TMEM70 facilitates biogenesis of mammalian ATP synthase by promoting subunit c incorporation into the rotor structure of the enzyme

Biogenesis of F1F0 ATP synthase, the key enzyme of mitochondrial energy provision, depends on transmembrane protein 70 (TMEM70), localized in the inner mitochondrial membrane of higher eukaryotes. TMEM70 absence causes severe ATP-synthase deficiency and leads to a neonatal mitochondrial encephalocardiomyopathy in humans. However, the exact biochemical function of TMEM70 remains unknown. Using TMEM70 conditional knockout in mice, we show that absence of TMEM70 impairs the early stage of enzyme biogenesis by preventing incorporation of hydrophobic subunit c into rotor structure of the enzyme. This results in the formation of an incomplete, pathologic enzyme complex consisting of F-1 domain and peripheral stalk but lacking F-o proton channel composed of subunits c and a. We demonstrated direct interaction between TMEM70 and subunit c and showed that overexpression of subunit c in TMEM70(-/-) cells partially rescued TMEM70 defect. Accordingly, TMEM70 knockdown prevented subunit c accumulation otherwise observed in F-1-deficient cells. Altogether, we identified TMEM70 as specific ancillary factor for subunit c. The biologic role of TMEM70 is to increase the low efficacy of spontaneous assembly of subunit c oligomer, the key and rate-limiting step of ATP-synthase biogenesis, and thus to reach an adequately high physiologic level of ATP synthase in mammalian tissues.