Debates about how eukaryotes get the mitochondria.

 I would like to share with you the debates about eukaryotic evolution. The evolution of eukaryotes is something very hard to understand the process of evolution. One of them that always debate, how they acquired the membranebound organelle, mitochondria. Mitochondria produce energy in nearly all eukaryotic cells and regulate cell metabolism by controlling the flow of factors such as ions, amino acids, and carbohydrates between themselves and the cytoplasm. Mitochondria evolved from a bacterial endosymbiont (an α-proteobacterium), and this process depended on the establishment of new pathways that facilitated the import of proteins into and across the double membrane (inner and outer) of the ancestral endosymbiont. Herein lies a debate: How did the process of protein import in mitochondria—which facilitated the evolution of this organelle, and thus, eukaryotic cell evolution— arise? Was the process driven by the ancestral host cell or by the prokaryotic endosymbiont, or by both?


Recently, Gross and Bhattacharya discussed the possibility that evolution of protein import into mitochondria was driven by the host cell (see the fi gure).In this paradigm, to capitalize on energy production by the ancestral endosymbiont, a protein sorting and importing mechanism was necessary to relocate host cell proteins to the endosymbiont. Thus, in the earliest evolutionary stages of mitochondria, host proteins were “imposed” on the ancestral endosymbiont.


It was argued that because the endosymbiont’s outer membrane had greatest access to host factors in the cytoplasm, evolution of mitochondrial protein import began at the outer membrane. Once established there, host proteins could then gain access to the intermembrane space and the inner membrane in an “outsideto- inside” trajectory of evolution.

Two views of mitochondrial evolution. The “Inside” view of the transition from intracellular bacterium to mitochondrion acknowledges that the ancestral endosymbiont’s genome encoded protein complexes (SecYEG and YidC) that facilitated the assembly of bacterial proteins into its inner membrane (e.g., metabolite carriers) and outer membrane (the bacterial β-barrel protein BAM). An early outer-membrane protein of the TOM complex (Tom40) may have arisen from a bacterial β-barrel protein with affinity for basic, amphipathic amino acid sequences on host cell proteins. Progenitors of modern protein import components (TIM) were also encoded by the endosymbiont’s genome. The “Outside” view proposes that proteins of the ancestral host cell were imported into the endosymbiont. It assumes that the endosymbiont’s genome is reduced (and unable to encode progenitor proteins of the import machinery). In this case, some host cell β-barrel protein is imposed into the endosymbiont’s outer membrane. It then facilitates the import of other protein import machinery components from the host cell.

To read further please refer this article on Science Magazine 5 february 2010 titled "Tinkering Inside the Organelle".