Clostridium septicum is a gram-positive, spore-forming anaerobe that causes

a variety of disease syndromes in humans and animals [1, 2]. This organism produces several extracellular factors, including deoxyribonuclease, hyaluronidase, neuraminidase and alpha-toxin [3]. Alpha-toxin, the major virulent factor, has hemolytic, lethal and necrotizing activities [4, 5]. Alpha-toxin is a pore-forming toxin that belongs to the same family as aerolysin from Aeromonas hydrophila [6] and epsilon-toxin from Clostridium perfringens [7]. Alpha-toxin is secreted by the organism as an inactivated 46 kDa protoxin [8, 9]. The protoxin binds to GPI-anchored proteins on cell surfaces with high affinity [10, 11]. The protoxin is then cleaved to its 43 kDa active form by host cell proteases, such as furin [5, 8]. Activated toxin Fulvestrant concentration monomers interact with each other on DRMs to form oligomeric prepore complexes [12, 13]. The prepore complexes ultimately insert into the plasma membrane, generating pores that are approximately 1.3–1.6 nm in diameter [4, 8]. Alpha-toxin and aerolysin show structural and functional similarities, at the level of 72%, with 27% identity [6, 9]. Although GPI-anchored Akt inhibitor proteins also act as receptors for aerolysin, each toxin binds to different subsets of GPI-anchored proteins [10]. Furthermore, the most striking difference between

alpha-toxin and aerolysin is that D1 of aerolysin is missing from the amino terminus in alpha-toxin, implying that alpha-toxin is a single-lobed structure consisting of three domains, D1, D2, and D3, which are homologous to D2, D3, and D4 of aerolysin, respectively (Fig. 1) [6, 14, 15]. The functional domains and

amino acids of alpha-toxin involved in receptor binding, oligomerization and pore formation have been identified by Melton et al. [14, 16]. Binding of alpha-toxin to GPI-anchored proteins is restricted to D1 [16]. Because cholesterol is essential Anidulafungin (LY303366) to binding and stability on the cellular membrane for many kinds of pore-forming toxins from gram-positive bacteria, these toxins have been named CDCs. CDCs that bind specifically to membrane cholesterol, such as perfringolysin O [17], streptolysin O [18], pneumolysin [19] and listeriolysin O [20], have a region of 11 highly conserved amino acid residues, ECTGLAWEWWR (tryptophan-rich motif) that is located in the C-terminal region of each toxin. In perfringolysin O, three tryptophan residues in the tryptophan-rich motif have an important role in binding to the cell membrane [21]. Although C. septicum alpha-toxin does not bind to cholesterol but to GPI-anchored proteins on the cell surface, alpha-toxin also has a tryptophan-rich region lying within an 11 amino acid sequence in D1 near the C terminus (NGYSEWDWKWV; residues 302–312; Fig. 1) [6]. In a previous study, Melton-Witt et al.