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(2+). This channel comprises two independent pores and closely apposed Ca(2+) 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(2+) concentration due to the alleviation of energy barriers for anion conduction. This phenomenon originates from Coulombic interactions between the bound Ca(2+) 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(2+) binding to enable an otherwise sub-conductive pore to conduct with full capacity.