Fig. 2: Electrostatic profiles of the Ca²⁺ binding site and their effect on the activation sequence.

a Pore region of a TMEM16A subunit in the Ca²⁺-free conformation (PDB: 5OYG) in Cα representation. Ca²⁺-binding residues are shown as sticks. Yellow spheres indicate the points at which the electrostatic potential is plotted. b Electrostatic potential (Φ) along the path for the indicated channel configurations (insets). Left, α6-loose conformation in the absence of Ca²⁺. Center, α6-loose conformation with the upper Ca²⁺ binding site occupied. Right, α6-tight conformation with the upper Ca²⁺ binding site occupied. Green spheres correspond to bound Ca²⁺ ions and small spheres display the trajectory at which the electrostatic potential was plotted (colors indicate the local electrostatic potential). The data point at zero (*) on the x-axis corresponds to the position at the vacant lower Ca²⁺ binding site. c Relationship between the calculated electrostatic potential and the estimated k_on for Ca²⁺ binding (displayed in Fig. 1e). Errors are 95% confidence intervals. d Relative probability of the two possible activation sequences (top, highlighted in brown and green) as a function of Ca²⁺ concentration (bottom), calculated using Eqs. 28 and 29 with the parameters estimated from Fig. 1. The shaded region corresponds to physiologically relevant intracellular Ca²⁺ concentrations.