Imaging CFTR: A Tail to Tail Dimer with a Central Pore

The cystic fibrosis transmembrane conductance regulator (CFTR) is a protein that belongs to the superfamily of ATP binding cassette (ABC) transporters. Mutations in the CFTR gene cause cystic fibrosis, an autosomal recessive disorder. The function of CFTR is versatile. It can serve as a regulatory protein, as a membrane transporter and as an ion channel. Dimerization of CFTR is necessary for full ion channel function although structural details of CFTR in native membrane are yet unknown. In order to identify CFTR in native plasma membrane we applied atomic force microscopy (AFM) to inside-out oriented membrane patches of CFTR-expressingXENOPUS LAEVIS oocytes after cAMP stimulation. First, oocytes were injected with CFTR-cRNA and, three days later, voltage-clamped verifying successful CFTR expression and incorporation into the plasma membrane. Then, plasma membrane patches were isolated, placed inside-out on appropriate substrate and incubated with gold-labelled antibodies against the C-terminus of CFTR. Finally, the intracellular surface of the plasma membrane was scanned by AFM. In close vicinity to the immunogold labels we detected ring-like structures with bipartite symmetry. The substructure of the ring, formed by the extramembrane protein domains of CFTR, is consistent with the model of a CFTR dimer. Derived from AFM molecular mass analysis of the intramolecular domains we conclude that two CFTR molecules line up in parallel, tail by tail, forming a pore in its center. This molecular arrangement could represent the CFTR chloride channel configuration, operative in native plasma membrane.