Na,K-ATPase [alpha]1 and [beta]-subunits show distinct localizations in the nervous tissue of the large milkweed bug

The Na,K-ATPase (NKA) is an essential ion transporter and signaling molecule in all animal tissues and believed to consist at least one [alpha] and one ß-subunit to form a functional enzyme. In the large milkweed bug, Oncopeltus fasciatus, adaptation to dietary cardiac glycosides (CGs), which can fatally block the NKA, has resulted in gene duplications leading to four [alpha]1-subunits. These differ in sensitivity to CGs, but resistance trades off against ion pumping activity, thus influencing the [alpha]1-subunits' suitability for specific tissues. Besides, O. fasciatus possesses four different ß-subunits that can alter the NKA's kinetics and should play an essential role in the formation of cellular junctions. Proteomic analyses revealed the distribution and composition of [alpha]1/ß-complexes in the nervous tissue of O. fasciatus. The highly CG-resistant, but less active [alpha]1B and the highly active, but less resistant [alpha]1C predominated in the nervous tissue and co-occurred with ß2 and ß3, partly forming larger complexes than just heterodimers. Immunohistochemical analyses provided a fine scale resolution of the subunits' distribution in different morphological structures of the nervous tissue. This may suggest that [alpha]1 as well as ß-subunits occur in isolation without the other subunit, which contradicts the present understanding that the two types of subunits have to associate to form functional complexes. An isolated occurrence was especially prominent for ß3 and [beta]x, the enigmatic fourth and N-terminally largely truncated ß-subunit. We hypothesize that dimerization of these ß-subunits plays a role in cell-cell contacts.