Electrostatic Tuning of Anion Attraction from the Cytoplasm to the Pore of the CFTR Chloride Channel

Anions enter from the cytoplasm into the channel pore of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel not via a central pathway but via a single lateral portal or fenestration. High Cl⁻ conductance is dependent on electrostatic attraction of cytoplasmic Cl⁻ ions by four positively charged amino acid side-chains located within this portal. Here we use a mutagenic approach to investigate the functional effects of transplanting or supplementing these positive charges at nearby portal-lining sites. Using patch clamp recording, we find that the functionally important positive charges at K190 and R303 can be transplanted to four nearby sites (N186, L197, W356, and A367) with little loss of Cl⁻ conductance. Introduction of additional positive charge at these sites had almost no effect on Cl⁻ conductance, but did increase the sensitivity to channel block by intracellular suramin and Pt(NO₂)₄ ²⁻ anions. We suggest that it is the number of positive charges within the portal, rather than their exact location, that is the most important factor influencing Cl⁻ conductance. The portal appears well optimized in terms of charge distribution to maximize Cl⁻ conductance.