(
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 Ca
2+-bound (
A) and Ca
2+-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 Ca
2+. In the Ca
2+-free structure, this residue is positioned at the TM4-TM6 contact point, suggesting that this substitution might prevent closure of the pathway.