THE PORE-FORMING PEPTIDE OF ENTAMOEBA-HISTOLYTICA, THE PROTOZOAN PARASITE CAUSING HUMAN AMEBIASIS

Amoebapore, the pore-forming peptide of E histolytica has been isolate d and its structure elucidated on the cDNA and protein level. The pept ide is composed of 77 amino acid residues including six cysteine resid ues and has a molecular mass of 8244 Da. The primary translation produ ct contains a signal sequence of 21 mostly hydrophobic amino acid resi dues. The active peptide has been located in the cytoplasmic granules of the amoebae. Circular dichroism spectroscopy revealed an all alpha- helical conformation and computer-aided secondary structure prediction yielded a structure of four helices. The helical conformation and thr ee intramolecular disulfide bonds impart a highly compact and rigid st ructure upon the molecule. The activity of amoebapore, measured by a l iposome depolarization assay, is resistant to heating at 100-degrees-C in the absence of reducing agents. Synthetic peptides corresponding t o the helices 1 and 3 exhibited pore-forming activity. Two minor, biol ogically active isoforms of amoebapore have amino acid sequence identi ty of 57% and 47%, respectively. Whereas amoebapore is a constituent o f pathogenic E histolytica isolates, nonpathogenic E histolytica produ ce a structurally very similar peptide, the specific activity of which is approximately one third that of amoebapore. The biological signifi cance of amoebapore for the pathogenicity of E histolytica and specifi cally for its cytolytic activity remains to be determined.