Molecular and functional characterization of the cystic fibrosis transmembrane conductance regulator from the Australian common brushtail possum, Trichosurus vulpecula
Unlike eutherian mammals, the colon of the Australian common brushtail possum, Trichosurus vulpecula, a metatherian mammal, is incapable of electrogenic Cl− secretion and has elevated levels of electrogenic Na+ absorption, while the ileum secretes HCO3− rather than Cl−. In eutherian mammals, the cystic fibrosis transmembrane conductance regulator (CFTR) is essential for both Cl− and HCO3− secretion and the regulation of Na+ absorption. Therefore, we have sequenced possum (p)CFTR, described its distribution and characterized the properties of cloned pCFTR expressed in Fischer rat thyroid (FRT) cells. pCFTR (GenBank accession No. AY916796) has a 1,478 amino acid open reading frame, which has >90% identity with CFTR from other marsupials and >80% identity with non-rodent eutherian mammals. In pCFTR, there is a high level of conservation of the transmembrane and nucleotide binding domains although, with the exception of other marsupials, there is considerable divergence from other species in the R domain. FRT cells transfected with pCFTR express mature CFTR protein which functions as a small Cl− channel activated by cAMP-dependent phosphorylation. In whole-cell recordings it has a linear, time and voltage-independent conductance, with a selectivity sequence $$ P_{\text{Br}} > P_{\text{Cl}} > P_{\text{I}} > P_{{{\text{HCO}}_{ 3} }} \gg P_{\text{Gluconate}} . $$ pCFTR transcript is present in a range of epithelia, including the ileum and the colon. The presence of pCFTR in the ileum and its measured HCO3− permeability suggest that it may be involved in ileal HCO3− secretion. Why the possum colon does not secrete Cl− and has elevated electrogenic Na+ absorption, despite the apparent expression of CFTR, remains to be determined.