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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 by mechanisms not well understood. 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).

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 use 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)

This family belongs to the: RTX-toxin Superfamily.

References associated with 1.C.11 family:

Balashova, N.V., J.A. Crosby, L. Al Ghofaily, and S.C. Kachlany. (2006). Leukotoxin confers β-hemolytic activity to Actinobacillus actinomycetemcomitans. Infect. Immun. 74: 2015-2021. 16552030
Benz, R., E. Maier, S. Bauer, and A. Ludwig. (2014). The deletion of several amino acid stretches of Escherichia coli α-hemolysin (HlyA) suggests that the channel-forming domain contains β-strands. PLoS One 9: e112248. 25463653
Bhakdi, S., H. Bayley, A. Valeva, I. Walev, B. Walker, U. Weller, M. Kehoe, and M. Palmer. (1996). Staphylococcal α-toxin, streptolysin-O and Escherichia coli hemolysin: prototypes of pore-forming bacterial cytolysins. Arch. Microbiol. 165: 73-79. 8593102
Braun, V. and T. Focareta. (1991). Pore-forming bacterial protein hemolysins (cytolysins). Crit. Rev. Microbiol. 18: 115-158. 1930675
Brown, A.C., N.V. Balashova, R.M. Epand, R.F. Epand, A. Bragin, S.C. Kachlany, M.J. Walters, Y. Du, K. Boesze-Battaglia, and E.T. Lally. (2013). Aggregatibacter actinomycetemcomitans leukotoxin utilizes a cholesterol recognition/amino acid consensus site for membrane association. J. Biol. Chem. 288: 23607-23621. 23792963
Davies, R.L., S. Campbell, and T.S. Whittam. (2002). Mosaic structure and molecular evolution of the leukotoxin operon (lktCABD) in Mannheimia (Pasteurella) haemolytica, Mannheimia glucosida, and Pasteurella trehalosi. J. Bacteriol. 184: 266-277. 11741868
Davies, R.L., T.S. Whittam, and R.K. Selander. (2001). Sequence diversity and molecular evolution of the leukotoxin (lktA) gene in bovine and ovine strains of Mannheimia (Pasteurella) haemolytica. J. Bacteriol. 183: 1394-1404. 11157953
Fagerberg, S.K., M.R. Jakobsen, M. Skals, and H.A. Praetorius. (2016). Inhibition of P2X receptors protects human monocytes against damage by leukotoxin from Aggregatibacter actinomycetemcomitans and α-hemolysin from Escherichia coli. Infect. Immun. [Epub: Ahead of Print] 27528275
Fiser, R. and I. Konopásek. (2009). Different modes of membrane permeabilization by two RTX toxins: HlyA from Escherichia coli and CyaA from Bordetella pertussis. Biochim. Biophys. Acta. 1788: 1249-1254. 19348784
Juntapremjit S., Thamwiriyasati N., Kurehong C., Prangkio P., Shank L., Powthongchin B. and Angsuthanasombat C. (2015). Functional importance of the Gly cluster in transmembrane helix 2 of the Bordetella pertussis CyaA-hemolysin: Implications for toxin oligomerization and pore formation. Toxicon. 106:14-9. 26363293
Knapp, O., E. Maier, J. Masín, P. Sebo, and R. Benz. (2008). Pore formation by the Bordetella adenylate cyclase toxin in lipid bilayer membranes: Role of voltage and pH. Biochim. Biophys. Acta. 1778(1): 260-269. 17976530
Kristensen BM., Frees D. and Bojesen AM. (2010). GtxA from Gallibacterium anatis, a cytolytic RTX-toxin with a novel domain organisation. Vet Res. 41(3):25. 19954731
Kurehong, C., C. Kanchanawarin, B. Powthongchin, P. Prangkio, G. Katzenmeier, and C. Angsuthanasombat. (2017). Functional Contributions of Positive Charges in the Pore-Lining Helix 3 of the Bordetella pertussis CyaA-Hemolysin to Hemolytic Activity and Ion-Channel Opening. Toxins (Basel) 9:. 28300777
Martín, C., M.-A. Requero, J. Masin, I. Konopasek, F.M. Goñi, P. Sebo, and H. Ostolaza. (2004). Membrane restructuring by Bordetella pertussis adenylate cyclase toxin, a member of the RTX toxin family. J. Bacteriol. 186: 3760-3765. 15175289
Osickova, A., R. Osicka, E. Maier, R. Benz, and P. Sebo. (1999). An amphipathic α-helix including glutamates 509 and 516 is crucial for membrane translocation of adenylate cyclase toxin and modulates formation and cation selectivity of its membrane channels. J. Biol. Chem. 274: 37644-37650. 10608820
Powthongchin, B. and C. Angsuthanasombat. (2009). Effects on haemolytic activity of single proline substitutions in the Bordetella pertussis CyaA pore-forming fragment. Arch. Microbiol. 191: 1-9. 18712361
Soloaga, A., M.P. Veiga, L.M. Garcia-Segura, H. Ostolaza, R. Brasseur, and F.M. Goñi. (1999). Insertion of Escherichia coli α-haemolysin in lipid bilayers as a non-transmembrane integral protein: prediction and experiment. Mol. Microbiol. 31: 1013-1024. 10096071
Svedova, M., J. Masin, R. Fiser, O. Cerny, J. Tomala, M. Freudenberg, L. Tuckova, M. Kovar, G. Dadaglio, I. Adkins, and P. Sebo. (2016). Pore-formation by adenylate cyclase toxoid activates dendritic cells to prime CD8+ and CD4+ T cells. Immunol Cell Biol 94: 322-333. 26437769
Wald, T., A. Osickova, J. Masin, P.M. Liskova, I. Petry-Podgorska, T. Matousek, P. Sebo, and R. Osicka. (2016). Transmembrane segments of complement receptor 3 do not participate in cytotoxic activities but determine receptor structure required for action of Bordetella adenylate cyclase toxin. Pathog Dis 74:. 26802078
Westrop, G., K. Hormozi, N. da Costa, R. Parton, and J. Coote. (1997). Structure-function studies of the adenylate cyclase toxin of Bordetella pertussis and the leukotoxin of Pasteurella haemolytica by heterologous C protein activation and construction of hybrid proteins. J. Bacteriol. 179: 871-879. 9006045
Wiles, T.J. and M.A. Mulvey. (2013). The RTX pore-forming toxin α-hemolysin of uropathogenic Escherichia coli: progress and perspectives. Future Microbiol 8: 73-84. 23252494