Evolutionary Conservation in Biogenesis of β-Barrel Proteins Allows Mitochondria to Assemble a Functional Bacterial Trimeric Autotransporter Protein

Background: -Barrel proteins are found in the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts. Results: Mitochondria are able to assemble the bacterial trimeric autotransporter YadA in a functional form. Conclusion: The lipoproteins of the BAM machinery are not absolutely required for the biogenesis of autotransporter protein. Significance: The evolutionary link of mitochondria to bacteria allows the former to process even prokaryotic-specific proteins. Yersinia adhesin A (YadA) belongs to a class of bacterial adhesins that form trimeric structures. Their mature form contains a passenger domain and a C-terminal -domain that anchors the protein in the outer membrane (OM). Little is known about how precursors of such proteins cross the periplasm and assemble into the OM. In the present study we took advantage of the evolutionary conservation in the biogenesis of -barrel proteins between bacteria and mitochondria. We previously observed that upon expression in yeast cells, bacterial -barrel proteins including the transmembrane domain of YadA assemble into the mitochondrial OM. In the current study we found that when expressed in yeast cells both the monomeric and trimeric forms of full-length YadA were detected in mitochondria but only the trimeric species was fully integrated into the OM. The oligomeric form was exposed on the surface of the organelle in its native conformation and maintained its capacity to adhere to host cells. The co-expression of YadA with a mitochondria-targeted form of the bacterial periplasmic chaperone Skp, but not with SurA or SecB, resulted in enhanced levels of both forms of YadA. Taken together, these results indicate that the proper assembly of trimeric autotransporter can occur also in a system lacking the lipoproteins of the BAM machinery and is specifically enhanced by the chaperone Skp.