7.11 Mitochondria-free parasites harvest ATP directly from hosts

http://arstechnica.com/journals/science.ars/2008/05/01/mitochondria-free-parasites-harvest-atp-directly-from-hosts

By John Timmer | Published: May 01, 2008 - 09:45AM CT

One of the defining features of the eukaryotes is the presence of mitochondria, which burn energy-rich molecules like sugars and fats to generate the ATP that runs most cellular processes. A few rare eukaryotes, however, appear to lack mitochondria, as well as the small genome the organelles posses. The best-studied example of these organisms are the microsporidia, parasites that live in animal cells, where they cause diarrhea and bronchitis. These organisms have a mitochondrial remnant, called a mitosome, but few of the genes normally involved with its activity.

A publication in yesterday's edition of Nature describes how these organisms get ahold of ATP. Researchers had previously noted that, once inside a victim's cells, the microsporidian Encephalitozoon cuniculi would congregate among the mitochondria of their host cells. In the new work, the researchers scanned the genomes of these organisms for the presence of a protein that could import ATP. They found it in the form of a 12-pass transmembrane protein family called a nucleotide transporter/translocase, or NTT, which is otherwise only present in bacteria or plants,. This suggests E. cuniculi picked the gene up via horizontal gene transfer.

The researchers demonstrated that the four NTT proteins could pull ATP into cells, and found that three of the proteins were located on the surface of the cells. This led them to conclude that these three transporters help to keep the parasites supplied with ATP from their hosts. They also propose that they would make good drug targets, since the human genome doesn't have copies of these proteins, and shutting them down would make it hard for the parasite to make a living.

There's a weird aside towards the end of the paper regarding the fourth protein, the one that isn't on the cell surface. The authors found it studding the surface of the mitosome, where it appears to be sending ATP into the organelle. This is strange on a number of levels. For starters, the primary role of a mitochondria is to ship ATP out, so it seems that the setup has been completely reversed in microsporidia. If it isn't making ATP, then it's not clear what the mitosome of these organisms is doing, raising questions as to what it might need the energy for. It's technically possible that the mitosome has turned the table on E. cuniculi and has become a DNA-free parasite that does nothing more than siphon energy from its host.

Nature, 2008. DOI: 10.1038/nature06903

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