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Targeting and translocation of proteins into the hydrogenosome of the protist Trichomonas: similarities with mitochondrial protein import (Bradley et al., 1997)

This article shows that the protein import mechanisms between hydrogenosomes and mitochondria are strikingly similar. Although this is attributed to their putative common endosymbiotic origin, I believe it is more in line with a eukaryotic descent, where the import mechanisms were developed before true mitochondria arose. Sequence: nucleus, ER, Golgi, metabolic vesicles single membraned, double membraned, mitochondria. Also interesting references to kinetoplasts, apparently mitochondrial-like plastids that share features with hydrogenosomes. 

Abstract: Trichomonads are early-diverging eukaryotes that lack both mitochondria and peroxisomes. They do contain a double membrane-bound organelle, called the hydrogenosome, that metabolizes pyruvate and produces ATP. To address the origin and biological nature of hydrogenosomes, we have established an in vitro protein import assay. Using purified hydrogenosomes and radiolabeled hydrogenosomal precursor ferredoxin (pFd), we demonstrate that protein import requires intact organelles, ATP and N-ethylmaleimide-sensitive cytosolic factors. Protein import is also affected by high concentrations of the protonophore, m-chlorophenylhydrazone (CCCP). Binding and translocation of pFd into hydrogenosomes requires the presence of an eight amino acid N-terminal presequence that is similar to presequences found on all examined hydrogenosomal proteins. Upon import, pFd is processed to a size consistent with cleavage of the presequence. Mutation of a conserved leucine at position 2 in the presequence to a glycine disrupts import of pFd into the organelle. Interestingly, a comparison of hydrogenosomal and mitochondrial protein presequences reveals striking similarities. These data indicate that mechanisms underlying protein targeting and biogenesis of hydrogenosomes and mitochondria are similar, consistent with the notion that these two organelles arose from a common endosymbiont.

Intriguingly, the first paragraph gives exactly the driving force that I imagine for the evolution of mitochondria: the separation of functionalities, in this case compartmentalization. Also, the lack of mitochondria in primitive cells is an indication for a eukaryote origin of mitos/

Compartmentalization of specialized cellular functions into organelles is a characteristic of all eukaryotic cells. Some organelles, such as mitochondria and peroxisomes, are common to most eukaryotes. There are also a number of unusual organelles, such as the hydrogenosome of trichomonads and the glycosome of trypanosomatids, that exist primarily in very early-evolving eukaryotes. Interestingly, eukaryotes that possess these organelles typically lack either mitochondria or peroxisomes or both, raising questions regarding the origin of these unusual organelles.

As the site of pyruvate fermentation in trichomonads, hydrogenosomes play a central role in carbohydrate metabolism of these amitochondriate protists. Within the organelle, pyruvate is metabolized to the end-products of acetate, CO2 and molecular hydrogen. This process is coupled to ATP formation via substrate-level phosphorylation. In this regard, hydrogenosomes can be considered the anaerobic equivalent of mitochondria. Although biochemical analyses of hydrogenosomes reveal properties similar to those of mitochondria, there are also significant differences. For example, the enzyme that mediates decarboxylation of pyruvate in hydrogenosomes, pyruvate:ferredoxin oxidoreductase, is completely different from its counterpart in mitochondria, the pyruvate dehydrogenase complex. Unlike mitochondria, hydrogenosomes contain hydrogenases and produce molecular hydrogen and do not contain cytochromes, DNA or ribosomes (Lloyd et al., 1979b; Müller, 1993; Johnson et al., 1995). Hydrogenosomes are similar to mitochondria in their abilities to metabolize pyruvate and produce ATP. The enzyme that catalyzes substrate-level production of ATP in hydrogenosomes, succinyl CoA synthetase, is in fact located exclusively in mitochondria in higher eukaryotic cells. Hydrogenosomes have also been demonstrated to contain a [2Fe-2S]-ferredoxin that is structurally similar to mitochondrial ferredoxins (Gorrell et al., 1984; Johnson et al., 1990). Although bounded by a double membrane, the inner membrane of the hydrogenosome does not form cristae as observed in mitochondria (Honigberg et al., 1984).

Comparison of protein import into hydrogenosomes and mitochondria

A comparison of hydrogenosomal and mitochondrial protein presequences reveals many similarities, although there are also notable differences. Presequences of both hydrogenosomal and mitochondrial proteins are absolutely required for binding and translocation of proteins into the matrix of the organelle in vitro. Hydrogenosomal and mitochondrial targeting sequences are also similar as they both have an N-terminal location, are cleaved from the mature protein found in the organelle and have similar amino acid composition.

The most conspicuous difference between hydrogenosomal presequences and those of yeast and mammalian mitochondrial proteins is length. The presequences identified thus far for hydrogenosomal proteins are 5–14 amino acids (Table I), whereas presequences of yeast and mammalian mitochondrial proteins are typically 20–80 amino acids (Hendrick et al., 1989). Nonetheless, presequences as short as 7–12 amino acids are capable of targeting proteins to yeast mitochondria (Hurt et al., 1984; Verner and Lemire, 1989). Interestingly, the length of hydrogenosomal presequences resembles that found on mitochondrial proteins of kinetoplastids, the only protists for which mitochondrial presequences have been examined (Giambiagi-de Marval et al., 1993; Peterson et al., 1993; Searle et al., 1993; Xu and Ray, 1993; Olson et al., 1994). These presequences range in size from 8 to 20 amino acids and function to target proteins to the kinetoplast, an unusual mitochondrion found in the earliest-diverging eukaryotes known to contain mitochondria (Sogin, 1991; Leipe et al., 1993).

Protein import requirements are consistent with a common evolutionary origin for hydrogenosomes and mitochondria

The general parallels between mechanisms used for targeting and translocation of hydrogenosomal and mitochondrial proteins are consistent with recent phylogenetic analyses of hydrogenosomal heat shock proteins (Hsps) that strongly support a common origin for hydrogenosomes and mitochondria. These analyses show that hydrogenosomal Hsps branch within monophyletic groups that otherwise contain exclusively mitochondrial Hsps (Bui et al., 1996; Germot et al., 1996; Horner et al., 1996; Roger et al., 1996; Palmer, 1997).

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