Evolution of mitochondrial protein import - lessons from trypanosomes

The evolution of mitochondrial protein import and the systems that mediate it marks the boundary between the endosymbiotic ancestor of mitochondria and a true organelle that is under the control of the nucleus. Protein import has been studied in great detail in Saccharomyces cerevisiae. More recently, it has also been extensively investigated in the parasitic protozoan Trypanosoma brucei, making it arguably the second best studied system. A comparative analysis of the protein import complexes of yeast and trypanosomes is provided. Together with data from other systems, this allows to reconstruct the ancestral features of import complexes that were present in the last eukaryotic common ancestor (LECA) and to identify which subunits were added later in evolution. How these data can be translated into plausible scenarios is discussed, providing insights into the evolution of (i) outer membrane protein import receptors, (ii) proteins involved in biogenesis of a-helically anchored outer membrane proteins, and (iii) of the intermembrane space import and assembly system. Finally, it is shown that the unusual presequence-associated import motor of trypanosomes suggests a scenario of how the two ancestral inner membrane protein translocases present in LECA evolved into the single bifunctional one found in extant trypanosomes.