Na,K-ATPase β₁-subunit increases the translation efficiency of the α₁-subunit in MSV-MDCK cells

The Na,K-ATPase consists of an α- and β-subunit. Moloney sarcoma virus-transformed MDCK cells (MSV-MDCK) express low levels of Na,K-ATPase β₁-subunit. Ectopic expression of Na,K-ATPase β₁-subunit in these cells increased the protein levels of the α₁-subunit of Na,K-ATPase. This increase was not due to altered transcription of the α₁-subunit gene or half-life of the α₁-subunit protein because both α₁-subunit mRNA levels and half-life of the α₁-subunit protein were comparable in MSV-MDCK and β₁-subunit expressing MSV-MDCK cells. However, short pulse labeling revealed that the initial translation rate of the α₁-subunit in β₁-subunit expressing MSV-MDCK cells was six- to sevenfold higher compared with MSV-MDCK cells. The increased translation was specific to α₁-subunit because translation rates of occludin and β-catenin, membrane and cytosolic proteins, respectively, were not altered. In vitro cotranslation/translocation experiments using rabbit reticulocyte lysate and rough microsomes revealed that the α₁-subunit mRNA is more efficiently translated in the presence of β₁-subunit. Furthermore, sucrose density gradient analysis revealed significantly more α₁-subunit transcript associated with the polysomal fraction in β₁-subunit expressing MSV-MDCK cells compared with MSV-MDCK cells, indicating that in mammalian cells the Na,K-ATPase β₁-subunit is involved in facilitating the translation of the α₁-subunit mRNA in the endoplasmic reticulum.