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Microsporidian mitosomes retain elements of the general mitochondrial targeting system (Burri et al., 2006)

This article regards the microsporidia as highly derived organisms and all their claims have to be seen in this context. Still, this article provides interesting data how a precursor mitochondrion, in this case the mitosome could have looked like. They find simpler import mechanisms based on a a few subunits of the mitochondrial ones and find that the targeting sequences are also similar. Thus import mechanisms of true mitochondria can be based in existing ones used for mitosomes.

From the abstract: Microsporidia were long considered to be amitochondriate, but recently a tiny mitochondrion-derived organelle called the mitosome was detected. The molecular function of this organelle remains poorly understood. The mitosome has no genome, so it must import all its proteins from the cytosol. In other 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. Here, we look at targeting sequences of the microsporidian mitosomal import system and show that mitosomes do in some cases still use N-terminal and internal targeting sequences that are recognizable by import systems of mitochondria in yeast. Furthermore, we have examined the function of the inner membrane peptidase processing enzyme and demonstrate that mitosomal substrates of this enzyme are processed to mature proteins in one species with a simplified processing complex, Antonospora locustae. However, in Encephalitozoon cuniculi, the processing complex is lost altogether, and the preprotein substrate functions with the targeting leader still attached.

Also some nice overview of the proteins in it.

The number of proteins estimated to be acting within yeast mitochondria is 800-1,000 (19, 20). In contrast, the number of putative mitochondrial proteins identified within all microsporidian species amounts to 21 thus far (21, 22). Based on the whole genome sequence of Encephalitozoon cuniculi, the only clear function of the organelle from these proteins is iron-sulfur cluster (ISC) assembly, and it seems to have lost the capacity for ATP production through oxidative phosphorylation (21).

The identified proteins can largely be categorized into those that act in protein and metabolite import (TOM70, TIM22, TOM40, Imp2, mitochondrial Hsp70, and ATM1-ABC transporter proteins) and those involved in ISC assembly and export (frataxin, ferredoxin, ISCU, ISCS, ERV1, and ferredoxin NADPH oxido-reductase [FNR]). The two subunits of pyruvate dehydrogenase, PDH and -, and mitochondrial glycerol-3-phosphate dehydrogenase (mtG3PDH) are involved in metabolic processes, and manganese-containing superoxide dismutase (MnSOD) is involved in protection against oxidative stress (Fig. 1).

Protein import mechanisms are in a more simplified form in mitosomes, in line with gradual evolution from a simpler to a more complex form: mitosomes –> mitochondria.

Of the identified proteins in microsporidia, few of the mitochondrial import and processing proteins are present (Fig. 1). In addition, very few of the microsporidian mitosomal proteins have N-terminal sequences with characteristics expected of a targeting sequence, so it is unclear how import is achieved. Here we have tested to what extent the common mitochondrial targeting system has been retained in microsporidia and how the complexity of the system is degenerating. We show that many 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. Using the inner-membrane peptidase (IMP) as a model, we also show the complexity of the system has been progressively reduced during microsporidian evolution. IMP usually consists of two catalytic subunits (Imp1 and Imp2) and a noncatalytic regulator (Som1), but this complex is apparently reduced to a single functional protein in Antonospora locustae and lost altogether in E. cuniculi. The protein substrates of this complex are processed in A. locustae but not in E. cuniculi, where they function as unprocessed preproteins. The reduction of the IMP complex is typical of how complex molecular systems have degenerated in microsporidia.

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