In this section, we compute the electric field at two locations: at the position of a water molecule close to the selectivity filter and the position of another close to the intracellular gate (Figure ). We average the electric fields at these two locations and project the resulting field along the main direction of the channel pore (Equation ). We define the projection axis as the unitary vector between the oxygens of the two water molecules (⃗ u O pw O pw in Figure ). The projected electric field is then decomposed into contributions from individual protein residues, membrane lipids, water, and ions. The magnitude and standard deviation of each residue contribution across our conformational ensemble, is given in Figure . Note that we orient the projection axis such that positive (negative) projections indicate that the electric field points down (up).