Evolutionary conservation of biogenesis of β-barrel membrane proteins

Abstract

The outer membranes of mitochondria and chloroplasts are distinguished by the presence of β-barrel membrane proteins^1,2. The outer membrane of Gram-negative bacteria also harbours β-barrel proteins³. In mitochondria these proteins fulfil a variety of functions such as transport of small molecules (porin/VDAC), translocation of proteins (Tom40) and regulation of mitochondrial morphology (Mdm10)^4,5,6,7. These proteins are encoded by the nucleus, synthesized in the cytosol, targeted to mitochondria as chaperone-bound species, recognized by the translocase of the outer membrane, and then inserted into the outer membrane where they assemble into functional oligomers^8,9,10,11. Whereas some knowledge has been accumulated on the pathways of insertion of proteins that span cellular membranes with α-helical segments, very little is known about how β-barrel proteins are integrated into lipid bilayers and assembled into oligomeric structures¹². Here we describe a protein complex that is essential for the topogenesis of mitochondrial outer membrane β-barrel proteins (TOB). We present evidence that important elements of the topogenesis of β-barrel membrane proteins have been conserved during the evolution of mitochondria from endosymbiotic bacterial ancestors¹³.