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Vol. 11, Issue 5, 1657-1672, May 2000
Subunit and in Its Stabilization by 
Subunit Assembly
Institute of Pharmacology and Toxicology, University of Lausanne,
CH-1005 Lausanne, Switzerland
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 
subunits alone or together with
subunits, we find that in unassembled 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. assembly with an domain comprising at least
D902 up to Y910 in the extracytoplasmic M7/M8 loop is necessary to
stabilize Na,K-ATPase 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 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.
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