Primary and secondary structure of the pore‐forming peptide of pathogenic Entamoeba histolytica.

A pore‐forming peptide is implicated in the potent cytolytic activity of pathogenic Entamoeba histolytica. Using NH2‐terminal sequence information of this peptide, the corresponding cDNA was isolated. The cDNA‐deduced amino acid sequence revealed a putative signal peptide and a mature peptide of 77 amino acids including six cysteine residues. Computer‐aided secondary structure analysis predicted that the peptide would be composed of four adjacent alpha‐helices, and CD spectroscopy indicated an all alpha‐helical conformation. The tertiary structure appears to be stabilized by three disulfide bonds; the pore‐forming activity was not sensitive to heat but was lost in the presence of reducing agents. Sequence homology was found to the saposins and to surfactant‐associated protein B, both mammalian polypeptides of similar size and secondary structure but of non‐lytic function. In particular, the six cysteine residues were found to be conserved, suggesting a common motif for stabilizing a favourable tertiary structure. Compared with previously characterized toxic peptides also containing three disulfide bonds, the amoeba peptide may represent a distinct class of biologically active peptides.