A transmembrane segment determines the steady-state localization of an ion-transporting adenosine triphosphatase

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.