Giardia intestinalis: A microaerophilic parasite with mitochondrial ancestry

Highly derived mitochondrion-related organelles originally found in Entamoeba histolytica and known as mitosomes (or cryptons) are heterogeneous in morphology and exist in a range of anaerobic protists including Trachipleistophora hominis and Cryptosporidium parvum. The iron-binding scaffold protein (IscU) and its functional partner IscS participate in the biosynthesis of the iron-sulphur (FeS) cluster. Specific antibodies raised against both proteins detected both IscS and IscU in high-speed pellets obtained from the fractionation of total extracts of Giardia. In an immunoelectron microscopy study using those antibodies labelled, we detected these proteins associated with double-membrane organelles distributed throughout the cytoplasm. Most of the mitochondrial proteins are synthesized as preproteins and are targeted to the organelle by a cleavage of the N-terminal extension composed of positively charged amino acids. The Giardia proteins IscU and ferredoxin have those amino-terminal targeting presequences. Transgenic parasites expressing GFP fusions of the amino terminal presequences of both proteins revealed that such extensions are essential for mitosome targeting. Additionally, the mitosomal targeting presequences were recognized by the mitochondrial and Trichomonas virginalis import machinery. Cpn60 and hsp70 were identified in the mitosomes of transfected trophozoites as well.