Peeping at TOMs-Diverse Entry Gates to Mitochondria Provide Insights into the Evolution of Eukaryotes

Mol Biol Evol. 2016 Feb;33(2):337-51. doi: 10.1093/molbev/msv219. Epub 2015 Oct 15.


Mitochondria are essential for eukaryotic life and more than 95% of their proteins are imported as precursors from the cytosol. The targeting signals for this posttranslational import are conserved in all eukaryotes. However, this conservation does not hold true for the protein translocase of the mitochondrial outer membrane that serves as entry gate for essentially all precursor proteins. Only two of its subunits, Tom40 and Tom22, are conserved and thus likely were present in the last eukaryotic common ancestor. Tom7 is found in representatives of all supergroups except the Excavates. This suggests that it was added to the core of the translocase after the Excavates segregated from all other eukaryotes. A comparative analysis of the biochemically and functionally characterized outer membrane translocases of yeast, plants, and trypanosomes, which represent three eukaryotic supergroups, shows that the receptors that recognize the conserved import signals differ strongly between the different systems. They present a remarkable example of convergent evolution at the molecular level. The structural diversity of the functionally conserved import receptors therefore provides insight into the early evolutionary history of mitochondria.

Keywords: TOM complex; eukaryotes; mitochondria; protein import; trypanosome.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Biological Evolution*
  • Eukaryota / classification
  • Eukaryota / genetics*
  • Mitochondria / metabolism*
  • Mitochondrial Membrane Transport Proteins / chemistry
  • Mitochondrial Membrane Transport Proteins / metabolism*
  • Mitochondrial Proteins / metabolism
  • Multigene Family
  • Multiprotein Complexes
  • Plants / metabolism
  • Porins / chemistry
  • Porins / metabolism
  • Protein Binding
  • Protein Subunits / metabolism
  • Protein Transport
  • Receptors, Cell Surface / chemistry
  • Receptors, Cell Surface / metabolism
  • Symbiosis
  • Trypanosoma / metabolism
  • Yeasts / metabolism


  • Mitochondrial Membrane Transport Proteins
  • Mitochondrial Proteins
  • Multiprotein Complexes
  • Porins
  • Protein Subunits
  • Receptors, Cell Surface