MOLECULAR-CLONING OF 3 DISTINCT FORMS OF THE NA+,K+-ATPASE ALPHA-SUBUNIT FROM RAT-BRAIN

Rat brain and kidney cDNA libraries were constructed and screened with a cDNA insert corresponding to the mRNA for the sheep kidney Na+,K+-ATPase catalytic subunit. The .alpha.-subunit cDNAs isolated from the kidney library were derived from a single class of messenger RNA, and the brain cDNAs were derived from three classes of messenger RNA. The most abundant brain cDNA, which spans 5.1 kilobases, encodes the .alpha.(+) form of the enzyme. The second most abundant brain cDNA, which spans 3.65 kilobases, is identical with that of the kidney form and therefore encodes the .alpha. isoform. The third class of cDNA, which spans 3.55 kilobases, was present at low abundance and enodes an isoform of the .alpha.-subunit, designated, .alpha.III, which has not been identified previously. The complete nucleotide sequence and deduced amino acid sequence for each of the brain and kidney cDNAs have been determined. In addition, we have identified a lysine-rich sequence that may function as a movable, ion-selective gate during cation binding and occlusion and have also identified several amino acid sequence variations that appear to explain some of the well-known species and tissue differences in cardiac glucoside sensitivity.