Abstract
Opening and closing of the cystic fibrosis transmembrane conductance regulator are controlled by ATP binding and hydrolysis by the cytoplasmic nucleotide-binding domains. Different conformational changes in the channel pore have been described during channel opening and closing; however, the relative importance of these changes to the process of gating the pore is not known. We have used patch clamp recording to identify high affinity Cd(2+) bridges formed between pairs of pore-lining cysteine residues introduced into different transmembrane α-helices (TMs). Seven Cd(2+) bridges were identified forming between cysteines in TMs 6 and 12. Interestingly, each of these Cd(2+) bridges apparently formed only in closed channels, and their formation stabilized the closed state. In contrast, a single Cd(2+) bridge identified between cysteines in TMs 1 and 12 stabilized the channel open state. Analysis of the pattern of Cd(2+) bridge formation in different channel states suggests that lateral separation and convergence of different TMs, rather than relative rotation or translation of different TMs, is the key conformational change that causes the channel pore to open and close.