Calcium-dependent electrostatic control of anion access to the pore of the calcium-activated chloride channel TMEM16A.

TMEM16A is a ligand-gated anion channel that is activated by intracellular Ca. This channel comprises two independent pores and closely apposed Ca binding sites that are contained within each subunit of a homodimeric protein. Previously we characterized the influence of positively charged pore-lining residues on anion conduction (Paulino et al., 2017a). Here, we demonstrate the electrostatic control of permeation by the bound calcium ions in mouse TMEM16A using electrophysiology and Poisson-Boltzmann calculations. The currents of constitutively active mutants lose their outward rectification as a function of Ca concentration due to the alleviation of energy barriers for anion conduction. This phenomenon originates from Coulombic interactions between the bound Ca and permeating anions and thus demonstrates that an electrostatic gate imposed by the vacant binding site present in the sterically open pore, is released by Ca binding to enable an otherwise sub-conductive pore to conduct with full capacity.