Skip to main content
. 2019 Jan 16;8:e43229. doi: 10.7554/eLife.43229

Figure 3. Ca2+-dependent rearrangements of the lipid permeation pathway.

(A) Structural alignment of the lipid pathway with afTMEM16 in the presence (maroon) or absence (cyan) of 0.5 mM Ca2+. The color scheme is the same throughout the figure. Arrows indicate direction of movement from the Ca2+-free to the Ca2+-bound conformations. The lipid permeation pathway is constricted by rearrangements of TM4 around P324 and P333 (shown as yellow spheres in both structures) and TM6 at A437 (shown as a red sphere in both structures). (B) Close-up view of the closed permeation pathway, residues at the interface with the membrane are shown as yellow sticks. (C) Top view of the permeation pathway in the absence of Ca2+. Residues pointing inside the pathway are shown as yellow sticks. The interacting charged pair E305 and R425 are shown as orange sticks. (D) Diameter of the afTMEM16 lipid pathway in the presence (maroon) and absence (cyan) of Ca2+. The diameter was estimated using the HOLE program (Smart et al., 1996). (E–F) Accessibility of the lipid permeation pathway estimated using the program HOLE in the presence (E) or absence (F) of Ca2+. Purple denotes areas of diameter d > 5.5 Å, yellow areas where 5.5 < d < 2.75 Å and red areas with d < 2.75 Å.

Figure 3.

Figure 3—figure supplement 1. Residues important for scrambling are mapped onto afTMEM16 open and closed permeation pathways.

Figure 3—figure supplement 1.

(A–B) The position of recently identified residues important for lipid scrambling by nhTMEM16 (red sticks) (Jiang et al., 2017; Lee et al., 2018) are mapped onto the afTMEM16 Ca2+-bound (A) and Ca2+-free (B) structures. Mutation of theses residues reduces the rate of scrambling by >100 fold. Critical positions cluster at the rearranging interfaces between helices TM3-6. Of note is V337 (S329 in afTMEM16, shown in yellow) whose mutation to tryptophan increases scrambling activity in the absence of Ca2+. In the Ca2+-free structure, this residue is positioned at the TM4-TM6 contact point, suggesting that this substitution might prevent closure of the pathway.
Figure 3—figure supplement 2. Ca2+-induced changes in TMEM16A.

Figure 3—figure supplement 2.

Structural alignment of TMEM16A in the presence of 0.5 mM Ca2+ (PDBID: 5OYB, red) and absence of Ca2+ (PDBID: 5OYG, blue). Left: conformational changes in ion permeation pathway. Right: view of cytosolic domains showing a lack of movement upon Ca2+-binding.