Convergent evolution of the mitochondrial protein translocation machinery.
Andrew Perry, Joanne Hulett, Nickie Chan, Peter Walsh, Vladimir Likic, Paul Gooley, Trevor Lithgow
Biochem. & Mol. Biol., University of Melbourne, Flemington Road, Parkville, 3010, Australia
Mitochondria evolved from an endosymbiotic proteobacterium in a process that required bacterial gene transfer to the host cell nucleus, and the translocation of proteins from the host cell cytosol into the organelle. The development of a primitive translocation channel in the outer membrane of the endosymbiont was a significant hurdle to initiate the evolution of mitochondria. Work done with various species of fungi has shown the receptors Tom20 and Tom70 transfer these precursor proteins to the core TOM (translocase in the outer mitochondrial membrane) complex. Comparative genomic analysis suggests that Tom20 and Tom70 evolved multiple times independently in the radiation of eukaryotes. Three other subunits, Tom40, Tom7 and Tom22, are common elements of the TOM complex in diverse extant eukaryotes. This strongly suggests Tom40, Tom7 and Tom22 functioned as the protein translocase in the earliest mitochondria. In bacteria with outer membranes, Omp85 is an essential component of the protein insertion and assembly machinery in the outer membrane. Recently, members of the Omp85 protein family were discovered in eukaryotes and studies in yeast show the gene is essential for cell viability with conditional omp85 mutants incapable of assembling a functional TOM complex. We suggest a preexisitng Omp85 organized a primitive TOM complex in the outer membrane of endosymbiotic bacteria, thereby opening a gateway for protein import into mitochondria.