cAMP-dependent protein kinase-mediated phosphorylation of cystic fibrosis transmembrane conductance regulator residue Ser-753 and its role in channel activation

Hormonal regulation of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel is largely mediated via cAMP-dependent protein kinase (PKA). CFTR contains 10 dibasic consensus sites for potential PKA phosphorylation ((R/K)(R/K)X(S(*)/T(*))). Previous studies (Chang, X.-B. Tabcharani, J. A., Hou, Y.-X., Jensen, T. J., Kartner, N., Alon, N., Hanrahan, J. W., and Riordan, J. R (1993) J. Biol. Chem. 268, 11304-11311) showed that approximately 25% of the CFTR wild-type response to PKA activation remained upon inhibition of most detectable phosphorylation by in vitro mutagenesis of all 10 dibasic consensus sites (10SA CFTR). To identify potential additional sites responsible for the residual activity, large amounts of this mutant CFTR were phosphorylated with PKA using high specific activity [γ-³²P]ATP. Cyanogen bromide cleavage indicated that a large portion of the observed PKA phosphorylation occurred within a 5.8-kDa fragment of the R domain between residues 722-773. Removal of serines at potential PKA sites in this fragment showed that Ser-753 accounted for all of the γ-³²P labeling of the 5.8-kDa peptide. Replacement of Ser-753 with alanine reduced the level of residual CFTR activity by a further 40%, indicating that phosphorylation at this previously unidentified site contributes to the activation of 10SA CFTR.