Inter-subunit interaction of gastric H+,K+-ATPase prevents reverse reaction of the transport cycle.

The gastric H+,K+-ATPase is an ATP-driven proton pump responsible for generating a million-fold proton gradient across the gastric membrane. We present the structure of gastric H+,K+-ATPase at 6.5 Å resolution as determined by electron crystallography of two-dimensional crystals. The structure shows the catalytic α-subunit and the non-catalytic β-subunit in a pseudo-E2P conformation. Different from Na+,K+-ATPase, the N-terminal tail of the β-subunit is in direct contact with the phosphorylation domain of the α-subunit. This interaction may hold the phosphorylation domain in place, thus stabilizing the enzyme conformation and preventing the reverse reaction of the transport cycle. Indeed, truncation of the β-subunit N-terminus allowed the reverse reaction to occur. These results suggest that the β-subunit N-terminus prevents the reverse reaction from E2P to E1P, which is likely to be relevant for the generation of a large H+ gradient in vivo situation. [ABSTRACT FROM AUTHOR]