Figure 6. Insertion depth and orientation of a cholesterol at the interface between two aquaporin-0 (AQP0) tetramers.
(A) Schematic figure illustrating how the insertion depth, d, and orientation angle, θ, of the cholesterol were measured. The cholesterol insertion depth was defined as the distance in z direction of the cholesterol oxygen atom (red stick representation) from the center of mass of the phosphorus atoms of the nearby sphingomyelin (SM) molecules in the extracellular leaflet (top green horizontal line). The cholesterol orientation was defined as the angle between the membrane normal (simulation box z-vector) and the vector along the rings of the cholesterol molecule (black dashed line). (B–E) Right panels: The three different systems that were simulated, namely (B) ‘No AQP0’, a pure lipid membrane without AQP0, (C, D) ‘AQP0+EC lipids S2/1’, a membrane with one AQP0 tetramer surrounded by the annular lipids seen in the AQP01SM:2Chol structure, and (E) ‘2×AQP0’, a membrane containing a pair of AQP0 tetramers together with the lipids in between them from a hybrid AQP02SM:1Chol structure that replaces the two central SM molecules with the EC deep cholesterol molecules found in the AQP01SM:2Chol structure. Left panels: Graphs showing normalized histograms for the insertion depth, d, and orientation angle, θ, for the monitored deep cholesterol in membranes with different SM:Chol ratios (see color code in panel A), except for the 2×AQP0 system, which was only simulated in a pure SM membrane. For the simulations with one AQP0 tetramer, insertion and orientation were computed separately for the deep cholesterol located at surface S2 (C) and S1 (D). The vertical line indicates the most probable cholesterol position in the 2×AQP0 system (E). See also Figure 6—figure supplement 1.
Figure 6—figure supplement 1. Time trace of the insertion depth of cholesterol.
