What controls the hydrolytic activity of Polytomella parva ATP synthase?

The mitochondrial ATP synthase catalyzes the formation of ATP from ADP and Pi. In the colorless alga Polytomella parva, this enzyme displays an atypical composition in the subunits that build the peripheral arm and in the ones involved in its dimerization. In addition to ten extra subunits (Asa1-10) apparently absent from other ATP synthases, the catalytic α and β subunits possess amino-acid extensions in their N- and C-terminal regions, respectively. The δ subunit-homologous to the bacterial ε subunit and responsible for linking the hydrophilic and hydrophobic sectors of the enzyme-also contains an atypical N-terminal extension. The ATP synthase of P. parva appears to lack an IF₁ peptide, the natural inhibitor of ATP hydrolysis. Due to the potential closeness of the δ subunit N-terminal extension to the DELSEED region of the catalytical core of the enzyme, we hypothesized that the δ subunit could regulate the hydrolytic activity of the algal ATP synthase and evaluated this possibility experimentally. We also investigated whether the S. cerevisiae IF₁ polypeptide could exert a cross-species inhibitory effect on the algal enzyme. Biochemical results, complexome profiling analysis, and 3D-structural data indicate the absence of a peptide with inhibitory capabilities in the algal ATP synthase and suggest that the enzyme hydrolytic activity may be regulated by ADP levels.