Targeting of Neisserial PorB to the mitochondrial outer membrane: an insight on the evolution of β-barrel protein assembly machines.

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.