DISEASE-ASSOCIATED MUTATIONS IN CYTOPLASMIC LOOP-1 AND LOOP-2 OF CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR IMPEDE PROCESSING OR OPENING OF THE CHANNEL

Since little is known about the contribution to function of the N-term inal cytoplasmic loops (CL1, residues 139-194; CL2, residues 242-307) of cystic fibrosis transmembrane conductance regulator (CFTR), all nin e point mutations identified in CLs 1 and 2 from patients with cystic fibrosis were reconstructed in the expression vector pcDNA3-CFTR and e xpressed transiently in COS-1 and HEK-293 cells and stably in Chinese hamster ovary (CHO) cells, Four amino acid substitutions retarded prod uction of mature, fully glycosylated CFTR, suggesting that misprocessi ng of the channel causes the disease symptoms in the affected patients , Protein maturation could not be promoted by cell culture conditions of reduced temperature (26 degrees C). When properly processed mutants were evaluated for functional defects by the iodide efflux method, th e G178R- and E193K-CFTR-expressing cell lines showed impaired anion tr anslocation activities. Patch-clamp studies of single channels reveale d that E193K variants had a significantly decreased open probability, which resulted from an increase in the mean closed time of the channel s, This contrasted with a previous study of disease-associated point m utations in CL3 that mainly affected the mean open time. None of the m aturation-competent CL 1 and 2 mutants had altered conductance. Thus, the N-terminal CLs appear not to contribute to the anion translocation pathway of CFTR; rather, mutations in CL1 can impede transition to th e open state, Interestingly, the ability of the non-hydrolyzable ATP a nalogue adenylyl imidodiphosphate (AMP-PNP) to lock the channel into o pen bursts was abolished by the I148T and G178R amino acid substitutio ns.