A SINGLE ISOFORM OF THE NA+/K+-ATPASE ALPHA-SUBUNIT IN DIPTERA - EVIDENCE FROM CHARACTERIZATION OF THE FIRST EXTRACELLULAR DOMAIN

The first extracellular domain of the alpha-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 alpha-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 alpha-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 alpha-subunit of Na+/K+-ATPase is present in Diptera.