Correction of Both NBD1 Energetics and Domain Interface Is Required to Restore ΔF508 CFTR Folding and Function

Department of Physiology, McGill University, Montréal, Quebec H3E 1Y6, Canada.
Cell (Impact Factor: 32.24). 01/2012; 148(1-2):150-63. DOI: 10.1016/j.cell.2011.11.024
Source: PubMed


The folding and misfolding mechanism of multidomain proteins remains poorly understood. Although thermodynamic instability of the first nucleotide-binding domain (NBD1) of ΔF508 CFTR (cystic fibrosis transmembrane conductance regulator) partly accounts for the mutant channel degradation in the endoplasmic reticulum and is considered as a drug target in cystic fibrosis, the link between NBD1 and CFTR misfolding remains unclear. Here, we show that ΔF508 destabilizes NBD1 both thermodynamically and kinetically, but correction of either defect alone is insufficient to restore ΔF508 CFTR biogenesis. Instead, both ΔF508-NBD1 energetic and the NBD1-MSD2 (membrane-spanning domain 2) interface stabilization are required for wild-type-like folding, processing, and transport function, suggesting a synergistic role of NBD1 energetics and topology in CFTR-coupled domain assembly. Identification of distinct structural deficiencies may explain the limited success of ΔF508 CFTR corrector molecules and suggests structure-based combination corrector therapies. These results may serve as a framework for understanding the mechanism of interface mutation in multidomain membrane proteins.

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    • "F508del, in addition to disrupting CL4:NBD1 interactions, also destabilizes the NBD1 domain [14], [15]. The mutations that can partially suppress the folding defects of F508del NBD1 are scattered across NBD1– V510D near CL4 in the channel, I539T directly opposite the NBD1:NBD2 interface, and Q637R near the start of the R region [12], hinting at the complex allosteric nature of the NBD1 folding landscape. As with protein folding, mutations such as S492F and F508del in NBD1 and G1069R in the CL4 coupling helix perturb the gating of the channel [6], [7], [75]. "
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    ABSTRACT: Cystic fibrosis is caused by mutations in CFTR (cystic fibrosis transmembrane conductance regulator), leading to folding and processing defects and to chloride channel gating misfunction. CFTR is regulated by ATP binding to its cytoplasmic nucleotide-binding domains, NBD1 and NBD2, and by phosphorylation of the NBD1 regulatory insert (RI) and the regulatory extension (RE)/R region. These regulatory effects are transmitted to the rest of the channel via NBD interactions with intracellular domain coupling helices (CL), particularly CL4. Using a sensitive method for detecting inter-residue correlations between chemical shift changes in NMR spectra, an allosteric network was revealed within NBD1, with a construct lacking RI. The CL4-binding site couples to the RI-deletion site and the C-terminal residues of NBD1 that precede the R region in full-length CFTR. Titration of CL4 peptide into NBD1 perturbs the conformational ensemble in these sites with similar titration patterns observed in F508del, the major CF-causing mutant, and in suppressor mutants F494N, V510D and Q637R NBD1, as well as in a CL4-NBD1 fusion construct. Reciprocally, the C-terminal mutation, Q637R, perturbs dynamics in these three sites. This allosteric network suggests a mechanism synthesizing diverse regulatory NBD1 interactions and provides biophysical evidence for the allosteric coupling required for CFTR function.
    PLoS ONE 09/2013; 8(9):e74347. DOI:10.1371/journal.pone.0074347 · 3.23 Impact Factor
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    • "Together, this suggests that in the F508del patient population significant differences in the kinetics of processing and recycling of CFTR can occur. It seems likely, therefore that for clinical benefit in specific F508del CF patients we need to define an optimal combination of agonists, correctors, potentiators and possibly proteasome inhibitors, which act on different levels of the CFTR processing and recycling machinery [10], [51], [52]. "
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    ABSTRACT: Most patients with Cystic Fibrosis (CF) carry at least one allele with the F508del mutation, resulting in a CFTR chloride channel protein with a processing, gating and stability defect, but with substantial residual activity when correctly sorted to the apical membranes of epithelial cells. New therapies are therefore aimed at improving the folding and trafficking of F508del CFTR, (CFTR correctors) or at enhancing the open probability of the CFTR chloride channel (CFTR potentiators). Preventing premature breakdown of F508del CFTR is an alternative or additional strategy, which is investigated in this study. We established an ex vivo assay for murine F508del CFTR rescue in native intestinal epithelium that can be used as a pre-clinical test for candidate therapeutics. Overnight incubation of muscle stripped ileum in modified William's E medium at low temperature (26°C), and 4 h or 6 h incubation at 37°C with different proteasome inhibitors (PI: ALLN, MG-132, epoxomicin, PS341/bortezomib) resulted in fifty to hundred percent respectively of the wild type CFTR mediated chloride secretion (forskolin induced short-circuit current). The functional rescue was accompanied by enhanced expression of the murine F508del CFTR protein at the apical surface of intestinal crypts and a gain in the amount of complex-glycosylated CFTR (band C) up to 20% of WT levels. Sustained rescue in the presence of brefeldin A shows the involvement of a post-Golgi compartment in murine F508del CFTR degradation, as was shown earlier for its human counterpart. Our data show that proteasome inhibitors are promising candidate compounds for improving rescue of human F508del CFTR function, in combination with available correctors and potentiators.
    PLoS ONE 12/2012; 7(12):e52070. DOI:10.1371/journal.pone.0052070 · 3.23 Impact Factor
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    • "Given its profound effect on CFTR folding, it was initially surprising that the ΔF508 mutation has little effect on NBD1 crystal structure (Lewis et al., 2004, 2005). However, recent work has revealed that ΔF508 significantly disrupts both kinetic and thermodynamic stability of NBD1 as well as increasing local backbone dynamics at residues 507–511 (Hoelen et al., 2010; Lewis et al., 2010; Wang et al., 2010; Rabeh et al., 2012). Moreover, the specific folding defect induced by ΔF508 appears to reside at least in part within the α-helical subdomain (Hoelen et al., 2010; Wang et al., 2010) as well as C-terminal β-strands, S9 and S10 (Hudson et al., 2012). "
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    ABSTRACT: In the past decade much has been learned about how Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) folds and misfolds as the etiologic cause of cystic fibrosis (CF). CFTR folding is complex and hierarchical, takes place in multiple cellular compartments and physical environments, and involves several large networks of folding machineries. Insertion of transmembrane (TM) segments into the endoplasmic reticulum (ER) membrane and tertiary folding of cytosolic domains begin cotranslationally as the nascent polypeptide emerges from the ribosome, whereas posttranslational folding establishes critical domain-domain contacts needed to form a physiologically stable structure. Within the membrane, N- and C-terminal TM helices are sorted into bundles that project from the cytosol to form docking sites for nucleotide binding domains, NBD1 and NBD2, which in turn form a sandwich dimer for ATP binding. While tertiary folding is required for domain assembly, proper domain assembly also reciprocally affects folding of individual domains analogous to a jig-saw puzzle wherein the structure of each interlocking piece influences its neighbors. Superimposed on this process is an elaborate proteostatic network of cellular chaperones and folding machineries that facilitate the timing and coordination of specific folding steps in and across the ER membrane. While the details of this process require further refinement, we finally have a useful framework to understand key folding defect(s) caused by ΔF508 that provides a molecular target(s) for the next generation of CFTR small molecule correctors aimed at the specific defect present in the majority of CF patients.
    Frontiers in Pharmacology 12/2012; 3:201. DOI:10.3389/fphar.2012.00201 · 3.80 Impact Factor
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