CFTR - Cystic Fibrosis Transmembrane Receptor Protein
In the cell membrane of the epithelial cells that line the lungs there is a transmembrane protein called CFTR. This helps regulate the consistency of mucus in the lungs.
If the mucus is too watery Chloride and Sodium ions are pumped away from the mucus, this creates a change in water potential and water leaves the mucus.
If mucus has too little water, chloride and sodium ions pump in and water follows, hydrating the mucus.
In CF the CFTR which regulates the transport of Chloride ions into the mucus is defective, so it is not possible to hydrate the mucus.
Links:
CFTR gene
CFTR review page