Cftr Channel Gating

Two papers, one in this issue (Weinreich et al., 1999) and the other in the April issue of The Journal (Zelt-wanger et al., 1999), help clarify the gating mechanisms of cystic fibrosis transmembrane conductance regulator (CFTR) Cl Ϫ channels, the products of the gene found mutated in cystic fibrosis patients. CFTR is a most unusual ion channel. It is a prominent member of the ABC transporter superfamily and comprises two ho-mologous halves, each with a probably hexa-helical transmembrane domain followed, in the primary sequence , by a cytoplasmic nucleotide-binding domain (NBD); the two halves are linked by an ‫ف‬ 20-kD intra-cellular regulatory (R) domain loaded with sites that can be phosphorylated by PKA and/or PKC (Riordan et al., 1989). Initially dubbed " regulator " because of its transporter family relatives, and more recently fingered as a bona fide modulator, somehow, of ENaC and possibly other channels and transporters, CFTR is, nevertheless , indubitably an ‫ف‬ 10 pS Cl Ϫ channel, albeit with byzantine gating habits (for recent reviews, see Shep-pard and Welsh, 1999; Gadsby and Nairn, 1999). Unlike other ion channels, CFTR channels won't open until they've been phosphorylated by PKA, presumably within the R domain, and even then they won't open unless they're supplied with ATP or other hydrolyzable nucleoside triphosphates (Anderson et al., 1991), a result leading to the proposal that channel opening might be energized by ATP hydrolysis. Shortly thereafter , in mixtures of hydrolyzable and nonhydrolyzable nucleoside triphosphates, CFTR channels were found to open but, instead of always closing in 1 s or less as normal, they often became locked in the open confor-mation for minutes (Gunderson and Kopito, 1994; Hwang et al., 1994), implying that both channel opening and closing might involve ATP hydrolysis. In line with that interpretation, ATPase activity at an appropriately stately pace (comparable with the ‫ف‬ 1 s Ϫ 1 channel opening and closing rates) was demonstrated for individual NH 2-(NBD1) and COOH-terminal (NBD2) NBD fusion proteins (Ko and Pedersen, 1995; Randak et al., 1997), as well as for full-length CFTR (Li et al., 1996). Rounding out the picture, NBD1 was tentatively assigned the principal role in channel opening, and NBD2 that in closing, on the basis of the marked pro-longation of channel openings seen after mutating the conserved Walker A lysine (believed critical for ATP hy-drolysis) in NBD2, K1250, but not after the corresponding mutation of K464 in NBD1, which, …