Biogenesis of β-barrel membrane proteins of mitochondria

β-Barrel membrane proteins have several important functions in outer membranes of Gram-negative bacteria and in the organelles of endosymbiotic origin, mitochondria and chloroplasts. The biogenesis of β-barrel membrane proteins was, until recently, an unresolved process. A breakthrough was achieved when a specific pathway for the insertion of β-barrel outer-membrane proteins was identified in both mitochondria and Gram-negative bacteria. The key component of this pathway is Tob55 (also known as Sam50) in mitochondria and Omp85 in bacteria, both β-barrel membrane proteins themselves. Tob55 is part of the hetero-oligomeric TOB (topogenesis of mitochondrial outer-membrane β-barrel proteins) or SAM (sorting and assembly of mitochondria) complex, which is present in the mitochondrial outer membrane. Tob55 belongs to an evolutionarily conserved protein family, the members of which are present in almost all eukaryotes and in Gram-negative bacteria and chloroplasts. Thus, is it emphasized that the insertion pathway of mitochondrial β-barrel membrane proteins was conserved during evolution of mitochondria from endosymbiotic bacterial ancestors.