Identification of the mammalian Na,K-ATPase β3 subunit

We have isolated and characterized cDNA clones encoding the human and rat Na,K-ATPase β3 subunit isoform. The human cDNA encodes a polypeptide of 279 amino acids that exhibits primary sequence and secondary structure similarities to Na,K-ATPase β subunit isoforms. Sequence comparisons showed that the human β3 subunit closely resembles the β3 subunit of Xenopus laevis (59% amino acid identity) and is less similar to the human Na,K- ATPase β1 and β2 subunits (38% and 48% amino acid identity, respectively). By analyzing the segregation of restriction fragment length polymorphisms among recombinant inbred strains of mice, we localized the β3 subunit gene to murine chromosome 7. Northern blot analysis revealed that the β3 subunit gene encodes two transcripts that are expressed in a variety of rat tissues including testis, brain, kidney, lung, stomach, small intestine, colon, spleen, and liver. Identification of the mammalian β3 subunit suggests an even greater potential for Na,K-ATPase isoenzyme diversity than previously realized.