More than fifty percent of the world’s population is infected with Helicobacter pylori. Chronic infection with the Gram-negative pathogen is associated with the development of peptic ulcers and is linked to an increased risk of gastric cancer. H. pylori secrete many proteinaceous factors that are important for initial colonization and subsequent persistence in the host stomach. One of the major protein toxins secreted by H. pylori is the bipartite toxin, Vacuolating cytotoxin A (VacA). After secretion from the bacteria via a type V autotransport secretion system, the two VacA subunits (p33 and p55) enter host cells and cause severe “vacuolation”. This is the accumulation of large vesicles that possess hallmarks of both late endosomes and early lysosomes. The development of “vacuoles” has been attributed to the formation of VacA anion selective channels in membranes. Apart from its vacuolating effects, it has recently become clear that VacA also directly affects mitochondrial function. Earlier studies suggested that the p33 subunit, but not the p55 subunit of VacA, could enter mitochondria to modulate organelle function. This raised the possibility that a mechanism separate from pore formation may be responsible for the effects of VacA on mitochondria, as crystallography studies and structural modeling predict that both subunits are required for a physiologically stable pore. It has also been suggested that the mitochondrial effects observed are due to indirect effects on pro-apoptotic proteins and more direct effects on mitochondrial morphology related processes. Other studies have shown that both the p55 and p33 subunits can indeed be efficiently imported into mammalian derived mitochondria raising the possibility that they could re-assemble to form a pore. Our review summarizes and consolidates the recent advances in VacA toxin research, with focus on the outstanding controversies in the field and the key remaining questions that need to be addressed.

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.

Bacterial infection has enormous global social and economic impacts stemming from effects on human health and agriculture. Although there are still many unanswered questions, decades of research has uncovered many of the pathogenic mechanisms at play. It is now clear that bacterial pathogens produce a plethora of proteins known as “toxins” and “effectors” that target a variety of physiological host processes during the course of infection. One of the targets of host targeted bacterial toxins and effectors are the mitochondria. The mitochondrial organelles are major players in many biological functions, including energy conversion to ATP and cell death pathways, which inherently makes them targets for bacterial proteins. We present a summary of the toxins targeted to mitochondria and for those that have been studied in finer detail, we also summarize what we know about the mechanisms of targeting and finally their action at the organelle. © 2012 IUBMB IUBMB Life, 2012

Mitochondria originated from Gram-negative bacteria through endosymbiosis. In modern day mitochondria, the Sorting and Assembly Machinery (SAM) is responsible for eukaryotic β-barrel protein assembly in the mitochondrial outer membrane. The SAM is the functional equivalent of the β-barrel assembly machinery found in the outer membrane of Gram-negative bacteria. In this study we examined the import pathway of a pathogenic bacterial protein, PorB, which is targeted from pathogenic Neisseria to the host mitochondria. We have developed a new method for measurement of PorB assembly into mitochondria that relies on the mobility shift exhibited by bacterial β-barrel proteins once folded and separated under semi-native electrophoretic conditions. We show that PorB is targeted to the outer mitochondrial membrane with a dependence on the intermembrane space shuttling chaperones and the core component of the SAM, Sam50, which is a functional homologue of BamA that is required for PorB assembly in bacteria. The peripheral subunits of the SAM, Sam35 and Sam37, which are essential for eukaryotic β-barrel protein assembly but do not have distinguishable functional homologues in bacteria, are not required for PorB assembly in eukaryotes. This shows that PorB uses an evolutionary conserved ‘bacterial like’ mechanism to infiltrate the host mitochondrial outer membrane.