CFTR (ABCC7) is a hydrolyzable-ligand-gated channel

Abstract

As the product of the gene mutated in cystic fibrosis, the most common genetic disease of Caucasians, CFTR is an atypical ABC protein. From an evolutionary perspective, it is apparently a relatively young member of the ABC family, present only in metazoans where it plays a critical role in epithelial salt and fluid homeostasis. Functionally, the membrane translocation process it mediates, the passive bidirectional diffusion of small inorganic anions, is simpler than the vectorial transport of larger more complex substrates ("allocrites") by most ABC transporters. However, the control of the permeation pathway which cannot go unchecked is necessarily more stringent than in the case of the transporters. There is tight regulation by the phosphorylation/dephosphorylation of the unique CFTR R domain superimposed on the basic ABC regulation mode of ATP binding and hydrolysis at the dual nucleotide binding sites. As with other ABCC subfamily members, only the second of these sites is hydrolytic in CFTR. The phosphorylation and ATP binding/hydrolysis events do not strongly influence each other; rather, R domain phosphorylation appears to enable transduction of the nucleotide binding allosteric signal to the responding channel gate. ATP hydrolysis is not required for either the opening or closing gating transitions but efficiently clears the ligand-binding site enabling a new gating cycle to be initiated.

State-dependent Access of Anions to the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel Pore

“…This would be consistent with the idea that activation of CFTR is a two-step process; the first step is controlled by phosphorylation of the regulatory domain, and the second step is controlled by ATP interactions with the NBDs (2, 3, 6, 7). Once the channel has been "activated" it will cycle between the closed and open states because of ATP-dependent channel gating governed by the NBDs (6,7). This gating cycle also appears to involve changes in accessibility to both MTSET and MTSES, at least as reported by a cysteine substituted for Arg-334 (18), although in this case there is no apparent charge dependence of accessibility.…”

“…Phosphorylation of the regulatory domain by cAMP-dependent protein kinase A (PKA) is a prerequisite for channel opening (2,3). Following phosphorylation, channel opening and closing are controlled by ATP interactions with the two intracellular nucleotide binding domains (NBDs) (6,7). How these cytoplasmic processes lead to a structural rearrangement of the TM regions that results in opening of the Cl Ϫ permeation pathway is not known.…”

CFTR: Break a pump, make a channel

“…Conventional equilibrium gating mechanisms also remain in contention (8,9). The present study nails the coffin closed on such mechanisms by recording Cl − currents through single CFTR channels, documenting in unprecedented detail a property that violates any equilibrium gating scheme.…”

An Electrostatic Interaction at the Tetrahelix Bundle Promotes Phosphorylation-dependent Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Channel Opening

“…Most ABC transporters are pumps that utilize the free energy released from ATP hydrolysis to actively transport substrates across cell membranes (13,14). CFTR is an ATP-gated ion channel for which ATP hydrolysis controls ligand occupancy, not permeation per se (15,16). Despite this thermodynamic distinction CFTR shares substantial sequence homology with other ABC transporters especially in the NBDs and to a lesser extent in the intracellular cytosolic loops (ICLs) that couple the NBDs to the TMs that form the translocation pathway.…”