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. 2018 Sep 28;1:154. doi: 10.1038/s42003-018-0153-0

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — CFTR wild-type and V232D mutant TM3/4 hairpin folding probed by single-molecule FRET. a Structure of CFTR³ highlighting the position of the V232D mutation in TM3/4 (yellow/red). b Schematic representation of the wild-type (left) and V232D (right) TM3/4 helical-hairpin motifs comprising CFTR’s transmembrane helices TM3 (yellow) and TM4 (red). c Schematic of the single-molecule FRET approach for investigating hairpin conformations. Shown are single fluorescently labeled TM3/4 hairpin molecules reconstituted into phospholipid vesicles freely diffusing through the observation volume of the confocal microscope. d FRET efficiency histograms of wild-type (blue) and V232D TM3/4 (orange) in PC lipid vesicles with 12:0, 14:1, 16:1, 16:0–18:1 (POPC), 18:1, and 20:1 acyl chains. Distances between the acyl chain C-2 atoms are indicated as measures of hydrophobic thicknesses⁴⁸. PDA fits to the histograms are shown as red cityscapes. e Fraction of folded hairpin as function of hydrophobic thickness for wild-type TM3/4 (blue) and V232D TM3/4 (orange) as determined by PDA fits. Errors are standard deviations of the PDA chi-square minimization algorithm calculated from ten iterations. f Closed-state (black dashed) and open-state (orange solid) interfluorophore distance (R_DA) distributions for V232D TM3/4 in POPC determined using PDA (left panel), in accordance with a fully extended interfacially bound hairpin or a partially inserted hairpin with TM3 being inserted and TM4 positioned atop the bilayer (right panels)