From evolution to pathogenesis: the link between β-barrel assembly machineries in the outer membrane of mitochondria and gram-negative bacteria

Abstract

β-barrel proteins are the highly abundant in the outer membranes of Gram-negative bacteria and the mitochondria in eukaryotes. The assembly of β-barrels is mediated by two evolutionary conserved machineries; the β-barrel Assembly Machinery (BAM) in Gram-negative bacteria; and the Sorting and Assembly Machinery (SAM) in mitochondria. Although the BAM and SAM have functionally conserved roles in the membrane integration and folding of β-barrel proteins, apart from the central BamA and Sam50 proteins, the remaining components of each of the complexes have diverged remarkably. For example all of the accessory components of the BAM complex characterized to date are located in the bacterial periplasm, on the same side as the N-terminal domain of BamA. This is the same side of the membrane as the substrates that are delivered to the BAM. On the other hand, all of the accessory components of the SAM complex are located on the cytosolic side of the membrane, the opposite side of the membrane to the N-terminus of Sam50 and the substrate receiving side of the membrane. Despite the accessory subunits being located on opposite sides of the membrane in each system, it is clear that each system is functionally equivalent with bacterial proteins having the ability to use the eukaryotic SAM and vice versa. In this review, we summarize the similarities and differences between the BAM and SAM complexes, highlighting the possible selecting pressures on bacteria and eukaryotes during evolution. It is also now emerging that bacterial pathogens utilize the SAM to target toxins and effector proteins to host mitochondria and this will also be discussed from an evolutionary perspective.

Keywords: bacteria; evolution; mitochondria; outer membrane; protein folding; protein transport; β-barrel proteins.

Figure 1

Comparison of the β-barrel protein assembly machineries in Gram-negative bacteria and eukaryotes (A) In Gram-negative bacteria, BamA is the core subunit of the BAM complex. BamA forms a complex with lipoproteins, BamB, C, D and E in E. coli. The BAM complex plays an important role in outer membrane biogenesis including the insertion of autotransporters; (B) In eukaryotes, the core channel of SAM, Sam50, is the homologue of BamA. In mitochondria, Sam50 forms a complex with the metaxins, Sam35 and Sam37. Mdm10 interacts with SAM to mediate the biogenesis of TOM. Mim1 transiently interacts with the SAM complex and is important for the biogenesis of α-helical transmembrane domain containing proteins. BamA has five repeat POTRA domains (P1–P5) while Sam50 only has one (P1). In eukaryotes the accessory subunits are located on the cytosolic side of the outer membrane whilst in bacteria BamB to E are located on the periplasmic face of the membrane. Color key-grey denotes components not conserved between prokaryotes and eukaryotes, blue denotes conserved proteins.