Signals in bacterial β-barrel proteins are functional in eukaryotic cells for targeting to and assembly in mitochondria

Abstract

The outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts harbor β-barrel proteins. The signals that allow precursors of such proteins to be targeted to mitochondria were not characterized so far. To better understand the mechanism by which β-barrel precursor proteins are recognized and sorted within eukaryotic cells, we expressed the bacterial β-barrel proteins PhoE, OmpA, Omp85, and OmpC in Saccharomyces cerevisiae and demonstrated that they were imported into mitochondria. A detailed investigation of the import pathway of PhoE revealed that it is shared with mitochondrial β-barrel proteins. PhoE interacts initially with surface import receptors, and its further sorting depends on components of the TOB/SAM complex. The bacterial Omp85 and PhoE integrated into the mitochondrial outer membrane as native-like oligomers. For the latter protein this assembly depended on the C-terminal Phe residue, which is important also for the correct assembly of PhoE into the bacterial outer membrane. Collectively, it appears that mitochondrial β-barrel proteins have not evolved eukaryotic-specific signals to ensure their import into mitochondria. Furthermore, the signal for assembly of β-barrel proteins into the bacterial outer membrane is functional in mitochondria. © 2009 by The National Academy of Sciences of the USA.