Cystic Fibrosis and the CFTR protein

Cystic fibrosis (CF) is caused by mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein, altering chloride efflux in epithelial cells. CFTR belongs to the large ABC transporter superfamily, but is the only member known to function as an ion channel. Therefore, it can be considered as a “broken” ABC transporter, having an atrophied or uncoupled cytoplasmic-side gate. However, the structural features associated with this specific evolution remained for a long time poorly understood in absence of experimental 3D structure of the full-length protein. These experimental 3D structures were particularly hard to obtain due to the low solubility of the protein and its limited thermal stability. Contributing to this are the dynamics fluctuations of the protein, exacerbated by intrinsic disorder associated with specific regions, especially the large regulatory (R) region, linking the two halves of the protein and regulating the protein activity through phosphorylation. Understanding the CFTR 3D structure and its conformational and functional landscape, within its interaction network, could however provide insights into the way the CFTR folding and functional defects could be rescued.

We have made several contributions to the CFTR field, by first modelling the 3D structure of the nucleotide-binding domains (NBDs) heterodimer (Callebaut et al. 2004, Eudes et al 2005) and then, that of the assembly of NBDs and Membrane-Spanning Domains (MSDs), once experimental 3D structures of ABC exporters NBDs:MSDs assemblies were available (Mornon et al. 2008, Mornon et al. 2009).

Furthermore, by using refined sequence alignments and molecular dynamics simulations (now including advanced methods, such as metadynamics), we were able to understand the specific evolution of this member of the ABC transporter family towards a channel function, especially by highlighting the presence of cytoplasmic portals allowing ion flux from the cytosol. Our models have been supported by various experimental data, including the recent cryo-EM 3D structures (Liu et al. 2017 Cell 169:85-95, Zhang and Chen 2017 Cell 170: 483-91). They are used to understand the molecular basis of CFTR function (see for instance Cai et al. 2015, Billet et al. 2013) and the impact of CF-causing mutations (see for instance Hinzpeter et al. 2017, Sharma et al. 2015) and are considered to design specific therapeutical approaches (WO/2016/087665, Zelli et al. 2018).

This expertise is now also applied to the study of other ABC transporters, such as the lipid transporter ABCB4 (Delaunay et al. 2017), for which similarities in rescuing mechanisms were identified with CFTR.

This work is supported by the French Association Vaincre La Mucoviscidose

People involved at IMPMC :

Isabelle Callebaut

Jean-Paul Mornon

Fabio Pietrucci

Post-doctoral fellows: Brice Hoffmann (untill 2017), Ahmad Elbahnsi

References : Pubmed