Subcellular distribution of amebapain, the major cysteine proteinase of Entamoeba histolytica.

Amebapain, the major cysteine proteinase of E. histolytica, appears to be important both for digestion and as a cytotoxic factor. By immunocytology we established that amebapain was bound to matrix-like structures both on the cell surface and within subcellular vesicles; the amebapain co-localized with pinocytized iron oxide, suggesting that the enzyme recycles between plasma membrane and pinocytic vesicles. We conclude from these data that (i) the pinocytic vesicles in E. histolytica perform functions that in higher eukaryotes are taken over by specialized organelles such as lysosomes and cytotoxic granules, and (ii) the vesicles contain a matrix that helps retain the protease upon exocytosis of vesicle contents.