Cystic fibrosis transmembrane conductance regulator has an altered structure when its maturation is inhibited

Inefficient maturation and trafficking to the cell surface of the cystic fi brosis transmembrane conductance regulator (CFTR) is the primary cause of c ystic fibrosis. CFTR protein that fails to mature accumulates as an immatur e core-glycosylated protein and is rapidly degraded. To determine how the s tructures of mature and immature CFTR are different, we compared the proper ties of CFTR that had been expressed in the presence or absence of the prot easome inhibitor, MG-132 (carbobenzoxy-L-leucyl-L-leucyl-L-leucinal)) Trans ient expression of wild-type CFTR in the presence of submicromolar concentr ations of MG-132 blocks maturation of the protein. We found that expression of CFTR in the presence of MG-132 trapped the protein in a trypsin-sensiti ve conformation. In addition, the structure of the carboxyl-terminus of imm ature and mature CFTR differed as histidine-tagged mature CFTR was preferen tially recovered by metal-chelate chromatography. No chloride channel activ ity was detected when membranes containing immature CFTR were fused with pl anar lipid bilayers. These results show that expression of CFTR in the pres ence of MG-132 traps the protein in an altered conformation that may be ina ctive.