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. Severl findings argue that hydrolysis of ATP 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 ATP 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.
Gadsby
D C
DC
Laboratory of Cardiac/Membrane Physiology, Rockefeller University, New York, NY 10021-6399, USA. gadsby@rockvax.rockefeller.edu
Dousmanis
A G
AG
Nairn
A C
AC
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000266:Adenylyl Imidodiphosphate / Q000378:metabolism; D000818:Animals; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000737:chemistry / Q000378:metabolism*; D006168:Guinea Pigs; D006868:Hydrolysis; D015640:Ion Channel Gating / Q000502:physiology*; D008274:Magnesium / Q000378:metabolism; D011487:Protein Conformation
D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D000266:Adenylyl Imidodiphosphate; D000255:Adenosine Triphosphate; D008274:Magnesium