Functional organization of cytoplasmic portals controlling access to the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel that apparently has evolved from an ancestral active transporter. Key to the CFTR's switch from pump to channel function may have been the appearance of one or more lateral portals. Such portals connect the cytoplasm to the transmembrane channel pore, allowing a continuous pathway for the electrodiffusional movement of Cl- ions. However, these portals remain the least well-characterized part of the Cl- transport pathway; even the number of functional portals is uncertain, and if multiple portals do exist, their relative functional contributions are unknown. Here, we used patch-clamp recording to identify the contributions of positively charged amino acid side chains located in CFTR's cytoplasmic transmembrane extensions to portal function. Mutagenesis-mediated neutralization of several charged side chains reduced single-channel Cl- conductance. However, these same mutations differentially affected channel blockade by cytoplasmic suramin and Pt(NO2)(4)(2-) anions. We considered and tested several models by which the contribution of these positively charged side chains to one or more independent or non-independent portals to the pore could affect Cl- conductance and interactions with blockers. Overall, our results suggest the existence of a single portal that is lined by several positively charged side chains that interact electrostatically with both Cl- and blocking anions. We further propose that mutations at other sites indirectly alter the function of this single portal. Comparison of our functional results with recent structural information on CFTR completes our picture of the overall molecular architecture of the Cl- permeation pathway.