ATP hydrolysis cycles and the gating of CFTR Cl- channels

The gene defective in cystic fibrosis encodes a Cl- channel named CFTR, which belongs to the family of transport proteins identified by their cytoplasmic domains that bind and hydrolyse ATP. CFTR channels require phosphorylation by protein kinase A at one or more serine residues in the large central regulatory domain before they will open. Several findings argue that hydrolysis of Am at the N-terminal nucleotide binding domain is the rate-limiting step for opening a phosphorylated CFTR channel. Although AMP-PNP the non-hydrolysable, but close structural, analog of ATP fails to open phosphorylated CFTR channels, once a channel has been opened, AMP-PNP can bind tightly to the channel and "lock" it into the open conformation for several minutes. This tight binding of AMP-PNP presumably occurs at CFTR's C-terminal nucleotide binding domain. Because it structurally resembles AMP-PNP, ATP must also bind tightly there, which suggests that hydrolysis of that ATP normally prompts channel closing. That conclusion is supported by the finding that free [Mg2+] level controls the rate of CFTR channel closure. A normal closed-open-closed gating cycle of a CFTR channel thus seems to involve hydrolysis of one ATP molecule to open it, and hydrolysis of a second Am to close it. Stabilization of an active state by tight binding of a nucleotide, and termination of that state by hydrolysis of the nucleotide, are characteristics reminiscent of G proteins. Indeed, CFTR's nucleotide binding domains share with G proteins not only this functional similarity, but also some sequence homology, at least in certain highly conserved motifs.