Yeast mitochondria can process de novo designed β-barrel proteins

Mitochondrial outer membrane beta-barrel proteins are encoded in the nucleus, translated in the cytosol and then targeted to and imported into the respective organelles. Detailed studies have uncovered the mechanisms involved in the import of these proteins and identified the targeting signals and the cytosolic factors that govern their proper biogenesis. Recently, de novo designed eight-stranded beta-barrel proteins (Tmb2.3 and Tmb2.17) were shown to fold and assemble into lipid membranes. To better understand the general aspects of the biogenesis of beta-barrel proteins, we investigated the fate of these artificial proteins upon their expression in yeast cells. We demonstrate that although these proteins are de novo designed and are not related to bona fide mitochondrial beta-barrel proteins, they were targeted to mitochondria and integrated into the organelle outer membrane. We further studied whether this integration requires components of the yeast mitochondrial import machinery like Tom20, Tom70, Tob55/Sam50 and Mas37/Sam37. Whereas it seems that none of the import receptors was required for the biogenesis of the artificial beta-barrel proteins, we observed a strong dependency on the TOB/SAM complex. Collectively, our findings demonstrate that the mitochondrial outer membrane is the preferential location in yeast cells for any membrane-embedded beta-barrel protein.