A multifunctional aqueous channel formed by CFTR

The cystic fibrosis gene product (CFTR) is a complex protein that functions as an adenosine 3,5-monophosphate (cAMP)-stimulated ion channel and possibly as a regulator of intracellular processes. In order to determine whether the CFTR molecule contains a functional aqueous pathway, anion, water, and urea transport were measured in Xenopus oocytes expressing CFTR. Cyclic AMP agonists induced a Cl^- conductance of 94 microsiemens and an increase in water permeability of 4 × 10^-4 centimeter per second that was inhibited by a Cl^- channel blocker and was dependent on anion composition. CFTR has a calculated single channel water conductance of 9 × 10^-13 cubic centimeter per second, suggesting a pore-like aqueous pathway. Oocytes expressing CFTR also showed cAMP-stimulated transport of urea but not the larger solute sucrose. Thus CFTR contains a cAMP-stimulated aqueous pore that can transport anions, water, and small solutes. The results also provide functional evidence for water movement through an ion channel.