The occurrence of a tripeptidyl peptidase I in trophozoites of Entamoeba histolytica.

Abstract

Tripeptidyl aminopeptidases are exopeptidases involved in intracellular protein degradation downstream of endopeptidases [for review, see (1)]. Mammalian tripeptide-releasing enzymes identified to date have been designated as tripeptidyl peptidases I (TPP I), which are localized in lysosomes, and tripeptidyl peptidases II (TPP II), which are cytoplasmatic. Additionally, evolutionarily unrelated TPPs have been identified in bacteria (1). Tripeptidases II and homologues belong to the subtilisin family of serine proteases and, like the proteasome, seem to have an ubiquitous distribution in eukaryotes, suggesting that they cooperate with the proteasome and with dipeptidases in degrading substrate proteins the entire way down into amino acids. Lysosomal TPP I, which further splits peptides generated by cathepsins, may also be a serine-type peptidase. Monomers of the enzyme, which generally are glycosylated, have a molecular mass between 48 kDa (human) and 107 kDa ( Dictyostelium ), and can form aggregates exhibiting molecular masses from 200–700 kDa (2). In addition to its role in lysosomal protein degradation, tripeptidase I has been suggested to be involved in more specific functions, such as generating signaling peptides or proteins for developmental processes (2). Entamoeba histolytica contains vesicular endopeptidases, the genes of which have been well characterized and have been shown to be cysteine proteinases (EhCP1, EhCP2, EhCP5, histolysain, and amebapain). Their activities contribute to lysosomal protein digestion and, extracellularly, to the attack of host structural proteins (3). The cytoplasm contains proteasomes that probably function in the endopeptidolytic cleavage of substrate proteins (4). By contrast, nothing is known about exopeptidases acting downstream of either cathepsins or of the proteasome in this parasite. Here, we report first data on a lysosomal/extracellular tripeptidyl peptidase I, with activity against AAF-X or AFP-X (X being a peptidolytically released fluorophor), in E. histolytica.