Oocyst wall formation and composition in coccidian parasites

摘要

The oocyst wall of coccidian parasites is a robust structure that is resistant to a variety of environmental andchemical insults. This resilience allows oocysts to survive for long periods, facilitating transmission from host tohost. The wall is bilayered and is formed by the sequential release of the contents of two specialized organelles - wallforming body 1 and wall forming body 2 - found in the macrogametocyte stage of Coccidia. The oocyst wall is over90% protein but few of these proteins have been studied. One group is cysteine-rich and may be presumed to crosslinkvia disulphide bridges, though this is yet to be investigated. Another group of wall proteins is rich in tyrosine.These proteins, which range in size from 8-31 kDa, are derived from larger precursors of 56 and 82 kDa found inthe wall forming bodies. Proteases may catalyze processing of the precursors into tyrosine-rich peptides, which arethen oxidatively crosslinked in a reaction catalyzed by peroxidases. In support of this hypothesis, the oocyst wall hashigh levels of dityrosine bonds. These dityrosine crosslinked proteins may provide a structural matrix for assemblyof the oocyst wall and contribute to its resilience.