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

CFTR Folding: From Structure and Proteostasis to Cystic Fibrosis Personalized Medicine

Cystic fibrosis (CF) is a lethal genetic disease caused by mutations in the chloride ion channel cystic fibrosis transmembrane conductance regulator (CFTR). Class-II mutants of CFTR lack intermolecular interactions important for CFTR structural stability and lead to misfolding. Misfolded CFTR is detected by a diverse suite of proteostasis factors that preferentially bind and route mutant CFTR toward premature degradation, resulting in reduced plasma membrane CFTR levels and impaired chloride ion conductance associated with CF. CF treatment has been vastly improved over the past decade by the availability of small molecules called correctors. Correctors directly bind CFTR, stabilize its structure by conferring thermodynamically favorable interactions that compensate for mutations, and thereby lead to downstream folding fidelity. However, each of over 100 Class-II CF causing mutations causes unique structural defects and shows a unique response to drug treatment, described as theratype. Understanding CFTR structural defects, the proteostasis factors evaluating those defects, and the stabilizing effects of CFTR correctors will illuminate a path toward personalized medicine for CF. Here, we review recent advances in our understanding of CFTR folding, focusing on structure, corrector binding sites, the mechanisms of proteostasis factors that evaluate CFTR, and the implications for CF personalized medicine.

Role of Protein Kinase A-Mediated Phosphorylation in CFTR Channel Activity Regulation

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel expressed on the apical membrane of epithelial cells, where it plays a pivotal role in chloride transport and overall tissue homeostasis. CFTR constitutes a unique member of the ATP-binding cassette transporter superfamily, due to its distinctive cytosolic regulatory (R) domain carrying multiple phosphorylation sites that allow the tight regulation of channel activity and gating. Mutations in the CFTR gene cause cystic fibrosis, the most common lethal autosomal genetic disease in the Caucasian population. In recent years, major efforts have led to the development of CFTR modulators, small molecules targeting the underlying genetic defect of CF and ultimately rescuing the function of the mutant channel. Recent evidence has highlighted that this class of drugs could also impact on the phosphorylation of the R domain of the channel by protein kinase A (PKA), a key regulatory mechanism that is altered in various CFTR mutants. Therefore, the aim of this review is to summarize the current knowledge on the regulation of the CFTR by PKA-mediated phosphorylation and to provide insights into the different factors that modulate this essential CFTR modification. Finally, the discussion will focus on the impact of CF mutations on PKA-mediated CFTR regulation, as well as on how small molecule CFTR regulators and PKA interact to rescue dysfunctional channels.

Protein kinase A phosphorylation potentiates cystic fibrosis transmembrane conductance regulator gating by relieving autoinhibition on the stimulatory C terminus of the regulatory domain

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel activated by protein kinase A (PKA) phosphorylation on the regulatory (R) domain. Phosphorylation at several R domain residues stimulates ATP-dependent channel openings and closings, termed channel gating. To explore the protein segment responsible for channel potentiation and PKA-dependent activation, deletion mutations were constructed by removing one to three protein segments of the R domain including residues 708-759 (ΔR_708-759), R_760-783, and R_784-835, each of which contains one or two PKA phosphorylation sites. Deletion of R_708-759 or R_760-783 had little effect on CFTR gating, whereas all mutations lacking R_784-835 reduced CFTR activity by decreasing the mean burst duration and increasing the interburst interval (IBI). The data suggest that R_784-835 plays a major role in stimulating CFTR gating. For ATP-associated regulation, ΔR_784-835 had minor impact on gating potentiation by 2'dATP, CaATP, and pyrophosphate. Interestingly, introducing a phosphorylated peptide matching R_809-835 shortened the IBI of ΔR_708-835-CFTR. Consistently, ΔR_815-835, but not ΔR_784-814, enhanced IBI, whereas both reduced mean burst duration. These data suggest that the entirety of R_784-835 is required for stabilizing the open state of CFTR; however, R_815-835, through interactions with the channel, is dominant for enhancing the opening rate. Of note, PKA markedly decreased the IBI of ΔR_708-783-CFTR. Conversely, the IBI of ΔR_708-814-CFTR was short and PKA-independent. These data reveal that for stimulating CFTR gating, PKA phosphorylation may relieve R_784-814-mediated autoinhibition that prevents IBI shortening by R_815-835 This mechanism may elucidate how the R domain potentiates channel gating and may unveil CFTR stimulation by other protein kinases.

Inhaled Biologicals for the Treatment of Cystic Fibrosis

Background:

Cystic Fibrosis (CF), one of the most frequent genetic diseases, is characterized by the production of viscous mucus in several organs. In the lungs, mucus clogs the airways and traps bacteria, leading to recurrent/resistant infections and lung damage. For cystic fibrosis patients, respiratory failure is still lethal in early adulthood since available treatments display incomplete efficacy.

Objective:

The objective of this review is to extend the current knowledge in the field of available treat-ments for cystic fibrosis. A special focus has been given to inhaled peptide-based drugs.

Methods:

The current review is based on recent and/or relevant literature and patents already available in various scientific databases, which include PubMed, PubMed Central, Patentscope and Science Direct. The information obtained through these diverse databases is compiled, critically interpreted and presented in the current study. An in-depth but not systematic approach to the specific research question has been adopted.

Results:

Recently, peptides have been proposed as possible pharmacologic agents for the treatment of respiratory diseases. Of note, peptides are suitable to be administered by inhalation to maximize efficacy and reduce systemic side effects. Moreover, innovative delivery carriers have been developed for drug administration through inhalation, allowing not only protection against proteolysis, but also a prolonged and controlled release.

Conclusion:

Here, we summarize newly patented peptides that have been developed in the last few years and advanced technologies for inhaled drug delivery to treat cystic fibrosis.

Changes in the R-region interactions depend on phosphorylation and contribute to PKA and PKC regulation of the cystic fibrosis transmembrane conductance regulator chloride channel

The CFTR chloride channel is regulated by phosphorylation at PKA and PKC consensus sites within its regulatory region (R-region) through a mechanism, which is still not completely understood. We used a split-CFTR construct expressing the N-term-TMD1-NBD1 (Front Half; FH), TMD2-NBD2-C-Term (Back Half; BH), and the R-region as separate polypeptides (Split-R) in BHK cells, to investigate in situ how different phosphorylation conditions affect the R-region interactions with other parts of the protein. In proximity ligation assays, we studied the formation of complexes between the R-region and each half of the Split-CFTR. We found that at basal conditions, the density of complexes formed between the R-region and both halves of the split channel were equal. PKC stimulation alone had no effect, whereas PKA stimulation induced the formation of more complexes between the R-region and both halves compared to basal conditions. Moreover, PKC + PKA stimulation further enhanced the formation of FH-R complexes by 40% from PKA level. In cells expressing the Split-R with the two inhibitory PKC sites on the R-region inactivated (SR-S641A/T682A), density of FH-R complexes was much higher than in Split-R WT expressing cells after PKC or PKC + PKA stimulation. No differences were observed for BH-R complexes measured at all phosphorylation conditions. Since full-length CFTR channels display large functional responses to PKC + PKA in WT and S641A/T682A mutant, we conclude that FH-R interactions are important for CFTR function. Inactivation of consensus PKC site serine 686 (S686A) significantly reduced the basal BH-R interaction and prevented the PKC enhancing effect on CFTR function and FH-R interaction. The phospho-mimetic mutation (S686D) restored basal BH-R interaction and the PKC enhancing effect on CFTR function with enhanced FH-R interaction. As the channel function is mainly stimulated by PKA phosphorylation of the R-region, and this response is known to be enhanced by PKC phosphorylation, our data support a model in which the regulation of CFTR activation results from increased interactions of the R-region with the N-term-TMD1-NBD1. Also, serine S686 was found to be critical for the PKC enhancing effect which requires a permissive BH-R interaction at basal level and increased FH-R interaction after PKC + PKA phosphorylation.

Cryo-EM Visualization of an Active High Open Probability CFTR Anion Channel

The cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, crucial to epithelial salt and water homeostasis, and defective due to mutations in its gene in patients with cystic fibrosis, is a unique member of the large family of ATP-binding cassette transport proteins. Regulation of CFTR channel activity is stringently controlled by phosphorylation and nucleotide binding. Structural changes that underlie transitions between active and inactive functional states are not yet fully understood. Indeed the first 3D structures of dephosphorylated, ATP-free, and phosphorylated ATP-bound states were only recently reported. Here we have determined the structure of inactive and active states of a thermally stabilized CFTR, the latter with a very high channel open probability, confirmed after reconstitution into proteoliposomes. These structures, obtained at nominal resolution of 4.3 and 6.6 Å, reveal a unique repositioning of the transmembrane helices and regulatory domain density that provide insights into the structural transition between active and inactive functional states of CFTR. Moreover, we observe an extracellular vestibule that may provide anion access to the pore due to the conformation of transmembrane helices 7 and 8 that differs from the previous orthologue CFTR structures. In conclusion, our work contributes detailed structural information on an active, open state of the CFTR anion channel.

Structural mechanisms of CFTR function and dysfunction

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

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

The gating of the CFTR channel

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel expressed in the apical membrane of epithelia. Mutations in the CFTR gene are the cause of cystsic fibrosis. CFTR is the only ABC-protein that constitutes an ion channel pore forming subunit. CFTR gating is regulated in complex manner as phosphorylation is mandatory for channel activity and gating is directly regulated by binding of ATP to specific intracellular sites on the CFTR protein. This review covers our current understanding on the gating mechanism in CFTR and illustrates the relevance of alteration of these mechanisms in the onset of cystic fibrosis.

Origin and evolution of the cystic fibrosis transmembrane regulator protein R domain

The Cystic Fibrosis Transmembrane Conductance Regulator protein (CFTR) is a member of the ABC transporter superfamily. CFTR is distinguished from all other members of this superfamily by its status as an ion channel as well as the presence of its unique regulatory (R) domain. We investigated the origin and subsequent evolution of the R domain along the CFTR evolutionary lineage. The R domain protein coding sequence originated via the loss of a splice donor site at the 3' end of exon 14, leading to the subsequent read-through and capture of formerly intronic sequence as novel coding sequence. Inclusion of the remaining part of the R domain coding sequence in the CFTR transcript involved a lineage-specific gain of exonic sequence with no homology to protein coding sequences outside of CFTR and loss of two exons conserved among ABC family members. These events occurred at the base of the Gnathostome evolutionary lineage ~550-650 million years ago. The apparent origination of the R domain de novo from previously non-coding sequence is consistent with its lack of sequence similarity to other domains as well as its intrinsically disordered structure, which has important implications for its function. In particular, this lack of structure may provide for a dynamic and inducible regulatory activity based on transient physical interactions with more structured domains of the protein. Since its acquisition along the CFTR evolutionary lineage, the R domain has evolved more rapidly than any other CFTR domain; however, there is no evidence for positive (adaptive) selection in the evolution of the domain. The R domain does show a distinct pattern of relative evolutionary rates compared to other CFTR domains, which sheds additional light on the connection between its function and evolution. The regulatory function of the R domain is dependent upon a fairly small number of sites that are subject to phosphorylation, and these sites were fixed very early in R domain evolution and have remained largely invariant since that time. In contrast, the rest of the R domain has been free to drift in sequence space leading to a more star-like phylogeny than seen for the other CFTR domains. The case of the R domain suggests that domain acquisition via the de novo creation of coding sequence, and the novel functional utility that such an event would seemingly entail, can be one route by which neo-functionalization is favored to occur.

A SAXS-based ensemble model of the native and phosphorylated regulatory domain of the CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR), the defective protein in cystic fibrosis, is an anion channel activated by protein kinase A phosphorylation. The regulatory domain (RD) of CFTR has multiple phosphorylation sites, and is responsible for channel activation. This domain is intrinsically disordered, rendering the structural analysis a difficult task, as high-resolution techniques are barely applicable. In this work, we obtained a biophysical characterization of the native and phosphorylated RD in solution by employing complementary structural methods. The native RD has a gyration radius of 3.25 nm, and a maximum molecular dimension of 11.4 nm, larger than expected for a globular protein of the same molecular mass. Phosphorylation causes compaction of the structure, yielding a significant reduction of the gyration radius, to 2.92 nm, and on the maximum molecular dimension to 10.2 nm. Using an ensemble optimization method, we were able to generate a low-resolution, three-dimensional model of the native and the phosphorylated RD based on small-angle X-ray scattering data. We have obtained the first experiment-based model of the CFTR regulatory domain, which will be useful to understand the molecular mechanisms of normal and pathological CFTR functioning.

Dynamics intrinsic to cystic fibrosis transmembrane conductance regulator function and stability

The cystic fibrosis transmembrane conductance regulator (CFTR) requires dynamic fluctuations between states in its gating cycle for proper channel function, including changes in the interactions between the nucleotide-binding domains (NBDs) and between the intracellular domain (ICD) coupling helices and NBDs. Such motions are also linked with fluctuating phosphorylation-dependent binding of CFTR's disordered regulatory (R) region to the NBDs and partners. Folding of CFTR is highly inefficient, with the marginally stable NBD1 sampling excited states or folding intermediates that are aggregation-prone. The severe CF-causing F508del mutation exacerbates the folding inefficiency of CFTR and leads to impaired channel regulation and function, partly as a result of perturbed NBD1-ICD interactions and enhanced sampling of these NBD1 excited states. Increased knowledge of the dynamics within CFTR will expand our understanding of the regulated channel gating of the protein as well as of the F508del defects in folding and function.

Regulation of activation and processing of the cystic fibrosis transmembrane conductance regulator (CFTR) by a complex electrostatic interaction between the regulatory domain and cytoplasmic loop 3

BACKGROUND

NEG2 regulates CFTR gating but the mechanism is unknown.

RESULTS

A putative NEG2-CL3 electrostatic attraction, possibly weakened by Arg-764/Arg-766 of the R domain, prohibited CFTR activation. A charge exchange between NEG2 and CL3 caused misprocessing.

CONCLUSION

Electrostatic regulation of CFTR activation and processing may be asymmetric at the CL3-R interface.

SIGNIFICANCE

The CL3-R interface is optimally designed for multiple regulations of CFTR functions. NEG2, a short C-terminal segment (817-838) of the unique regulatory (R) domain of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, has been reported to regulate CFTR gating in response to cAMP-dependent R domain phosphorylation. The underlying mechanism, however, is unclear. Here, Lys-946 of cytoplasmic loop 3 (CL3) is proposed as counter-ion of Asp-835, Asp-836, or Glu-838 of NEG2 to prevent the channel activation by PKA. Arg-764 or Arg-766 of the Ser-768 phosphorylation site of the R domain is proposed to promote the channel activation possibly by weakening the putative CL3-NEG2 electrostatic attraction. First, not only D835A, D836A, and E838A but also K946A reduced the PKA-dependent CFTR activation. Second, both K946D and D835R/D836R/E838R mutants were activated by ATP and curcumin to a different extent. Third, R764A and R766A mutants enhanced the PKA-dependent activation. However, it is very exciting that D835R/D836R/E838R and K946D/H950D and H950R exhibited normal channel processing and activity whereas D835R/D836R/E838R/K946D/H950D was fractionally misprocessed and silent in response to forskolin. Further, D836R and E838R played a critical role in the asymmetric electrostatic regulation of CFTR processing, and Ser-768 phosphorylation may not be involved. Thus, a complex interfacial interaction among CL3, NEG2, and the Ser-768 phosphorylation site may be responsible for the asymmetric electrostatic regulation of CFTR activation and processing.

CFTR-SLC26 transporter interactions in epithelia

Transport mechanisms that mediate the movements of anions must be coordinated tightly in order to respond appropriately to physiological stimuli. This process is of paramount importance in the function of diverse epithelial tissues of the body, such as, for example, the exocrine pancreatic duct and the airway epithelia. Disruption of any of the finely tuned components underlying the transport of anions such as Cl(-), HCO(3) (-), SCN(-), and I(-) may contribute to a plethora of disease conditions. In many anion-secreting epithelia, the interactions between the cystic fibrosis transmembrane conductance regulator (CFTR) and solute carrier family 26 (SLC26) transporters determine the final exit of anions across the apical membrane and into the luminal compartment. The molecular identification of CFTR and many SLC26 members has enabled the acquisition of progressively more detailed structural information about these transport molecules. Studies employing a vast array of increasingly sophisticated approaches have culminated in a current working model which places these key players within an interactive complex, thereby setting the stage for future work.

Functional interaction between CFTR and the sodium-phosphate co-transport type 2a in Xenopus laevis oocytes

Background

A growing number of proteins, including ion transporters, have been shown to interact with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). CFTR is an epithelial chloride channel that is involved in Cystic Fibrosis (CF) when mutated; thus a better knowledge of its functional interactome may help to understand the pathophysiology of this complex disease. In the present study, we investigated if CFTR and the sodium-phosphate co-transporter type 2a (NPT2a) functionally interact after heterologous expression of both proteins in Xenopus laevis oocytes.

Methodology/Findings

NPT2a was expressed alone or in combination with CFTR in X. laevis oocytes. Using the two-electrode voltage-clamp technique, the inorganic phosphate-induced current (IPi) was measured and taken as an index of NPT2a activity. The maximal IPi for NPT2a substrates was reduced when CFTR was co-expressed with NPT2a, suggesting a decrease in its expression at the oolemna. This was consistent with Western blot analysis showing reduced NPT2a plasma membrane expression in oocytes co-expressing both proteins, whereas NPT2a protein level in total cell lysate was the same in NPT2a- and NPT2a+CFTR-oocytes. In NPT2a+CFTR- but not in NPT2a-oocytes, IPi and NPT2a surface expression were increased upon PKA stimulation, whereas stimulation of Exchange Protein directly Activated by cAMP (EPAC) had no effect. When NPT2a-oocytes were injected with NEG2, a short amino-acid sequence from the CFTR regulatory domain that regulates PKA-dependent CFTR trafficking to the plasma membrane, IPi values and NPT2a membrane expression were diminished, and could be enhanced by PKA stimulation, thereby mimicking the effects of CFTR co-expression.

Conclusion/Perspectives

We conclude that when both CFTR and NPT2a are expressed in X. laevis oocytes, CFTR confers to NPT2a a cAMPi-dependent trafficking to the membrane. This functional interaction raises the hypothesis that CFTR may play a role in phosphate homeostasis.

The inhibition mechanism of non-phosphorylated Ser768 in the regulatory domain of cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette transporters but serves as a chloride channel dysfunctional in cystic fibrosis. The activity of CFTR is tightly controlled not only by ATP-driven dimerization of its nucleotide-binding domains but also by phosphorylation of a unique regulatory (R) domain by protein kinase A (PKA). The R domain has multiple excitatory phosphorylation sites, but Ser(737) and Ser(768) are inhibitory. The underlying mechanism is unclear. Here, sulfhydryl-specific cross-linking strategy was employed to demonstrate that Ser(768) or Ser(737) could interact with outwardly facing hydrophilic residues of cytoplasmic loop 3 regulating channel gating. Furthermore, mutation of these residues to alanines promoted channel opening by curcumin in an ATP-dependent manner even in the absence of PKA. However, mutation of Ser(768) and His(950) with different hydrogen bond donors or acceptors clearly changed ATP- and PKA-dependent channel activity no matter whether curcumin was present or not. More importantly, significant activation of a double mutant H950R/S768R needed only ATP. Finally, in vitro and in vivo single channel recordings suggest that Ser(768) may form a putative hydrogen bond with His(950) of cytoplasmic loop 3 to prevent channel opening by ATP in the non-phosphorylated state and by subsequent cAMP-dependent phosphorylation. These observations support an electron cryomicroscopy-based structural model on which the R domain is closed to cytoplasmic loops regulating channel gating.

State-dependent regulation of cystic fibrosis transmembrane conductance regulator (CFTR) gating by a high affinity Fe3+ bridge between the regulatory domain and cytoplasmic loop 3

The unique regulatory (R) domain differentiates the human CFTR channel from other ATP-binding cassette transporters and exerts multiple effects on channel function. However, the underlying mechanisms are unclear. Here, an intracellular high affinity (2.3 × 10(-19) M) Fe(3+) bridge is reported as a novel approach to regulating channel gating. It inhibited CFTR activity by primarily reducing an open probability and an opening rate, and inhibition was reversed by EDTA and phenanthroline. His-950, His-954, Cys-832, His-775, and Asp-836 were found essential for inhibition and phosphorylated Ser-768 may enhance Fe(3+) binding. More importantly, inhibition by Fe(3+) was state-dependent. Sensitivity to Fe(3+) was reduced when the channel was locked in an open state by AMP-PNP. Similarly, a K978C mutation from cytoplasmic loop 3 (CL3), which promotes ATP-independent channel opening, greatly weakened inhibition by Fe(3+) no matter whether NBD2 was present or not. Therefore, although ATP binding-induced dimerization of NBD1-NBD2 is required for channel gating, regulation of CFTR activity by Fe(3+) may involve an interaction between the R domain and CL3. These findings may support proximity of the R domain to the cytoplasmic loops. They also suggest that Fe(3+) homeostasis may play a critical role in regulating pathophysiological CFTR activity because dysregulation of this protein causes cystic fibrosis, secretary diarrhea, and infertility.

NMR evidence for differential phosphorylation-dependent interactions in WT and DeltaF508 CFTR

The most common cystic fibrosis (CF)-causing mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) is deletion of Phe508 (DeltaF508) in the first of two nucleotide-binding domains (NBDs). Nucleotide binding and hydrolysis at the NBDs and phosphorylation of the regulatory (R) region are required for gating of CFTR chloride channel activity. We report NMR studies of wild-type and DeltaF508 murine CFTR NBD1 with the C-terminal regulatory extension (RE), which contains residues of the R region. Interactions of the wild-type NBD1 core with the phosphoregulatory regions, the regulatory insertion (RI) and RE, are disrupted upon phosphorylation, exposing a potential binding site for the first coupling helix of the N-terminal intracellular domain (ICD). Phosphorylation of DeltaF508 NBD1 does not as effectively disrupt interactions with the phosphoregulatory regions, which, along with other structural differences, leads to decreased binding of the first coupling helix. These results provide a structural basis by which phosphorylation of CFTR may affect the channel gating of full-length CFTR and expand our understanding of the molecular basis of the DeltaF508 defect.

PKC phosphorylation modulates PKA-dependent binding of the R domain to other domains of CFTR

Activity of the CFTR channel is regulated by phosphorylation of its regulatory domain (RD). In a previous study, we developed a bicistronic construct called DeltaR-Split CFTR, which encodes the front and back halves of CFTR as separate polypeptides without the RD. These fragments assemble to form a constitutively active CFTR channel. Coexpression of the third fragment corresponding to the missing RD restores regulation by PKA, and this is associated with dramatically enhanced binding of the phosphorylated RD. In the present study, we examined the effect of PKC phosphorylation on this PKA-induced interaction. We report here that PKC alone enhanced association of the RD with DeltaR-Split CFTR and that binding was further enhanced when the RD was phosphorylated by both kinases. Mutation of all seven PKC consensus sequences on the RD (7CA-RD) did not affect its association under basal (unphosphorylated) conditions but abolished phosphorylation-induced binding by both kinases. Iodide efflux responses provided further support for the essential role of RD binding in channel regulation. The basal activity of DeltaR-Split/7CA-RD channels was similar to that of DeltaR-Split/wild type (WT)-RD channels, whereas cAMP-stimulated iodide efflux was greatly diminished by removal of the PKC sites, indicating that 7CA-RD binding maintains channels in an inactive state that is unresponsive to PKA. These results suggest a novel mechanism for CFTR regulation in which PKC modulates PKA-induced domain-domain interactions.

Regulation of CFTR trafficking by its R domain

Phosphorylation of the R domain is required for cystic fibrosis transmembrane conductance regulator (CFTR) channel gating, and cAMP/protein kinase A (PKA) simulation can also elicit insertion of CFTR into the plasma membrane from intracellular compartments (Bertrand, C. A., and Frizzell, R. A. (2003) Am. J. Physiol. 285, C1-C18). We evaluated the structural basis of regulated CFTR trafficking by determining agonist-evoked increases in plasma membrane capacitance (Cm) of Xenopus oocytes expressing CFTR deletion mutants. Expression of CFTR as a split construct that omitted the R domain (Deltaamino acids 635-834) produced a channel with elevated basal current (Im) and no DeltaIm or trafficking response (DeltaCm) upon cAMP/PKA stimulation, indicating that the structure(s) required for regulated CFTR trafficking are contained within the R domain. Additional deletions showed that removal of amino acids 817-838, a 22-amino acid conserved helical region having a net charge of -9, termed NEG2 (Xie, J., Adams, L. M., Zhao, J., Gerken, T. A., Davis, P. B., and Ma, J. (2002) J. Biol. Chem. 277, 23019-23027), produced a channel with regulated gating that lacked the agonist-induced increase in CFTR trafficking. Injection of NEG2 peptides into oocytes expressing split DeltaNEG2 CFTR prior to stimulation restored the agonist-evoked DeltaCm, consistent with the concept that this sequence mediates the regulated trafficking event. In support of this idea, DeltaNEG2 CFTR escaped from the inhibition of wild type CFTR trafficking produced by overexpression of syntaxin 1A. These observations suggest that the NEG2 region at the C terminus of the R domain allows stabilization of CFTR in a regulated intracellular compartment from which it traffics to the plasma membrane in response to cAMP/PKA stimulation.

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.

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.

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.

Methods to study CFTR protein in vitro

CFTR is a cyclic AMP and nucleotide-related chloride-selective channel with a low unitary conductance. Many of the physiological roles of CFTR are effectively studied in intact cells and tissues. However, there are also several clear advantages to the application of cell-free technologies to the study of the biochemical and biophysical properties of CFTR. When expressed in heterologous cells, CFTR is processed relatively poorly, depending, however, on the cell-type analysed. In some cells, only 20-25% of the protein which is initially synthesized exits the endoplasmic reticulum to insert into the cell membrane [Cell 83 (1995) 121; EMBO J. 13 (1994) 6076]. Further, many of the disease-causing mutants of CFTR result in even lower processing efficiencies. Therefore, several procedures have been developed to study regulated CFTR channel function expressed in microsomal membranes and following its purification and reconstitution. These experimental approaches and their application are discussed here.

Intermolecular interaction between R domains of cystic fibrosis transmembrane conductance regulator

The function of the R domain of cystic fibrosis transmembrane conductance regulator (CFTR) has not yet been fully established. The cis-trans proline isomerase cyclophilin A stimulates channel activity, and stimulation depends on the presence of highly conserved prolines at positions 740, 750, and 759. When the prolines at these positions, which normally exist in the cis conformation, are locked into the trans conformation by mutation to alanine (the P3A mutant), the open probability of P3A is high and is not further increased by cyclophilin A. We speculated that one mechanism by which this could occur was by promoting CFTR dimerization, which has been shown to increase open probability, and that the P3A-CFTR might favor dimerization more strongly than the native sequence. To test the hypothesis that R-R interaction occurs and is stronger in the P3A-R mutants, we investigated R-R interactions. GST-R and StrepII-R proteins expressed in Escherichia coli could interact with R domain protein translated in vitro as well as with full-length CFTR. In similar assays, the P3A mutant of R domain also interacts with R domain and P3A-R. The P3A-R-P3A-R interaction is stronger than the R-R interaction, which corroborates our data from the channel study and supports our hypothesis. Studies of deletion constructs of the isolated R domain and of full-length CFTR localize the region of interaction to the C-terminal portion of R (after amino acid 708). Particularly, the last 22 a.a. residues (838-859) of R are essential for this binding. R-R interaction possibly plays a role in channel gating.