Endoplasmic Reticulum Quality Control of Oligomeric Membrane Proteins: Topogenic Determinants Involved in the Degradation of the Unassembled Na,K-ATPase a Subunit and in Its Stabilization by b Subunit Assembly

The molecular nature of determinants that mediate degradation of unassembled, polytopic subunits of oligomeric membrane proteins and their stabilization after partner subunit assembly is largely unknown. Expressing truncated Na,K-ATPase a subunits alone or together with b subunits, we find that in unassembled a subunits neither the four N-terminal transmembrane segments acting as efficient alternating signal anchor–stop transfer sequences nor the large, central cytoplasmic loop exposes any degradation signal, whereas poor membrane insertion efficiency of C-terminal membrane domains M5, M7, and M9 coincides with the transient exposure of degradation signals to the cytoplasmic side. b assembly with an a domain comprising at least D902 up to Y910 in the extracytoplasmic M7/M8 loop is necessary to stabilize Na,K-ATPase a subunits by favoring M7/M8 membrane pair formation and by protecting a degradation signal recognized from the endoplasmic reticulum (ER) lumenal side. Thus our results suggest that ER degradation of Na,K-ATPase a subunits is 1) mainly mediated by folding defects caused by inefficient membrane insertion of certain membrane domains, 2) a multistep process, which involves proteolytic and/or chaperone components acting from the ER lumenal side in addition to cytosolic, proteasome-related factors, and 3) prevented by partner subunit assembly because of direct protection and retrieval of degradation signals from the cytoplasm to the ER lumenal side. These results likely represent a paradigm for the ER quality control of unassembled, polytopic subunits of oligomeric membrane proteins.