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. 2024 Sep 2;12:RP90851. doi: 10.7554/eLife.90851

Figure 4. The 1:2 sphingomyelin/cholesterol bilayer surrounding aquaporin-0 (AQP0) and comparison with the 2:1 sphingomyelin/cholesterol bilayer.

(A) The five sphingomyelins (dark green sticks) and four cholesterol (red sticks) molecules surrounding an AQP0 subunit (gray surface). The arrows between the orange and blue lines indicate the average distances between the phosphorus atoms of the phosphodiester groups and the nitrogen atoms of the amide groups in the two leaflets, respectively. (B) The AQP02SM:1Chol structure shown for comparison with the AQP01SM:2Chol structure in (A). Arrows as in (A). (C) Overlay of the lipid bilayers in the AQP02SM:1Chol and AQP01SM:2Chol structures. (D) Location of the four cholesterols (red sticks) in the AQP01SM:2Chol structure with respect to AQP0 surface characteristics. Color coding: yellow, aromatic residues; cyan, hydrophobic residues; and light green, polar and charged residues. (E) Position of cholesterol Chol3 (red sticks) in the AQP01SM:2Chol structure and its interaction with residues of two adjacent AQP0 tetramers (brown sticks). The dotted lines indicate the distance between the two adjacent AQP0 tetramers at the positions of the ring system (~8.5 Å) and the acyl chain (~2.5 Å). See also Figure 4—figure supplements 1 and 2 and Table 3.

Figure 4.

Figure 4—figure supplement 1. Interactions of cholesterol molecules in AQP01SM:2Chol with aquaporin-0 (AQP0).

Figure 4—figure supplement 1.

(A) The cholesterols in the extracellular leaflet, Chol1 and Chol2. (B) The cholesterol in the cytoplasmic leaflet, Chol4. All cholesterol molecules interact with hydrophobic residues on the AQP0 surface. The ring systems of Chol1, Chol2, and Chol4 also make π-stacking interactions with aromatic residues Trp-205, Phe-198, and Trp-10, respectively.
Figure 4—figure supplement 2. Average B-factors of acyl chains in structures of aquaporin-0 (AQP0) in different lipid bilayers.

Figure 4—figure supplement 2.

(A) AQP0 in a bilayer formed with a molar sphingomyelin:cholesterol ratio of 1:2. (B) AQP0 in a bilayer formed with a molar sphingomyelin:cholesterol ratio of 2:1. (C) AQP0 in a bilayer formed with E. coli polar lipids. The two acyl chains of each lipid are labeled in the top panel with subscripts and their average B-factors are tabulated below.
Figure 4—figure supplement 3. The hydroxyl head group of Chol3 makes a hydrogen bond with a water molecule.

Figure 4—figure supplement 3.

The distance of the water molecule from the Chol3 hydroxyl group is 3.2 Å. Aquaporin-0 (AQP0) is shown in gray ribbon and sticks representation, Chol3 is shown as red sticks, and water as a red sphere. The 2Fo-Fc map for Chol3 and the water molecule is shown in green and the composite omit map in blue.