Figure 3.

Effects of membrane tension on the up-down difference vector and on the TM2 helix angle. (A) Shown is a projection of simulations at different membrane tensions projected onto the difference vector of the crystallographic states (excluding the cap). On the x axis, the membrane tension is shown. The figure at the bottom represents the simulation setup where the channel is located inside the membrane and is shown with some ions surrounding them. The arrows represent the stretch applied to the membrane. The lines in (A) represent the configuration of the crystallographic state (x-ray down, PDB: 4XDJ; x-ray up, PDB: 4BW5). The violins represent the probability of the simulations starting from the up and down structures to be at the position on the difference vector. The dot in the violin represents the average over all simulations, and the bars represent the variance of the data points summarized in the respective violins. MD simulations using the CHARMM force field are highlighted. Going from a negative (compression) to a positive (stretch) tension of the membrane, the configuration of TREK-2 moves along the difference vector from a more down-like configuration to a more up-like configuration. Only the CHARMM simulations are flexible enough to reach the up state. (B) Quantification of the helix angle between the transmembrane helix 2 (TM2) of the two subunits. The violins show the distribution of the helix angle for different membrane tensions. The helix flattens out in the membrane with stretch applied (represented by a larger TM2 angle between the helices), whereas a compression of the membrane results in a steeper position of the helix in the membrane, which is represented by a smaller angle between the TM2 helices of the subunits. Up and down simulations show a minor difference, with the up simulations having a tendency toward a larger helix angle. deg, degree. To see this figure in color, go online.