Ecto-F1-ATPase: A moonlighting protein complex and an unexpected apoA-I receptor

Figure 1 Mitochondrial ATP synthase. In eukaryotic cells, F₁F_O ATP synthase is a complex molecular motor composed of 16 unique subunits (α, β and c being present in multiple copies in the whole complex). It comprises the activity-bearing subunits (three αβ heterodimers), a rotor (in orange: subunits γ, δ, ε and the c ring) and a stator (in green: subunits a, e, f, g, A6L, b, F6, d and OSCP) which holds the αβ heterodimers. It is organized into two major domains: a soluble F₁ domain (α, β, γ, δ, ε) and a membrane-associated F_O domain. The F₁ domain bears the catalytic activity while the F_O domain is a proton channel. The F_O domain uses the proton gradient created by the four other complexes of the respiratory chain to drive the rotation of the central stalk, which will alternatively change the conformation of the three αβ heterodimers, leading to the synthesis of ATP from ADP and Pi. In the case of the eukaryotic ATP synthase, the translocation of 10 protons through the c ring will lead to a full turn, and the synthesis of 3 ATP molecules. In the absence of a proton gradient, or if the F₁ domain is isolated from the F_O domain, the enzyme behaves as an ATP hydrolase.