The CFTR ion channel: gating, regulation, and anion permeation

Abstract

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated anion channel with two remarkable distinctions. First, it is the only ATP-binding cassette (ABC) transporter that is known to be an ion channel--almost all others function as transport ATPases. Second, CFTR is the only ligand-gated channel that consumes its ligand (ATP) during the gating cycle--a consequence of its enzymatic activity as an ABC transporter. We discuss these special properties of CFTR in the context of its evolutionary history as an ABC transporter. Other topics include the mechanisms by which CFTR gating is regulated by phosphorylation of its unique regulatory domain and our current view of the CFTR permeation pathway (or pore). Understanding these basic operating principles of the CFTR channel is central to defining the mechanisms of action of prospective cystic fibrosis drugs and to the development of new, rational treatment strategies.

Figure 1.

CFTR topology and alternating access model for ABC transporters. (A) Simplified topological model emphasizing the domain structure of a CFTR chloride channel, including cytoplasmic amino (N) and carboxyl (C) termini, two nucleotide-binding domains (NBD1, NBD2), the regulatory (R) domain, and the predicted transmembrane domains (TMD1 and TMD2). (B) Alternating access model for the ABC transporter that extrudes its substrate (i.e., exporter). In the apo state of exporters, the configuration of the TMDs is inward facing. The two NBDs are far apart. Once ATP binds to the Walker A and B motifs of the “head” subdomain, two NBDs dimerize in a head-to-tail configuration and this NBD dimerization provides the driving force to flip-flop TMDs so that the substrate binding site is now outward facing.

Figure 2.

State-dependent modulation of CFTR gating by PPi. (A) A continuous macroscopic recording of WT-CFTR in an inside-out patch showing a slow response to PPi 10 s after removal of ATP. However, once the channels were opened by PPi, they closed very slowly with a relaxation time constant (τ) of ∼30 s. (B) Three minutes after ATP washout, CFTR channels responded to PPi in a dose-dependent manner, but with a much shorter open time (τ = 1.5 s). However, robust response to PPi can be restored by ATP priming again (not shown). Cartoons depict proposed configurations of NBDs. (Modified from Tsai et al. 2009; reprinted, with permission, from the author.)

Figure 3.

A simplified model for CFTR gating. The diagram illustrates dominant gating transitions (C₂ ↔ O) and a second gating cycle (C₁ → C₂′ → O → C₂ → C₁) involving complete disengagement of the two NBDs. It should be noted that the transition from O to C₂ likely contains many transitional states that await identification. The model also implicates a closed state wherein the two NBDs are separated with only one ATP molecule bound to NBD1. (Adapted from Tsai et al. 2010; reprinted, with permission, from the author.)

Figure 4.

Three proposed mechanisms for CFTR channel regulation by PKA phosphorylation. PKA sites are indicated by the red crosses. (1) Phosphorylation of the R domain (in red) and/or distal NBD1 modulates NBD dimerization. (2) Phosphorylation more directly regulates the flexibility or packing of the TMs by direct physical interactions with the long cytosolic loops (in yellow) or with the TMs themselves. (3) Phosphorylation of sites within NBD1 stabilizes the coupling between the NBDs and the TMDs.

Figure 5.

Schematic view of the CFTR permeation pathway. TM6 is indicated as a likely pore-lining helix. The contributions of other TM helices are less clear (see text).