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.
Highlights
Gram-negative bacteria and the mitochondria in eukaryotes
Assembly of β-barrels occurs from the same face of the membrane in bacteria and mitochondria as substrates are first imported by the the Outer Mitochondrial membrane (TOM) complex into the intermembrane space before they engage with the Sorting and Assembly Machinery (SAM)
We propose that some pathogens, through selective forces, have continued to produce β-barrel toxins that can be transported to mitochondria with high efficiency
Summary
Gram-negative bacteria contain two lipid bilayers which envelope the periplasm: the inner membrane (IM) and outer membrane (OM). Mitochondria are derived from an ancient α-proteobacterium through an endosymbiotic relationship that was initiated billions of years ago [3,4,5] They have retained their double membrane structure that forms the inner and outer membranes, which enclose two aqueous environments, the intermembrane space and the matrix. In Gram-negative bacteria, nascent polypeptides are synthesized with an amino-terminal signal peptide by cytoplasmic ribosomes These precursors are bound by factors such as SecB and SecA or the chaperone DnaK and transported to the secretion machinery (Sec). In mitochondria, precursor β-barrel proteins are synthesized by cytosolic ribosomes and are bound by chaperones in the cytosol [11] These precursor substrates are transported to the receptor subunits of the Translocase of the Outer Mitochondrial membrane (TOM). The small chaperones Tim9/10 or Tim8/13 associate with substrates, in a manner which is functionally analogous to that performed by SurA and Skp in bacteria, for the transfer to the Sorting and Assembly Machinery (SAM) complex at the mitochondrial outer membrane [12,13,14,15,16,17]
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