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

Proteases play an important role in the pathogenic mechanisms and different iation events of protozoan parasites; the proteasome/ubiquitin system is es sential for maintaining the differentiation state of many cell types. A sin gle input of the specific inhibitor of proteasomes, lactacystin, prevented encystation of the protozoan parasite Entameoba invadens, whereas a cystein e protease inhibitor, E64, only delayed encystation. The ameba target of la ctacystin 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 t ransfer of amoebae to encystation induction medium, and reappeared at the s ame time during encystation as other encystation-specific transcripts. Thes e results demonstrate that proteasome function is required during the conve rsion of the disease-causing trophozoite into the infectious cyst stage of Entamoeba parasites, and that ubiquitin transcript levels undergo an unusua l decrease during the early stages of this differentiation process.