INA complex liaises the FF-ATP synthase membrane motor modules.

The FF-ATP synthase translates a proton flux across the inner mitochondrial membrane into a mechanical rotation, driving anhydride bond formation in the catalytic portion. The complex's membrane-embedded motor forms a proteinaceous channel at the interface between Atp9 ring and Atp6. To prevent unrestricted proton flow dissipating the H-gradient, channel formation is a critical and tightly controlled step during ATP synthase assembly. Here we show that the INA complex (INAC) acts at this decisive step promoting Atp9-ring association with Atp6. INAC binds to newly synthesized mitochondrial-encoded Atp6 and Atp8 in complex with maturation factors. INAC association is retained until the F-portion is built on Atp6/8 and loss of INAC causes accumulation of the free F. An independent complex is formed between INAC and the Atp9 ring. We conclude that INAC maintains assembly intermediates of the F F-ATP synthase in a primed state for the terminal assembly step-motor module formation.