Molecular Structure of the Human CFTR Ion Channel

Area of Science:

• Structural Biology
• Ion Channel Physiology
• Biochemistry

Background:

• The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-binding cassette (ABC) transporter with unique ion channel function.
• Understanding CFTR structure is crucial for elucidating its gating mechanism and developing therapeutics.

Purpose of the Study:

• To determine the high-resolution structure of dephosphorylated human CFTR.
• To identify structural features that regulate CFTR channel activity.
• To propose a mechanism for CFTR activation based on its structure and kinetics.

Main Methods:

• Electron cryomicroscopy (cryo-EM) was used to determine the 3.9 Å structure of dephosphorylated human CFTR.
• Comparison with zebrafish CFTR and other ABC transporters (e.g., MRP1) was performed.
• Analysis of the sigmoid time course of CFTR current activation kinetics.

Main Results:

• A 3.9 Å cryo-EM structure of dephosphorylated human CFTR was obtained.
• A previously unresolved R domain helix was found docked in the intracellular vestibule, preventing channel opening.
• A unique helix-loop transition in transmembrane helix 8 distinguishes CFTR from other ABC transporters.

Conclusions:

• The docked R domain helix explains the closed state of dephosphorylated CFTR.
• PKA phosphorylation likely facilitates R domain disengagement, enabling channel opening.
• The helix-loop transition in TM8 is proposed as the structural basis for CFTR's ion channel function.