Mitochondrion-related organelles in eukaryotic protists

Abstract

The discovery of mitochondrion-type genes in organisms thought to lack mitochondria led to the demonstration that hydrogenosomes share a common ancestry with mitochondria, as well as the discovery of mitosomes in multiple eukaryotic lineages. No examples of examined eukaryotes lacking a mitochondrion-related organelle exist, implying that the endosymbiont that gave rise to the mitochondrion was present in the first eukaryote. These organelles, known as hydrogenosomes, mitosomes, or mitochondrion-like organelles, are typically reduced, both structurally and biochemically, relative to classical mitochondria. However, despite their diversification and adaptation to different niches, all appear to play a role in Fe-S cluster assembly, as observed for mitochondria. Although evidence supports the use of common protein targeting mechanisms in the biogenesis of these diverse organelles, divergent features are also apparent. This review examines the metabolism and biogenesis of these organelles in divergent unicellular microbes, with a focus on parasitic protists.

Figure 1

Mitochondrion-related organelles are found in all eukaryotes. Eukaryotes can be divided into several major groups, all of which contain mitochondria (red), mitosomes or MLO (blue), or hydrogenosomes (green). Three of these groups have lineages with organisms that contain combinations of these organelles. This tree is adapted from Keeling et al. 2005.

Figure 2

Fe-S cluster formation. The biogenesis of Fe-S clusters within mitochondria is similar to the bacterial ISC process. IscS is a cysteine desulferase that donates the S, and Fe is often donated by frataxin in mitochondria. Ferredoxin acts in electron transfer, perhaps to reduce the donated S. IscU is a scaffold protein on which the initial FeS cluster is formed before transfer to the apoprotein.