The cystic fibrosis transmembrane conductance regulator chloride channel. Iodide block and permeation

Cystic fibrosis (CF) is a common autosomal-recessivediseasecharacterizedby abnormallylow chlorideconduc­tance in epithelial cell membranes (1). The CF genesequence predicts a protein having two sets of sixtransmembranespanningregions, two nucleotide (ATP)binding folds, and a highly charged regulatory domainwith concensus phosphorylation sites for protein kinasesA and C (2,3). An ohmic, low-conductance chloridechannel is generated when this protein (named CFTRfor cystic fibrosis transmembrane conductance regula­tor) is expressed in insect cells, and the channels andCFTR protein appear with the same time course (4).The same CI channel is also produced when the gene isexpressed in CFTR-transfectedCHO cells (5) and inXenopus oocytes injected with cRNA (6). Moreover, amacroscopic CI conductance having properties consis­tent with this channel is generated in HeLa cellstransiently expressing CFTR, and the apparent selectiv­ity of this conductance can be altered by mutatingresidues within the first and sixth transmembrane span­ning regions (7). Taken together, the single channel andmutagenesis data strongly suggest CFTR is the cAMP­activated Cl channel which has been characterizedpreviously in human pancreatic duct (8), thyroid (12),and thecolonic cell line T84 (9).The channel generated in CFTR-expressingcells canbe activated in excised patches by exposure to proteinkinases (5), making it possible to study single channelselectivity in detail. Iodide selectivity has recently beenused for identifying this channel macroscopically(7,10,11), but previous studies of iodide permeabilityhave led toconflicting results. In this paperwe use CHOcells stably transfected with the CFTR gene to studyiodide effects at the single channel level under biionicconditions. We report that iodide blocks the channel atnormal (150 mM) ionic strengthand thatpermeabilitytoiodide is higher than to chloride when measured fromthe cytoplasmic side.