Stimulation of CFTR activity by its phosphorylated R domain

Computational studies reveal phosphorylation-dependent changes in the unstructured R domain of CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent chloride channel that is mutated in cystic fibrosis, an inherited disease of high morbidity and mortality. The phosphorylation of its approximately 200 amino acid R domain by protein kinase A is obligatory for channel gating under normal conditions. The R domain contains more than ten PKA phosphorylation sites. No individual site is essential but phosphorylation of increasing numbers of sites enables progressively greater channel activity. In spite of numerous studies of the role of the R domain in CFTR regulation, its mechanism of action remains largely unknown. This is because neither its structure nor its interactions with other parts of CFTR have been completely elucidated. Studies have shown that the R domain lacks well-defined secondary structural elements and is an intrinsically disordered region of the channel protein. Here, we have analyzed the disorder pattern and employed computational methods to explore low-energy conformations of the R domain. The specific disorder and secondary structure patterns detected suggest the presence of molecular recognition elements (MoREs) that may mediate phosphorylation-regulated intra- and inter-domain interactions. Simulations were performed to generate an ensemble of accessible R domain conformations. Although the calculated structures may represent more compact conformers than occur in vivo, their secondary structure propensities are consistent with predictions and published experimental data. Equilibrium simulations of a mimic of a phosphorylated R domain showed that it exhibited an increased radius of gyration. In one possible interpretation of these findings, by changing its size, the globally unstructured R domain may act as an entropic spring to perturb the packing of membrane-spanning sequences that constitute the ion permeability pathway and thereby activate channel gating.

CFTR regulatory region interacts with NBD1 predominantly via multiple transient helices

The regulatory (R) region of the cystic fibrosis transmembrane conductance regulator (CFTR) is intrinsically disordered and must be phosphorylated at multiple sites for full CFTR channel activity, with no one specific phosphorylation site required. In addition, nucleotide binding and hydrolysis at the nucleotide-binding domains (NBDs) of CFTR are required for channel gating. We report NMR studies in the absence and presence of NBD1 that provide structural details for the isolated R region and its interaction with NBD1 at residue-level resolution. Several sites in the R region with measured fractional helical propensity mediate interactions with NBD1. Phosphorylation reduces the helicity of many R-region sites and reduces their NBD1 interactions. This evidence for a dynamic complex with NBD1 that transiently engages different sites of the R region suggests a structural explanation for the dependence of CFTR activity on multiple PKA phosphorylation sites.

The ABC protein turned chloride channel whose failure causes cystic fibrosis

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

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Insight in eukaryotic ABC transporter function by mutation analysis

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

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.

Phosphorylation of CFTR by PKA promotes binding of the regulatory domain

The unphosphorylated regulatory (R) domain of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) is often viewed as an inhibitor that is released by phosphorylation. To test this notion, we studied domain interactions using CFTR channels assembled from three polypeptides. Nucleotides encoding the R domain (aa 635-836) were replaced with an internal ribosome entry sequence so that amino- and carboxyl-terminal half-molecules would be translated from the same mRNA transcript. Although only core glycosylation was detected on SplitDeltaR, biotinylation, immunostaining, and functional studies clearly demonstrated its trafficking to the plasma membrane. SplitDeltaR generated a constitutive halide permeability, which became responsive to cAMP when the missing R domain was coexpressed. Each half-molecule was co-precipitated by antibody against the other half. Contrary to expectations, GST-R domain was pulled down only if prephosphorylated by protein kinase A, and coexpressed R domain was precipitated with SplitDeltaR much more efficiently when cells were stimulated with cAMP. These results indicate that phosphorylation regulates CFTR by promoting association of the R domain with other domains rather than by causing its dissociation from an inhibitory site.

Rainbow tags: a visual tag system for recombinant protein expression and purification

Visualization systems for tracking proteins are standard experimental tools in most areas of biological research apart from protein purification. Here, we have sought to plug this gap by producing red and yellow visual tags using the heme-binding domain of mosquito cytochrome b5 and the flavin mononucleotide (FMN)-binding domain of human P450 reductase. Tests with colorless glutathione-S-transferase (GST) show them to be simple and effective tools for visually identifying correctly folded protein and tracking protein molecules through protein expression and purification. Furthermore, the characteristic absorbance signatures of the colored tags can be used to quantify protein concentrations directly, which allows purification to be linked to colorimetric detection. This technology, which we call Rainbow Tagging, facilitates expression and downstream processing of recombinant proteins, paving the way for the development of automated high-throughput protein expression systems.

Curcumin Stimulates Cystic Fibrosis Transmembrane Conductance Regulator Cl– Channel Activity

Compounds that enhance either the function or biosynthetic processing of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel may be of value in developing new treatments for cystic fibrosis (CF). Previous studies suggested that the herbal extract curcumin might affect the processing of a common CF mutant, CFTR-DeltaF508. Here, we tested the hypothesis that curcumin influences channel function. Curcumin increased CFTR channel activity in excised, inside-out membrane patches by reducing channel closed time and prolonging the time channels remained open. Stimulation was dose-dependent, reversible, and greater than that observed with genistein, another compound that stimulates CFTR. Curcumin-dependent stimulation required phosphorylated channels and the presence of ATP. We found that curcumin increased the activity of both wild-type and DeltaF508 channels. Adding curcumin also increased Cl(-) transport in differentiated non-CF airway epithelia but not in CF epithelia. These results suggest that curcumin may directly stimulate CFTR Cl(-) channels.

ADP inhibits function of the ABC transporter cystic fibrosis transmembrane conductance regulator via its adenylate kinase activity

ADP interacts with the nucleotide-binding domains (NBDs) of the cystic fibrosis transmembrane conductance regulator (CFTR) to inhibit its Cl- channel activity. Because CFTR NBD2 has reversible adenylate kinase activity (ATP + AMP<==> ADP + ADP) that gates the channel, we asked whether ADP might inhibit current through this enzymatic activity. In adenylate kinases, binding of the two ADP molecules is cooperative. Consistent with this hypothesis, CFTR current inhibition showed positive cooperativity for ADP. We also found that ADP inhibition of current was attenuated when we prevented adenylate kinase activity with P1,P5-di(adenosine-5') pentaphosphate. Additional studies suggested that adenylate kinase-dependent inhibition involved phosphotransfer between two nucleotide diphosphates. These data indicate that the adenylate kinase reaction at NBD2 contributed to the inhibitory effect of ADP. Finding that ADP inhibits function via an adenylate kinase activity also helps explain the earlier observation that mutations that disrupt adenylate kinase activity also disrupt ADP inhibition. Thus, the results reveal a previously unrecognized mechanism by which ADP inhibits an ABC transporter.

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

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

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

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

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Phosphorylation by Protein Kinase CK2 Modulates the Activity of the ATP Binding Cassette A1 Transporter

In a previous characterization of the ABCA subfamily of the ATP-binding cassette (ABC) transporters, we identified potential protein kinase 2 (CK2) phosphorylation sites, which are conserved in eukaryotic and prokaryotic members of the ABCA transporters. These phosphorylation residues are located in the conserved cytoplamic R1 and R2 domains, downstream of the nucleotide binding domains NBD1 and NBD2. To study the possible regulation of the ABCA1 transporter by CK2, we expressed the recombinant cytoplasmic domains of ABCA1, NBD1+R1 and NBD2+R2. We demonstrated that in vitro ABCA1 NBD1+R1, and not NBD2+R2, is phosphorylated by CK2, and we identified Thr-1242, Thr-1243, and Ser-1255 as the phosphorylated residues in the R1 domain by mass spectrometry. We further investigated the functional significance of the threonine and serine phosphorylation sites in NBD1 by site-directed mutagenesis of the entire ABCA1 followed by transfection into Hek-293 Tet-Off cells. The ABCA1 flippase activity, apolipoprotein AI and AII binding, and cellular phospholipid and cholesterol efflux were enhanced by mutations preventing CK2 phosphorylation of the threonine and serine residues. This was confirmed by the effect of specific protein kinase CK2 inhibitors upon the activity of wild type and mutant ABCA1 in transfected Hek-293 Tet-Off cells. The activities of the mutants mimicking threonine phosphorylation were close to that of wild type ABCA1. Our data, therefore, suggest that besides protein kinase A and C, protein kinase CK2 might play an important role in vivo in regulating the function and transport activity of ABCA1 and possibly of other members of the ABCA subfamily.

Potentiation of effect of PKA stimulation of Xenopus CFTR by activation of PKC: role of NBD2

Activity of the human (h) cystic fibrosis transmembrane conductance regulator (CFTR) channel is predominantly regulated by PKA-mediated phosphorylation. In contrast, Xenopus (X)CFTR is more responsive to PKC than PKA stimulation. We investigated the interaction between the two kinases in XCFTR. We expressed XCFTR in Xenopus oocytes and maximally stimulated it with PKA agonists. The magnitude of activation after PKC stimulation was about eightfold that without pretreatment with PKC agonist. hCFTR, expressed in the same system, lacked this response. We name this phenomenon XCFTR-specific PKC potentiation effect. To ascertain its biophysical mechanism, we first tested for XCFTR channel insertion into the plasma membrane by a substituted-cysteine-accessibility method. No insertion was detected during kinase stimulation. Next, we studied single-channel properties and found that the single-channel open probability (Po) with PKA stimulation subsequent to PKC stimulation was 2.8-fold that observed in the absence of PKC preactivation and that single-channel conductance (gamma) was increased by approximately 22%. To ascertain which XCFTR regions are responsible for the potentiation, we constructed several XCFTR-hCFTR chimeras, expressed them in Xenopus oocytes, and tested them electrophysiologically. Two chimeras [hCFTR NH2-terminal region or regulatory (R) domain in XCFTR] showed a significant decrease in potentiation. In the chimera in which XCFTR nucleotide-binding domain (NBD)2 was replaced with the hCFTR sequence there was no potentiation whatsoever. The converse chimera (hCFTR with Xenopus NBD2) did not exhibit potentiation. These results indicate that potentiation by PKC involves a large increase in Po (with a small change in gamma) without CFTR channel insertion into the plasma membrane, that XCFTR NBD2 is necessary but not sufficient for the effect, and that the potentiation effect is likely to involve other CFTR domains.

Dibasic phosphorylation sites in the R domain of CFTR have stimulatory and inhibitory effects on channel activation

To better understand the mechanisms by which PKA-dependent phosphorylation regulates CFTR channel activity, we have assayed open probabilities (P(o)), mean open time, and mean closed time for a series of CFTR constructs with mutations at PKA phosphorylation sites in the regulatory (R) domain. Forskolin-stimulated channel activity was recorded in cell-attached and inside-out excised patches from transiently transfected Chinese hamster ovary cells. Wild-type CFTR and constructs with a single Ser-to-Ala mutation as well as octa (Ser-to-Ala mutations at 8 sites) and constructs with one or two Ala-to-Ser mutations were studied. In cell-attached patches, Ser-to-Ala mutations at amino acids 700, 795, and 813 decreased P(o), whereas Ser-to-Ala mutations at 737 and 768 increased P(o). In general, differences in P(o) were due to differences in mean closed time. For selected constructs with either high or low values of P(o), channel activity was measured in excised patches. With 1 mM ATP, P(o) was similar to that observed in cell-attached patches, but with 10 mM ATP, all constructs tested showed elevated P(o) values. ATP-dependent increases in P(o) were due to reductions in mean closed time. These results indicate that R-domain phosphorylation affects ATP binding and not the subsequent steps of hydrolysis and channel opening. A model was developed whereby R-domain phosphorylation, in a site-dependent manner, alters equilibrium between forms of CFTR with low and high affinities for ATP.

Protein kinase A regulates ATP hydrolysis and dimerization by a CFTR (cystic fibrosis transmembrane conductance regulator) domain

Gating of the CFTR Cl- channel is associated with ATP hydrolysis at the nucleotide-binding domains (NBD1, NBD2) and requires PKA (protein kinase A) phosphorylation of the R domain. The manner in which the NBD1, NBD2 and R domains of CFTR (cystic fibrosis transmembrane conductance regulator) interact to achieve a properly regulated ion channel is largely unknown. In this study we used bacterially expressed recombinant proteins to examine interactions between these soluble domains of CFTR in vitro. PKA phosphorylated a fusion protein containing NBD1 and R (NBD1-R-GST) on CFTR residues Ser-660, Ser-700, Ser-712, Ser-737, Ser-768, Ser-795 and Ser-813. Phosphorylation of these serine residues regulated ATP hydrolysis by NBD1-R-GST by increasing the apparent K(m) for ATP (from 70 to 250 microM) and the Hill coefficient (from 1 to 1.7) without changing the V(max). When fusion proteins were photolabelled with 8-azido-[alpha-32P]ATP, PKA phosphorylation increased the apparent k(d) for nucleotide binding and it caused binding to become co-operative. PKA phosphorylation also resulted in dimerization of NBD1-R-GST but not of R-GST, a related fusion protein lacking the NBD1 domain. Finally, an MBP (maltose-binding protein) fusion protein containing the NBD2 domain (NBD2-MBP) associated with and regulated the ATPase activity of PKA-phosphorylated NBD1-R-GST. Thus when the R domain in NBD1-R-GST is phosphorylated by PKA, ATP binding and hydrolysis becomes co-operative and NBD dimerization occurs. These findings suggest that during the activation of native CFTR, phosphorylation of the R domain by PKA can control the ability of the NBD1 domain to hydrolyse ATP and to interact with other NBD domains.

Phosphorylation-induced Conformational Changes of Cystic Fibrosis Transmembrane Conductance Regulator Monitored by Attenuated Total Reflection-Fourier Transform IR Spectroscopy and Fluorescence Spectroscopy

Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ABC protein superfamily. Phosphorylation of a regulatory domain of this protein is a prerequisite for activity. We analyzed the effect of protein kinase A (PKA) phosphorylation on the structure of purified and reconstituted CFTR protein. 1H/2H exchange monitored by attenuated total reflection Fourier transform IR spectroscopy demonstrates that CFTR is highly accessible to aqueous medium. Phosphorylation of the regulatory (R) domain by PKA further increases this accessibility. More specifically, fluorescence quenching of cytosolic tryptophan residues revealed that the accessibility of the cytoplasmic part of the protein is modified by phosphorylation. Moreover, the combination of polarized IR spectroscopy with 1H/2H exchange suggested an increase of the accessibility of the transmembrane domains of CFTR. This suggests that CFTR phosphorylation can induce a large conformational change that could correspond either to a displacement of the R domain or to long range conformational changes transmitted from the phosphorylation sites to the nucleotide binding domains and the transmembrane segments. Such structural changes may provide better access for the solutes to the nucleotide binding domains and the ion binding site.

Stimulatory and inhibitory protein kinase C consensus sequences regulate the cystic fibrosis transmembrane conductance regulator

Protein kinase C (PKC) phosphorylation stimulates the cystic fibrosis transmembrane conductance regulator (CFTR) channel and enhances its activation by protein kinase A (PKA) through mechanisms that remain poorly understood. We have examined the effects of mutating consensus sequences for PKC phosphorylation and report here evidence for both stimulatory and inhibitory sites. Sequences were mutated in subsets and the mutants characterized by patch clamping. Activation of a 4CA mutant (S707A/S790A/T791A/S809A) by PKA was similar to that of wild-type CFTR and was enhanced by PKC, whereas responses of 3CA (T582A/T604A/S641A) and 2CA (T682A/S686A) channels to PKA were both drastically reduced (>90%). When each mutation in the 3CA and 2CA constructs was studied individually in a wild-type background, T582, T604, and S686 were found to be essential for PKA activation. Responses were restored when these three residues were reintroduced simultaneously into a 9CA mutant lacking all nine PKC consensus sequences (R6CA revertant); however, PKC phosphorylation was not required for this rescue. Nevertheless, two of the sites (T604 and S686) were phosphorylated in vitro, and PKC alone partially activated wild-type CFTR, the 4CA mutant, and the point mutants T582A and T604A, but not S686A channels, indicating that PKC does act at S686. The region encompassing S641 and T682 is inhibitory, because S641A enhanced activation by PKA, and T682A channels had 4-fold larger responses to PKC compared to wild-type channels. These results identify functionally important PKC consensus sequences on CFTR and will facilitate studies of its convergent regulation by PKC and PKA.

Structure of nucleotide-binding domain 1 of the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-binding cassette (ABC) transporter that functions as a chloride channel. Nucleotide-binding domain 1 (NBD1), one of two ABC domains in CFTR, also contains sites for the predominant CF-causing mutation and, potentially, for regulatory phosphorylation. We have determined crystal structures for mouse NBD1 in unliganded, ADP- and ATP-bound states, with and without phosphorylation. This NBD1 differs from typical ABC domains in having added regulatory segments, a foreshortened subdomain interconnection, and an unusual nucleotide conformation. Moreover, isolated NBD1 has undetectable ATPase activity and its structure is essentially the same independent of ligand state. Phe508, which is commonly deleted in CF, is exposed at a putative NBD1-transmembrane interface. Our results are consistent with a CFTR mechanism, whereby channel gating occurs through ATP binding in an NBD1-NBD2 nucleotide sandwich that forms upon displacement of NBD1 regulatory segments.

Activation of AMP-activated protein kinase reduces cAMP-mediated epithelial chloride secretion

AMP-activated protein kinase (AMPK) is activated in response to fluctuations in cellular energy status caused by oxidative stress. One of its targets is the cystic fibrosis transmembrane conductance regulator (CFTR), which is the predominant Cl- secretory channel in colonic tissue. The aim of this study was to determine the role of AMPK in the modulation of colonic chloride secretion under conditions of oxidative stress and chronic inflammation. Chloride secretion and AMPK activity were examined in colonic tissue from adult IL-10-deficient and wild-type 129 Sv/Ev mice in the presence and absence of pharmacological AMPK inhibitors and activators, respectively. Apical levels of CFTR were measured in brush-border membrane vesicles. Cell culture studies in human colonic T84 monolayers examined the effect of hydrogen peroxide and pharmacological activation of AMPK on forskolin-stimulated chloride secretion. Inflamed colons from IL-10-deficient mice exhibited hyporesponsiveness to forskolin stimulation in association with reductions in surface CFTR expression and increased AMPK activity. Inhibition of AMPK restored tissue responsiveness to forskolin, whereas stimulation of AMPK with 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) induced tissue hyporesponsivness in wild-type mice. T84 cells exposed to hydrogen peroxide demonstrated a time-dependent increase in AMPK activity and reduction of forskolin-stimulated chloride secretion. Inhibition of AMPK prevented the reduction in chloride secretion. Treatment of cells with the AMPK activator, AICAR, resulted in a decreased chloride secretion. In conclusion, AMPK activation is linked with reductions in cAMP-mediated epithelial chloride flux and may be a contributing factor to the hyporesponsiveness seen under conditions of chronic inflammation.

Characterization of the ABCA transporter subfamily: identification of prokaryotic and eukaryotic members, phylogeny and topology

An alignment of the mammalian ABCA transporters enabled the identification of sequence segments, specific to the ABCA subfamily, which were used as queries to search for eukaryotic and prokaryotic homologues. Thirty-seven eukaryotic half and full-length transporters were found, and a close relationship with prokaryotic subfamily 7 transporters was detected. Each half of the ABCA full-transporters is predicted to comprise a membrane-spanning domain (MSD) composed of six helices and a large extracellular loop, followed by a nucleotide-binding domain (NBD) and a conserved cytoplasmic 80-residue sequence, which might have a regulatory function. The topology predicted for the ABCA transporters was compared to the crystal structures of the MsbA and BtuCD bacterial transporters. The alignment of the MSD and NBD domains provided an estimate of the degree of residue conservation in the cytoplasmic, extracellular and transmembrane domains of the ABCA transporter subfamily. The phylogenic tree of eukaryotic ABCA transporters based upon the NBD sequences, consists of three major clades, corresponding to the half-transporter single NBDs and to the full-transporter NBDls and NBD2s. A phylogenic tree of prokaryotic transporters and the eukaryotic ABCA transporters confirmed the evolutionary relationship between prokaryotic subfamily 7 transporters and eukaryotic ABCA half and full-transporters.

Regulation of channel gating by AMP-activated protein kinase modulates cystic fibrosis transmembrane conductance regulator activity in lung submucosal cells

Cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel activity is important for fluid and electrolyte transport in many epithelia including the lung, the site of most cystic fibrosis-associated morbidity. CFTR is unique among ion channels in requiring ATP hydrolysis for its gating, suggesting that its activity is coupled to cellular metabolic status. The metabolic sensor AMP-activated kinase (AMPK) binds to and phosphorylates CFTR, co-localizes with it in various tissues, and inhibits CFTR currents in Xenopus oocytes (Hallows, K. R., Raghuram, V., Kemp, B. E., Witters, L. A. & Foskett, J. K. (2000) J. Clin. Invest. 105, 1711-1721). Here we demonstrate that this AMPK-CFTR interaction has functional implications in human lung epithelial cells. Pharmacologic activation of AMPK inhibited forskolin-stimulated CFTR short circuit currents in polarized Calu-3 cell monolayers. In whole-cell patch clamp experiments, the activation of endogenous AMPK either pharmacologically or by the overexpression of an AMPK-activating non-catalytic subunit mutant (AMPK-gamma1-R70Q) dramatically inhibited forskolin-stimulated CFTR conductance in Calu-3 and CFTR-expressing Chinese hamster ovary cells. Plasma membrane expression of CFTR, assessed by surface biotinylation, was not affected by AMPK activation. In contrast, the single channel open probability of CFTR was strongly reduced in cell-attached patch clamp measurements of Calu-3 cells transfected with the AMPK-activating mutant, an effect due primarily to a substantial prolongation of the mean closed time of the channel. As a metabolic sensor in cells, AMPK may be important in tuning CFTR activity to cellular energy charge, thereby linking transepithelial transport and the maintenance of cellular ion gradients to cellular metabolism.

A macromolecular complex of beta 2 adrenergic receptor, CFTR, and ezrin/radixin/moesin-binding phosphoprotein 50 is regulated by PKA

It has been demonstrated previously that both the cystic fibrosis transmembrane conductance regulator (CFTR) and beta(2) adrenergic receptor (beta(2)AR) can bind ezrinradixinmoesin-binding phosphoprotein 50 (EBP50, also referred to as NHERF) through their PDZ motifs. Here, we show that beta(2) is the major adrenergic receptor isoform expressed in airway epithelia and that it colocalizes with CFTR at the apical membrane. beta(2)AR stimulation increases CFTR activity, in airway epithelial cells, that is glybenclamide sensitive. Deletion of the PDZ motif from CFTR uncouples the channel from the receptor both physically and functionally. This uncoupling is specific to the beta(2)AR receptor and does not affect CFTR coupling to other receptors (e.g., adenosine receptor pathway). Biochemical studies demonstrate the existence of a macromolecular complex involving CFTR-EBP50-beta(2)AR through PDZ-based interactions. Assembly of the complex is regulated by PKA-dependent phosphorylation. Deleting the regulatory domain of CFTR abolishes PKA regulation of complex assembly. This report summarizes a macromolecular signaling complex involving CFTR, the implications of which may be relevant to CFTR-dysfunction diseases.

Mutations in the nucleotide binding domain 1 signature motif region rescue processing and functional defects of cystic fibrosis transmembrane conductance regulator delta f508

The gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), an ATP binding cassette (ABC) transporter that functions as a phosphorylation- and nucleotide-regulated chloride channel, is mutated in cystic fibrosis (CF) patients. Deletion of a phenylalanine at amino acid position 508 (DeltaF508) in the first nucleotide binding domain (NBD1) is the most prevalent CF-causing mutation and results in defective protein processing and reduced CFTR function, leading to chloride impermeability in CF epithelia and heterologous systems. Using a STE6/CFTRDeltaF508 chimera system in yeast, we isolated two novel DeltaF508 revertant mutations, I539T and G550E, proximal to and within the conserved ABC signature motif of NBD1, respectively. Western blot and functional analysis in mammalian cells indicate that mutations I539T and G550E each partially rescue the CFTRDeltaF508 defect. Furthermore, a combination of both revertant mutations resulted in a 38-fold increase in CFTRDeltaF508-mediated chloride current, representing 29% of wild type channel activity. The G550E mutation increased the sensitivity of CFTRDeltaF508 and wild type CFTR to activation by cAMP agonists and blocked the enhancement of CFTRDeltaF508 channel activity by 2 mm 3-isobutyl-1-methylxanthine. The data show that the DeltaF508 defect can be significantly rescued by second-site mutations in the nucleotide binding domain 1 region, that includes the LSGGQ consensus motif.

A short segment of the R domain of cystic fibrosis transmembrane conductance regulator contains channel stimulatory and inhibitory activities that are separable by sequence modification

The regulatory (R) domain of the cystic fibrosis transmembrane conductance regulator (CFTR) contains consensus phosphorylation sites for cAMP-dependent protein kinase (PKA) that are the basis for physiological regulation of the CFTR chloride channel. A short peptide segment in the R domain with a net negative charge of B9 (amino acids 817-838, NEG2) and predicted helical tendency is shown to play a critical role in CFTR chloride channel function. Deletion of NEG2 from CFTR completely eliminates the PKA dependence of channel activity. Exogenous NEG2 peptide interacts with CFTR to exert both stimulatory and inhibitory effects on the channel function. The NEG2 peptide with sequence scrambled to remove helical tendencies also inhibits channel function, but does not stimulate. Similar results are found for a NEG2 peptide whose helical structure is disrupted by a proline residue. When six of the negatively charged carboxylic acid residues are replaced by their cognate amides, reducing net negative charge to B3, but increasing helical propensity as assessed by circular dichroism, the peptide stimulates CFTR channel function, but does not inhibit. We speculate that the NEG2 region interacts with other cytosolic domains of CFTR to control opening and closing transitions of the chloride channel.

Distinct Mg(2+)-dependent steps rate limit opening and closing of a single CFTR Cl(-) channel

The roles played by ATP binding and hydrolysis in the complex mechanisms that open and close cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels remain controversial. In this work, the contributions made by ATP and Mg(2+) ions to the gating of phosphorylated cardiac CFTR channels were evaluated separately by measuring the rates of opening and closing of single channels in excised patches exposed to solutions in which [ATP] and [Mg(2+)] were varied independently. Channel opening was found to be rate-limited not by the binding of ATP alone, but by a Mg(2+)-dependent step that followed binding of both ATP and Mg(2+). Once a channel had opened, sudden withdrawal of all Mg(2+) and ATP could prevent it from closing for tens of seconds. But subsequent exposure of such an open channel to Mg(2+) ions alone could close it, and the closing rate increased with [Mg(2+)] over the micromolar range (half maximal at approximately 50 microM [Mg(2+)]). A simple interpretation is that channel closing is stoichiometrically coupled to hydrolysis of an ATP molecule that remains tightly associated with the open CFTR channel despite continuous washing. If correct, that ATP molecule appears able to reside for over a minute in the catalytic site that controls channel closing, implying that the site must entrap, or have an intrinsically high apparent affinity for, ATP, even without a Mg(2+) ion. Such stabilization of the open-channel conformation of CFTR by tight binding, or occlusion, of an ATP molecule echoes the stabilization of the active conformation of a G protein by GTP.

Nucleoside triphosphate pentose ring impact on CFTR gating and hydrolysis

Alterations in the pentose ring of ATP have a major impact on cystic fibrosis transmembrane conductance regulator (CFTR) function. Both 2'- and 3'-deoxy-ATP (dATP) accelerate ion channel openings and stabilize open channel structure better than ATP. Purified wild-type CFTR hydrolyzes dATP. The apparent first-order rate constants for hydrolysis at low substrate concentration are the same for dATP and ATP. This suggests that product release and/or relaxation of the enzyme structure to the initial ligand free state is the rate-limiting step in the CFTR hydrolytic cycle. Circumvention of the normal requirement for protein kinase A phosphorylation of the R-domain for channel activation implies that the impact of the deoxyribonucleotide interaction with the nucleotide binding domains is transmitted to the channel-forming elements of the protein more readily than that of the ribonucleotide.

Domain-domain associations in cystic fibrosis transmembrane conductance regulator

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR is a chloride channel whose activity requires protein kinase A-dependent phosphorylation of an intracellular regulatory domain (R-domain) and ATP hydrolysis at the nucleotide-binding domains (NBDs). To identify potential sites of domain-domain interaction within CFTR, we expressed, purified, and refolded histidine (His)- and glutathione-S-transferase (GST)-tagged cytoplasmic domains of CFTR. ATP-binding to his-NBD1 and his-NBD2 was demonstrated by measuring tryptophan fluorescence quenching. Tryptic digestion of in vitro phosphorylated his-NBD1-R and in situ phosphorylated CFTR generated the same phosphopeptides. An interaction between NBD1-R and NBD2 was assayed by tryptophan fluorescence quenching. Binding among all pairwise combinations of R-domain, NBD1, and NBD2 was demonstrated with an overlay assay. To identify specific sites of interaction between domains of CFTR, an overlay assay was used to probe an overlapping peptide library spanning all intracellular regions of CFTR with his-NBD1, his-NBD2, and GST-R-domain. By mapping peptides from NBD1 and NBD2 that bound to other intracellular domains onto crystal structures for HisP, MalK, and Rad50, probable sites of interaction between NBD1 and NBD2 were identified. Our data support a model where NBDs form dimers with the ATP-binding sites at the domain-domain interface.

Lung infections associated with cystic fibrosis

While originally characterized as a collection of related syndromes, cystic fibrosis (CF) is now recognized as a single disease whose diverse symptoms stem from the wide tissue distribution of the gene product that is defective in CF, the ion channel and regulator, cystic fibrosis transmembrane conductance regulator (CFTR). Defective CFTR protein impacts the function of the pancreas and alters the consistency of mucosal secretions. The latter of these effects probably plays an important role in the defective resistance of CF patients to many pathogens. As the modalities of CF research have changed over the decades from empirical histological studies to include biophysical measurements of CFTR function, the clinical management of this disease has similarly evolved to effectively address the ever-changing spectrum of CF-related infectious diseases. These factors have led to the successful management of many CF-related infections with the notable exception of chronic lung infection with the gram-negative bacterium Pseudomonas aeruginosa. The virulence of P. aeruginosa stems from multiple bacterial attributes, including antibiotic resistance, the ability to utilize quorum-sensing signals to form biofilms, the destructive potential of a multitude of its microbial toxins, and the ability to acquire a mucoid phenotype, which renders this microbe resistant to both the innate and acquired immunologic defenses of the host.

CFTR with a partially deleted R domain corrects the cystic fibrosis chloride transport defect in human airway epithelia in vitro and in mouse nasal mucosa in vivo

In developing gene therapy for cystic fibrosis (CF) airways disease, a transgene encoding a partially deleted CF transmembrane conductance regulator (CFTR) Cl- channel could be of value for vectors such as adeno-associated virus that have a limited packaging capacity. Earlier studies in heterologous cells indicated that the CFTR R (regulatory) domain is predominantly random coil and that parts of the R domain can be deleted without abolishing channel function. Therefore, we designed a series of CFTR variants with shortened R domains (between residues 708 and 835) and expressed them in well-differentiated cultures of CF airway epithelia. All of the variants showed normal targeting to the apical membrane, and for the constructs we tested, biosynthesis was like wild type. Moreover, all constructs generated transepithelial Cl- current in CF epithelia. Comparison of the Cl- transport suggested that the length of the R domain, the presence of phosphorylation sites, and other factors contribute to channel activity. A variant deleting residues 708-759 complemented CF airway epithelia to the same extent as wild-type CFTR and showed no current in the absence of cAMP stimulation. In addition, expression in nasal mucosa of CF mice corrected the Cl- transport defect. These data provide insight into the structure and function of the R domain and identify regions that can be deleted with retention of function. Thus they suggest a strategy for shortening the transgene used in CF gene therapy.

A conditional probability analysis of cystic fibrosis transmembrane conductance regulator gating indicates that ATP has multiple effects during the gating cycle

ATP-binding cassette (ABC) transporters bind and hydrolyze ATP. In the cystic fibrosis transmembrane conductance regulator Cl(-) channel, this interaction with ATP generates a gating cycle between a closed (C) and two open (O1 and O2) conformations. To understand better how ATP controls channel activity, we examined gating transitions from the C to the O1 and O2 states and from these open states to the C conformation. We made three main observations. First, we found that the channel can open into either the O1 or O2 state, that the frequency of transitions to both states was increased by ATP concentration, and that ATP increased the relative proportion of openings into O1 vs. O2. These results indicate that ATP can interact with the closed state to open the channel in at least two ways, which may involve binding to nucleotide-binding domains (NBDs) NBD1 and NBD2. Second, ATP prolonged the burst duration and altered the way in which the channel closed. These data suggest that ATP also interacts with the open channel. Third, the channel showed runs of specific types of open-closed transitions. This finding suggests a mechanism with more than one cycle of gating transitions. These data suggest models to explain how ATP influences conformational transitions in cystic fibrosis transmembrane conductance regulator and perhaps other ABC transporters.

Cystic fibrosis transmembrane conductance regulator Cl- channels with R domain deletions and translocations show phosphorylation-dependent and -independent activity

Phosphorylation of the R domain regulates cystic fibrosis transmembrane conductance regulator Cl- channel activity. Earlier studies suggested that the R domain controls activity via more than one mechanism; a phosphorylated R domain may stimulate activity, and an unphosphorylated R domain may prevent constitutive activity, i.e. opening with ATP alone. However, the mechanisms responsible for these two regulatory properties are not understood. In this study we asked whether the two effects are dependent on its position in the protein and whether smaller regions from the R domain mediate the effects. We found that several portions of the R domain conferred phosphorylation-stimulated activity. This was true whether the R domain sequences were present in their normal location or were translocated to the C terminus. We also found that some parts of the R domain could be deleted without inducing constitutive activity. However, when residues 760-783 were deleted, channels opened without phosphorylation. Translocation of the R domain to the C terminus did not prevent constitutive activity. These results suggest that different parts of the phosphorylated R domain can stimulate activity and that their location within the protein is not critical. In contrast, prevention of constitutive activity required a short specific sequence that could not be moved to the C terminus. These results are consistent with a recent model of an R domain composed primarily of random coil in which more than one phosphorylation site is capable of stimulating channel activity, and net activity reflects interactions between multiple sites in the R domain and the rest of the channel.

Contribution of R domain phosphoserines to the function of CFTR studied in Fischer rat thyroid epithelia

The regulatory domain of cystic fibrosis transmembrane conductance regulator (CFTR) regulates channel activity when several serines are phosphorylated by cAMP-dependent protein kinase. To further define the functional role of individual phosphoserines, we studied CFTR containing previously studied and new serine to alanine mutations. We expressed these constructs in Fischer rat thyroid epithelia and measured transepithelial Cl(-) current. Mutation of four in vivo phosphorylation sites, Ser(660), Ser(737), Ser(795), and Ser(813) (S-Quad-A), substantially decreased cAMP-stimulated current, suggesting that these four sites account for most of the phosphorylation-dependent response. Mutation of either Ser(660) or Ser(813) alone significantly decreased current, indicating that these residues play a key role in phosphorylation-dependent stimulation. However, neither Ser(660) nor Ser(813) alone increased current to wild-type levels; both residues were required. Changing Ser(737) to alanine increased current above wild-type levels, suggesting that phosphorylation of Ser(737) may inhibit current in wild-type CFTR. These data help define the functional role of regulatory domain phosphoserines and suggest interactions between individual phosphoserines.

Definition of a "functional R domain" of the cystic fibrosis transmembrane conductance regulator

The R domain of the cystic fibrosis transmembrane conductance regulator (CFTR) was originally defined as 241 amino acids, encoded by exon 13. Such exon/intron boundaries provide a convenient way to define the R domain, but do not necessarily reflect the corresponding functional domain within CFTR. A two-domain model was later proposed based on a comparison of the R-domain sequences from 10 species. While RD1, the N-terminal third of the R domain is highly conserved, RD2, the large central region of the R domain has less rigid structural requirements. Although this two-domain model was given strong support by recent functional analysis data, the simple observation that two of the four main phosphorylation sites are excluded from RD2 clearly indicates that RD2 still does not satisfy the requirements of a "functional R domain." Nevertheless, knowledge of the CFTR structure and function accumulated over the past decade and reevaluated in the context of a comprehensive sequence comparison of 15 CFTR homologues made it possible to define such a "functional R domain," i.e., amino acids C647 to D836. This definition is validated primarily because it contains all of the important potential consensus phosphorylation sequences. In addition, it includes the highly charged motif from E822 to D836. Finally, it includes all of the deletions/insertions in this region. This definition also aids in understanding the effects of missense mutations occurring within this domain.

Severed channels probe regulation of gating of cystic fibrosis transmembrane conductance regulator by its cytoplasmic domains

Opening and closing of a CFTR Cl(-) channel is controlled by PKA-mediated phosphorylation of its cytoplasmic regulatory (R) domain and by ATP binding, and likely hydrolysis, at its two nucleotide binding domains. Functional interactions between the R domain and the two nucleotide binding domains were probed by characterizing the gating of severed CFTR channels expressed in Xenopus oocytes. Expression levels were assessed using measurements of oocyte conductance, and detailed functional characteristics of the channels were extracted from kinetic analyses of macroscopic current relaxations and of single-channel gating events in membrane patches excised from the oocytes. The kinetic behavior of wild-type (WT) CFTR channels was compared with that of split CFTR channels bearing a single cut (between residues 633 and 634) just before the R domain, of split channels with a single cut (between residues 835 and 837) just after the R domain, and of split channels from which the entire R domain (residues 634-836) between those two cut sites was omitted. The channels cut before the R domain had characteristics almost identical to those of WT channels, except for less than twofold shorter open burst durations in the presence of PKA. Channels cut just after the R domain were characterized by a low level of activity even without phosphorylation, strong stimulation by PKA, enhanced apparent affinity for ATP as assayed by open probability, and a somewhat destabilized binding site for the locking action of the nonhydrolyzable ATP analog AMPPNP. Split channels with no R domain (from coexpression of CFTR segments 1-633 and 837-1480) were highly active without phosphorylation, but otherwise displayed the characteristics of channels cut after the R domain, including higher apparent ATP affinity, and less tight binding of AMPPNP at the locking site, than for WT. Intriguingly, severed channels with no R domain were still noticeably stimulated by PKA, implying that activation of WT CFTR by PKA likely also includes some component unrelated to the R domain. As the maximal opening rates were the same for WT channels and split channels with no R domain, it seems that the phosphorylated R domain does not stimulate opening of CFTR channels; rather, the dephosphorylated R domain inhibits them.

Nucleotide-binding domain 1 of cystic fibrosis transmembrane conductance regulator production of a suitable protein for structural studies

Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This protein belongs to the large ATP-binding cassette (ABC) family of transporters. Most patients with cystic fibrosis bear a mutation in the nucleotide-binding domain 1 (NBD1) of CFTR, which plays a key role in the activation of the channel function of CFTR. Determination of the three dimensional structure of NBD1 is essential to better understand its structure-function relationship, and relate it to the biological features of CFTR. In this paper, we report the first preparation of recombinant His-tagged NBD1, as a soluble, stable and isolated domain. The method avoids the use of renaturing processes or fusion constructs. ATPase activity assays show that the recombinant domain is functional. Using tryptophan intrinsic fluorescence, we point out that the local conformation, in the region of the most frequent mutation DeltaF508, could differ from that of the nucleotide-binding subunit of histidine permease, the only available ABC structure. We have undertaken three dimensional structure determination of NBD1, and the first two dimensional 15N-1H NMR spectra demonstrate that the domain is folded. The method should be applicable to the structural studies of NBD2 or of other NBDs from different ABC proteins of major biological interest, such as multidrug resistance protein 1 or multidrug resistance associated protein 1.

Regulation of CFTR Cl- channel gating by ATP binding and hydrolysis

Opening and closing of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is regulated by the interaction of ATP with its two cytoplasmic nucleotide-binding domains (NBD). Although ATP hydrolysis by the NBDs is required for normal gating, the influence of ATP binding versus hydrolysis on specific steps in the gating cycle remains uncertain. Earlier work showed that the absence of Mg(2+) prevents hydrolysis. We found that even in the absence of Mg(2+), ATP could support channel activity, albeit at a reduced level compared with the presence of Mg(2+). Application of ATP with a divalent cation, including the poorly hydrolyzed CaATP complex, increased the rate of opening. Moreover, in CFTR variants with mutations that disrupt hydrolysis, ATP alone opened the channel and Mg(2+) further enhanced ATP-dependent opening. These data suggest that ATP alone can open the channel and that divalent cations increase ATP binding. Consistent with this conclusion, when we mutated an aspartate thought to bind Mg(2+), divalent cations failed to increase activity compared with ATP alone. Two observations suggested that divalent cations also stabilize the open state. In wild-type CFTR, CaATP generated a long duration open state, whereas ATP alone did not. With a CFTR variant in which hydrolysis was disrupted, MgATP, but not ATP alone, produced long openings. These results suggest a gating cycle for CFTR in which ATP binding opens the channel and either hydrolysis or dissociation leads to channel closure. In addition, the data suggest that ATP binding and hydrolysis by either NBD can gate the channel.

Identification of the cystic fibrosis transmembrane conductance regulator domains that are important for interactions with ROMK2

In addition to functioning as a cAMP-activated chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR) plays an important role in conferring regulatory properties on other ion channels. It is known, with respect to CFTR regulation of ROMK2 (renally derived K(ATP) channel), that the first transmembrane domain and the first nucleotide binding fold domain (NBF1) of CFTR are necessary for this interaction to occur. It has been shown that under conditions that promote phosphorylation, the ROMK2-CFTR interaction is attenuated. To elucidate the complex nature of this interaction, CFTR constructs were co-expressed with ROMK2 in Xenopus oocytes, and two microelectrode voltage clamp experiments were performed. Although the second half of CFTR can act as a functional chloride channel, our results suggest that it does not confer glibenclamide sensitivity on ROMK2, as does the first half of CFTR. The attenuation of the ROMK2-CFTR interaction under conditions that promote phosphorylation is dependent on at least the presence of the R domain of CFTR. We conclude that transmembrane domain 1, NBF1, and the R domain are the CFTR domains involved in the ROMK2-CFTR interaction and that NBF2 and transmembrane domain 2 are not essential. Lastly, the R domain of CFTR is necessary for the attenuation of the ROMK2-CFTR interaction under conditions that promote phosphorylation.

Stimulatory and Inhibitory Functions of the R Domain on CFTR Chloride Channel

CFTR is a chloride channel whose gating process involves coordinated interactions among the regulatory (R) domain and the nucleotide-binding folds (NBFs). Protein kinase A phosphorylation of serine residues renders the R domain from inhibitory to stimulatory and enables ATP binding and hydrolysis at the NBFs, which in turn control opening and closing of the chloride channel.

A functional R domain from cystic fibrosis transmembrane conductance regulator is predominantly unstructured in solution

Phosphorylation of the regulatory (R) domain initiates cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel activity. To discover how the function of this domain is determined by its structure, we produced an R domain protein (R8) that spanned residues 708-831 of CFTR. Phosphorylated, but not unphosphorylated, R8 stimulated activity of CFTR channels lacking this domain, indicating that R8 is functional. Unexpectedly, this functional R8 was predominantly random coil, as revealed by CD and limited proteolysis. The CD spectra of both phosphorylated and nonphosphorylated R8 were similar in aqueous buffer. The folding agent trimethylamine N-oxide induced only a small increase in the helical content of nonphosphorylated R8 and even less change in the helical content of phosphorylated R8. These data, indicating that the R domain is predominantly random coil, may explain the seemingly complex way in which phosphorylation regulates CFTR channel activity.

Deletion of phenylalanine 508 causes attenuated phosphorylation-dependent activation of CFTR chloride channels

In cell-attached patches stimulated with cAMP agonists, the single-channel open probability (Po) of the phenylalanine 508-deleted cystic fibrosis transmembrane conductance regulator (DeltaF508-CFTR) channel, the most common disease-associated mutation in cystic fibrosis, was abnormally low (a functional defect). To investigate the mechanism for the poor response of DeltaF508-CFTR to cAMP stimulation, we examined, in excised inside-out patches, protein kinase A (PKA)-dependent phosphorylation activation and ATP-dependent gating of wild-type (WT) and DeltaF508-CFTR channels expressed in NIH3T3 mouse fibroblasts. For WT-CFTR, the activation time course of CFTR channel current upon addition of PKA and ATP followed a sigmoidal function with time constants that decreased as [PKA] was increased. The curvilinear relationship between [PKA] and the apparent activation rate suggests an incremental phosphorylation-dependent activation of CFTR at multiple phosphorylation sites. The time course of PKA-dependent activation of DeltaF508-CFTR channel current also followed a sigmoidal function, but the rate of activation was at least 7-fold slower than that with WT channels. This result suggests that deletion of phenylalanine 508 causes attenuated PKA-dependent phosphorylation of the CFTR chloride channel. Once DeltaF508-CFTR channels were maximally activated with PKA, the mutant channel and WT channel had indistinguishable steady-state Po values, ATP dose-response relationships and single-channel kinetics, indicating that DeltaF508-CFTR is not defective in ATP-dependent gating. By measuring whole-cell current density, we compared the number of functional channels in WT- and DeltaF508-CFTR cell membrane. Our data showed that the estimated channel density for DeltaF508-CFTR was approximately 10-fold lower than that for WT-CFTR, but the cAMP-dependent whole-cell current density differed by approximately 200-fold. We thus conclude that the functional defect (a decrease in Po) of DeltaF508-CFTR is as important as the trafficking defect (a decrease in the number of functional channels in the plasma membrane) in cystic fibrosis pathogenesis.

Conformation, independent of charge, in the R domain affects cystic fibrosis transmembrane conductance regulator channel openings

The R domain of cystic fibrosis transmembrane conductance regulator (CFTR), when phosphorylated, undergoes conformational change, and the chloride channel opens. We investigated the contribution of R domain conformation, apart from the changes induced by phosphorylation, to channel opening, by testing the effect of the peptidyl-prolyl isomerase, cyclophilin A, on the CFTR channel. When it was applied after the channel had been opened by PKA phosphorylation, cyclophilin A increased the open probability of wild-type CFTR (from P(o) = 0.197 +/- 0.010 to P(o) = 0.436 +/- 0. 029) by increasing the number of channel openings, not open time. Three highly conserved proline residues in the R domain, at positions 740, 750, and 759, were considered as candidate targets for cyclophilin A. Mutations of these prolines to alanines (P3A mutant) resulted in a channel unresponsive to cyclophilin A but with pore properties similar to the wild type, under strict control of PKA and ATP, but with significantly increased open probability (P(o) = 0.577 +/- 0.090) compared to wild-type CFTR, again due to an increase in the number of channel openings and not open time. Mutation of each of the proline residues separately and in pairs demonstrated that all three proline mutations are required for maximal P(o). When P3A was expressed in 293 HEK cells and tested by SPQ assay, chloride efflux was significantly increased compared to cells transfected with wild-type CFTR. Thus, treatments favoring the trans-peptidyl conformation about conserved proline residues in the R domain of CFTR affect openings of CFTR, above and beyond the effect of PKA phosphorylation.

Influence of phosphorylation by protein kinase A on CFTR at the cell surface and endoplasmic reticulum

CFTR possesses a large cluster of strict dibasic consensus sites for phosphorylation by protein kinase A (PKA) in the R-domain and an obligatory dependence on phosphorylation is a hallmark of CFTR Cl(-) channel function. Removal of as many as 11 of these sites reduces the conformational change in the R-domain and the degree of channel activation in response to PKA. However, until recently a completely PKA-unresponsive CFTR variant has not been reported, leaving open the possibility that the residual response may be mediated by associating ancillary phosphoproteins. We traced the residual PKA-catalyzed (32)P-labelling of the variant with 11 sites mutagenized (11SA) to distinct CNBr phosphopeptides within the R-domain. Mutagenesis of 4 additional monobasic sites in these segments produced a 15SA variant in which Cl(-) channel response to PKA was abolished. Therefore, it can be concluded that ancillary phosphoproteins do not contribute to CFTR activation by PKA. Notably, however, the 15SA protein did exhibit a low level of constitutive channel activity not dependent on PKA, which might have reflected a down-regulating effect of phosphorylation of one or two of the 15 sites as suggested by others. However, this did not prove to be the case.Since immature CFTR has been claimed to be active in the endoplasmic reticulum (ER), we also examined whether it can be phosphorylated in cells and what influence if any this might have on its susceptibility to degradation. Teleologically, activation by phosphorylation of CFTR Cl(-) channels in the ER might be undesirable to the cell. Using various phosphorylation site mutants and kinase and phosphatase inhibitors in pulse-chase experiments, we have found that although nascent CFTR can be phosphorylated at the ER, this is without effect on its ability to mature and avoid proteolysis. Furthermore, we found that microsomes from cells expressing CFTR processing mutants such as DeltaF508 do not generate Cl(-) active channels when fused with planar bilayers unless maturation is promoted, e.g. by growth of cells at reduced temperature or other means. We conclude that the ER-retained mutant nascent chains which are incapable of maturation may be phosphorylated but do not form active channels. Stimulation by PKA of the insertion of CFTR containing vesicles into the plasma membrane as part of the mechanism of stimulation of chloride secretion has been reported, as has an influence of CFTR on the balance between endocytosis and exocytosis but these findings have not been universally confirmed.

CFTR chloride channel regulation by an interdomain interaction

The cystic fibrosis gene encodes a chloride channel, CFTR (cystic fibrosis transmembrane conductance regulator), that regulates salt and water transport across epithelial tissues. Phosphorylation of the cytoplasmic regulatory (R) domain by protein kinase A activates CFTR by an unknown mechanism. The amino-terminal cytoplasmic tail of CFTR was found to control protein kinase A-dependent channel gating through a physical interaction with the R domain. This regulatory activity mapped to a cluster of acidic residues in the NH(2)-terminal tail; mutating these residues proportionately inhibited R domain binding and CFTR channel function. CFTR activity appears to be governed by an interdomain interaction involving the amino-terminal tail, which is a potential target for physiologic and pharmacologic modulators of this ion channel.

Function of the second nucleotide-binding fold in the CFTR chloride channel

To test the role of nucleotide-binding fold (NBF) 2 and its interaction with the regulatory (R) domain in the function of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, we used three deletion mutants of CFTR: DeltaR(708-835), DeltaNBF2(1185-1349) and DeltaR-DeltaNBF2. In lipid bilayers, DeltaNBF2 channel activity is ATP- and cAMP-dependent protein kinase (PKA)-dependent, but unlike wild-type (wt) CFTR, it displays a reduced activity and insensitivity to 5'-adenylylimidodiphosphate (AMP-PNP). Both DeltaR and DeltaR-DeltaNBF2 channels are PKA-independent, but DeltaR activity is reduced whereas DeltaR-DeltaNBF2 activity is increased. Deletion of NBF2 from CFTR affects protein trafficking and channel gating kinetics. The data suggest that NBF2 could have inhibitory and stimulatory roles in CFTR activity by interaction with NBF1 directly or indirectly via the R domain.

Syntaxin 1A inhibits regulated CFTR trafficking in xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial cell Cl channel, whose gating activity and membrane trafficking are controlled by cAMP/protein kinase A (PKA)-mediated phosphorylation. CFTR Cl currents are regulated also by syntaxin 1A (A. P. Naren, D. J. Nelson, W. W. Xie, B. Jovov, J. Pevsner, M. K. Bennett, D. J. Benos, M. W. Quick, and K. L. Kirk. Nature 390: 302-305, 1997), a protein best known for its role in membrane trafficking and neurosecretion. To examine the mechanism of syntaxin 1A inhibition, we expressed these proteins in Xenopus oocytes and monitored agonist-induced changes in plasma membrane capacitance and cell surface fluorescence of CFTR that contains an external epitope tag. cAMP stimulation elicited large increases in membrane capacitance and in cell surface labeling of flag-tagged CFTR. Coexpression of CFTR with syntaxin 1A, but not syntaxin 3, inhibited cAMP-induced increases in membrane capacitance and plasma membrane CFTR content. Injection of botulinum toxin/C1 rapidly reversed syntaxin's effects on current and capacitance, indicating that they cannot be explained by an effect on CFTR synthesis. Functional expression of other integral membrane proteins, including Na-coupled glucose transporter hSGLT1, inwardly rectified K channel hIK1, P2Y2 nucleotide receptor, and viral hemagglutinin protein, was not affected by syntaxin 1A coexpression. These findings indicate that acute regulation of the number of CFTR Cl channels in plasma membrane is one mechanism by which cAMP/PKA regulates Cl currents. Inhibition of plasma membrane CFTR content by syntaxin 1A is consistent with the concept that syntaxin and other components of the SNARE machinery are involved in regulated trafficking of CFTR.