Proteasome-dependent cyst formation and stage-specific ubiquitin mRNA accumulation in Entamoeba invadens.

Proteases play an important role in the pathogenic mechanisms and differentiation events of protozoan parasites; the proteasome/ubiquitin system is essential for maintaining the differentiation state of many cell types. A single input of the specific inhibitor of proteasomes, lactacystin, prevented encystation of the protozoan parasite Entameoba invadens, whereas a cysteine protease inhibitor, E64, only delayed encystation. The ameba target of lactacystin was purified and it displayed the features typical of eukaryotic 20S proteasome complexes. In addition, transcripts encoding ubiquitin were detectable in trophozoites stage cells, disappeared immediately following transfer of amoebae to encystation induction medium, and reappeared at the same time during encystation as other encystation-specific transcripts. These results demonstrate that proteasome function is required during the conversion of the disease-causing trophozoite into the infectious cyst stage of Entamoeba parasites, and that ubiquitin transcript levels undergo an unusual decrease during the early stages of this differentiation process.