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Adherence of Entamoeba histolytica to host cells is considered essential for pathogenic tissue invasion [1]. The main receptor that mediates adherence of E. histolytica to a number of different cells was identified to be a galactose (Gal) and N-acetylgalactosamine (GalNAc) inhibitable lectin located on the surface of the amoebae [2]. The lectin is a membrane-associated glycoprotein with a molecular mass of about 220 000. It has been claimed that the molecule is a heterodimer comprising a large subunit of 170 kDa that is disulfide-linked to one or more small subunits of 35 kDa [3]. The primary structure of the 170-kDa subunit has recently been deduced from cDNA analysis [4,5]. Sequence information for the 35-kDa subunit so far available is limited to a stretch of 17 N-terminal amino acid residues [3]. It has been reported that the molecule may function as a fibronectin receptor [6]. In addition, reactivity to antibodies specific for the cross-reactive determinant on the glycosylphospatidyl inosi

Entamoeba histolytica is susceptible to complement attack in its lumen-dwelling state and develops complement resistance during pathogenic tissue invasion. As experimental evidence suggests that this change in phenotype is accompanied by a change in gene expression, we constructed a subtractive cDNA library to identify genes involved. Poly(A) + RNA from complement-sensitive trophozoites was subtracted from single stranded cDNA derived from complement-resistant ones. Transcripts enriched in the library were found to code for a putative polypeptide comprising all sequence elements characteristic for serine/threonine protein kinases. The gene contains an intron of 46 nucleotides and two polyadenylation sites. Northern-blot analyses confirmed that the gene is expressed in both tissue-derived and laboratory-grown forms of complement-resistant E. histolytica.

Similar to the findings obtained with pathogenic Entamoeba histolytica , nonpathogenic isolates were found to kill mammalian cells in vitro, and cell extract caused pore formation in liposome membranes. A pore-forming peptide termed APnp was isolated from a nonpathogenic isolate using the schedule developed for the purification of APp or amoebapore, the homologous peptide of the pathogenic isolate HM-1:IMSS. Compared to APp, the specific activity of APnp in pore formation was 60% lower, cDNA sequencing indicated 95% identity of the primary structures of APnp and APp, and secondary structure predictions revealed a high degree of similarity. Notably, a glutamic acid residue at position 2 of APp is in APnp replaced by proline, which shortens one of the two amphipathic α-belices considered crucial for the pore-forming function. This structural divergence of the two peptides might explain the difference in their pore-forming activities.

Superoxide dismutase (SOD) activity was determined in the cell lysate of the axenically culturedEntamoeba histolytica isolate HM-1:IMSS. Under anaerobic culture conditions, 18.7 (± 4.9) units SOD activity (mg protein)−1 were found. By inhibition studies the activity was attributed to an iron-containing type of SOD (FeSOD). Using degenerate oligonucleotide primers derived from regions highly conserved in prokaryotic FeSOD sequences, a genomic DNA fragment was amplified by the polymerase chain reaction. The fragment was used to isolate FeSOD specific cDNA clones from a pathogenic and a nonpathogenicE. histolytica isolate. A comparison of the 2 sequences revealed 5% nucleotide differences resulting in a single amino acid exchange. The primary structure showed the characteristics of an iron-containing type of SOD with a homology of approximately 55% with other FeSOD sequences. The enzyme was found to be encoded by single copy genes in both the pathogenic and the nonpathogenicE. histolytica, but restriction fragment lengths differed between the 2 groups. In 5 isolates studied, no correlation was found between pathogenic behavior of the amebae and the expression of FeSOD-related mRNA.