Loss, replacement and gain of proteins at the origin of the mitochondria

We review what has been inferred about the changes at the level of the proteome that accompanied the evolution of the mitochondrion from an alphaproteobacterium. We regard these changes from an alphaproteobacterial perspective: which proteins were lost during mitochondrial evolution? And, of the proteins that were lost, which ones have been replaced by other, non-orthologous proteins with a similar function? Combining literature-supported replacements with quantitative analyses of mitochondrial proteomics data we infer that most of the loss and replacements that separate current day mitochondria in mammals from alphaproteobacteria took place before the radiation of the eukaryotes. Recent analyses show that also the acquisition of new proteins to the large protein complexes of the oxidative phosphorylation and the mitochondrial ribosome occurred mainly before the divergence of the eukaryotes. These results indicate a significant number of pivotal evolutionary events between the acquisition of the endosymbiont and the radiation of the eukaryotes and therewith support an early acquisition of mitochondria in eukaryotic evolution. Technically, advancements in the reconstruction of the evolutionary trajectories of loss, replacement and gain of mitochondrial proteins depend on using profile-based homology detection methods for sequence analysis. We highlight the mitochondrial Holliday junction resolvase endonuclease, for which such methods have detected new "family members" and in which function differentiation is accompanied by the loss of catalytic residues for the original enzymatic function and the gain of a protein domain for the new function. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.