The large milkweed bugs' Na,K-ATPase β-subunits colocalize with septate junction proteins in a tissue-specific manner

The Na,K-ATPase is a vital transmembrane enzyme, which is important for maintaining cell membrane potentials and the general functionality of animal cells. The enzyme's minimal functional unit consists of one α and one β-subunit, whereas the number of existing paralogs varies in different insect species. The functional roles of different β-subunits, which can account for their diversity within a single species, are so far only partially explained. The emphasis of this study was to specifically elucidate the involvement in septate junctions of the four β-subunits of the new model system Oncopeltus fasciatus. Septate junctions function as a paracellular barrier controlling the flow of solutes across epithelia. So far, studies in Drosophila revealed that nervana2, the β2 homolog of Drosophila, is involved in septate junction formation. In O. fasciatus, we demonstrate that most of the Na,K-ATPase subunits colocalize with septate junction proteins. This agrees with our previous findings implying a role of β2 in the control of tracheal tube size in O. fasciatus, which according to the findings in Drosophila appears to be dependent on a stable formation of septate junctions. Finally, our data suggest a connection between the septate junction protein coracle and the enigmatic, N-terminally strongly truncated βx, which has no obvious homologs in other insects. Our study proposes that the four β-subunits form functional units with septate junction proteins, either allowing tissue-adjusted formation of cell-cell contacts or other yet unknown functions.