Organization of phospholipids (smooth grey areas) and cholesterol (darker patches) next to the transmembrane surfaces viewed in sections tangential to the tube axis. (a) At the level of the outer phospholipid headgroups, showing small cholesterol-rich patches next to M4, M1 and the adjacent M3 helices (red bars); also showing a larger patch (microdomain) bridging the central δ–δ dimer. (b) At the level of the inner phospholipid headgroups, showing cholesterol-rich patches next to the M3 and M4 helices (red bars); also showing two additional microdomains. (c), (d) Slabs encompassing the outer (c) and inner (d) headgroup regions, with structures superimposed to identify regions of the protein responsible for the features in (a) and (b). The microdomains A, B and C bridge dimer interfaces involving the δ, αδ/δ and αγ subunits, respectively. Both (a) and (b) are shown in inverted contrast; the numbering 1–4 identifies TM helices M1–M4. Subunit colours: αγ, red; αδ, orange; β, green; γ, cyan; and δ, blue.