Recently, a third subunit, the γ-subunit has been described, but in contrast to α- and β-subunit, which are ubiquitously expressed, γ-subunit expression is restricted to certain tissues ( Therien et al., 1997 Arystarkhova et al., 1999). The Na,K-ATPase is an oligomeric transmembrane protein consisting of a noncovalently linked α- and β-subunit. The sodium gradient generated by the enzyme provides the primary energy for uptake and extrusion of a wide variety of solutes by epithelial cells and is crucial for efficient functioning of other Na +-coupled transport systems ( Katz, 1988 Lingrel and Kuntzweiler, 1994). It catalyzes an ATP-dependent transport of three sodium ions out and two potassium ions into the cell per pump cycle, thereby generating a transmembrane sodium gradient.

Na,K-ATPase, also known as sodium pump, is a key enzyme that regulates intracellular Na + and K + homeostasis in animal cells. Furthermore, sucrose density gradient analysis revealed significantly more α 1-subunit transcript associated with the polysomal fraction in β 1-subunit expressing MSV-MDCK cells compared with MSV-MDCK cells, indicating that in mammalian cells the Na,K-ATPase β 1-subunit is involved in facilitating the translation of the α 1-subunit mRNA in the endoplasmic reticulum. In vitro cotranslation/translocation experiments using rabbit reticulocyte lysate and rough microsomes revealed that the α 1-subunit mRNA is more efficiently translated in the presence of β 1-subunit. The increased translation was specific to α 1-subunit because translation rates of occludin and β-catenin, membrane and cytosolic proteins, respectively, were not altered. However, short pulse labeling revealed that the initial translation rate of the α 1-subunit in β 1-subunit expressing MSV-MDCK cells was six- to sevenfold higher compared with MSV-MDCK cells.

This increase was not due to altered transcription of the α 1-subunit gene or half-life of the α 1-subunit protein because both α 1-subunit mRNA levels and half-life of the α 1-subunit protein were comparable in MSV-MDCK and β 1-subunit expressing MSV-MDCK cells. Ectopic expression of Na,K-ATPase β 1-subunit in these cells increased the protein levels of the α 1-subunit of Na,K-ATPase. Moloney sarcoma virus-transformed MDCK cells (MSV-MDCK) express low levels of Na,K-ATPase β 1-subunit.

The Na,K-ATPase consists of an α- and β-subunit.

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