channel. Potentiator drugs used in the treatment of cystic fibrosis act on the channel to increase overall channel function, by increasing the stability of its open state and/or decreasing the stability of its closed state. The structure of the channel in either the open state or the closed state is not currently known. However, changes in the conformation of the protein as it transitions between these two states have been studied using functional investigation and molecular modeling techniques. This review summarizes our current understanding of the architecture of the transmembrane channel pore that controls the movement of chloride and other small anions, both in the open state and in the closed state. Evidence for different kinds of changes in the conformation of the pore as it transitions between open and closed states is described, as well as the mechanisms by which these conformational changes might be controlled to regulate normal channel gating. The ways that key conformational changes might be targeted by small compounds to influence overall CFTR activity are also discussed. Understanding the changes in pore structure that might be manipulated by such small compounds is key to the development of novel therapeutic strategies for the treatment of cystic fibrosis.

Drug-induced secretory diarrhea: A role for CFTR

Many medications induce diarrhea as a side effect, which can be a major obstacle to therapeutic efficacy and also a life-threatening condition. Secretory diarrhea can be caused by excessive fluid secretion in the intestine under pathological conditions. The cAMP/cGMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) is the primary chloride channel at the apical membrane of intestinal epithelial cells and plays a major role in intestinal fluid secretion and homeostasis. CFTR forms macromolecular complexes at discreet microdomains at the plasma membrane, and its chloride channel function is regulated spatiotemporally through protein-protein interactions and cAMP/cGMP-mediated signaling. Drugs that perturb CFTR-containing macromolecular complexes in the intestinal epithelium and upregulate intracellular cAMP and/or cGMP levels can hyperactivate the CFTR channel, causing excessive fluid secretion and secretory diarrhea. Inhibition of CFTR chloride-channel activity may represent a novel approach to the management of drug-induced secretory diarrhea.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): CLOSED AND OPEN STATE CHANNEL MODELS

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily. CFTR controls the flow of anions through the apical membrane of epithelia. Dysfunctional CFTR causes the common lethal genetic disease cystic fibrosis. Transitions between open and closed states of CFTR are regulated by ATP binding and hydrolysis on the cytosolic nucleotide binding domains, which are coupled with the transmembrane (TM) domains forming the pathway for anion permeation. Lack of structural data hampers a global understanding of CFTR and thus the development of "rational" approaches directly targeting defective CFTR. In this work, we explored possible conformational states of the CFTR gating cycle by means of homology modeling. As templates, we used structures of homologous ABC transporters, namely TM(287-288), ABC-B10, McjD, and Sav1866. In the light of published experimental results, structural analysis of the transmembrane cavity suggests that the TM(287-288)-based CFTR model could correspond to a commonly occupied closed state, whereas the McjD-based model could represent an open state. The models capture the important role played by Phe-337 as a filter/gating residue and provide structural information on the conformational transition from closed to open channel.

The Fifth Transmembrane Segment of Cystic Fibrosis Transmembrane Conductance Regulator Contributes to Its Anion Permeation Pathway

Previous studies have identified several transmembrane segments (TMs), including TM1, TM3, TM6, TM9, TM11, and TM12, as pore-lining segments in cystic fibrosis transmembrane conductance regulator (CFTR), but the role of TM5 in pore construction remains controversial. In this study, we employed substituted cysteine accessibility methodology (SCAM) to screen the entire TM5 defined by the original topology model and its cytoplasmic extension in a Cysless background. We found six positions (A299, R303, N306, S307, F310, and F311) where engineered cysteines react to intracellular 2-sulfonatoethyl methanethiosulfonate (MTSES⁻). Quantification of the modification rate of engineered cysteines in the presence or absence of ATP suggests that these six residues are accessible in both the open and closed states. Whole-cell experiments with external MTSES⁻ identified only two positive positions (L323 and A326), resulting in a segment containing 11 consecutive amino acids, where substituted cysteines respond to neither internal nor external MTSES⁻, a unique feature not seen previously in CFTR's pore-lining segments. The observation that these positions are inaccessible to channel-permeant thiol-specific reagent [Au(CN)₂]⁻ suggests that this segment of TM5 between F311 and L323 is concealed from the pore by other TMs and/or lipid bilayers. In addition, our data support the idea that the positively charged arginine at position 303 poses a pure electrostatic action in determining the single-channel current amplitude of CFTR and the effect of an open-channel blocker glibencalmide. Collectively, we conclude that the cytoplasmic portion of CFTR's TM5 lines the pore. Our functional data are remarkably consistent with predicted structural arrangements of TM5 in some homology models of CFTR.

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.

Conformational changes opening and closing the CFTR chloride channel: insights from cysteine scanning mutagenesis

Cystic fibrosis, the most common lethal genetic disease affecting young people in North America, is caused by failure of the chloride ion channel known as CFTR (cystic fibrosis transmembrane conductance regulator). CFTR belongs to the large family of ATP-binding cassette (ABC) membrane transporters. In CFTR, ATP-driven events at the nucleotide-binding domains (NBDs) open and close a gate that controls chloride permeation. However, the conformational changes concomitant with opening and closing of the CFTR gate are unknown. Diverse techniques including substituted cysteine accessibility method, disulfide cross-linking, and patch-clamp recording have been used to explore CFTR channel structure. Here, we consider the architecture of both the open and the closed CFTR channel. We review how CFTR channel structure changes between the closed and the open channel conformations and portray the relative function of both cytoplasmic and vestigial gates during the gating cycle. Understanding how the CFTR channel gates chloride permeation is central for understanding how CFTR defects lead to CF. Such knowledge opens the door for novel ways to maximize CFTR channel activity in a CF setting.

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.

Functional architecture of the CFTR chloride channel

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of membrane transport proteins. CFTR is unique among ABC proteins in that it functions not as an active transporter but as an ATP-gated Cl(-) channel. As an ion channel, the function of the CFTR transmembrane channel pore that mediates Cl(-) movement has been studied in great detail. On the other hand, only low resolution structural data is available on the transmembrane parts of the protein. The structure of the channel pore has, however, been modeled on the known structure of active transporter ABC proteins. Currently, significant barriers exist to building a unified view of CFTR pore structure and function. Reconciling functional data on the channel with indirect structural data based on other proteins with very different transport functions and substrates has proven problematic. This review summarizes current structural and functional models of the CFTR Cl(-) channel pore, including a comprehensive review of previous electrophysiological investigations of channel structure and function. In addition, functional data on the three-dimensional arrangement of pore-lining helices, as well as contemporary hypotheses concerning conformational changes in the pore that occur during channel opening and closing, are discussed. Important similarities and differences between different models of the pore highlight current gaps in our knowledge of CFTR structure and function. In order to fill these gaps, structural and functional models of the membrane-spanning pore need to become better integrated.

Relative contribution of different transmembrane segments to the CFTR chloride channel pore

The membrane-spanning part of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel comprises 12 transmembrane (TM) α-helices, arranged in 2 symmetrical groups of 6. However, those TMs that line the channel pore are not completely defined. We used patch clamp recording to compare the accessibility of cysteine-reactive reagents to cysteines introduced into different TMs. Several residues in TM11 were accessible to extracellular and/or intracellular cysteine reactive reagents; however, no reactive cysteines were identified in TMs 5 or 11. Two accessible residues in TM11 (T1115C and S1118C) were found to be more readily modified from the extracellular solution in closed channels, but more readily modified from the intracellular solution in open channels, as previously reported for T338C in TM6. However, the effects of mutagenesis at S1118 (TM11) on a range of pore functional properties were relatively minor compared to the large effects of mutagenesis at T338 (TM6). Our results suggest that the CFTR pore is lined by TM11 but not by TM5 or TM7. Comparison with previous works therefore suggests that the pore is lined by TMs 1, 6, 11, and 12, suggesting that the structure of the open channel pore is asymmetric in terms of the contributions of different TMs. Although TMs 6 and 11 appear to undergo similar conformational changes during channel opening and closing, the influence of these two TMs on the functional properties of the narrowest region of the pore is clearly unequal.

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.

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.