Deletion of phenylalanine 508 causes attenuated phosphorylation-dependent activation of CFTR chloride channels

J Physiol. 2000 May 1;524 Pt 3(Pt 3):637-48. doi: 10.1111/j.1469-7793.2000.00637.x.


In cell-attached patches stimulated with cAMP agonists, the single-channel open probability (Po) of the phenylalanine 508-deleted cystic fibrosis transmembrane conductance regulator (DeltaF508-CFTR) channel, the most common disease-associated mutation in cystic fibrosis, was abnormally low (a functional defect). To investigate the mechanism for the poor response of DeltaF508-CFTR to cAMP stimulation, we examined, in excised inside-out patches, protein kinase A (PKA)-dependent phosphorylation activation and ATP-dependent gating of wild-type (WT) and DeltaF508-CFTR channels expressed in NIH3T3 mouse fibroblasts. For WT-CFTR, the activation time course of CFTR channel current upon addition of PKA and ATP followed a sigmoidal function with time constants that decreased as [PKA] was increased. The curvilinear relationship between [PKA] and the apparent activation rate suggests an incremental phosphorylation-dependent activation of CFTR at multiple phosphorylation sites. The time course of PKA-dependent activation of DeltaF508-CFTR channel current also followed a sigmoidal function, but the rate of activation was at least 7-fold slower than that with WT channels. This result suggests that deletion of phenylalanine 508 causes attenuated PKA-dependent phosphorylation of the CFTR chloride channel. Once DeltaF508-CFTR channels were maximally activated with PKA, the mutant channel and WT channel had indistinguishable steady-state Po values, ATP dose-response relationships and single-channel kinetics, indicating that DeltaF508-CFTR is not defective in ATP-dependent gating. By measuring whole-cell current density, we compared the number of functional channels in WT- and DeltaF508-CFTR cell membrane. Our data showed that the estimated channel density for DeltaF508-CFTR was approximately 10-fold lower than that for WT-CFTR, but the cAMP-dependent whole-cell current density differed by approximately 200-fold. We thus conclude that the functional defect (a decrease in Po) of DeltaF508-CFTR is as important as the trafficking defect (a decrease in the number of functional channels in the plasma membrane) in cystic fibrosis pathogenesis.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 3T3 Cells / chemistry
  • 3T3 Cells / enzymology
  • Adenosine Triphosphate / pharmacology
  • Animals
  • Colforsin / pharmacology
  • Cyclic AMP / analogs & derivatives
  • Cyclic AMP / pharmacology
  • Cyclic AMP-Dependent Protein Kinases / metabolism
  • Cystic Fibrosis / genetics
  • Cystic Fibrosis / metabolism
  • Cystic Fibrosis Transmembrane Conductance Regulator / chemistry
  • Cystic Fibrosis Transmembrane Conductance Regulator / genetics*
  • Cystic Fibrosis Transmembrane Conductance Regulator / metabolism*
  • Dose-Response Relationship, Drug
  • Enzyme Inhibitors / pharmacology
  • Gene Deletion*
  • Genistein / pharmacology
  • Ion Channel Gating / drug effects
  • Ion Channel Gating / genetics*
  • Mice
  • Patch-Clamp Techniques
  • Phenylalanine
  • Phosphorylation
  • Point Mutation
  • Protein Structure, Tertiary
  • Thionucleotides / pharmacology


  • Enzyme Inhibitors
  • Thionucleotides
  • Cystic Fibrosis Transmembrane Conductance Regulator
  • Colforsin
  • 8-((4-chlorophenyl)thio)cyclic-3',5'-AMP
  • Phenylalanine
  • Adenosine Triphosphate
  • Genistein
  • Cyclic AMP
  • Cyclic AMP-Dependent Protein Kinases