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protein targeting

Many of the essential proteins of mitochondria are imported into the organelle and all of the proteins of hydrogenosomes and mitosomes are imported. In a eukaryotic origin, one expects to see the gradual evolution from simpler organelles to mitochondria also at the level of targeting. A scenario could be targeting to ER membrane,  followed by targeting to organelle membranes, and finally the mitochondrion. We would expect to find similar targeting sequences that need only tweaking in order to be targeted to another membrane. We do see a good similarity between mitochondrion-, hydrogenosome-, mitosome- and ER-targeting. Also, I would expect a sort of backwards compatibility where older version (i.e. mitosomes, hydrogensomes) would still work with newer version, but not vice versa, in line with design-by-contract where old interfaces need to remain intact.

  • Proteins targeted into mitosomes of Giardia intestinalis that were expressed in Trichomonas vaginalis results in targeting to hydrogenosomes, a hydrogen-producing form of mitochondria. Both Giardia and Trichomonas share proteins related to the component of the translocase in the inner membrane from mitochondria and the processing peptidase. This supports the hypothesis that mitosomes, hydrogenosomes, and mitochondria represent different forms of the same fundamental organelle (here).
  • In fungi, the mitochondrial protein import machinery consists of a network series of heterooligomeric translocases and peptidases, but in microsporidia, only a few subunits of some of these complexes have been identified to date. Mitosomes do in some cases still use N-terminal and internal targeting sequences that are recognizable by import systems of mitochondria in yeast. Also, mitosomal substrates of this enzyme are processed to mature proteins in one species with a simplified processing complex, Antonospora locustae. Mitosomal proteins are targeted appropriately in yeast, confirming that homologous elements of the ancestral system are still used and that the N terminus is at least partly responsible for encoding targeting information (here).
  • The NH2-terminal presequence of a Cryptosporidium gene Cp-mtHSP70 can correctly target green fluorescent protein to the single mitochondrion of the apicomplexan Toxoplasma gondii and the mitochondrial network of the yeast Saccharomyces cerevisiae (here).
  • The hydrogenosomal of Trichomonas and mitochondrial protein presequences reveals striking similarities and mechanisms underlying protein targeting and biogenesis of hydrogenosomes and mitochondria are similar, requiring pre- and trageting sequences. Hydrogenosomal presequences are smaller of those of yeast and mammalian mitochondrial (5–14 versus 20–80 amino acids), indicating addition of extra regulators during evolution to mitochondria. Presequences as short as 7–12 amino acids are capable of targeting proteins to yeast mitochondria, indicating a backwards compatibility with an earlier (hydrogenosomal) sequence (here).
  • The length of hydrogenosomal presequences resembles that found on mitochondrial proteins of kinetoplastids eg in Trypanosoma, an unusual mitochondrion found in the earliest-diverging eukaryotes known to contain mitochondria, suggesting the presence of an intermediate targeting mechanism in these protists (see here).
  • The terminal sequence of the nuclear-encoded protein cox1 in plants has been acquired from the cytosolic GAPc protein by exon shuffling, and thought to have evolved into a targeting sequence (here), in line with the gradual acquisition of these sequences in the evolution to organelle and mitochondrial targeting during eukaryotic evolution.