Mutating the Conserved Q-loop Glutamine 1291 Selectively Disrupts Adenylate Kinase-dependent Channel Gating of the ATP-binding Cassette (ABC) Adenylate Kinase Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and Reduces Channel Function in Primary Human Airway Epithelia

The ATP-binding cassette (ABC) transporter cystic fibrosis transmembrane conductance regulator (CFTR) and two other non-membrane-bound ABC proteins, Rad50 and a structural maintenance of chromosome (SMC) protein, exhibit adenylate kinase activity in the presence of physiologic concentrations of ATP and AMP or ADP (ATP + AMP ⇆ 2 ADP). The crystal structure of the nucleotide-binding domain of an SMC protein in complex with the adenylate kinase bisubstrate inhibitor P¹,P⁵-di(adenosine-5′) pentaphosphate (Ap₅A) suggests that AMP binds to the conserved Q-loop glutamine during the adenylate kinase reaction. Therefore, we hypothesized that mutating the corresponding residue in CFTR, Gln-1291, selectively disrupts adenylate kinase-dependent channel gating at physiologic nucleotide concentrations. We found that substituting Gln-1291 with bulky side-chain amino acids abolished the effects of Ap₅A, AMP, and adenosine 5′-monophosphoramidate on CFTR channel function. 8-Azidoadenosine 5′-monophosphate photolabeling of the AMP-binding site and adenylate kinase activity were disrupted in Q1291F CFTR. The Gln-1291 mutations did not alter the potency of ATP at stimulating current or ATP-dependent gating when ATP was the only nucleotide present. However, when physiologic concentrations of ADP and AMP were added, adenylate kinase-deficient Q1291F channels opened significantly less than wild type. Consistent with this result, we found that Q1291F CFTR displayed significantly reduced Cl⁻ channel function in well differentiated primary human airway epithelia. These results indicate that a highly conserved residue of an ABC transporter plays an important role in adenylate kinase-dependent CFTR gating. Furthermore, the results suggest that adenylate kinase activity is important for normal CFTR channel function in airway epithelia.     

Role of CFTR’s intrinsic adenylate kinase activity in gating of the Cl− channel

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl^?channel in the ATP-binding cassette (ABC) transporter protein family. CFTR features the modular design characteristic of ABC transporters, which includes two membrane-spanning domains forming the channel pore, and two ABC nucleotide-binding domains that interact with ATP and contain the enzymatic activity coupled to normal gating. Like other ABC transporters CFTR is an ATPase (ATP + H₂O → ADP + P_i). Recent work has shown that CFTR also possesses intrinsic adenylate kinase activity (ATP + AMP ? ADP + ADP). This finding raises important questions: How does AMP influence CFTR gating? Why does ADP inhibit CFTR current? Which enzymatic activity gates CFTR in vivo? Are there implications for other ABC transporters? This minireview attempts to shed light on these questions by summarizing recent advances in our understanding of the role of the CFTR adenylate kinase activity for channel gating.     

The intact CFTR protein mediates ATPase rather than adenylate kinase activity

The two NBDs (nucleotide-binding domains) of ABC (ATP-binding-cassette) proteins function in a complex to mediate ATPase activity and this activity has been linked to their regulated transport activity. A similar model has been proposed for CFTR (cystic fibrosis transmembrane conductance regulator), the chloride channel defective in cystic fibrosis, wherein ATP binding and hydrolysis regulate the channel gate. Recently, it was shown that the individual NBDs isolated from CFTR primarily mediate adenylate kinase activity, raising the possibility that this activity may also contribute to gating of the CFTR channel. However, this present study shows that whereas the isolated NBDs exhibit adenylate kinase activity, the full-length purified and reconstituted CFTR protein functions as an ATPase, arguing that the enzymatic activity of the NBDs is dependent on their molecular context and appropriate domain-domain assembly. As expected, the disease-causing mutant bearing a mutation in the ABC signature motif, CFTR-G551D, exhibited a markedly reduced ATPase activity. Furthermore, mutation of the putative catalytic base in CFTR caused a reduction in ATPase activity, with the CFTR-E1371Q mutant supporting a low level of residual activity. Neither of these mutants exhibited detectable adenylate kinase activity. Together, these findings support the concept that the molecular mechanism of action of CFTR is dependent on ATP binding and hydrolysis, and that the structure of prokaryotic ABC ATPases provide a useful template for understanding their mechanism of action.     

ATP and AMP Mutually Influence Their Interaction with the ATP-binding Cassette (ABC) Adenylate Kinase Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) at Separate Binding Sites

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP-binding cassette (ABC) transporter protein family. In the presence of ATP and physiologically relevant concentrations of AMP, CFTR exhibits adenylate kinase activity (ATP + AMP ⇆ 2 ADP). Previous studies suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for this activity. Two other ABC proteins, Rad50 and a structural maintenance of chromosome protein, also have adenylate kinase activity. All three ABC adenylate kinases bind and hydrolyze ATP in the absence of other nucleotides. However, little is known about how an ABC adenylate kinase interacts with ATP and AMP when both are present. Based on data from non-ABC adenylate kinases, we hypothesized that ATP and AMP mutually influence their interaction with CFTR at separate binding sites. We further hypothesized that only one of the two CFTR ATP-binding sites is involved in the adenylate kinase reaction. We found that 8-azidoadenosine 5′-triphosphate (8-N₃-ATP) and 8-azidoadenosine 5′-monophosphate (8-N₃-AMP) photolabeled separate sites in CFTR. Labeling of the AMP-binding site with 8-N₃-AMP required the presence of ATP. Conversely, AMP enhanced photolabeling with 8-N₃-ATP at ATP-binding site 2. The adenylate kinase active center probe P¹,P⁵-di(adenosine-5′) pentaphosphate interacted simultaneously with an AMP-binding site and ATP-binding site 2. These results show that ATP and AMP interact with separate binding sites but mutually influence their interaction with the ABC adenylate kinase CFTR. They further indicate that the active center of the adenylate kinase comprises ATP-binding site 2.     

An intrinsic adenylate kinase activity regulates gating of the ABC transporter CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP binding cassette (ABC) transporter family. Like other ABC transporters, it can hydrolyze ATP. Yet while ATP hydrolysis influences channel gating, it has long seemed puzzling that CFTR would require this reaction because anions flow passively through CFTR. Moreover, no other ion channel is known to require the large energy of ATP hydrolysis to gate. We found that CFTR also has adenylate kinase activity (ATP + AMP <=> ADP + ADP) that regulates gating. When functioning as an adenylate kinase, CFTR showed positive cooperativity for ATP suggesting its two nucleotide binding domains may dimerize. Thus, channel activity could be regulated by two different enzymatic reactions, ATPase and adenylate kinase, that share a common ATP binding site in the second nucleotide binding domain. At physiologic nucleotide concentrations, adenylate kinase activity, rather than ATPase activity may control gating, and therefore involve little energy consumption.     

Demonstration of Phosphoryl Group Transfer Indicates That the ATP-binding Cassette (ABC) Transporter Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Exhibits Adenylate Kinase Activity

Cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane-spanning adenosine 5′-triphosphate (ATP)-binding cassette (ABC) transporter. ABC transporters and other nuclear and cytoplasmic ABC proteins have ATPase activity that is coupled to their biological function. Recent studies with CFTR and two nonmembrane-bound ABC proteins, the DNA repair enzyme Rad50 and a structural maintenance of chromosome (SMC) protein, challenge the model that the function of all ABC proteins depends solely on their associated ATPase activity. Patch clamp studies indicated that in the presence of physiologically relevant concentrations of adenosine 5′-monophosphate (AMP), CFTR Cl⁻ channel function is coupled to adenylate kinase activity (ATP+AMP ⇆ 2 ADP). Work with Rad50 and SMC showed that these enzymes catalyze both ATPase and adenylate kinase reactions. However, despite the supportive electrophysiological results with CFTR, there are no biochemical data demonstrating intrinsic adenylate kinase activity of a membrane-bound ABC transporter. We developed a biochemical assay for adenylate kinase activity, in which the radioactive γ-phosphate of a nucleotide triphosphate could transfer to a photoactivatable AMP analog. UV irradiation could then trap the ³²P on the adenylate kinase. With this assay, we discovered phosphoryl group transfer that labeled CFTR, thereby demonstrating its adenylate kinase activity. Our results also suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for adenylate kinase activity. These biochemical data complement earlier biophysical studies of CFTR and indicate that the ABC transporter CFTR can function as an adenylate kinase.     

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.     

Converting Nonhydrolyzable Nucleotides to Strong Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Agonists by Gain of Function (GOF) Mutations

Cystic fibrosis transmembrane conductance regulator (CFTR) is the only ligand-gated ion channel that hydrolyzes its agonist, ATP. CFTR gating has been argued to be tightly coupled to its enzymatic activity, but channels do open occasionally in the absence of ATP and are reversibly activated (albeit weakly) by nonhydrolyzable nucleotides. Why the latter only weakly activates CFTR is not understood. Here we show that CFTR activation by adenosine 5′-O-(thiotriphosphate) (ATPγS), adenosine 5′-(β,γ-imino)triphosphate (AMP-PNP), and guanosine 5′-3-O-(thio)triphosphate (GTPγS) is enhanced substantially by gain of function (GOF) mutations in the cytosolic loops that increase unliganded activity. This enhancement correlated with the base-line nucleotide-independent activity for several GOF mutations. AMP-PNP or ATPγS activation required both nucleotide binding domains (NBDs) and was disrupted by a cystic fibrosis mutation in NBD1 (G551D). GOF mutant channels deactivated very slowly upon AMP-PNP or ATPγS removal (τ_deac ∼ 100 s) implying tight binding between the two NBDs. Despite this apparently tight binding, neither AMP-PNP nor ATPγS activated even the strongest GOF mutant as strongly as ATP. ATPγS-activated wild type channels deactivated more rapidly, indicating that GOF mutations in the cytosolic loops reciprocally/allosterically affect nucleotide occupancy of the NBDs. A GOF mutation substantially rescued defective ATP-dependent gating of G1349D-CFTR, a cystic fibrosis NBD2 signature sequence mutant. Interestingly, the G1349D mutation strongly disrupted activation by AMP-PNP but not by ATPγS, indicating that these analogs interact differently with the NBDs. We conclude that poorly hydrolyzable nucleotides are less effective than ATP at opening CFTR channels even when they bind tightly to the NBDs but are converted to stronger agonists by GOF mutations.     

Electrophysiological evidence for the presence of cystic fibrosis transmembrane conductance regulator (CFTR) in mouse sperm

Mammalian sperm must undergo a maturational process, named capacitation, in the female reproductive tract to fertilize the egg. Sperm capacitation is regulated by a cAMP/protein kinase A (PKA) pathway and involves increases in intracellular Ca²⁺, pH, Cl^?, protein tyrosine phosphorylation, and in mouse and some other mammals a membrane potential hyperpolarization. The cystic fibrosis transmembrane conductance regulator (CFTR), a Cl^? channel modulated by cAMP/PKA and ATP, was detected in mammalian sperm and proposed to modulate capacitation. Our whole‐cell patch‐clamp recordings from testicular mouse sperm now reveal a Cl^? selective component to membrane current that is ATP‐dependent, stimulated by cAMP, cGMP, and genistein (a CFTR agonist, at low concentrations), and inhibited by DPC and CFTR_inh‐172, two well‐known CFTR antagonists. Furthermore, the Cl^? current component activated by cAMP and inhibited by CFTR_inh‐172 is absent in recordings on testicular sperm from mice possessing the CFTR ΔF508 loss‐of‐function mutation, indicating that CFTR is responsible for this component. A Cl^? selective like current component displaying CFTR characteristics was also found in wild type epididymal sperm bearing the cytoplasmatic droplet. Capacitated sperm treated with CFTR_inh‐172 undergo a shape change, suggesting that CFTR is involved in cell volume regulation. These findings indicate that functional CFTR channels are present in mouse sperm and their biophysical properties are consistent with their proposed participation in capacitation. J. Cell. Physiol. 228: 590–601, 2013. © 2012 Wiley Periodicals, Inc.     

Crystal structures of Bacillus stearothermophilus adenylate kinase with bound Ap5A,Mg2+ Ap5A,and Mn2+ Ap5A reveal an intermediate lid position and six coordinate octahedral geometry for bound Mg2+ and Mn2+

Crystal structures of Bacillus stearothermophilus adenylate kinase with bound Ap₅A, Mn²⁺ Ap₅A, and Mg²⁺ Ap₅A have been determined by X-ray crystallography to resolutions of 1.6 Å, 1.85 Å, and 1.96 Å, respectively. The protein's lid domain is partially open, being both rotated and translated away from bound Ap₅A. The flexibility of the lid domain in the ternary state and its ability to transfer force directly to the the active site is discussed in light of our proposed entropic mechanism for catalytic turnover. The bound Zn²⁺ atom is demonstrably structural in nature, with no contacts other than its ligating cysteine residues within 5 Å. The B. stearothermophilus adenylate kinase lid appears to be a truncated zinc finger domain, lacking the DNA binding finger, which we have termed a zinc knuckle domain. In the Mg²⁺ Ap₅A and Mn²⁺ Ap₅A structures, Mg²⁺ and Mn²⁺ demonstrate six coordinate octahedral geometry. The interactions of the Mg²⁺-coordinated water molecules with the protein and Ap₅A phosphate chain demonstrate their involvement in catalyzing phosphate transfer. The protein selects for β-γ (preferred by Mg²⁺) rather than α-γ (preferred by Mn²⁺) metal ion coordination by forcing the ATP phosphate chain to have an extended conformation. Proteins 32:276–288, 1998. © 1998 Wiley-Liss, Inc.     

Coupling of ATP Hydrolysis with Channel Gating by Purified, Reconstituted CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel situated on the apical membrane of epithelial cells. Our recent studies of purified, reconstituted CFTR revealed that it also functions as an ATPase and that there may be coupling between ATP hydrolysis and channel gating. Both the ATP turnover rate and channel gating are slow, in the range of 0.2 to 1 s^–1, and both activities are suppressed in a disease-causing mutation situated in a putative nucleotide binding motif. Our future studies using purified protein will be directed toward understanding the structural basis and mechanism for coupling between hydrolysis and channel function.     

A unified view of cystic fibrosis transmembrane conductance regulator (CFTR) gating: combining the allosterism of a ligand-gated channel with the enzymatic activity of an ATP-binding cassette (ABC) transporter

The cystic fibrosis transmembrane conductance regulator (CFTR) is a unique ion channel in that its gating is coupled to an intrinsic enzymatic activity (ATP hydrolysis). This enzymatic activity derives from the evolutionary origin of CFTR as an ATP-binding cassette transporter. CFTR gating is distinct from that of a typical ligand-gated channel because its ligand (ATP) is usually consumed during the gating cycle. However, recent findings indicate that CFTR gating exhibits allosteric properties that are common to conventional ligand-gated channels (e.g. unliganded openings and constitutive mutations). Here, we provide a unified view of CFTR gating that combines the allosterism of a ligand-gated channel with its unique enzymatic activity.     

Potentiation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl− currents by the chemical solvent tetrahydrofuran

The chemical solvent tetrahydrofuran (THF) increases short-circuit current (I_sc) in renal epithelia endogenously expressing the cystic fibrosis transmembrane conductance regulator (CFTR). To understand how THF increases I_sc, we employed the Ussing chamber and patch-clamp techniques to study cells expressing recombinant human CFTR. THF increased I_sc in Fischer rat thyroid (FRT) epithelia expressing wild-type CFTR with half-maximal effective concentration (K_D) of 134?mM. This THF-induced increase in I_sc was enhanced by forskolin (10 µM), inhibited by the PKA inhibitor H-89 (10 µM) and the thiazolidinone CFTR_inh-172 (10 µM) and attenuated greatly in FRT epithelia expressing the cystic fibrosis mutants F508del- and G551D-CFTR. By contrast, THF (100?mM) was without effect on untransfected FRT epithelia, while other solvents failed to increase I_sc in FRT epithelia expressing wild-type CFTR. In excised inside-out membrane patches, THF (100?mM) potentiated CFTR Cl^? channels open in the presence of ATP (1?mM) alone by increasing the frequency of channel openings without altering their duration. However, following the phosphorylation of CFTR by PKA (75?nM), THF (100?mM) did not potentiate channel activity. Similar results were obtained with the ?R-S660A-CFTR Cl^? channel that is not regulated by PKA-dependent phosphorylation and using 2′deoxy-ATP, which gates wild-type CFTR more effectively than ATP. Our data suggest that THF acts directly on CFTR to potentiate channel gating, but that its efficacy is weak and dependent on the phosphorylation status of CFTR.     

Channel Gating Regulation by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) First Cytosolic Loop

In this study, we present data indicating a robust and specific domain interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) first cytosolic loop (CL1) and nucleotide binding domain 1 (NBD1) that allows ion transport to proceed in a regulated fashion. We used co-precipitation and ELISA to establish the molecular contact and showed that binding kinetics were not altered by the common clinical mutation F508del. Both intrinsic ATPase activity and CFTR channel gating were inhibited severely by CL1 peptide, suggesting that NBD1/CL1 binding is a crucial requirement for ATP hydrolysis and channel function. In addition to cystic fibrosis, CFTR dysregulation has been implicated in the pathogenesis of prevalent diseases such as chronic obstructive pulmonary disease, acquired rhinosinusitis, pancreatitis, and lethal secretory diarrhea (e.g. cholera). On the basis of clinical relevance of the CFTR as a therapeutic target, a cell-free drug screen was established to identify modulators of NBD1/CL1 channel activity independent of F508del CFTR and pharmacologic rescue. Our findings support a targetable mechanism of CFTR regulation in which conformational changes in the NBDs cause reorientation of transmembrane domains via interactions with CL1 and result in channel gating.     

Accessory protein facilitated CFTR-CFTR interaction, a molecular mechanism to potentiate the chloride channel activity

The cystic fibrosis transmembrane conductance regulator (CFTR) gene encodes a chloride channel protein that belongs to the superfamily of ATP binding cassette (ABC) transporters. Phosphorylation by protein kinase A in the presence of ATP activates the CFTR-mediated chloride conductance of the apical membranes. We have identified a novel hydrophilic CFTR binding protein, CAP70, which is also concentrated on the apical surfaces. CAP70 consists of four PDZ domains, three of which are capable of binding to the CFTR C terminus. Linking at least two CFTR molecules via cytoplasmic C-terminal binding by either multivalent CAP70 or a bivalent monoclonal antibody potentiates the CFTR chloride channel activity. Thus, the CFTR channel can be switched to a more active conducting state via a modification of intermolecular CFTR-CFTR contact that is enhanced by an accessory protein.     

Cystic Fibrosis Transmembrane Conductance Regulator–associated ATP Release Is Controlled by a Chloride Sensor

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is defective in cystic fibrosis, and has also been closely associated with ATP permeability in cells. Using a Xenopus oocyte cRNA expression system, we have evaluated the molecular mechanisms that control CFTR-modulated ATP release. CFTR-modulated ATP release was dependent on both cAMP activation and a gradient change in the extracellular chloride concentration. Activation of ATP release occurred within a narrow concentration range of external Cl⁻ that was similar to that reported in airway surface fluid. Mutagenesis of CFTR demonstrated that Cl⁻ conductance and ATP release regulatory properties could be dissociated to different regions of the CFTR protein. Despite the lack of a need for Cl⁻ conductance through CFTR to modulate ATP release, alterations in channel pore residues R347 and R334 caused changes in the relative ability of different halides to activate ATP efflux (wtCFTR, Cl >> Br; R347P, Cl >> Br; R347E, Br >> Cl; R334W, Cl = Br). We hypothesize that residues R347 and R334 may contribute a Cl⁻ binding site within the CFTR channel pore that is necessary for activation of ATP efflux in response to increases of extracellular Cl⁻. In summary, these findings suggest a novel chloride sensor mechanism by which CFTR is capable of responding to changes in the extracellular chloride concentration by modulating the activity of an unidentified ATP efflux pathway. This pathway may play an important role in maintaining fluid and electrolyte balance in the airway through purinergic regulation of epithelial cells. Insight into these molecular mechanisms enhances our understanding of pathogenesis in the cystic fibrosis lung.     

Structural Basis for Adenylate Kinase Activity in ABC ATPases

ATP-binding cassette (ABC) enzymes are involved in diverse biological processes ranging from transmembrane transport to chromosome cohesion and DNA repair. They typically use ATP hydrolysis to conduct energy-dependent biological reactions. However, the cystic fibrosis transmembrane conductance regulator and the DNA repair protein Rad50 can also catalyze the adenylate kinase reaction (ATP + AMP ↔ 2ADP). To clarify and provide a mechanistic basis for the adenylate kinase activity of ABC enzymes, we report the crystal structure of the nucleotide-binding domain of the Pyrococcus furiosus structural maintenance of chromosome protein (pfSMC^nbd) in complex with the adenylate kinase inhibitor P¹,P⁵-di(adenosine-5′)pentaphosphate. We show that pfSMC^nbd possesses reverse adenylate kinase activity. Our results suggest that in adenylate kinase reactions, ATP binds to its canonical binding site while AMP binds to the Q-loop glutamine and a hydration water of the Mg²⁺ ion. Furthermore, mutational analysis indicates that adenylate kinase reaction occurs in the engaged pfSMC^nbd dimer and requires the Signature motif for phosphate transfer. Our results explain how ATP hydrolysis and adenylate kinase reactions can be catalyzed by the same functional motifs within the structural framework of ABC enzymes. Thus, adenylate kinase activity is likely to be a latent activity in many ABC enzymes.     

Nucleotide-binding domains of cystic fibrosis transmembrane conductance regulator, an ABC transporter, catalyze adenylate kinase activity but not ATP hydrolysis

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP-binding cassette (ABC) transporter family. CFTR consists of two transmembrane domains, two nucleotide-binding domains (NBD1 and NBD2), and a regulatory domain. Previous biochemical reports suggest NBD1 is a site of stable nucleotide interaction with low ATPase activity, whereas NBD2 is the site of active ATP hydrolysis. It has also been reported that NBD2 additionally possessed adenylate kinase (AK) activity. Knowledge about the intrinsic biochemical activities of the NBDs is essential to understanding the Cl(-) ion gating mechanism. We find that purified mouse NBD1, human NBD1, and human NBD2 function as adenylate kinases but not as ATPases. AK activity is strictly dependent on the addition of the adenosine monophosphate (AMP) substrate. No liberation of [(33)P]phosphate is observed from the gamma-(33)P-labeled ATP substrate in the presence or absence of AMP. AK activity is intrinsic to both human NBDs, as the Walker A box lysine mutations abolish this activity. At low protein concentration, the NBDs display an initial slower nonlinear phase in AK activity, suggesting that the activity results from homodimerization. Interestingly, the G551D gating mutation has an exaggerated nonlinear phase compared with the wild type and may indicate this mutation affects the ability of NBD1 to dimerize. hNBD1 and hNBD2 mixing experiments resulted in an 8-57-fold synergistic enhancement in AK activity suggesting heterodimer formation, which supports a common theme in ABC transporter models. A CFTR gating mechanism model based on adenylate kinase activity is proposed.     

Optimization of the Degenerated Interfacial ATP Binding Site Improves the Function of Disease-related Mutant Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Channels

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, an ATP binding cassette (ABC) protein whose defects cause the deadly genetic disease cystic fibrosis (CF), encompasses two nucleotide binding domains (NBD1 and NBD2). Recent studies indicate that in the presence of ATP, the two NBDs coalesce into a dimer, trapping an ATP molecule in each of the two interfacial composite ATP binding sites (site 1 and site 2). Experimental evidence also suggests that CFTR gating is mainly controlled by ATP binding and hydrolysis in site 2, whereas site 1, which harbors several non-canonical substitutions in ATP-interacting motifs, is considered degenerated. The CF-associated mutation G551D, by introducing a bulky and negatively charged side chain into site 2, completely abolishes ATP-induced openings of CFTR. Here, we report a strategy to optimize site 1 for ATP binding by converting two amino acid residues to ABC consensus (i.e. H1348G) or more commonly seen residues in other ABC proteins (i.e. W401Y,W401F). Introducing either one or both of these mutations into G551D-CFTR confers ATP responsiveness for this disease-associated mutant channel. We further showed that the same maneuver also improved the function of WT-CFTR and the most common CF-associated ΔF508 channels, both of which rely on site 2 for gating control. Thus, our results demonstrated that the degenerated site 1 can be rebuilt to complement or support site 2 for CFTR function. Possible approaches for developing CFTR potentiators targeting site 1 will be discussed.     

Potentiation of Disease-associated Cystic Fibrosis Transmembrane Conductance Regulator Mutants by Hydrolyzable ATP Analogs

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel belonging to the ATP-binding cassette transporter superfamily. CFTR is gated by ATP binding and hydrolysis at its two nucleotide-binding domains (NBDs), which dimerize in the presence of ATP to form two ATP-binding pockets (ABP1 and ABP2). Mutations reducing the activity of CFTR result in the genetic disease cystic fibrosis. Two of the most common mutations causing a severe phenotype are G551D and ΔF508. Previously we found that the ATP analog N⁶-(2-phenylethyl)-ATP (P-ATP) potentiates the activity of G551D by ∼7-fold. Here we show that 2′-deoxy-ATP (dATP), but not 3′-deoxy-ATP, increases the activity of G551D-CFTR by ∼8-fold. We custom synthesized N⁶-(2-phenylethyl)-2′-deoxy-ATP (P-dATP), an analog combining the chemical modifications in dATP and P-ATP. This new analog enhances G551D current by 36.2 ± 5.4-fold suggesting an independent but energetically additive action of these two different chemical modifications. We show that P-dATP binds to ABP1 to potentiate the activity of G551D, and mutations in both sides of ABP1 (W401G and S1347G) decrease its potentiation effect, suggesting that the action of P-dATP takes place at the interface of both NBDs. Interestingly, P-dATP completely rectified the gating abnormality of ΔF508-CFTR by increasing its activity by 19.5 ± 3.8-fold through binding to both ABPs. This result highlights the severity of the gating defect associated with ΔF508, the most prevalent disease-associated mutation. The new analog P-dATP can be not only an invaluable tool to study CFTR gating, but it can also serve as a proof-of-principle that, by combining elements that potentiate the channel activity independently, the increase in chloride transport necessary to reach a therapeutic target is attainable.