Aerolysin (β-hemolysin; cytolytic enterotoxin) precursor (Parker et al., 1994).  Upon transition  from the prepore to pore, the aerolysin heptamer shows a unique concerted swirling movement, accompanied by a vertical collapse of the complex, ultimately leading to the insertion of a transmembrane beta-barrel (Degiacomi et al. 2013).  Multiple conformational states lead to rotation of the core lysin to unleash the membrane spanning regions (Whisstock and Dunstone 2013).  Monomer activation, dependent on proteolysis, is the rate-limiting step for pore formation (Bischofberger et al. 2016). Cryo-electron microscopy structures of three conformational intermediates and the final aerolysin pore provide insight into the conformational changes that allow pore formation. The structures reveal a protein fold consisting of two concentric beta-barrels, tightly kept together by hydrophobic interactions. This fold suggests a basis for the prion-like ultrastability of aerolysin pore and its stoichiometry (Iacovache et al. 2016).

Gram-negative bacteria of the Aeromonas family

Aerolysin precursor of Aeromonas hydrophila

α-Toxin forms large ion permeable (slightly anion-selective) pores with no lipid specificity. It induces rapid cell necrosis in many cell types (Knapp et al., 2009). The structure of the membrane-spanning domain has been solved (Melton et al. 2004).

Gram-positive bacteria

α-toxin of Clostridium septicum (BAC54147)

A β-pore-forming cytolysin, Biomphalysin of 572 aas. It is involved in Biomphalaria glabrata immune defense against Schistosoma mansoni (Galinier et al. 2013).  Its binding properties have been studied (Wu et al. 2017).

Biomphalysin of Biomphalaria glabrata (Bloodfluke planorb) (Freshwater snail)

Hydralysin (Sher et al., 2005; Zhang et al., 2003).  Hydrolysins comprise a family of pore-forming proteins that are secreted into the gastrovascular cavity during feeding, probably helping in disintegration of the prey (Sher and Zlotkin 2009). Induces an immediate fast muscle contraction followed by flaccid paralysis when injected into blowfly larvae. The paralytic effect is lower in mice and fish. Has strong cytolytic activity against insect Sf9 cells and human HeLa cells. Binds to erythrocyte membranes and has weak hemolytic activity by mediating oligomerization and pore formation (Zhang et al. 2003; Sher et al. 2008).

Animals

Hydralysin of Hydra viridis (Q86LR2)

Spherulin 2A

Hemolytic lectin LSLc exhibits hemolytic and hemagglutinating activities. The structure at 2.6 Å resolution has been determined (Mancheño et al., 2005). The protein is hexameric. The monomer (35kDa) consists of two distinct modules: an N-terminal lectin module (a β-trefoil scaffold) and a pore-forming module (composed of domains 2 and 4) which resemble the β-pore-forming domains of aerolysin and ε-toxin (Mancheño et al., 2005).

Fungi

LSLc of Laetiporus sulphureus (BAC78490)

Parasporin-2 β-toxin (crystal structures are known) (Akiba et al., 2009; Akiba and Okumura 2016).

Bacteria

Paraspora-2 of Bacillus thuringiensis (Q7WZI1)

Mosquitocidal toxin [synthetic construct] of 304 aas; 84% identical to 1.C.4.4.4.  Cry46Ab from Bacillus thuringiensis TK-E6 is a mosquitocidal toxin with an aerolysin-type architecture (Hayakawa et al. 2020). Cry46Ab mutants were constructed by targeting the putative transmembrane beta-hairpin region showing that charged residues within the beta-hairpin control the flux of ions through channel pores and that channel-pore cation selectivity is correlated with insecticidal activity (Hayakawa et al. 2020).

Cry4Ab Toxin, synthetic construct

Fhb1 protein (PFT gene product) of 478 aas with two agglutinin domains followed by a DON (ETX/MTX2) domain that has the toxin activity (Rawat et al. 2016).  Counteracts Fusarium head blight (FHB), caused by Fusarium graminearum, a devastating disease of wheat and barley. The fungicidal mechanism of glabridin, an isoflavane, a type of isoflavonoid, against Fusarium graminearum, showed that it acts on ergosterol synthesis-related proteins to destroy the integrity of the cell membrane, resulting in abnormal transmembrane transport and an increased membrane potential (Yang et al. 2021).

Fhb1 of Triticum aestivum

Amaranthin agglutinin of 304 aas and 0 TMSs. The x-ray structure at 2.2 Å resolution of the homodimeric protein is available (1JLX) (Transue et al. 1997) Sequences containing amaranthin domains are widely distributed in plants (Dang et al. 2017).

Amaranthin agglutinin of Amaranthus caudatus (Love-lies-bleeding) (Inca-wheat)

Natterin-3 precursor (venom gland protein) of 364 aas and 1 N-terminal TMS possibly plus 1 - 3 semi-hydrophobic TMSs.  See family description for details about the Natterin family (Lima et al. 2021).

Animals

Natterin-3 precursor of Thalassophryne nattereri (AAU11824)

Natterin-like precursor of 315 aas from zebra fish, Dln1 or Aep1.  Aep1 is an innate immune molecule that prevents zebrafish from bacterial infections. Thus, Aep1 may be a pro-inflammatory protein that triggers the antimicrobial immune responses (Chen et al. 2018).

Animals

Natterin-like protein of Danio rerio

The Bin binary toxin, BinAB.  BinA is a toxic P42 protein (protein of 42 KDa) of 362 aas.  The 3-d structure of BinB (448 aas; 1.75 Å resolution) is available; it has two domains, an N-terminal sugar-binding lectin-like domain, and a C-terminal aerolysin-like β-barrel pore-forming domain. Although it shows low sequence identity with other members of the family, it is a member of the Aerolysin Family (Srisucharitpanit et al. 2014).  Protoxin subunits only form monomers, but in vitro activated toxin forms heterodimers. Maximal toxicity to mosquito larvae is achieved when the two subunits, BinA and BinB, are in a 1:1 molar ratio (Surya et al. 2016). An aromatic residue cluster in the C-terminal domain of BinB is critical for toxin insertion in membranes (Chooduang et al. 2018).

Firmicutes

BinAB of Lysinibacillus (Bacillus) sphaericus
BinA (P81935)
BinB (P10565)

Cry35 of 385 aas.  Shares a common strucure with ε-toxin, ETX (Moar et al. 2016).

Cry35 of Bacillus thuringiensis

Cellular endolysosome-modulating aerolysin-like pore-forming protein, ALP1, of 156 aas (Wang et al. 2020). The protein shows sequence similarity in its N-terminal half with family 1.C.73 members and in its C-terminal half with family 1.C.4 members. βγ-CAT is a complex of an ALP (BmALP1) and a trefoil factor (BmTFF3) in the firebelly toad (Bombina maxima). It is a secreted endogenous pore-forming protein that modulates the biochemical properties of endolysosomes by inducing pore formation. BmALP3, a paralog of BmALP1 that lacks membrane pore-forming capacity, like BmALP1, has a conserved cysteine in its C-terminal regions. BmALP3 is readily oxidized to a disulfide bond-linked homodimer, and this homodimer can oxidize BmALP1 via disulfide bond exchange, resulting in the dissociation of βγ-CAT subunits and elimination of its biological activity. BmALP3 senses environmental oxygen tension in vivo, leading to modulation of βγ-CAT activity. This C-terminal cysteine site is well conserved in numerous vertebrate ALPs, suggesting that it is a regulatory ALP (BmALP3) that modulates the activity on the active ALP (BmALP1) in a redox-dependent manner (Wang et al. 2020).

ALP of Bombina maxima (firebelly toad)

Epidermal differentiation-specific protein, EDP, of 335 aas.

EDP of Cynops pyrrhogaster (Japanese firebelly newt)

Epidermal differentiation-specific protein-like, EDP-L, of 341 aas.

EDP-L of Erpetoichthys calabaricus (reedfish)

Epidermal differentiation-specific protein, EDP, of 406 aas.

EDP of Bagarius yarrelli (goonch)