Functional architecture of the cytoplasmic entrance to the cystic fibrosis transmembrane conductance regulator chloride channel pore

As an ion channel, the cystic fibrosis transmembrane conductance regulator must form a continuous pathway for the movement of Cl^- and other anions between the cytoplasm and the extracellular solution. Both the structure and the function of the membrane-spanning part of this pathway are well defined. In contrast, the structure of the pathway that connects the cytoplasm to the membrane-spanning regions is unknown, and functional roles for different parts of the protein forming this pathway have not been described. We used patch clamp recording and substituted cysteine accessibility mutagenesis to identify positively charged amino acid side chains that attract cytoplasmic Cl^- ions to the inner mouth of the pore. Our results indicate that the side chains of Lys-190, Arg-248, Arg-303, Lys-370, Lys-1041, and Arg-1048, located in different intracellular loops of the protein, play important roles in the electrostatic attraction of Cl^- ions. Mutation and covalent modification of these residues have charge-dependent effects on the rate of Cl^- permeation, demonstrating their functional role in maximization of Cl^- flux. Other nearby positively charged side chains were not involved in electrostatic interactions with Cl^-. The location of these Cl^--attractive residues suggests that cytoplasmic Cl^- ions enter the pore via a lateral portal located between the cytoplasmic extensions to the fourth and sixth transmembrane helices; a secondary, functionally less relevant portal might exist between the extensions to the 10^th and 12^th transmembrane helices. These results define the cytoplasmic mouth of the pore and show how it attracts Cl^- ions from the cytoplasm.