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. 2018 Sep 28;9:3978. doi: 10.1038/s41467-018-06414-8

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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. 2 — Characterization of the SthK 2D-crystals in presence of cAMP. a HS-AFM image of a 2D-crystal viewed from the extracellular side. Each channel appears as a square-shaped tetramer. Each protruding turret of the pore domain within the channel tetramer is clearly resolved. Inset: The alternating packing of the channels is highlighted with dashed outlines (scale bar: 15 nm). b Four-fold symmetrized correlation average of a. The unit cell (dashed square, dimensions a = b = 11.6 nm, γ = 90°) comprises two tetramers (scale bar: 10 nm). c Left: Height profiles of SthK molecules along the dashed and solid lines in a are shown by the black and grey traces, respectively. Right: Height histogram of the two classes of molecules with 0.3 and 0.6 nm average protrusion heights. d Packing model of the 2D-crystal (PDB 6CJQ). e, f, g, h same as a, b, c, and d, but the 2D-crystal is imaged from the intracellular side exposing the CNBDs. Dashed circles in a and e highlight the alternating packing of the tetramers in the 2D-crystal. The stronger-protruding CNBDs on the intracellular side (e, f) are the less protruding ones from the extracellular side (a, b). White arrows in h indicate the CW rotation that the CNBDs should undergo upon cAMP-binding to match the HS-AFM data. The dotted white and cyan circles in h show the position of the protrusions in the data, which do not match the packing of the resting state SthK model. i Structural model with 25° CW rotated CNBDs matches the experimental data (compare with h). The white and cyan dashed circles in f highlight the two neighboring tetramers of high and low height, respectively. The 25° CW rotation of the CNBDs in the model brings the model CNBD back into the white and cyan dashed circles (i)