Divergent Cytochrome Maturation System in Kinetoplastid Protists.

In eukaryotes, heme attachment through two thioether bonds to mitochondrial cytochromes and is catalyzed by either multisubunit cytochrome maturation system I or holocytochrome synthetase (HCCS). The former was inherited from the alphaproteobacterial progenitor of mitochondria; the latter is a eukaryotic innovation for which prokaryotic ancestry is not evident. HCCS provides one of a few exemplars of protein innovation in eukaryotes, but structure-function insight of HCCS is limited. Uniquely, euglenozoan protists, which include medically relevant kinetoplastids and parasites, attach heme to mitochondrial -type cytochromes by a single thioether linkage. Yet the mechanism is unknown, as genes encoding proteins with detectable similarity to any proteins involved in cytochrome maturation in other taxa are absent. Here, a bioinformatics search for proteins conserved in all hemoprotein-containing kinetoplastids identified kinetoplastid cytochrome synthetase (KCCS), which we reveal as essential and mitochondrial and catalyzes heme attachment to trypanosome cytochrome KCCS has no sequence identity to other proteins, apart from a slight resemblance within four short motifs suggesting relatedness to HCCS. Thus, KCCS provides a novel resource for studying eukaryotic cytochrome maturation, possibly with wider relevance, since mutations in human HCCS leads to disease. Moreover, many examples of mitochondrial biochemistry are different in euglenozoans compared to many other eukaryotes; identification of KCCS thus provides another exemplar of extreme, unusual mitochondrial biochemistry in an evolutionarily divergent group of protists. Cytochromes are essential proteins for respiratory and photosynthetic electron transfer. They are posttranslationally modified by covalent attachment of a heme cofactor. Kinetoplastids include important tropical disease-causing parasites; many aspects of their biology differ from other organisms, including their mammalian or plant hosts. Uniquely, kinetoplastids produce cytochromes with a type of heme attachment not seen elsewhere in nature and were the only cytochrome bearing taxa without evidence of protein machinery to attach heme to the apocytochrome. Using bioinformatics, biochemistry, and molecular genetics, we report how kinetoplastids make their cytochromes Unexpectedly, they use a highly diverged version of an enzyme used for heme-protein attachment in many eukaryotes. Mutations in the human enzyme lead to genetic disease. Identification of kinetoplastid cytochrome synthetase, thus, solves an evolutionary unknown, provides a possible target for antiparasite drug development, and an unanticipated resource for studying the mechanistic basis of a human genetic disease.