Na, K-ATPase and gastric H, K-ATPase are primary active transporters that belong to the superfamily of P-type ATPases. These membrane proteins utilize the energy released by the hydrolysis of ATP to transport their cationic substrates against electrochemical gradients. Despite more than 50 years of extensive research on P-type ATPases, the molecular details of their ion transport mechanisms are not fully understood. Especially for oligomeric P-type ATPases, such as Na, K- and H, K-ATPases, which require an accessory -subunit for their transport activity, many open questions remain, since only...