Modifications and innovations in the evolution of mitochondrial protein import pathways

Eukaryotic cells are defined by their mitochondria, organelles that were derived through endosymbiosis. The development of this organelle from a bacterial endosymbiont required establishment of effective protein import pathways so that much of the genetic capacity of the bacterium could be relocated to the host cell. Two realms of study have delivered insight into the early evolution of these mitochondrial pathways: (1) considering the starting material based on what can be observed of protein trafficking pathways in extant species of bacteria and (2) analysing the protein import pathways of parasites whose mitochondria have undergone secondary reduction and now offer insight into minimal functional pathways. These approaches have illuminated what components of bacterial trafficking pathways were co-opted in the developing mitochondrion and what further innovations occurred within the eukaryote host. Now comparative analysis of model mitochondrial systems, with organelles found in a broad diversity of eukaryotes (namely protists), shows when in eukaryotic radiation these major innovations took place and what lineage-specific changes have since occurred to mitochondrial import systems in eukaryotes.