A single isoform of the Na+/K+-ATPase α-subunit in Diptera: evidence from characterization of the first extracellular domain

The first extracellular domain of the α-subunit of the Na⁺/K⁺-ATPase (sodium/potassium pump) is functionally important, affecting sensitivity of the enzyme to cardiac glycosides (e.g. ouabain) and being implicated in the transport of K⁺. This domain is also variable among mammalian isoforms of the α-subunit. Using PCR, we have isolated from seven insect species with contrasting physiologies a DNA fragment containing this region, in order to help determine whether tissue-specific expression might be associated with isoforms encoded by a gene family, as it is in mammals. A single sequence (with one ORF) characteristic of Na⁺/K⁺-ATPase was obtained from genomic DNA of each species. Only the fragment from Manduca sexta contained an intron, but at a location different to that found in mammals. For all Diptera so far characterized, the species phylogeny is the same as the α-subunit gene phylogeny (based on the sequences of the first extracellular domain and flanking transmembrane domains). The results strongly indicate a single, ouabain-sensitive isoform of the α-subunit of Na⁺/K⁺-ATPase is present in Diptera.