CFTR (ABCC7) is a phospho-regulated chloride channel that is found in the apical membranes of epithelial cells, is gated by ATP and the activity of the protein is crucial in the homeostasis of the extracellular liquid layer in many organs [Annu. Rev. Biochem. (2008) 77, 701–726; Science (1989) 245, 1066–1073]. Mutations in CFTR cause the inherited disease cystic fibrosis (CF), the most common inherited condition in humans of European descent [Science (1989) 245, 1066–1073; Pflugers Arch. (2007) 453, 555–567]. The structural basis of CF will be discussed in this article.

Introduction

CFTR is a member of the ATP-binding cassette (ABC) family of proteins [13]. As such, CFTR has the standard four-domain structure consisting of two transmembrane domains and two soluble nucleotide-binding domains (NBDs) [4]. The ABC family members are predominantly active transporters. In accord with the Jardetzky model for transporter action [5], the ATP is employed to drive the transition of the transmembrane domains from an inward-facing to an outward-facing conformation (or vice versa for ABC importers). However, for CFTR it is thought that ATP drives a conformational change that opens a channel in the transmembrane domains, through which chloride ions can diffuse passively [6]. CFTR is the only member of the ABC family that has this channel-type functionality, although some members do act as regulatory switches for other channels [7,8].

Initial CFTR structural data

Initial structural studies were carried out over a decade ago on the soluble CFTR domains studied in isolation from the transmembrane portions [9,10]. In these early studies, the first soluble ATP-binding domain (nucleotide-binding domain 1, NBD1) containing the most common CF-causing mutation (F508 deletion) was crystallised in the presence and absence of this phenylalanine residue. The F508 deletion was found to have almost no effect on the structure of the isolated domain apart from some local rearrangements around the F508 position. This was unexpected: prior studies of the mutation in various cells had been interpreted in terms of its protein folding, with the F508del mutation discussed as a misfolding mutation that led to its recognition by the endoplasmic reticulum quality control machinery [1114]. However, crystallisation of the domain, as well as the second NBD (NBD2), required an exhaustive search for mutations that promoted solubility and stability. In the case of NBD1, these same mutations were later demonstrated to partially rescue the defect caused by F508 deletion [15]. Isolated domains were also studied by NMR although, as above, mutations that increased the protein solubility were needed to assign the NMR resonances; however, later studies of the effects of F508 deletion were with NBDs with background wild-type sequences [1619].

NMR was also employed to study the interactions of the third soluble ‘domain’ in CFTR, the 200 residue-long Regulatory-region or R-region [18,20]. These interactions were studied with several potential partner proteins, including the two soluble NBDs. Whether this region was intrinsically disordered (as suggested by the NMR data and predictions based on its amino acid sequence), or a structured domain, or some intermediate between the two, remained a moot point until recently. When cryo-EM data for single CFTR particles emerged [2124], the data implied that the R-region existed in an intermediate state in the absence of phosphorylation — i.e. it displayed weak, but localised, Coulomb-scattering density in the cryo-EM maps. This weak density was observed between the two NBDs and in one map it protruded into the aqueous compartment generated by the two transmembrane domains in their inward-facing configuration ([23], see Figure 1). Upon phosphorylation, the weak density for the R-region could no longer be observed [22,24], implying that the addition of ∼7–9 negative charges to Serine and Threonine residues in this region caused its dissociation from the CFTR core domains and resulted in an increase in disorder throughout this portion of the protein. Such a phosphorylation dependence on R-region/NBD association was observed by the NMR interaction studies using the isolated domains [20].

Context of CF-causing mutations and the importance of surface charges.

Figure 1.
Context of CF-causing mutations and the importance of surface charges.

(A) Zebrafish CFTR in the outward-facing state (PDBID-5W81) shows F508 (highlighted green), which is present in NBD1 (taupe backbone) and is at the interface (solid line) with ICL4 of TMD2 (grey). The ribbon trace shows the paths of the long cytoplasmic extensions of transmembrane helices 10 and 11. The human CFTR structure (sky blue) in the inward-facing state (PDBID-5UAK) is shown superimposed after alignment. There is strict conservation of the interface across species and it is retained despite the global conformational changes involved in the transit from inward- to outward-facing states. (B) Position of G551 in the signature sequence of NBD1 (right) and at the NBD1/NBD2 interface (black line). Nearby charged residues in NBD2 and the phosphate groups of ATP (yellow/red space-fill atoms) imply that mutation of G to D will disfavour NBD dimerisation and hence channel opening. The zebrafish CFTR in the outward-facing state is shown (PDBID-5W81). (C) Surface charge distribution in the inward-facing human CFTR structure (PDBID-5UAK) with no nucleotide and unassigned R-region residues removed. Blue and red shades represent varying positive or negative charge, respectively, and white, no charge. Positively charged patches on the inner surface (arrows) must be brought together for formation of the outward-facing state.

Figure 1.
Context of CF-causing mutations and the importance of surface charges.

(A) Zebrafish CFTR in the outward-facing state (PDBID-5W81) shows F508 (highlighted green), which is present in NBD1 (taupe backbone) and is at the interface (solid line) with ICL4 of TMD2 (grey). The ribbon trace shows the paths of the long cytoplasmic extensions of transmembrane helices 10 and 11. The human CFTR structure (sky blue) in the inward-facing state (PDBID-5UAK) is shown superimposed after alignment. There is strict conservation of the interface across species and it is retained despite the global conformational changes involved in the transit from inward- to outward-facing states. (B) Position of G551 in the signature sequence of NBD1 (right) and at the NBD1/NBD2 interface (black line). Nearby charged residues in NBD2 and the phosphate groups of ATP (yellow/red space-fill atoms) imply that mutation of G to D will disfavour NBD dimerisation and hence channel opening. The zebrafish CFTR in the outward-facing state is shown (PDBID-5W81). (C) Surface charge distribution in the inward-facing human CFTR structure (PDBID-5UAK) with no nucleotide and unassigned R-region residues removed. Blue and red shades represent varying positive or negative charge, respectively, and white, no charge. Positively charged patches on the inner surface (arrows) must be brought together for formation of the outward-facing state.

New structural data

There have been several cryo-EM-derived structures of CFTR published recently, with the studies based on zebrafish, chicken and human versions of CFTR expressed in mammalian cells [2124]. Reflecting the major improvement in cryo-EM technology, all these structures contain structural data with a resolution considerably better than was previously possible [25]. However, there are features of the new structures that are not readily explained by prior models of CFTR structure–function relationships [6,26]. For example, dephosphorylated CFTR (studied without ATP) and phosphorylated CFTR (studied with ATP) both displayed an inward-facing configuration in the structures of chicken CFTR described by Fay et al. [24]. Minimal changes in conformation were reported upon phosphorylation and nucleotide binding, a major departure from the prior models. However, zebrafish CFTR in the dephosphorylated/ATP-free condition was shown by cryo-EM to be inward-facing [21], while after phosphorylation and addition of ATP there was a major conformational shift to the outward-facing state [22]. This latter conformation was obtained with a version of the protein where the catalytic Walker B glutamate residue (E1372) was mutated to glutamine. This mutation's effects can be rationalised in terms of prior models in that prevention of ATP hydrolysis should prolong channel opening and the outward-facing state. However, the phosphorylated/ATP bound chicken CFTR structure which was in the inward-facing state was also obtained with mutations that stabilised the open state of the channel [24]. Hence the new structures may demand a new model of CFTR structure–function relationships that can accommodate all the current structural data.

Disease-causing mutations

The multiple cryo-EM-derived structures for CFTR at medium resolution (3–9 Å resolution) represent structures equivalent to the wild-type protein as well as structures with mutations that were incorporated to stabilise the protein or to favour a particular conformation [2125]. The structural effects of mutations that are cystic fibrosis (CF)-causing in a significant number of patients (such as F508del) still remain to be studied using cryo-EM, although small-angle X-ray scattering (SAXS) has been employed to study this most prevalent disease-causing mutation [27]. The structural data so far imply that F508 deletion causes significant changes in the overall structure of the purified full-length protein as detected by SAXS. This is in accord with the biochemical data that show that F508 deletion has a major effect on the stability of the protein [28,29]. F508 sits in a buried position in NBD1 at its interface with the 2nd intracellular loop of transmembrane domain 2 (Figure 1). This loop links transmembrane helices 10 and 11, which cross over from the opposite side of the molecule in a domain swap-type arrangement. Hinge-like movements of these transmembrane helices are closely associated with the transition from inward- to outward-facing conformations and this may explain why F508del CFTR has poor channel activity. However, the position of F508 at a crucial and exquisitely conserved domain–domain interface may explain why it has a large effect on the overall stability of the structure of the protein (Figure 1A).

The second most common missense mutation that causes CF is G551D, which results in a predominantly closed channel. The open probability of fully phosphorylated G551D CFTR channels in the presence of milliMolar ATP concentrations is 10-fold lower (Po = 0.04) compared with the WT channel (Po = 0.40) [29]. G551 is part of the ‘signature’ sequence of NBD1 which jointly binds ATP in association with the Walker A and Walker B residues from NBD2 (Figure 1B). The signature sequence is a characteristic of all ABC proteins [3032]. The incorporation of an additional negative charge at this position as well as the –CH2–COO side-chain of aspartate is likely to impede ATP binding and make the formation of the NBD1–NBD2 sandwich dimer much less likely. Not only are the negatively charged phosphate groups of the ATP in close proximity to G551, but also the E1371 and D1370 residues in the Walker B region of NBD2. A drug to treat G551D patients was developed over the last decade (Ivacaftor [33,34]) and this therapeutic compound greatly increases the open probability of the channel to close to WT levels [33]. Ivacaftor has been shown to be effective at increasing the channel function of many, but not all CF-causing mutations studied in vitro [35], and these mutations are distributed throughout the 3D structure of the protein. Of note is that many of the mutations that Ivacaftor can work on involve a change in the charge of the mutated residue [35] akin to G551D. Hence elucidation of the mode of action of the drug is challenging but we can infer that charge effects appear to be implicated. Examination of the distribution of charged residues across the water-exposed surface of CFTR allows the calculation of the Coulombic surface potential, which is displayed using colour-coding in Figure 1C. This shows that the inner surfaces of the intracytoplasmic loops are strongly positively charged, a feature of significance for the above discussion since these surfaces must be pressed together in order to reach the outward-facing state and hence open the channel.

Integration of the biochemical and structural data for CFTR

We propose a new model to explain these observations (Figure 2). This model assumes that nucleotide will remain bound to CFTR for most of its lifetime in the cell, with a slow rate of ATP hydrolysis. Nucleotide-free CFTR will never exist unless ATP levels drop in the cell due to cell death, but note that this state can be produced and studied experimentally (e.g. in inside-out membrane patches [36]). For CFTR activity to be regulated, phosphorylation by PKA and dephosphorylation by protein phosphatase will be needed (step 1). Once phosphorylation of the R-region occurs, then this region can dissociate from its interfering position between the NBDs and take up a more disordered structure. In this new situation, inward-facing CFTR with no channel activity can alternate with an outward-facing conformation that can form a channel for chloride ions (step 2). Production of the outward-facing state may have a significant energy barrier, possibly because of the long-range influence of electrostatic forces due to the positively charged residues that cluster in the inward-facing surface of the intracytoplasmic loops as well as the negative charge of the ATP molecules in the NBDs (Figure 1B,C). The ‘heavy-lifting’ needed to produce the outward-facing state leads to the naming of this conceptualisation as the ‘Grunt model’. Once the outward-facing state is formed, short-range interactions (H-bonding, van der Waals) are likely to dominate and stabilise it. Mutations such as G551D that reduce the open probability of the channel are likely to affect the equilibrium at step 2 as well as binding of ATP at step 5, and we predict that Ivacaftor also acts at step 2, but in this case it will promote the outward-facing state. We expect the drug will be acting in a similar fashion to ABC transporter allocrites (transported substrates), which are known to stimulate ATPase activity when they bind and hence may consequently lead to the outward-facing state. For exporters in the same family as CFTR, allocrites bind to the inward-facing state with higher affinity [3] and for some transporters, the binding sites have been characterised by mutagenesis and structural studies. For example in P-glycoprotein (ABCB1), which can transport xenobiotics and drugs out of the cell, there are two drug-binding sites at the apex of the inner vestibule formed by the two transmembrane domains in the inward-facing conformation [37,38]. It is known that Ivacaftor is one of the many transported substrates of P-glycoprotein [39].

‘Grunt’ model for CFTR structure–function relationships.

Figure 2.
‘Grunt’ model for CFTR structure–function relationships.

Here, nucleotide-free CFTR is considered non-physiological (dashed circle). Inactivated CFTR (bottom right) has an R-region (green) impeding NBD dimerisation. Activation by PKA (reversible by protein phosphatase) is via phosphorylation of the R-region and removal of the impediment (Step 1). NBD dimerisation and production of the open channel (Step 2) is not inevitable because of the need to overcome longer-range charge repulsion as the outward-facing state is approached (red, blue circles, ‘Grunt’). Mutations affecting channel open probability are predicted to exert their influence at Step 2, along with potentiator compounds such as Ivacaftor [35]. Once formed, the outward-facing state is stable due to short-range interactions, but is destabilised in some way by ATP hydrolysis by NBD2 Walker B residues (Step 3). Mutations affecting Step 3 may prolong channel opening. Dissociation of ADP and inorganic phosphate (Step 4) and re-binding of ATP (Step 5) must now occur, and mutations affecting the kinetics of these steps may also affect the overall open probability of the channel. Structural studies by cryo-EM have so far revealed three of these states for CFTR (tick symbols). The relative thickness of the arrows at each step is only indicative of the likely proportions of the participants.

Figure 2.
‘Grunt’ model for CFTR structure–function relationships.

Here, nucleotide-free CFTR is considered non-physiological (dashed circle). Inactivated CFTR (bottom right) has an R-region (green) impeding NBD dimerisation. Activation by PKA (reversible by protein phosphatase) is via phosphorylation of the R-region and removal of the impediment (Step 1). NBD dimerisation and production of the open channel (Step 2) is not inevitable because of the need to overcome longer-range charge repulsion as the outward-facing state is approached (red, blue circles, ‘Grunt’). Mutations affecting channel open probability are predicted to exert their influence at Step 2, along with potentiator compounds such as Ivacaftor [35]. Once formed, the outward-facing state is stable due to short-range interactions, but is destabilised in some way by ATP hydrolysis by NBD2 Walker B residues (Step 3). Mutations affecting Step 3 may prolong channel opening. Dissociation of ADP and inorganic phosphate (Step 4) and re-binding of ATP (Step 5) must now occur, and mutations affecting the kinetics of these steps may also affect the overall open probability of the channel. Structural studies by cryo-EM have so far revealed three of these states for CFTR (tick symbols). The relative thickness of the arrows at each step is only indicative of the likely proportions of the participants.

ATP hydrolysis is likely to occur at step 3 and the model implies that catalysis requires the NBDs to be dimerised, as they exist in the outward-facing conformation, for this hydrolysis to be efficient. Mutations that affect this step (such as those in the Walker B region) may have some influence on step 2 as well. Indeed, mutation E1371Q in CFTR was used to generate a sufficient proportion of outward-facing CFTR molecules to allow that state to be studied by single-particle cryo-EM [22]. Steps 4 and 5 are typical of the subset of ABC transporter proteins where one of the ATP-binding sites has evolved to be catalytically inactive. There are many ABC transporters that show this feature, including CFTR where the site formed between the NBD1 Walker motifs and the NBD2 signature motif is non-consensus [1]. Mutations that affect these steps may also influence the balance at step 2 for the same reasons as above.

Perspectives

Both inward-facing and outward-facing conformations of CFTR in the presence of ATP have recently been reported for phosphorylated protein [18,22]. The inward-facing conformation was also observed in a study of CFTR interacting with the NHERF1-PDZ1 domain, which is known to bind at the CFTR C-terminus [40]. Moreover, a detailed study of negatively stained single particles of two purified ABC transporters found a spectrum of conformations in the presence of ATP, with 1–2% in the outward-facing configuration even in the absence of ATP [41]. Similarly, structure-based calculations suggest that the free energy changes needed to affect NBD dimerisation may be relatively small [42], and surprisingly, CFTR channel activity can be detected in constructs completely lacking NBD2 [43]. Hence there seems to be a remarkable plasticity in ABC protein conformations and structures that are emerging. The ‘Grunt’ model we propose seems to be consistent with the (otherwise perplexing) diversity of CFTR structures as well as observations concerning channel-disrupting and ATPase-disrupting mutations [44] and the broad mutation specificity of potentiators such as Ivacaftor. Future structural studies will highlight the effects of disease-causing mutations and the most significant of these will be structures that include information about the binding site(s) of the new therapeutics.

Abbreviations

     
  • ATP

    ATP-binding cassette

  •  
  • CF

    cystic fibrosis

  •  
  • NBD

    nucleotide-binding domain

  •  
  • NBD1

    nucleotide-binding domain 1

  •  
  • SAXS

    small-angle X-ray scattering

Author Contribution

R.C.F. wrote the paper. X.M., J.C., A.D.C. and E.R.M. discussed and edited the manuscript and contributed to the concepts therein.

Funding

The authors acknowledge the Cystic Fibrosis Foundation [CFF FORD13XX0] and the Cystic Fibrosis Trust [F508del CFTR SRC] for funding.

Acknowledgments

We thank Alessandro Barbieri, Nopnithi Thonghin and Talha Shafi for useful discussions.

Competing Interests

R.C.F. consulted for Vertex Inc. in 2016 and 2017.

References

References
1
Riordan
,
J.R.
(
2008
)
CFTR function and prospects for therapy
.
Annu. Rev. Biochem.
77
,
701
726
2
Riordan
,
J.R.
,
Rommens
,
J.M.
,
Kerem
,
B.
,
Alon
,
N.
,
Rozmahel
,
R.
,
Grzelczak
,
Z.
et al. 
(
1989
)
Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA
.
Science
245
,
1066
1073
3
Linton
,
K.J.
and
Higgins
,
C.F.
(
2007
)
Structure and function of ABC transporters: the ATP switch provides flexible control
.
Pflugers Arch.
453
,
555
567
4
Linton
,
K.J
. (
2007
)
Structure and function of ABC transporters
.
Physiology (Bethesda)
22
,
122
130
5
Jardetzky
,
O.
(
1966
)
Simple allosteric model for membrane pumps
.
Nature
211
,
969
970
6
Gadsby
,
D.C.
,
Vergani
,
P.
and
Csanády
,
L.
(
2006
)
The ABC protein turned chloride channel whose failure causes cystic fibrosis
.
Nature
440
,
477
483
7
Mikhailov
,
M.V.
,
Campbell
,
J.D.
,
de Wet
,
H.
,
Shimomura
,
K.
,
Zadek
,
B.
,
Collins
,
R.F.
et al. 
(
2005
)
3-D structural and functional characterization of the purified KATP channel complex Kir6.2-SUR1
.
EMBO J.
24
,
4166
4175
8
Bryan
,
J.
and
Aguilar-Bryan
,
L.
(
1999
)
Sulfonylurea receptors: ABC transporters that regulate ATP-sensitive K(+) channels
.
Biochim. Biophys. Acta
1461
,
285
303
9
Lewis
,
H.A.
,
Buchanan
,
S.G.
,
Burley
,
S.K.
,
Conners
,
K.
,
Dickey
,
M.
,
Dorwart
,
M.
et al. 
(
2004
)
Structure of nucleotide-binding domain 1 of the cystic fibrosis transmembrane conductance regulator
.
EMBO J.
23
,
282
293
10
Lewis
,
H.A.
,
Zhao
,
X.
,
Wang
,
C.
,
Sauder
,
J.M.
,
Rooney
,
I.
,
Noland
,
B.W.
et al. 
(
2005
)
Impact of the deltaF508 mutation in first nucleotide-binding domain of human cystic fibrosis transmembrane conductance regulator on domain folding and structure
.
J. Biol. Chem.
280
,
1346
1353
11
Cyr
,
D.M.
(
2011
)
Quality control of nascent Cftr in the endoplasmic reticulum
.
Pediatr. Pulm.
46
,
147
148
12
Ren
,
H.Y.
,
Grove
,
D.E.
,
De La Rosa
,
O.
,
Houck
,
S.A.
,
Sopha
,
P.
,
Van Goor
,
F.
et al. 
(
2013
)
VX-809 corrects folding defects in cystic fibrosis transmembrane conductance regulator protein through action on membrane-spanning domain 1
.
Mol. Biol. Cell
24
,
3016
3024
13
Ward
,
C.L.
,
Omura
,
S.
and
Kopito
,
R.R.
(
1995
)
Degradation of CFTR by the ubiquitin-proteasome pathway
.
Cell
83
,
121
127
14
Kopito
,
R.R.
(
1999
)
Biosynthesis and degradation of CFTR
.
Physiol. Rev.
79
,
S167
S173
15
Roxo-Rosa
,
M.
,
Xu
,
Z.
,
Schmidt
,
A.
,
Neto
,
M.
,
Cai
,
Z.
,
Soares
,
C.M.
et al. 
(
2006
)
Revertant mutants G550E and 4RK rescue cystic fibrosis mutants in the first nucleotide-binding domain of CFTR by different mechanisms
.
Proc. Natl Acad. Sci. U.S.A.
103
,
17891
17896
16
Kanelis
,
V.
,
Chong
,
P.A.
and
Forman-Kay
,
J.D.
(
2011
)
NMR spectroscopy to study the dynamics and interactions of CFTR
.
Methods Mol. Biol.
741
,
377
403
17
Kanelis
,
V.
,
Hudson
,
R.P.
,
Thibodeau
,
P.H.
,
Thomas
,
P.J.
and
Forman-Kay
,
J.D.
(
2010
)
NMR evidence for differential phosphorylation-dependent interactions in WT and Delta F508 CFTR
.
EMBO J.
29
,
263
277
18
Baker
,
J.M.R.
,
Hudson
,
R.P.
,
Kanelis
,
V.
,
Choy
,
W.Y.
,
Thibodeau
,
P.H.
,
Thomas
,
P.J.
et al. 
(
2007
)
CFTR regulatory region interacts with NBD1 predominantly via multiple transient helices
.
Nat. Struct. Mol. Biol.
14
,
738
745
19
Mendoza
,
J.L.
,
Schmidt
,
A.
,
Li
,
Q.
,
Nuvaga
,
E.
,
Barrett
,
T.
,
Bridges
,
R.J.
et al. 
(
2012
)
Requirements for efficient correction of DeltaF508 CFTR revealed by analyses of evolved sequences
.
Cell
148
,
164
174
20
Bozoky
,
Z.
,
Krzeminski
,
M.
,
Muhandiram
,
R.
,
Birtley
,
J.R.
,
Al-Zahrani
,
A.
,
Thomas
,
P.J.
et al. 
(
2013
)
Regulatory R region of the CFTR chloride channel is a dynamic integrator of phospho-dependent intra- and intermolecular interactions
.
Proc. Natl Acad. Sci. U.S.A.
110
,
E4427
E4436
21
Zhang
,
Z.
and
Chen
,
J.
(
2016
)
Atomic structure of the cystic fibrosis transmembrane conductance regulator
.
Cell
167
,
1586
1597 e1589
22
Zhang
,
Z.
,
Liu
,
F.
and
Chen
,
J.
(
2017
)
Conformational changes of CFTR upon phosphorylation and ATP binding
.
Cell
170
,
483
491 e488
23
Liu
,
F.
,
Zhang
,
Z.
,
Csanady
,
L.
,
Gadsby
,
D.C.
and
Chen
,
J.
(
2017
)
Molecular structure of the human CFTR ion channel
.
Cell
169
,
85
95 e88
24
Fay
,
J.F.
,
Aleksandrov
,
L.
,
Jensen
,
T.J.
,
Kousouros
,
J.N.
,
He
,
L.
,
Aleksandrov
,
A.A.
et al. 
(
2017
)
Cryo-Em visualization of an active phosphorylated Cftr channel
.
Pediatr. Pulm.
52
,
S214
S215
25
Rosenberg
,
M.F.
,
O'Ryan
,
L.P.
,
Hughes
,
G.
,
Zhao
,
Z.
,
Aleksandrov
,
L.A.
,
Riordan
,
J.R.
et al. 
(
2012
)
The cystic fibrosis transmembrane conductance regulator (CFTR): three-dimensional structure and localization of a channel gate
.
J. Biol. Chem.
286
,
42647
42654
26
Hwang
,
T.C.
and
Kirk
,
K.L.
(
2013
)
The CFTR ion channel: gating, regulation, and anion permeation
.
Cold Spring Harb. Perspect. Med.
3
,
a009498
27
Pollock
,
N.L.
,
Satriano
,
L.
,
Zegarra-Moran
,
O.
,
Ford
,
R.C.
and
Moran
,
O.
(
2016
)
Structure of wild type and mutant F508del CFTR: a small-angle X-ray scattering study of the protein-detergent complexes
.
J. Struct. Biol.
194
,
102
111
28
Meng
,
X.
,
Clews
,
J.
,
Kargas
,
V.
,
Wang
,
X.
and
Ford
,
R.C.
(
2017
)
The cystic fibrosis transmembrane conductance regulator (CFTR) and its stability
.
Cell. Mol. Life Sci.
74
,
23
38
29
Meng
,
X.
,
Wang
,
Y.
,
Wang
,
X.
,
Wrennall
,
J.A.
,
Rimington
,
T.L.
,
Li
,
H.
et al. 
(
2017
)
Two small molecules restore stability to a sub-population of the cystic fibrosis transmembrane conductance regulator with the predominant disease-causing mutation
.
J. Biol. Chem.
292
,
3706
3719
30
Hopfner
,
K.P.
,
Karcher
,
A.
,
Shin
,
D.S.
,
Craig
,
L.
,
Arthur
,
L.M.
,
Carney
,
J.P.
et al. 
(
2000
)
Structural biology of Rad50 ATPase: ATP-driven conformational control in DNA double-strand break repair and the ABC-ATPase superfamily
.
Cell
101
,
789
800
31
Higgins
,
C.F.
,
Haag
,
P.D.
,
Nikaido
,
K.
,
Ardeshir
,
F.
,
Garcia
,
G.
and
Ames
,
G.F.
(
1982
)
Complete nucleotide sequence and identification of membrane components of the histidine transport operon of S. typhimurium
.
Nature
298
,
723
727
32
Higgins
,
C.F.
(
1992
)
ABC transporters: from microorganisms to man
.
Annu. Rev. Cell Biol.
8
,
67
113
33
Van Goor
,
F.
,
Hadida
,
S.
and
Grootenhuis
,
P.
(
2007
)
VX-770, a potent, selective, and orally bioavailable potentiator of CFTR gating
.
Pediatr. Pulm.
42
,
289
289
34
Hadida
,
S.
,
Van Goor
,
F.
,
Dinehart
,
K.
,
Looker
,
A.R.
,
Mueller
,
P.
and
Grootenhuis
,
P.D.J.
(
2014
)
Case history: Kalydeco (R) (VX-770, Ivacaftor), a CFTR potentiator for the treatment of patients with cystic fibrosis and the G551D-CFTR mutation
.
Annu. Rep. Med. Chem.
49
,
383
398
35
Van Goor
,
F.
,
Yu
,
H.H.
,
Burton
,
B.
and
Hoffman
,
B.J.
(
2014
)
Effect of ivacaftor on CFTR forms with missense mutations associated with defects in protein processing or function
.
J. Cyst. Fibros.
13
,
29
36
36
Sheppard
,
D.N.
,
Gray
,
M.A.
,
Gong
,
X.
,
Sohma
,
Y.
,
Kogan
,
I.
,
Benos
,
D.J.
et al. 
(
2004
)
The patch-clamp and planar lipid bilayer techniques: powerful and versatile tools to investigate the CFTR Cl- channel
.
J. Cyst. Fibros.
3
(
Suppl. 2
),
101
108
37
Alam
,
A.
,
Kung
,
R.
,
Kowal
,
J.
,
McLeod
,
R.A.
,
Tremp
,
N.
,
Broude
,
E.V.
et al. 
(
2018
)
Structure of a zosuquidar and UIC2-bound human-mouse chimeric ABCB1
.
Proc. Natl Acad. Sci. U.S.A.
115
,
E1973
E1982
38
Szewczyk
,
P.
,
Tao
,
H.
,
McGrath
,
A.P.
,
Villaluz
,
M.
,
Rees
,
S.D.
,
Lee
,
S.C.
et al. 
(
2015
)
Snapshots of ligand entry, malleable binding and induced helical movement in P-glycoprotein
.
Acta Crystallogr. D Biol. Crystallogr.
71
,
732
741
39
Lingam
,
S.
,
Thonghin
,
N.
and
Ford
,
R.C.
(
2017
)
Investigation of the effects of the CFTR potentiator ivacaftor on human P-glycoprotein (ABCB1)
.
Sci. Rep.
7
,
17481
40
Al-Zahrani
,
A.
,
Cant
,
N.
,
Kargas
,
V.
,
Rimington
,
T.
,
Aleksandrov
,
L.
,
Riordan
,
J.R.
et al. 
(
2015
)
Structure of the cystic fibrosis transmembrane conductance regulator in the inward-facing conformation revealed by single particle electron microscopy
.
AIMS Biophys.
2
,
131
152
41
Moeller
,
A.
,
Lee
,
S.C.
,
Tao
,
H.
,
Speir
,
J.A.
,
Chang
,
G.
,
Urbatsch
,
I.L.
et al. 
(
2015
)
Distinct conformational spectrum of homologous multidrug ABC transporters
.
Structure
23
,
450
460
42
Kos
,
V.
and
Ford
,
R.C.
(
2009
)
The ATP-binding cassette family: a structural perspective
.
Cell. Mol. Life Sci.
66
,
3111
3126
43
Cui
,
L.
,
Aleksandrov
,
L.
,
Chang
,
X.B.
,
Hou
,
Y.X.
,
He
,
L.
,
Hegedus
,
T.
et al. 
(
2007
)
Domain interdependence in the biosynthetic assembly of CFTR
.
J. Mol. Biol.
365
,
981
994
44
Ikuma
,
M.
and
Welsh
,
M.J.
(
2000
)
Regulation of CFTR Cl- channel gating by ATP binding and hydrolysis
.
Proc. Natl Acad. Sci. U.S.A.
97
,
8675
8680