Preview

PDF, English Download (7MB) | Terms of use

Abstract

Throughout evolution most of the mitochondrial genes have been transferred to the nuclear genome. Thus, proteins targeting one of the four mitochondrial compartments have to be translocated after their synthesis in the cytosol. This process is essential for mitochondrial biogenesis in all eukaryotes. Nevertheless, mitochondrial protein import machineries of parasitic protists differ significantly from the established models in opisthokonts (e.g. mammals and yeast), although import signals are functionally conserved. In yeast and mammals, Mia40/CHCHD4 and the sulfhydryl electron transferase Erv1/ALR are the essential components for the import and oxidative folding of sulphur-containing proteins in the mitochondrial intermembrane space. Substrates of this pathway carry conserved cysteine motifs that are recognised by Mia40 and subsequently oxidized to the formation of intramolecular disulphide bonds. In a disulphide relay system, electrons are transported from Mia40 to Erv1 and on to cytochrome c, which finally introduces them into the respiratory chain. However, a Mia40 homologue could not be identified in the genome of important apicomplexan and kinetoplastid parasites, while Erv1 is ubiquitously conserved. Both the Erv homologues from the kinetoplastid parasite Leishmania tarentolae (LtErv) and the malaria parasite Plasmodium falciparum (PfErv) were imported into yeast mitochondria but failed to replace their yeast counterpart. Here I analyse structure-function relationships of LtErv in the endogenous as well as the opisthokont model system Saccharomyces cerevisiae. I characterise particularities of the overall structure and the cysteine motifs that distinguish the protein from other homologues that actually interact with Mia40. The most obvious features comprise an additional C-terminal domain (KISS) restricted to this group of parasites and a partially conserved cysteine residue close to the N-terminus (C17). Yeast complementation assays suggest that the presence of the KISS domain does not inactivate the protein in the yeast system. In contrast, LtErv gained functionality when C17 was removed or the yeast shuttle arm was N-terminally fused to the protein. Residue C17 is therefore most likely the reason why LtErv does not interact with yeast Mia40 and interrupts the oxidative protein folding machinery in yeast. However, the translocation of LtErv itself into yeast mitochondria is not affected. Unlike soluble ScErv1, the protist protein is presumably imported due to its membrane association. Whether the translocation of LtErv is structurally connected to its dysfunction in yeast and controlled by a conserved trans side receptor or by protein-lipid interactions remains to be further investigated. Additionally, the data strongly support the existence of a Mia40 replacement in parasitic protists because both LtErv and PfErv were unable to initiate substrate import on their own in yeast mitochondria. Potential replacement candidates were enriched by trapping mixed disulphide intermediates with a model substrate and have to be evaluated in the future.