La. Dunbar et al., A transmembrane segment determines the steady-state localization of an ion-transporting adenosine triphosphatase, J CELL BIOL, 148(4), 2000, pp. 769-778
The H,K-adenosine triphosphatase (ATPase) of gastric parietal cells is targ
eted to a regulated membrane compartment that fuses with the apical plasma
membrane in response to secretagogue stimulation. Previous work has demonst
rated that the a subunit of the H,K-ATPase encodes localization information
responsible for this pump's apical distribution, whereas the beta subunit
carries the signal responsible for the cessation of acid secretion through
the retrieval of the pump from the surface to the regulated intracellular c
ompartment. By analyzing the sorting behaviors of a number of chimeric pump
s composed of complementary portions of the H,K-ATPase alpha subunit and th
e highly homologous Na,K-ATPase alpha subunit, we have identified a portion
of the gastric H,K-ATPase, which is sufficient to redirect the normally ba
solateral Na,K-ATPase to the apical surface in transfected epithelial cells
. This motif resides within the fourth of the H,K-ATPase alpha subunit's te
n predicted transmembrane domains, Although interactions with glycosphingol
ipid-rich membrane domains have been proposed to play an important role in
the targeting of several apical membrane proteins, the apically located chi
meras are not found in detergent-insoluble complexes, which are typically e
nriched in glycosphingolipids. Furthermore, a chimera incorporating the Na,
K-ATPase alpha subunit fourth transmembrane domain is apically targeted whe
n both of its flanking sequences derive from H,K-ATPase sequence. These res
ults provide the identification of a defined apical localization signal in
a polytopic membrane transport protein, and suggest that this signal functi
ons through conformational interactions between the fourth transmembrane sp
anning seg ment and its surrounding sequence domains.