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. 2017 Dec 1;7:16741. doi: 10.1038/s41598-017-16842-z

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© The Author(s) 2017

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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QUAD opsin scrambles lipids as a monomer. (A) Schematic representation of the fluorescence-based scramblase activity assay. (B) Representative fluorescence traces of scrambling by WT opsin, reconstituted at different protein/phospholipid ratios (PPR) into POPC:POPG (9:1) vesicles containing a trace amount of NBD-PC. (C) As in panel B, for QUAD opsin. (D) The extent of fluorescence reduction in the scramblase assay was determined for vesicles reconstituted with QUAD and WT opsin over a range of PPR values, 0–3 g/mol and the data were transformed into plots of p(≥1) scramblase (the probability of a vesicle having at least one scramblase) vs PPR* (related to measured PPR, see Experimental Procedures). The solid lines are data fits (Poisson analysis, see Experimental Procedures and Table 2), and the dashed lines are the 95% confidence interval for the fits. (E) Molar mass of the functionally reconstituted scramblase deduced from the data shown in panel D (see Table 2). (F) Schematic illustration showing that whereas both WT and QUAD opsin are monomers when added to DDM-destabilized phospholipid vesicles (based on Fig. 4), WT opsin dimerizes (multimerization is not shown here for simplicity) en route to reconstitution whereas QUAD opsin reconstitutes as a monomer (based on this figure, panel E). Direct evidence for dimerization of WT opsin during detergent withdrawal was previously obtained through co-immunoprecipitation studies¹³.