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Pore-forming toxin

Pore-forming toxin is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Pore-forming toxin rather than just read about it. In short: Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthworms, who produce lysenin. They are frequently cytotoxic (i.e., they kill cells), as they create unregulated pores in the membrane of targeted cells.

Pore-forming toxin — main illustration
Pore-forming toxin — illustration

Key takeaways

  • Pore-forming toxin belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Pore-forming toxin to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Pore-forming toxin from memory before moving on to harder problems.

Reference excerpt

Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthworms, who produce lysenin. They are frequently cytotoxic (i.e., they kill cells), as they create unregulated pores in the membrane of targeted cells.

Types PFTs can be divided into two categories, depending on the alpha-helical or beta-barrel architecture of their transmembrane channel that can consist either of

Alpha-pore-forming toxins e.g., Haemolysin E family, actinoporins, Corynebacterial porin B, Cytolysin A of E. coli. Beta-barrel pore-forming toxins e.g. α-Hemolysin (Fig 1), PVL – Panton-Valentine leukocidin, various insecticidal toxins. Other categories:

Large beta-barrel pore-forming toxins MACPF and Cholesterol-dependent cytolysins (CDCs), gasdermin Binary toxins e.g., Anthrax toxin, Pleurotolysin Small pore-forming toxins e.g., Gramicidin A According to TCDB, there are following families of pore-forming toxins:

1.C.3 α-Hemolysin (αHL) family: 1.C.4 Aerolysin family 1.C.5 ε-Toxin family 1.C.11 RTX-toxin superfamily 1.C.12 Membrane attack complex/perforin superfamily 1.C.13 Leukocidin family 1.C.14 Cytohemolysin (CHL) family 1.C.39 Thiol-activated cholesterol-dependent cytolysin family 1.C.43 Lysenin family 1.C.56 Pseudomonas syringae HrpZ cation channel family 1.C.57 Clostridial cytotoxin family 1.C.74 Snake cytotoxin (SCT) family 1.C.97 Pleurotolysin pore-forming family

Beta-pore-forming toxins

β-PFTs are so-named because of their structural characteristics: they are composed mostly of β-strand-based domains. They have divergent sequences, and are classified by Pfam into a number of families including Leukocidins, Etx-Mtx2, Toxin-10, and aegerolysin. X-ray crystallographic structures have revealed some commonalities: α-hemolysin and Panton-Valentine leukocidin S are structurally related. Similarly, aerolysin and clostridial epsilon-toxin. and Mtx2 are linked in the Etx/Mtx2 family. The ß-PFTs include a number of toxins of commercial interest for the control of pest insects. These toxins are potent but also highly specific to a limited range of target insects, making them safe biological control agents. Insecticidal members of the Etx/Mtx2 family include Mtx2 and Mtx3 from Lysinibacillus sphaericus that can control mosquito vectors of human diseases and also Cry15, Cry23, Cry33, Cry38, Cry45, Cry51, Cry60, Cry64 and Cry74 from Bacillus thuringiensis that control a range of insect pests that can cause great losses to agriculture. Insecticidal toxins in the Toxin_10 family show an overall similarity to the aerolysin and Etx/Mtx2 toxin structures but differ in two notable features. While all of these toxins feature a head domain and a larger, extended beta-sheet tail domain, in the Toxin_10 family, the head is formed exclusively from the N-terminal region of the primary amino acid sequence whereas regions from throughout the protein sequence contribute to the head domain in Etx/Mtx2 toxins. In addition, the head domains of the Toxin_10 proteins show lectin-like features of carbohydrate binding domains. The only reported natural targets of Toxin_10 proteins are insects. With the exception of Cry36 and Cry78, the Toxin_10 toxins appear to act as two-part, binary toxins. The partner proteins in these combinations may belong to different structural groups, depending on the individual toxin: two Toxin_10 proteins (BinA and BinB) act together in the Bin mosquitocidal toxin of Lysinibacillus sphaericus; the Toxin_10 Cry49 is co-dependent on the 3-domain toxin family member Cry48 for its activity against Culex mosquito larvae; and the Bacillus thuringiensis Toxin_10 protein Cry35 interacts with the aegerolysin family Cry34 to kill Western Corn Rootworm. This toxin pair has been included in insect resistant plants such as SmartStax corn.

Mode of action

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 PDB: 3O44​ in the pore form is also heptameric; however, Staphylococcus aureus gamma-hemolysin PDB: 3B07​ reveals an octomeric pore, consequently with a 16-strand 'stalk'. The Panton-Valentine leucocidin S structure PDB: 1T5R​ 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. While the Bin toxin of Lysinibacillus sphaericus is able to form pores in artificial membranes and mosquito cells in culture, it also causes a series of other cellular changes including the uptake of toxin in recycling endosomes and the production of large, autophagic vesicles and the ultimate cause of cell death may be apoptotic. Similar effects on cell biology are also seen with other Toxin_10 activities but the roles of these events in toxicity remain to be established.

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.

… excerpt ends here. Continue reading the full article.

Illustrations

Pore-forming toxin: α-Hemolysin from S.aureus (PDB: 7AHL​)
α-Hemolysin from S.aureus (PDB: 7AHL​)
Pore-forming toxin: 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. (PDB: 7AHL, 1T5R​)
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. (PDB: 7AHL, 1T5R​)
Pore-forming toxin: EM reconstruction of a pneumolysin pre-pore
EM reconstruction of a pneumolysin pre-pore
Pore-forming toxin: a) The structure of perfringolysin O[31] and b) the structure of PluMACPF.[32] In both proteins, the two small clusters of α-helices that unwind and pierce the membrane are in pink. (PDB: 1PFO, 2QP2​)
a) The structure of perfringolysin O[31] and b) the structure of PluMACPF.[32] In both proteins, the two small clusters of α-helices that unwind and pierce the membrane are in pink. (PDB: 1PFO, 2QP2​)

Worked examples

Example 1 — a first encounter with Pore-forming toxin

Start with the simplest possible case. Write down what Pore-forming toxin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Pore-forming toxin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Pore-forming toxin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Pore-forming toxin

In research
Pore-forming toxin appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Pore-forming toxin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Pore-forming toxin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Oligomers, Peripheral membrane proteins, Protein toxins, so understanding it makes those chapters shorter.
In everyday life
Look for Pore-forming toxin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Pore-forming toxin in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Pore-forming toxin means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Pore-forming toxin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Pore-forming toxin in simple terms?

Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthworms, who produce lysenin. They are frequently cytotoxic…

Why does Pore-forming toxin matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Pore-forming toxin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Pore-forming toxin.

Tags

  • Oligomers
  • Peripheral membrane proteins
  • Protein toxins

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