Alternate translation initiation codons can create functional forms of cystic fibrosis transmembrane conductance regulator

To evaluate the function of transmembrane domain 1 (TMD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) and the methionines that function in translation initiation, a series of progressive 5' truncations in TMD1 were created to coincide with residues that might serve as translation initiation codons. Expression of the mutants in Xenopus oocytes demonstrated that internal sites in TMD1 can function as initiation codons. In addition, all of the mutants that progressively removed the first four transmembrane segments (M1-M4) of TMD1 expressed functional cAMP-regulated Cl^- channels with ion selectivity identical to wild-type CFTR but with reduced open probability and single channel conductance. Further removal of transmembrane segments did not produce functional Cl^- channels. These data suggest that segments M1-M4 are not essential components of the conduction pore or the selectivity filter of CFTR.