Diversity and reductive evolution of mitochondria among microbial eukaryotes

Abstract

All extant eukaryotes are now considered to possess mitochondria in one form or another. Many parasites or anaerobic protists have highly reduced versions of mitochondria, which have generally lost their genome and the capacity to generate ATP through oxidative phosphorylation. These organelles have been called hydrogenosomes, when they make hydrogen, or remnant mitochondria or mitosomes when their functions were cryptic. More recently, organelles with features blurring the distinction between mitochondria, hydrogenosomes and mitosomes have been identified. These organelles have retained a mitochondrial genome and include the mitochondrial-like organelle of Blastocystis and the hydrogenosome of the anaerobic ciliate Nyctotherus. Studying eukaryotic diversity from the perspective of their mitochondrial variants has yielded important insights into eukaryote molecular cell biology and evolution. These investigations are contributing to understanding the essential functions of mitochondria, defined in the broadest sense, and the limits to which reductive evolution can proceed while maintaining a viable organelle.

Figure 1.

Detection of E. cuniculi mitosomes using electron and light microscopy and functional evidence that mitosomes use a novel route for ATP acquisition. Transmission electron micrographs of fixed rabbit kidney cell (RK-13) cultures infected with E. cuniculi showing double-membrane-bounded organelles inside the parasite. (a) One parasite shows a group of three double-membrane-bounded organelles (red arrows) situated close to the parasite nucleus (N). The inner and outer membranes of the parasite nuclear envelope are indicated with blue arrowheads. The double-membrane-bounded structures are associated with a rather homogeneous zone, which has a higher electron density than the surrounding cytoplasm (asterisk), and which in turn is applied closely to the nuclear envelope. The inset shows another field where putative microtubules (blue arrowheads) radiate from this zone (asterisk), and associated mitosomes are indicated by red arrows. (b) Higher magnification of the three mitosomes displayed in (a). The outer and inner membranes of one mitosome are indicated with red arrows. Scale bars, (a) 200 nm and (b) 100 nm. Localization and expression of E. cuniculi mitosomal Hsp70 and a novel nucleotide transport protein (EcNTT3). Mitosomal Hsp70 shares structural features with characterized mtHsp70 proteins that require ATP for their activities (Williams et al. 2002; Tsaousis et al. 2008). (c) Western blot analyses using published antisera (Tsaousis et al. 2008) demonstrate that the EcNTT3 protein is present in E. cuniculi spores (lane 1) and RK-13 cells infected with E. cuniculi (lane 2), but it is not detected in control non-infected RK-13 cells (lane 3). The corresponding signal for the mitosomal E. cuniculi Hsp70 is shown for comparison. The reasons for the different signal intensities for the two proteins in the different preparations (lanes 1 and 2) are unknown. (d–f) Three E. cuniculi cells isolated from infected RK-13 cells were colabelled with the anti-Hsp70 sera (red signal) and anti-EcNTT3 sera (green signal). The signal for EcNTT3 clearly colocalizes with the signal for the mitosomal marker Hsp70. The transport properties of the NTT3 protein were investigated in E. coli cells expressing the complete NTT3 protein fused to a His-tag. (g) Back-exchange properties of E. coli cells pre-loaded with [α³²P]-ATP and expressing recombinant NTT3 protein. The E. coli cells took up [α³²P]-ATP in an NTT3-dependent manner (data not shown; Tsaousis et al. 2008). Efflux of radio-labelled adenine nucleotides was observed when the external substrate was removed, indicating that NTT3 can equilibrate nucleotide pools across a gradient. Addition of cold ADP or ATP further stimulated efflux, confirming that NTT3 is an exchanger of adenine nucleotides. Data points represent the mean and s.d. of three independent experiments. Open circles, none; filled triangles, ADP; filled squares, ATP. Adapted from Tsaousis et al. (2008). Copyright © Macmillan Publishers Ltd.