The Mitochondrion-Related Organelles of Cryptosporidium Species

Amongst all apicomplexans, the mitochondrion-related organelle of Cryptosporidium species is the least studied. To date, most of our knowledge on this tiny organelle stems from observations on the remnant mitochondrion, mitosome, of Cryptosporidium parvum. In C. parvum the mitosome is structurally distinguished from the hydrogenosomes and mitosomes of other anaerobic protists by its (1) close association with the crystalloid body, an organelle unique to this apicomplexan and the function of which is currently unknown; (2) close association with the outer nuclear membrane, and possibly nuclear pores; (3) envelopment by rough endoplasmic reticulum, and in some cases an apparent direct tethering to ribosomes; and (4) atypical internal membranous compartments that lack well-defined crista junctions with the mitochondrial inner membrane, a characteristic that defines most aerobic mitochondria. Like most hydrogenosome- and other mitosome-bearing anaerobic protists, however, C. parvum lacks a mitochondrial genome, i.e. proteins are encoded by the nucleus and targeted back to the mitosome. As a consequence of this reductive evolution, there are no genes for electron transport or oxidative phosphorylation, and the only function so far ascribed to this tiny organelle is one common to all eukaryotic mitochondria, the assembly and maturation of iron-sulphur clusters. The ultrastructure and tomography of the C. parvum mitosome and crystalloid body, as well as the probable functions of these organelles, are the primary topics herein. An overview of iron-sulphur cluster biosynthesis, likely mechanisms for import into and export from the mitosome, as well as core carbohydrate and energy metabolism are also discussed. Similarities and differences in the structure and function of both organelles in the genus Cryptosporidium, with anaerobic protists in general, and with other apicomplexans specifically, are described.