Cloning of an Epithelial Chloride Channel from Bovine Trachea

Abstract

We have isolated and cloned a novel epithelial Cl- channel protein from a bovine tracheal cDNA expression library using an antibody probe. The antibody (alpha p38) was raised against a 38-kDa component of a homopolymeric protein that behaves as a Ca2+/calmodulin kinase II-, DIDS-, and dithiothreitol (DTT)-sensitive, anion-selective channel when incorporated into planar lipid bilayers. The full-length cDNA is 3001 base pairs long and codes for a 903-amino acid protein. The clone does not show any significant homology to any other previously reported Cl- channel sequence. Northern analysis of bovine tracheal mRNA with a cDNA probe corresponding to the cloned sequence revealed a band at 3.1 kilobases, suggesting that close to the full-length sequence has been cloned. The full-length open reading frame (2712 base pairs) has been expressed in Xenopus oocytes and in mammalian COS-7 cells. In oocytes, expression of the clone was associated with the appearance of a novel DIDS-, and DTT-sensitive, anion-selective conductance that was outwardly rectified and exhibited a reversal potential close to 0 mV. Whole-cell patch clamp studies in COS-7 cells transfected with the clone identified an ionomycin-, and DTT-sensitive chloride conductance that was not apparent in mock-transfected or control cells. In vitro translation studies have shown that the primary transcript codes for a protein migrating at 140 kDa under reduced conditions, significantly larger than the polypeptide recognized by alpha p38. We therefore suggest that either the 140-kDa translated product is a prepro form of the 38-kDa subunit of the previously identified bovine tracheal anion channel and that the primary transcript is post-translationally cleaved to yield the final product, or that the cloned channel and the previously identified bovine tracheal anion channel protein share an epitope that is recognized by the alpha p38 antibody.