Amoebapores, the pore-forming polypeptides of the protozoan parasite Entamoeba histolytica, and NK-lysin, an effector molecule of porcine NK (natural killer) and cytotoxic T cells, belong to the same protein family, the saposin-like proteins. As both types of protein are implicated in the killing of microbes in vivo, it appears that phylogenetically diverse organisms such as amoebae and mammals use similar effector molecules to fulfil a comparable task. However, structural features have led to the assumption that the proteins display their activities according to different modes of action. To address this question, we analysed the antibacterial, cytotoxic and pore-forming activities of these proteins in parallel and in comprehensive detail. Interestingly, the comparison of activities revealed significant differences. Whereas NK-lysin, recombinantly expressed, is efficient at a broad range of pH values, the amoebapores exhibited a pronounced pH dependence of all their activities, with markedly decreased activity at pH values above 6. Moreover, increasing salinity affects amoebapores more drastically than NK-lysin. All of the proteins compared were found to be potently active against Gram-positive bacteria, but only NK-lysin was equally efficient against Gram-negative bacteria. However, the amoebapores displayed five times higher pore-forming activity than NK-lysin, which is in accordance with the more hydrophobic character of the amoebapores compared with the essentially cationic NK-lysin.
Abbreviations used: BCECF AM, 2′,7′-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester; LB medium, Luria–Bertani medium; MALDI-TOF, matrix-assisted laser-desorption ionization–time-of-flight; MIC, minimal inhibitory concentration; NK cells, natural killer cells; rNK-lysin, recombinant NK-lysin; SAPLIP, saposin-like protein; TFA, trifluoroacetic acid.
Present address: Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London, U.K.