Characterization of the internalization pathways for the cystic fibrosis transmembrane conductance regulator

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Abstract
Mutations in the gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel give rise to the most common lethal genetic disease of Caucasian populations, CF. Although the function of CFTR is primarily related to the regulation of apical membrane chloride permeability, biochemical, immunocytochemical, and functional studies indicate that CFTR is also present in endosomal andtrans Golgi compartments. The molecular pathways by which CFTR is internalized into intracellular compartments are not fully understood. To define the pathways for CFTR internalization, we investigated the association of CFTR with two specialized domains of the plasma membrane, clathrin-coated pits and caveolae. Internalization of CFTR was monitored after cell surface biotinylation and quantitation of cell surface CFTR levels after elution of cell lysates from a monomeric avidin column. Cell surface levels of CFTR were determined after disruption of caveolae or clathrin-coated vesicle formation. Biochemical assays revealed that disrupting the formation of clathrin-coated vesicles inhibited the internalization of CFTR from the plasma membrane, resulting in a threefold increase in the steady-state levels of cell surface CFTR. In contrast, the levels of cell surface CFTR after disruption of caveolae were not different from those in control cells. In addition, although our studies show the presence of caveolin at the apical membrane domain of human airway epithelial cells, we were unable to detect CFTR in purified caveolae. These results suggest that CFTR is constitutively internalized from the apical plasma membrane via clathrin-coated pits and that CFTR is excluded from caveolae.