Conservation of mitochondrial targeting sequence function in mitochondrial and hydrogenosomal proteins from the early-branching eukaryotes Crithidia, Trypanosoma and Trichomonas.

Kinetoplastid protozoa are the earliest-branching eukaryotes to possess a true mitochondrion. This organelle is host to a variety of intriguing and unique features, including RNA editing. We examined the characteristics of protein import into mitochondria of Trypanosoma brucei. Dihydrofolate reductase (DHFR) carrying a yeast mitochondrial targeting signal was correctly translocated into trypanosome mitochondria in vivo, as were DHFR fusion proteins bearing two unusually short (7-9 amino acids) presequences from trypanosomatids. The short trypanosomal targeting signals were functional in Saccharomyces cerevisiae as well, but their targeting efficiency was lower and processing was absent. Trichomonads branched even earlier than kinetoplastids in eukaryotic evolution and contain energy-generating organelles called hydrogenosomes. The origin of hydrogenosomes has been controversial, but most evidence suggests that they are related to mitochondria. Putative hydrogenosomal targeting signals from Trichomonas vaginalis are short (5-12 amino acids). Three such sequences were capable of targeting a passenger protein to mitochondria both in yeast and in trypanosomes, and one of the hydrogenosomal presequences was efficiently processed in both organisms. These findings suggest a resemblance between the import machineries of mitochondria and hydrogenosomes.