1.C.11 The Pore-forming RTX Toxin (RTX-toxin) Family
The RTX-toxin family is a large family of multidomain Gram-negative bacterial pore-forming exotoxins. They are secreted from the bacteria, and after processing, they insert into the membranes of animal cells. They exert both cell type- and species-specific effects (e.g., the leukotoxin of M. haemolytica interacts only with alveolar macrophages, neutrophils, and lymphocytes of ruminants and is believed to promote bacterial proliferation by killing or incapacitating these cells) (Davies et al., 2001). These toxins recognize protein receptors such as the β2-integrins, form pores at high concentrations, and cause cell rupture. Three transmembrane domains are believed to be involved in pore formation which in the E. coli HlyA protein are at residues 299-319, 361-381 and 383-403. However, at low, sublytic concentrations, leukotoxin causes activation of neutrophils, production of inflammatory cytokines, degranulation, generation of oxygen-derived free radicals, and morphologic changes consistent with apoptosis (Davies et al., 2002). Pore-forming mechanisms of various pore-forming toxins (PFTs) based on cryoEM structures have been reviewed (Mondal et al. 2022).
The C-terminal domain of the adenylate cyclase toxin (ACT or CyaA) of Bordetella pertussis forms a small cation-selective channel, disrupting the permeability barrier. This channel probably delivers the N-terminal adenylate cyclase to the host cell cytoplasm. Mutations in residues in an amphipathic α-helix (Glu509 and Glu516) in the pore-forming domain block adenylate cyclase translocation and modulate cation selectivity of the membrane channel (Osickova et al., 1999). ACT does not require a protein receptor and inserts into liposomes. Phosphatidylethanolamine and cholesterol stimulate ACT insertion. ACT also promotes lipid flip-flop suggesting that ACT forms trans-bilayer nonlamellar lipid structures when it inserts into the membrane (Martin et al., 2004). CyaA may form two different types of pore-like structures, dependent on the orientation of the membrane potential and the pH (Knapp et al., 2008).
Members of the RTX superfamily (RTX (1.C.11); CCT (1.C.57), and S-PFT (1.C.75)) contain repeat sequences that are also found in autotransporters (e.g., 1.B.12.10.1 and 1.B.40.1.2) as well as TolA (2.C.1.2.1). These domains probably mediate protein-protein interactions.
The generalized transport reaction proposed for members of the RTX-toxin family is:
Small molecules (in) small molecules (out)