Expression of genes for the α and β subunits of the Na,K-ATPase in normal and drug resistant cells

The studies described in this report show that the genes for sodium, potassium-ATPase exhibit more complexity than was originally expected on the basis of studies of the Na,K-ATPase proteins or their transport activities. First, there are at least three isoforms of the α subunit. Second, expression of the β subunit, although apparently always leading to the same protein product, involves complex variability in the sizes and predominance of different mRNA classes. The biologic basis for this heterogeneity of mRNA transcripts is not known. Third, ouabain resistance in at least one cell line, the human Hela C⁺ cell line, involves amplification of two independent genetic units, the α and β subunit genes, as well as high levels of expression of the mRNAs. These results serve as strong evidence for the importance of the β subunit, although no discrete functions have yet been assignable to this protein. It is otherwise difficult to understand why the α and β subunits are both amplified in these cells, and both decline upon withdrawal of the selective agent. On the basis of evidence gathered in our laboratory and that of other groups, we conclude that the genetic basis for sodium transport in eukaryotic tissues is far more complex than previously anticipated. There appears to be a need for specialization of sodium, potassium-ATPases in different tissues and for complex generation of multiple β-subunit mRNA species. Our molecular genetic approach has also allowed us to demonstrate that there is probably an important role played by the β-subunit in Na,K-ATPase function. Our current efforts focus on more detailed characterization of the α subunit isoforms, and reconstruction of the various Na,K-ATPase activities by production of recombinant proteins from cloned templates.