Thermal denaturation of the Na,K-ATPase provides evidence for α-α oligomeric interaction and γ subunit association with the C-terminal domain

Thermal denaturation can help elucidate protein domain substructure. We previously showed that the Na,K-ATPase partially unfolded when heated to 55 degreesC (Arystarkhova, E,, Gibbons, D. L,, and Sweadner, K, J. (1995) J, Biol, Chem, 270, 8785-8796). The beta subunit unfolded without leaving the membrane, but three transmembrane spans (M8-M10) and the C terminus of the alpha subunit were extruded, while the rest of alpha retained its normal topology with respect to the lipid bilayer, Here we investigated thermal denaturation further, with several salient results. First, trypsin sensitivity at both surfaces of alpha was increased, but not sensitivity to V8 protease, suggesting that the cytoplasmic domains and extruded domain were less tightly packed but still retained secondary structure. Second, thermal denaturation was accompanied by SDS-resistant aggregation of a subunits as dimers, trimers, and tetramers without beta or gamma subunits, This implies specific alpha-alpha contact. Third, the gamma subunit, like the C-terminal spans of alpha, was selectively lost from the membrane. This suggests its association with M8-M10 rather than the more firmly anchored transmembrane spans. The picture that emerges is of a Na,K-ATPase complex of alpha, beta, and gamma subunits in which a can associate in assemblies as large as tetramers via its cytoplasmic domain, while beta and gamma subunits associate with alpha primarily in its C-terminal portion, which has a unique structure and thermal instability.