ABSTRACTThe encystation ofAcanthamoebaleads to the formation of resilient cysts from vegetative trophozoites. This process is essential for parasite survival under unfavorable conditions, such as those associated with starvation, low temperatures, and biocides. Furthermore, cysteine proteases have been implicated in the massive turnover of intracellular components required for encystation. Thus, strict modulation of the activities of cysteine proteases is required to protectAcanthamoebafrom intracellular damage. However, mechanisms underlying the control of protease activity during encystation have not been established inAcanthamoeba. In the present study, we identified and characterizedAcanthamoebacysteine protease inhibitor (AcStefin), which was found to be highly expressed during encystation and to be associated with lysosomes by fluorescence microscopy. Recombinant AcStefin inhibited various cysteine proteases, including human cathepsin B, human cathepsin L, and papain. Transfection with small interfering RNA against AcStefin increased cysteine protease activity during encystation and resulted in incomplete cyst formation, reduced excystation efficiency, and a significant reduction in cytoplasmic area. Taken together, these results indicate that AcStefin is involved in the modulation of cysteine proteases and that it plays an essential role during the encystation ofAcanthamoeba.
Human cathepsin X: A novel cysteine protease of the papain family with a very short proregion and unique insertions
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