We carried out whole-genome homozygosity mapping, gene expression analysis and DNA sequencing in individuals with isolated mitochondrial ATP synthase deficiency and identified disease-causing mutations in TMEM70. Complementation of the cell lines of these individuals with wild-type TMEM70 restored biogenesis and metabolic function of the enzyme complex. Our results show that TMEM70 is involved in mitochondrial ATP synthase biogenesis in higher eukaryotes.

TMEM70 protein represents a novel ancillary factor of mammalian ATP synthase. We have investigated import and processing of this factor in human cells using GFP- and FLAG-tagged forms of TMEM70 and specific antibodies. TMEM70 is synthesized as a 29kDa precursor protein that is processed to a 21kDa mature form. Immunocytochemical detection of TMEM70 showed mitochondrial colocalization with MitoTracker Red and ATP synthase. Western blot of subcellular fractions revealed the highest signal of TMEM70 in isolated mitochondria and mitochondrial location was confirmed by mass spectrometry analysis. Based on analysis of submitochondrial fractions, TMEM70 appears to be located in the inner mitochondrial membrane, in accordance with predicated transmembrane regions in the central part of the TMEM70 sequence. Two-dimensional electrophoretic analysis did not show direct interaction of TMEM70 with assembled ATP synthase but indicated the presence of dimeric form of TMEM70. No TMEM70 protein could be found in cells and isolated mitochondria from patients with ATP synthase deficiency due to TMEM70 c.317-2A>G mutation thus confirming that TMEM70 biosynthesis is prevented in these patients.

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