Pore-forming toxin

Fig 1. α-hemolysin from S.aureus. PDB entry 7ahl

Pore-forming toxins (PFTs) are protein exotoxins, typically (but not exclusively) produced by bacteria, such as C.septicum and S.aureus. They are frequently cytotoxic (i.e., they kill cells), as they create unregulated pores in the membrane of targeted cells.

Types of PFTs

PFTs can be divided into the following subcategories:

• Alpha-pore-forming toxins
  • e.g., Cytolysin A of E. coli.
• Beta-pore-forming toxins
  • e.g., α-hemolysin (Fig 1), PVL- Panton-Valentine leukocidin.

Above are the two main distinctions of PFTs. They differ in the suspected mode of membrane integration, either by alpha-helical or beta-sheet elements.^[1]

Other Categories:

• Binary toxins
  • e.g., Anthrax toxin
• Cholesterol-dependent cytolysins (CDCs)
  • e.g., Pneumolysin
• Small pore-forming toxins
  • e.g., Gramicidin A

Beta-pore-forming toxins

β-PFTs are so-named because of their structural characteristics: they are composed mostly of β-strand-based domains. Whilst they frequently have divergent sequences, many are classified by Pfam as Leukocidins. X-ray crystallographic structures have revealed some commonalities: α-hemolysin^[2] and Panton-Valentine leukocidin S^[3] are structurally related, as are aerolysin^[4] and Clostridial Epsilon-toxin.^[5]

Mode of action

Fig 2. Structural comparison of pore-form α-Hemolysin (pink/red) and soluble-form PVL (pale green/green). It is postulated that the green section in PVL 'flips out' to the 'red' conformation as seen in α-Haemolysin

β-PFTs are dimorphic proteins that exist as soluble monomers and then assemble to form multimeric assemblies that constitute the pore. Figure 1 shows the pore-form of α-Hemolysin, the first crystal structure of a β-PFT in its pore-form. 7 α-Hemolysin monomers come together to create the mushroom-shaped pore. The 'cap' of the mushroom sits on the surface of the cell, and the 'stalk' of the mushroom penetrates the cell membrane, rendering it permeable (see later). The 'stalk' is composed of a 14-strand β-barrel, with two strands donated from each monomer.

A structure of the Vibrio cholerae Cytolysin^[6] in the pore form is also heptameric, however Staphylococcus aureus gamma-hemolysin ^[7] reveals an octomeric pore, consequently with a 16-strand 'stalk'.

The Panton-Valentine Leucocidin S structure ^[8] shows a highly related structure, but in its soluble monomeric state. This shows that the strands involved in forming the 'stalk' are in a very different conformation - shown in Fig 2.

Assembly

The transition between soluble monomer and membrane-associated protomer to oligomer is not a trivial one: It is believed that β-PFTs, follow as similar assembly pathway as the CDCs (see Cholesterol-dependent cytolysins later), in that they must first assemble on the cell-surface (in a receptor-mediated fashion in some cases) in a pre-pore state. Following this, the large-scale conformational change occurs in which the membrane spanning section is formed and inserted into the membrane. The portion entering the membrane, referred to as the head, is usually apolar and hydrophobic, this produces an energetically favorable insertion of the Pore-forming toxin.^[1]

Specificity

Some β-PFTs such as clostridial ε-toxin and Clostridium perfringens Enterotoxin (CPE) bind to the cell membrane via specific receptors - possibly certain claudins for CPE,^[9] possibly GPI anchors or other sugars for ε-toxin - these receptors help raise the local concentration of the toxins, allowing oligomerisation and pore formation.

The Cyto-lethal effects of the pore

When the pore is formed, the tight regulation of what can and cannot enter/leave a cell is disrupted. Ions and small molecules, such as amino acids and nucleotides within the cell, flow out, and water from the surrounding tissue enters. The loss of important small molecules to the cell can disrupt protein synthesis and other crucial cellular reactions. The loss of ions, especially calcium, can cause cell signaling pathways to be spuriously activated or deactivated. The uncontrolled entry of water into a cell can cause the cell to swell up uncontrollably: this causes a process called blebbing, wherein large parts of the cell membrane are distorted and give way under the mounting internal pressure. In the end, this can cause the cell to burst.

Binary toxins

See the main article for more information on Anthrax toxins.

Binary toxins,^[10] such as Anthrax lethal & edema toxins, C.perfringens Iota toxin and C.difficile cyto-lethal toxins consist of two components (hence binary):

• an enzymatic component - A
• a membrane-altering component - B

The B component facilitates the entry of the enzymatic 'payload' into the target cell, by forming homooligomeric pores, as shown above for βPFTs. The A component then enters the cytosol and inhibits normal cell functions by one of the following means:

Mono-ADP-Ribosylation of G-actin

ADP-Ribosylation is a common enzymatic methods used by various bacterial toxins from various species. These toxins (including C.perfringens Iota toxin & C.Botulinum C2 toxin) attach a ribosyl-ADP moiety to surface Arginine residue 177 of G-actin. This prevents G-actin assembling to form F-actin, and, thus, the cytoskeleton breaks down, resulting in cell death.

Proteolysis of Mitogen-activated protein kinase kinases (MAPKK)

The A component of Anthrax toxin lethal toxin is zinc-metalloprotease, which shows specificity for a conserved family of Mitogen-activated protein kinase kinases. The loss of these proteins results in a breakdown of cell signaling, which, in turn, renders the cell insensitive to outside stimuli - therefore no immune response is triggered.

Increasing intracellular levels of cAMP

Anthrax toxin Edema toxin triggers a calcium ion influx into the target cell. This subsequently elevates intracellular cAMP levels. This can profoundly alter any sort of immune response, by inhibiting leucocyte proliferation, phagocytosis, and proinflammatory cytokine release.

Cholesterol-dependent cytolysins

CDCs, such as pneumolysin, from S.pneumoniae, form pores as large as 260Å (26 nm), containing between 30 and 44 monomer units.^[11] Electron Microscopy studies of Pneumolysin show that it assembles into large multimeric peripheral membrane complexes before undergoing a conformational change in which a group of α-helices in each monomer change into extended, amphipathic β-hairpins that span the membrane, in a manner reminiscent of α-haemolysin, albeit on a much larger scale (Fig 3). CDCs are homologous to the MACPF family of pore-forming toxins, and it is suggested that both families utilise a common mechanism (Fig 4).^[12] Eukaryote MACPF proteins function in immune defence and are found in proteins such as perforin and complement C9.^[13]

Fig 4: a) The structure of perfringolysin O ^[14] and b) the structure of PluMACPF ^[12]. In both proteins, the two small clusters of α-helices that unwind and pierce the membrane are in pink.

Purpose

Bacteria invest much time and energy in making these toxins: CPE can account for up to 15% of the dry mass of C.perfringens at the time of sporulation. The purpose of toxins is thought to be one of the following:

• Defense against phagocytosis, e.g., by a macrophage.^[15]
• Inside a host, provoking a response which is beneficial for the proliferation of the bacteria, for example in cholera.^[15]
• Food: After the target cell has ruptured and released its contents, the bacteria can scavenge the remains for nutrients.
• Environment: The mammalian immune response helps create the anaerobic environment that anaerobic bacteria require.

See also

• Exotoxin

References

Further reading

• F. Gisou van der Goot, Pore-forming toxins, Springer, 2001, ISBN 3-540-41386-3
• A deadly toxin with a romantic name: Panton-Valentine Leukocidin complex. PDBe Quips

External links

Wikimedia Commons has media related to Pore forming cytotoxic proteins.

• Pore Forming Cytotoxic Proteins at the US National Library of Medicine Medical Subject Headings (MeSH)