Mitochondrial OXA Translocase Plays a Major Role in Biogenesis of Inner-Membrane Proteins

The mitochondrial inner membrane harbors three protein translocases. Presequence translocase and carrier translocase are essential for importing nuclear-encoded proteins. The oxidase assembly (OXA) translocase is required for exporting mitochondrial-encoded proteins; however, different views exist about its relevance for nuclear-encoded proteins. We report that OXA plays a dual role in the biogenesis of nuclear-encoded mitochondrial proteins. First, a systematic analysis of OXA-deficient mitochondria led to an unexpected expansion of the spectrum of OXA substrates imported via the presequence pathway. Second, biogenesis of numerous metabolite carriers depends on OXA, although they are not imported by the presequence pathway. We show that OXA is crucial for the biogenesis of the Tim18-Sdh3 module of the carrier translocase. The export translocase OXA is thus required for the import of metabolite carriers by promoting assembly of the carrier translocase. We conclude that OXA is of central importance for the biogenesis of the mitochondrial inner membrane. [Display omitted] •Systematic search for substrates of mitochondrial inner-membrane OXA translocase•OXA translocase has different functions in presequence and carrier pathways•Many cleavable preproteins use the presequence translocase-OXA import-export pathway•OXA promotes carrier import via assembly of Tim18-Sdh3 module of carrier translocase Stiller et al. report that the sorting of many nuclear-encoded mitochondrial proteins depends on the OXA translocase, a homolog of bacterial YidC. The conservative sorting of transmembrane segments into the mitochondrial inner membrane is required for assembly of the carrier translocase and thus for the biogenesis of numerous metabolite carriers.