Abstract
β-Barrel proteins are present only in the outer membranes of Gram-negative bacteria, chloroplasts and mitochondria. Fungal mitochondria were shown to readily import and assemble bacterial β-barrel proteins, but human mitochondria exhibit certain selectivity. Whereas enterobacterial β-barrel proteins are not imported, neisserial ones are. Of those, solely neisserial Omp85 is integrated into the outer membrane of mitochondria. In this study, we wanted to identify the signal that targets neisserial β-barrel proteins to mitochondria. We exchanged parts of neisserial Omp85 and PorB with their Escherichia coli homologues BamA and OmpC. For PorB, we could show that its C-terminal quarter can direct OmpC to mitochondria. In the case of Omp85, we could identify several amino acids of the C-terminal β-sorting signal as crucial for mitochondrial targeting. Additionally, we found that at least two POTRA (polypeptide-transport associated) domains and not only the β-sorting signal of Omp85 are needed for its membrane integration and function in human mitochondria. We conclude that the signal that directs neisserial β-barrel proteins to mitochondria is not conserved between these proteins. Furthermore, a linear mitochondrial targeting signal probably does not exist. It is possible that the secondary structure of β-barrel proteins plays a role in directing these proteins to mitochondria.
Highlights
Β-Barrel proteins can be found in the outer membranes of Gram-negative bacteria, chloroplasts and mitochondria
BamA/Omp85 is accompanied by several other accessory lipoproteins required for the assembly of bacterial β-barrel proteins, but their number and significance vary among different bacteria [2,4]
C-terminal part of Omp85Ngo is important for its mitochondrial localization The outer mitochondrial membrane (OMM) proteins Omp85 of N. gonorrhoeae (Ngo) and BamA of Escherichia coli (Eco) are homologous proteins
Summary
Β-Barrel proteins can be found in the outer membranes of Gram-negative bacteria, chloroplasts and mitochondria. Whereas in mammalian cells the β-sorting signal has to be located at the very end of a β-barrel protein to be recognized, in yeast cells this is not an absolute requirement [11,12] It has been proposed by several experiments performed with fungal mitochondria that the biogenesis of β-barrel proteins is evolutionary conserved in such a way that mitochondria will recognize and import bacterial β-barrel precursors, as well as integrate them into the OMM [13]. Likewise, it seems that mitochondrial β-barrel proteins can be recognized by the BAM complex and integrated into the bacterial outer membrane [14]. Mitochondria-localized neisserial Omp was able to insert PorB molecules into the OMM, showing that Omp is capable of functioning in a lipid bilayer without any accessory lipoproteins [15]
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