Abstact

F-type ATP synthase (F1FO) catalyzes proton motive force-driven ATP synthesis in mitochondria, chloroplasts, and bacteria. Different from the mitochondrial and bacterial enzymes, F1FO from photosynthetic organisms have evolved diverse structural and mechanistic details to adapt to the light-dependent reactions. Although complete structure of chloroplast F1FO has been reported, no high-resolution structure of an F1FO from photosynthetic bacteria has been available. Here, we report cryo-EM structures of an intact and functionally competent F1FO from Chloroflexus aurantiacus (CaF1FO), a filamentous anoxygenic phototrophic bacterium from the earliest branch of photosynthetic organisms. The structures of CaF1FO in its ADP-free and ADP-bound forms for three rotational states reveal a previously unrecognized architecture of ATP synthases. A pair of peripheral stalks connect to the CaF1 head through a dimer of δ-subunits, and associate with two membrane-embedded a-subunits that are asymmetrically positioned outside and clamp CaFO's c10-ring. The two a-subunits constitute two proton inlets on the periplasmic side and two proton outlets on the cytoplasmic side, endowing CaF1FO with unique proton translocation pathways that allow more protons being translocated relative to single a-subunit F1FO. Our findings deepen understanding of the architecture and proton translocation mechanisms of F1FO synthases and suggest innovative strategies for modulating their activities by altering the number of a-subunit.