Membrane insertion, processing, and topology of cystic fibrosis transmembrane conductance regulator (CFTR) in microsomal membranes

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated Cl- channel. Malfunction of CFTR causes cystic fibrosis (CF). CFTR belongs to an ATP-binding cassette (ABC) transporter superfamily which includes P-glycoprotein (Pgp), the molecule that is responsible for multidrug resistance in cancer cells. P-glycoprotein molecules have been suggested to have more than one topology and function. In this study, we analysed the early stages of membrane insertion, processing, and topology of human CFTR using rabbit reticulocyte lysate and wheat germ extract translation systems supplemented with canine pancreatic microsomal membranes. Our results suggest that CFTR contains an uncleavable signal sequence and its membrane targeting and insertion may depend on the signal recognition particle (SRP) and SRP receptor. The topology of CFTR in microsomal membranes is the same as the one predicted based on hydropathy plot analysis. These results, together with our previous findings on Pgp, indicate that (1) the topologies of mammalian ABC transporters can be dissected and studied using protein fusion chimeras in a cell-free system; and (2) the membrane targeting and insertion of CFTR and Pgp may take the same pathway, i.e., the SRP-dependent pathway, but the membrane folding mechanism of these two proteins in microsomal membranes is probably different.