Escherichia coli H⁺-ATPase: Role of the δ subunit in binding F₁ to the F₀ sector

The roles of the Escherichia coli H⁺-ATPase (F₀F₁) δ subunit (177 amino acid residues) was studied by analyzing mutants. The membranes of nonsense (Gln-23 → end, Gln-29 → end, Gln-74 → end) and missense (Gly-150 → Asp) mutants had very low ATPase activities, indicating that the δ subunit is essential for the binding of the F₁ portion to F₀. The Gln-176 → end mutant had essentially the same membrane-bound activity as the wild type, whereas in the Val-174 → end mutant most of the ATPase activity was in the cytoplasm. Thus Val-174 (and possibly Leu-175 also) was essential for maintaining the structure of the subunit, whereas the two carboxyl terminal residues Gln-176 and Ser-177 were dispensable. Substitutions were introduced at various residues (Thr-11, Glu-26, Asp-30, Glu-42, Glu-82, Arg-85, Asp-144, Arg-154, Asp-161, Ser-163), including apparently conserved hydrophilic ones. The resulting mutants had essentially the same phenotypes as the wild type, indicating that these residues do not have any significant functional role(s). Analysis of mutations (Gly-150 → Asp, Pro, or Ala) indicated that Gly-150 itself was not essential, but that the mutations might affect the structure of the subunit. These results suggest that the overall structure of the δ subunit is necessary, but that individual residues may not have strict functional roles.