[Fe–S] biogenesis and unusual assembly of the ISC scaffold complex in the Plasmodium falciparum mitochondrion

Abstract

The malaria parasite harbors two [Fe–S] biogenesis pathways of prokaryotic origin–the SUF and ISC systems in the apicoplast and mitochondrion, respectively. While the SUF machinery has been delineated, there is little experimental evidence on the ISC pathway. We confirmed mitochondrial targeting of Plasmodium falciparum ISC proteins followed by analyses of cysteine desulfurase, scaffold, and [Fe–S]-carrier components. PfIscU functioned as the scaffold in complex with the PfIscS–PfIsd11 cysteine desulfurase and could directly assemble [4Fe–4S] without prior [2Fe–2S] formation seen in other homologs. Small angle X-ray scattering and spectral studies showed that PfIscU, a trimer, bound one [4Fe–4S]. In a deviation from reported complexes from other organisms, the P. falciparum desulfurase-scaffold complex assembled around a PfIscS tetramer instead of a dimer, resulting in a symmetric hetero-hexamer [2× (2PfIscS–2PfIsd11–2PfIscU)]. PfIscU directly transferred [4Fe–4S] to the apo-protein aconitase B thus abrogating the requirement of intermediary proteins for conversion of [2Fe–2S] to [4Fe–4S] before transfer to [4Fe–4S]-recipients. Among the putative cluster-carriers, PfIscA2 was more efficient than PfNifU-like protein; PfIscA1 primarily bound iron, suggesting its potential role as a Fe²⁺ carrier/donor. Our results identify the core P. falciparum ISC machinery and reveal unique features compared with those in bacteria or yeast and human mitochondria.