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

The Na,K-ATPase consists of an and -subunit. Moloney sarcoma virus-transformed MDCK cells (MSV-MDCK) express low levels of Na,K-ATPase 1-subunit. Ectopic expression of Na,K-ATPase 1-subunit in these cells increased the protein levels of the 1-subunit of Na,K-ATPase. This increase was not due to altered transcription of the 1-subunit gene or half-life of the 1-subunit protein because both 1-subunit mRNA levels and half-life of the 1-subunit protein were comparable in MSV-MDCK and 1-subunit expressing MSV-MDCK cells. However, short pulse labeling revealed that the initial translation rate of the 1-subunit in 1-subunit expressing MSV-MDCK cells was sixto sevenfold higher compared with MSV-MDCK cells. The increased translation was specific to 1-subunit because translation rates of occludin and -catenin, membrane and cytosolic proteins, respectively, were not altered. In vitro cotranslation/translocation experiments using rabbit reticulocyte lysate and rough microsomes revealed that the 1-subunit mRNA is more efficiently translated in the presence of 1-subunit. Furthermore, sucrose density gradient analysis revealed significantly more 1-subunit transcript associated with the polysomal fraction in 1-subunit expressing MSV-MDCK cells compared with MSV-MDCK cells, indicating that in mammalian cells the Na,K-ATPase 1-subunit is involved in facilitating the translation of the 1-subunit mRNA in the endoplasmic reticulum.