Gastric proton pump with two occluded K(+) engineered with sodium pump-mimetic mutations

The gastric H(+),K(+)-ATPase mediates electroneutral exchange of 1H(+)/1K(+) per ATP hydrolysed across the membrane. Previous structural analysis of the K(+)-occluded E2-P transition state of H(+),K(+)-ATPase showed a single bound K(+) at cation-binding site II, in marked contrast to the two K(+) ions occluded at sites I and II of the closely-related Na(+),K(+)-ATPase which mediates electrogenic 3Na(+)/2K(+) translocation across the membrane. The molecular basis of the different K(+) stoichiometry between these K(+)-counter-transporting pumps is elusive. We show a series of crystal structures and a cryo-EM structure of H(+),K(+)-ATPase mutants with changes in the vicinity of site I, based on the structure of the sodium pump. Our step-wise and tailored construction of the mutants finally gave a two-K(+) bound H(+),K(+)-ATPase, achieved by five mutations, including amino acids directly coordinating K(+) (Lys791Ser, Glu820Asp), indirectly contributing to cation-binding site formation (Tyr340Asn, Glu936Val), and allosterically stabilizing K(+)-occluded conformation (Tyr799Trp). This quintuple mutant in the K(+)-occluded E2-P state unambiguously shows two separate densities at the cation-binding site in its 2.6 Å resolution cryo-EM structure. These results offer new insights into how two closely-related cation pumps specify the number of K(+) accommodated at their cation-binding site.