1.C.4 The Aerolysin Channel-forming Toxin (Aerolysin) Family
The aerolysins are a closely related group of channel-forming toxins secreted by members of the family Aeromonas, important human and animal pathogens. They are activated by host and bacterial proteases which remove a C-terminal fragment of about 40 amino acyl residues. The activated monomeric toxin then binds to a receptor glycosyl phosphatidylinositol (GPI)-anchored protein on the surface of the target cell. Because GPI anchored proteins are incorporated into the envelope membrane of human immunodeficiency virus type I (HIV-1), aerolysin can neutralize the virus in a process that depends on channel formation. The dual chaperone role of the C-terminal propeptide of aerolysin participates in folding and oligomerization of the pore-forming toxin (Iacovache et al., 2011). Monomer activation is possibly the rate-limiting step for the entire pore formation process, probably through release of a propeptide (Bischofberger et al. 2016). A loop that lines the aerolysin channel has an alternating pattern of charged and uncharged residues, suggesting that this transmembrane region has a beta-barrel configuration, as observed for Staphylococcal alpha-toxin. The turn of the beta-hairpin is composed of a stretch of five hydrophobic residues which drives membrane insertion of the developing channel. Possibly once the lipid bilayer has been crossed, it folds back parallel to the plane of the membrane in a rivet-like fashion (Iacovache et al. 2006).
Aerolysin-like pore-forming proteins are characterized by a domain organization and mechanism of action that involves extensive conformational rearrangements. The structures of the membrane integraed pores, well-defined beta-barrels, and their mechanism of assembly are fairly well understood (Podobnik et al. 2017). The cell surface-binding domains present high variability within the family to provide membrane receptor specificity (Cirauqui et al. 2017). However, the novel concentric double β-barrel structure found in aerolysin is highly conserved in terms of sequence, structure and conformational dynamics, which likely contribute to preserve a common transition mechanism from the prepore to the mature pore within the family. The key role of several amino acids in the conformational changes needed for oligomerization and further pore formation, include Y221, W227, P248, Q263 and L277, which may be involved in the release of the stem loop and the two adjacent β-strands to form the transmembrane β-barrel (Cirauqui et al. 2017).
Membrane binding of the monomeric toxin promotes oligomerization to a stable heptamer (as is known for the homologous α-hemolysin (αHL) family (TC #1.C.3)). Heptamerization converts the protein from a soluble form to a membrane insertion-competent form, and the oligomer penetrates the membrane producing channels that destroy the permeability barrier of the membrane, thereby killing the cell. The membrane-associated channel-forming protein may comprise a β-barrel. The three-dimensional structure of the soluble form of aerolysin from the Gram-negative bacterium, Aeromonas hydrophila, has been determined by x-ray crystallography (2.8 Å resolution) (Parker et al., 1994, 1996). The closely related aerolysins are distantly related to many other toxins including the α-toxin of the Gram-positive bacterium, Clostridium septicum, enterolobin, a cytolysin of the plant, Enterolobium contortisiliquum, the ε-toxin of Clostridium perfringens (1.C.5.1.1), and the α-hemolysin of Staphylococcus aureus (1.C.3.1.1). Members of the aerolysin family are therefore found in both bacteria and eukaryotes.
Hydralysins (1.C.4.2.1) are β pore-forming toxins in cnidaria, venomous animals such as Hydra vulgaris, and Chlorohydra viridissima (Sher et al., 2005). The soluble monomers are rich in β-structure and bind to erythrocyte membranes to form pores with an inner diameter of about 1.2 nm (Sher et al., 2005). Cytolysis is cell type-specific, suggesting the involvement of specific receptors. These toxins share some motif similarity around the pore-forming domains of the toxins. They induce immediate fast muscle contraction followed by flaccid paralysis when injected into blowfly larvae (Zhang et al., 2003). They have strong hemolytic activity against certain insect cells. Other toxins, including the pore-forming actinoporins, but not hydralysins, are stored in sting cells called nematocytes.
The binary toxin (Bin), produced by Lysinibacillus (Bacillus) sphaericus, is composed of BinA (42 kDa) and BinB (51 kDa) proteins, which are both required for full toxicity against Culex and Anopheles mosquito larvae. Specificity of Bin toxin is determined by the binding of BinB to a receptor present on the midgut epithelial membranes, while BinA is proposed to be a toxic component. Srisucharitpanit et al. 2014 determined the crystal structure of the active form of BinB at a resolution of 1.75 A. It possesses two distinct structural domains in its N- and C-termini. The globular N-terminal domain has a beta-trefoil scaffold which is a highly conserved architecture of some sugar binding lectins, suggesting a role of this domain in receptor-binding. The BinB beta-rich C-terminal domain shares similar three-dimensional folding with aerolysin type beta-pore forming toxins, despite a low sequence identity. The BinB structure, therefore, is a new member of the aerolysin-like toxin family, with probably similarities in the cytolytic mechanism that takes place via pore formation.
The generalized transport reaction catalyzed by members of the aerolysin family is:
Small molecules (in) small molecules (out)