Control of the CFTR channel's gates

Area of Science:

• Biochemistry
• Molecular Biology
• Ion Channel Physiology

Background:

• The cystic fibrosis transmembrane conductance regulator (CFTR), or ABCC7, is an ion channel unique among ATP-binding cassette (ABC) proteins.
• CFTR's function as an anion-selective channel relies on ATP binding and hydrolysis at its two nucleotide-binding domains (NBDs): NBD1 and NBD2.
• Prokaryotic ABC protein NBDs form homodimers with ATP, dissociating after hydrolysis, a behavior contrasted with CFTR's presumed heterodimerization.

Purpose of the Study:

• To elucidate the relationship between ATP-mediated events in CFTR's NBDs and the opening/closing of the ion channel gates.
• To investigate the role of NBD1-NBD2 interactions in CFTR channel gating.
• To determine if the NBD dimerization mechanism in CFTR is conserved within the broader ABC protein superfamily.

Main Methods:

• Site-directed mutagenesis of CFTR.
• Single-channel electrophysiological recordings.
• Nucleotide photolabelling assays on intact CFTR molecules.

Main Results:

• Two specific CFTR residues at the predicted NBD1-NBD2 interface become energetically coupled during channel opening.
• These residues are independent in closed CFTR channels, indicating a dynamic restructuring of the NBD heterodimer interface.
• The energetic coupling directly links tight NBD dimerization, driven by ATP, to the opening of the transmembrane channel gates.

Conclusions:

• ATP-driven tight dimerization of CFTR's cytoplasmic NBDs is essential for opening the ion channel.
• A conserved molecular mechanism involving dynamic NBD dimer interface restructuring links ATP hydrolysis to ABC transporter function.
• This mechanism, characterized by energetically coupled residues forming hydrogen bonds, is likely shared across the entire ABC protein superfamily.