Giardia mitosomes and trichomonad hydrogenosomes share a common mode of protein targeting

Proc. Natl. Acad. Sci. USA (2005) 10924-10929

Speaker: 余婧萱                              Date:05/10/19

Commentator: 胥直利 老師                    Time: 13:10~14:00

Abstract:

Mitosomes were identified in the human intestinal parasite Giardia intestinalis. The presence of mitosomes in Giardia provides evidence that even if Giardia really is an early branching eukaryote, it nevertheless split from other eukaryotes after the mitochondrial endosymbiosis event ¹. This view is further supported by identification of several genes of putative mitochondrial origin on the Giardiagenome. The presence of a common type of FeS assembly machinery in Giardia mitosomes, trichomonad hydrogenosomes, and mitochondria argues for a common evolutionary history of these organelles.

To provide the biogenesis of Giardia mitosomes, the authors investigated and compared targeting of GiiscS, GiiscU, and ferredoxin to Giardia mitosomes and to hydrogenosomes in Trichomonas vaginalis. GiiscU and Gifdx can make use of N-terminal sequence as a signal peptide. They found that GiiscU and Gifdx present exclusively in the mitosome-rich fragment and N-terminal targeting sequence is independent of import of GiiscS. To determine whether the mitosomal targeting sequences on GiiscU and Gifdx can function to target proteins to hydrogenosomes, the giardial genes were overexpressed in T. vaginalis. Immunofluorescence labeling of trichomonad cells expressing tagged GiiscU, Gifdx, and GiiscS localized these proteins to hydrogenosomes. The size of tagged GiiscU detected in mitosomes of the cells expressing the complete giiscU was identical to its truncated form expressed in _△giiscU transformants. These observations indicated processing of N-terminal targeting sequence within the target organelles. The cleavage was inhibited by the addition of EDTA, indicating that a metalloprotease is involved.

The fact that hydrogenosomes and mitochondria recognize the targeting signals of mitosomal proteins indicates that these organelles possess a common protein import mechanism and suggests that all these organelles share, through common descent, what must have been among the earliest features of the first mitochondriate organisms.

References:

1.      Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation

Nature (2003) 172-176

2.  Evolution of the hydrogenosome

   FEMS Microbiol. Lett. (1997) 133-140