Amoebapore A is a 77-residue protein from the protozoan parasite and human pathogen Entamoeba histolytica. Amoebapores lyse both bacteria and eukaryotic cells by pore formation and play a pivotal role in the destruction of host tissues during amoebiasis, one of the most life-threatening parasitic diseases. Amoebapore A belongs to the superfamily of saposin-like proteins that are characterized by a conserved disulfide bond pattern and a fold consisting of five helices. Membrane-permeabilizing effector molecules of mammalian lymphocytes such as porcine NK-lysin and the human granulysin share these structural attributes. Several mechanisms have been proposed to explain how saposin-like proteins form membrane pores. All mechanisms indicate that the surface charge distribution of these proteins is the basis of their membrane binding capacity and pore formation. Here, we have solved the structure of amoebapore A by NMR spectroscopy. We demonstrate that the specific activation step of amoebapore A depends on a pH-dependent dimerization event and is modulated by a surface-exposed histidine residue. Thus, histidine-mediated dimerization is the molecular switch for pore formation and reveals a novel activation mechanism of pore-forming toxins.The protozoan parasite, Entamoeba histolytica, inhabits the colon of infected humans. It is the causative agent of human amoebiasis that often leads to tissue damage, colitis, and extraintestinal abscesses (1). Amoebiasis is the second leading cause of death from parasitic diseases worldwide (2). About 50 million people suffer from invasive amoebiasis, of whom up to 100,000 die annually (3).In the amoebic trophozoite, several factors have been identified that are involved in pathogenesis. In addition to a galactose-/N-acetylgalactosamine-specific lectin on the amoebic surface that mediates adhesion to colonic mucus and host cells (4) and secreted cysteine proteinases that disintegrate tissues by cleaving extracellular matrix proteins (5), a family of membrane-active polypeptides have been discovered (6). These polypeptides exist as three isoforms and are named amoebapore A, B, and C, respectively. They are capable of lysing a broad spectrum of target cells, including human host cells and bacteria. It has been recently shown that trophozoites of E. histolytica lacking amoebapore A, due to transcriptional silencing of the encoding gene, became avirulent (7), demonstrating that this protein is a key pathogenicity factor of the parasite. All three amoebapore isoforms have been isolated and biochemically characterized, and their primary structure has been elucidated by molecular cloning of the genes of their precursors (8 -10). The mature proteins consist of 77 amino acid residues each and are localized within cytoplasmic granules. The overall sequence identity between the three amoebapores is between 35 and 57% (10). Despite the substantial sequence divergence, they possess a characteristic disulfide bond pattern and a single conserved C-terminal histidine residue. The secondary structur...

Background: SH3 domains are small protein modules of 60-85 amino acids that bind to short proline-rich sequences with moderate-to-low affinity and specificity. Interactions with SH3 domains play a crucial role in regulation of many cellular processes (some are related to cancer and AIDS) and have thus been interesting targets in drug design. The decapeptide APSYSPPPPP (p41) binds with relatively high affinity to the SH3 domain of the Abl tyrosine kinase (Abl-SH3), while it has a 100 times lower affinity for the α-spectrin SH3 domain (Spc-SH3).

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