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Giardia mitosomes and trichomonad hydrogenosomes share a common mode of protein targeting (Dolezal et al., 2005)

This article shows that mitosomes and hydrogenosomes have targeting mechanisms that are similar. This is expected when mitochondria and related organelles were both derived from the endomembrane system and where the anaerobic organelles preceded the mitochondria in evolution.

Abstract: […] by the discovery of mitochondria-like double membrane-bound organelles called mitosomes. Here, we report that proteins targeted into mitosomes of Giardia intestinalis have targeting signals necessary and sufficient to be recognized by the mitosomal protein import machinery. Expression of these mitosomal proteins in Trichomonas vaginalis results in targeting to hydrogenosomes, a hydrogen-producing form of mitochondria. We identify, in Giardia and Trichomonas, proteins related to the component of the translocase in the inner membrane from mitochondria and the processing peptidase. A shared mode of protein targeting supports the hypothesis that mitosomes, hydrogenosomes, and mitochondria represent different forms of the same fundamental organelle having evolved under distinct selection pressures.

The presence of a common type of FeS assembly machinery in Giardia mitosomes, trichomonad hydrogenosomes, and mitochondria argues for a common evolutionary history of these organelles (4); however, it does not refute contentions that these organelles each arose independently from related species of bacterial endosymbionts (15). One problem is the absence of knowledge concerning the biogenesis of the mitosomes, the evidence that provided strong arguments for a common progenitor of hydrogenosomes and mitochondria (16, 17). Proteins targeted into the mitochondria are synthesized in cytosol with an N-terminal extension for protein targeting; however, many have internal targeting signals. Both sorts of targeting information are recognized by the outer (TOM) and inner (TIM) membrane translocases (18, 19). The mitochondrial matrix proteins are further translocated through the TIM23 complex, with energy supplied by a PAM complex. The PAM complex includes an integral membrane protein with a J domain referred to either as Pam18 (20) or Tim14 (21). After translocation, N-terminal presequences are then cleaved by a matrix-located processing peptidase (MPP) (22). Proteins targeted to hydrogenosomes have N-terminal extensions that carry targeting information (23).

To provide insight into the biogenesis of Giardia mitosomes, we investigated and compared targeting of GiiscS, GiiscU, and [2Fe2S] ferredoxin to Giardia mitosomes and to hydrogenosomes in Trichomonas vaginalis. We show that mitosomes and hydrogenosomes share a common mode of protein targeting that, like protein import into mitochondria, can make use of N-terminal or internal targeting signals. Initial sequence analysis and cell localization studies suggests that Giardia and Trichomonas have protein import machinery that shares common components with the protein import machinery of mitochondria and mitochondria-like processing peptidases.

Conservation of Protein Targeting in Mitosomes and Hydrogenosomes. To determine whether the mitosomal targeting sequences on GiiscU and Gifdx can function to target proteins to hydrogenosomes, the giardial genes were overexpressed in T. vaginalis. Immunofluorescence labeling of trichomonad cells expressing tagged GiiscU, Gifdx, and GiiscS localized these proteins to discrete structures surrounding trichomonad nuclei and cytoskeletal structures, the cell distribution typical for hydrogenosomes (Fig. 3A). The labeling of tagged proteins also colocalized with malic enzyme, a marker protein for hydrogenosomes. Stronger malic enzyme signal corresponds to its abundance in hydrogenosomes (30). In contrast, the absence of N-terminal leader sequences on GiiscU and Gifdx abrogated the delivery of the proteins into the target organelle with the majority of each protein accumulating in the cytosol (Fig. 3B).
 

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