Mitochondria use transport proteins of the eukaryotic mitochondrial carrier family (MCF) to mediate the exchange of diverse substrates, including ATP, with the host cell cytosol. According to classical endosymbiosis theory, insertion of a host-nuclear-encoded MCF transporter into the protomitochondrion was the key step that allowed the host cell to harvest ATP from the enslaved endosymbiont1. Notably the genome of the microsporidian Encephalitozoon cuniculi has lost all of its genes for MCF proteins2. This raises the question of how the recently discovered microsporidian remnant mitochondrion, called a mitosome, acquires ATP to support protein import and other predicted ATP-dependent activities2,3,4. The E. cuniculi genome does contain four genes for an unrelated type of nucleotide transporter used by plastids and bacterial intracellular parasites, such as Rickettsia and Chlamydia, to import ATP from the cytosol of their eukaryotic host cells5,6,7. The inference is that E. cuniculi also uses these proteins to steal ATP from its eukaryotic host to sustain its lifestyle as an obligate intracellular parasite. Here we show that, consistent with this hypothesis, all four E. cuniculi transporters can transport ATP, and three of them are expressed on the surface of the parasite when it is living inside host cells. The fourth transporter co-locates with mitochondrial Hsp70 to the E. cuniculi mitosome. Thus, uniquely among eukaryotes, the traditional relationship between mitochondrion and host has been subverted in E. cuniculi, by reductive evolution and analogous gene replacement. Instead of the mitosome providing the parasite cytosol with ATP, the parasite cytosol now seems to provide ATP for the organelle.
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We thank T. Booth for help with confocal microscopy, J. James for help with electron microscopy, I. Haferkamp for providing details about the transport assay for NTTs expressed in E. coli and S. Harris for advice on bioinformatics analyses. A.D.T. was supported by a scholarship from the Division of Biology, Newcastle University. T.M.E. acknowledges support from the British Royal Society and the Leverhulme Trust. E.R.S.K. is supported by the Medical Research Council UK.
Author Contributions A.D.T. cloned the EcNTT1–4 and mtHsp70 genes, generated the antisera and performed the western blots and IFAs; E.R.S.K. and A.D.T. performed the transport assays and their analysis; A.V.G. supervised and contributed to the cloning and antisera generation, western blots and IFAs; J.M.L. performed the electron microscopy work; and R.P.H. and T.M.E. designed the project, supervised the project on a day-to-day basis, and wrote the paper. All authors discussed the results and commented on the manuscript.
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Tsaousis, A., Kunji, E., Goldberg, A. et al. A novel route for ATP acquisition by the remnant mitochondria of Encephalitozoon cuniculi. Nature 453, 553–556 (2008). https://doi.org/10.1038/nature06903
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