Role of the amino terminal region of the ϵ subunit of Escherichia coli H+-ATPase (FoF1)

Escherichia coli strain KF148(SD^-) defective in translation of the uncC gene for the ε{lunate} subunit of H⁺-ATPase could not support growth by oxidative phosphorylation due to lack of F₁ binding to Fo (M. Kuki, T. Noumi, M. Maeda, A. Amemura, and M. Futai, 1988, J. Biol. Chem. 263, 17,437-17,442). Mutant uncC genes for ε{lunate} subunits lacking different lengths from the amino terminus were constructed and introduced into strain KF148(SD^-). F₁ with an ε{lunate} subunit lacking the 15 amino-terminal residues could bind to Fo in a functionally competent manner, indicating that these amino acid residues are not absolutely necessary for formation of a functional enzyme. However, mutant F₁ in which the ε{lunate} subunit lacked 16 amino-terminal residues showed defective coupling between ATP hydrolysis (synthesis) and H⁺-translocation, although the mutant F₁ showed partial binding to Fo. These findings suggest that the ε{lunate} subunit is essential for binding of F₁ to Fo and for normal H⁺-translocation. Previously, Kuki et al. (cited above) reported that 60 residues were not necessary for a functional enzyme. However, the mutant with an ε{lunate} subunit lacking 15 residues from the amino terminus and 4 residues from the carboxyl terminus was defective in oxidative phosphorylation, suggesting that both terminal regions affect the conformation of the region essential for a functional enzyme.