Leukotoxin, HlaA or LktA of 955 aas and 2 TMSs. Cytolysin LktA is one of the major pathogenicity factors of Mannheimia haemolytica (formerly Pasteurella haemolytica) that is the cause of pasteurellosis, also known as shipping fever pneumonia, causing substantial loss of sheep and cattle during transport. LktA belongs to the family of RTX-toxins (Repeats in ToXins) that are produced as pathogenicity factors by a variety of Gram-negative bacteria. Sublytic concentrations of LktA cause inflammatory responses of ovine leukocytes while higher concentrations result in formation of transmembrane channels in target cells that may cause cell lysis and apoptosis. Channel formation by LktA occurs in artificial lipid bilayer membranes made of different lipids. LktA channels had a single-channel conductance of about 60 pS in 0.1 M KCl, which is about one tenth of the conductance of most RTX-toxins with the exception of the adenylate cyclase toxin of Bordetella pertussis (Benz et al. 2019). The LktA channels are highly cation-selective, and the channel diameter is around 1.5 nm.

Gram-negative bacteria

HlaA of Mannheimia (Pasteurella) haemolytica

Haemolysin A, HlyA (α-haemolysin) (Wiles and Mulvey 2013). The channel-forming domain may contain β-strands, possibly in addition to alpha-helical structures (Benz et al. 2014).  Although homologous, HlyA and CyaA (1.C.11.1.4) exhibit different modes of permeabilization (Fiser and Konopásek 2009). HlyA triggered an increase in mitochondrial Ca²⁺ levels and manipulated mitochondrial dynamics by causing fragmentation of the mitochondrial network. Alterations in mitochondrial dynamics resulted in severe impairment of mitochondrial functions by loss of membrane potential, increase in reactive oxygen species production, and ATP depletion. HlyA also caused disruption of plasma membrane integrity (Lu et al. 2018).

Proteobacteria

HlyA of E. coli

Bifunctional adenylate cyclase-haemolysin toxin precursor, CyaA.  Although homologous, HlyA (1.C.11.1.3) and CyaA  exhibit different modes of permeabilization (Fiser and Konopásek 2009).  A pore model comprising three alpha2-loop-alpha3 hairpins suggested that Gly530XXGly533XXXGly537  in TMS2 could function in toxin oligomerization (Juntapremjit et al. 2015).  Structural integrity of TMSs 1, 2, 3 and 5, but not 4, is important for haemolytic activity, particularly for transmembrane helices 2 and 3 that might form the pore (Powthongchin and Angsuthanasombat 2009). CyaA forms small cation-selective membrane pores that permeabilize cells for potassium efflux, contributing to cytotoxicity of CyaA and eventually provoking colloid-osmotic cell lysis (Wald et al. 2016).  The toxin penetrates myeloid phagocytes expressing the complement receptor 3 and delivers into the cytosol its N-terminal adenylate cyclase enzyme domain (~400 residues). In parallel, the ~1300 residue-long RTX hemolysin moiety of CyaA permeabilizes target cell membranes for efflux of cytosolic potassium ions (Svedova et al. 2016).  Positively-charged side-chains substituted at positions Gln574 and Glu581 in the pore-lining alpha3 enhance hemolytic activity and ion-channel opening, mimicing the highly-active RTX (repeat-in-toxin) cytolysins (Kurehong et al. 2017). Residues 529 to 549 participate in membrane penetration and pore-forming activity (Roderova et al. 2019). Two distinct conformers of CyaA appear to accomplish its two parallel activities within target cell membranes. The translocating conformer would deliver the N-terminal adenylyl cyclase domain into the cytosol of cells, while the pore precursor conformer would assemble into oligomeric cation-selective pores and permeabilize cellular membrane. Both toxin activities involve a membrane-interacting 'AC-to-Hly-linking segment' (residues 400 to 500). Two clusters of negatively charged residues within this linking segment (Glu419 to Glu432 and Asp445 to Glu448) regulate the balance between the AC domain translocating and pore-forming capacities of CyaA as a function of the calcium concentration (Sukova et al. 2020). Four cholesterol-recognition motifs in the pore-forming and translocation domains of CyaA are essential for invasion of eukaryotic cells and lysis of erythrocytes (Amuategi et al. 2022).

Proteobacteria

CyaA of Bordetella pertussis

Cytolytic RTX-toxin, GtxA (causes salpingitis and peritonitis in birds (Kristensen et al., 2009)

Gram-negative bacterium

GtxA of Gallibacterium anatis

Enterohemolysin EhxA of 998 aas

Proteobacteria

EhxA of E. coli

Leukotoxin A, LtxA pore-forming toxin of 1055 aas, exhibiting β-hemolytic activity.  Plays a role in immune evasion by lysing human lymphocytes and monocytes. It binds to the LFA-1 integrin on the surface of the host cell and to cholesterol-containing membranes, resulting in large LtxA-LFA-1 clusters in lipid rafts (Balashova et al. 2006; Brown et al. 2013).  Blocking P2X receptors protects monocytes from LtxA (Fagerberg et al. 2016).

LtxA of Aggregatibacter (Actinobacillus) actinomycetemcomitans (Haemophilus actinomycetemcomitans)

Leukotoxin, RtxA or IktA, of 956 aas and 3 or 4 TMSs in a 1 + 1 + 2 TMS arrangement. An interaction between the toxin and cholesterol occurs via two cholesterol recognition/interaction amino acid consensus motifs located in the C-terminal portion of the pore-forming domain of the toxin, and the cytotoxic activity of RtxA depends on post-translational acylation of the K558 and/or K689 residues as well as on the toxin binding to cholesterol in the membrane (Osickova et al. 2018).

RtxA if Kingella kingae

FrpC of 1492 aas and 1 or 2 TMSs, one N-terminal and one at about residue 270. 

FrpC of Vibrio anguillarum