Chapter 13 : Membrane-Damaging Toxins:Pore Formation

Roles of PFTs. A variety of nonmutually exclusive roles for PFTs have been proposed. Certain PFTs punch holes in the plasma membrane of the target cell with the possible purpose of releasing nutrients or killing the host cell (step 1 example: aerolysin). Others might serve at the tip of a type III secretion system to perforate the host cell plasma membrane and allow injection of bacterial effectors into the host cytoplasm (step 2 example: streptolysin O). In the case of invading bacteria, production of PFT within the phagocytic vacuole allows rupture of the vacuolar membrane and release of the bacterium in the host cytoplasm (step 3 example: listeriolysin O). Once the invading bacterium has sufficiently multiplied, egress from the host cell can require the secretion of a PFT (step 4 example: IcmS).

General mode of action of PFTs. All β-barrel PFTs have a similar overall mode of action. They are secreted as soluble proteins that bind, often with great specificity, to the host cell membranes. There, upon encounter with other toxin molecules, they undergo circular polymerization into a ring-like structure called the prepore; a subsequent conformational change leads to the exposure of hydrophobic surfaces. The complex then inserts into the membrane and forms a pore. Pore formation at the plasma membrane can have a number of consequences that might be dependent on the toxin, the toxin concentration, and the cell type.

Structure of transmembrane proteins. (A) Soluble proteins, although they might have a hydrophobic interior (depicted in grayblack), only expose hydrophilic surfaces. (B) Transmembrane proteins have a surface of mixed hydrophobicity. Indeed, regions of the protein surface (depicted in gray) must interact with the acyl chains of the lipids while other regions of the surface must interact with the intracellular or lumenal aqueous environment (depicted in white). (C) Photosystem I, an example of a transmembrane α-helical protein complex (from reference 2 with permission). (D) Side views of bacterial outer membrane proteins OmpA, OmpF, and FhuA (from left to right) (from reference 4 with permission).

Pore formation by staphylococcal PFTs. A spaced filled model of the structure of leucocidin F illustrates the structure of monomeric α- hemolysin. The heptameric pore-forming complex is shown in a top (B) and a side (A) view in a ribbon diagram. One monomer is, however, shown as a spaced filled model to illustrate the conformational changes associated with the transition from water-soluble to transmembrane. Two regions of the protein undergo major and apparently concomitant conformational changes. The amino-latch (black) is initially folded against the core of the soluble monomer. In the heptamer it has folded out and interacts with the following monomer in the heptameric complex. Similarly, the stem domain (light gray) is initially folded onto the core of the protein to hide hydrophobic residues. In the heptamer this region has completely unfolded away from the rest of the protein into a curved β-hairpin, forming with the same region in neighboring monomers a closed circular β-sheet (from reference 3 with permission).

Pore formation by CDTs. (A) Interaction of cholesterol-dependent toxins with the lipid bilayer. CDT can be divided into four domains. Domain 4 interacts first with the membrane. This interaction triggers a conformational change in the molecule, resulting in membrane contact with domain 3. As shown in (B), unfolding of domain 3 occurs with a concomitant α-helix-to-β-sheet transition that leads to the insertion of the membrane. The inset shows a schematic top view of the channel. Monomers associate in a circular membrane. What remains a mystery is the fate of the lipids inside the ring-like structure. (B) Conformational changes in domain 3 of perfringolysin O that accompany pore formation. As shown on the left, domain 3 is composed by a four-stranded β-sheet that terminates with two bundles of three helices. It is proposed that, upon membrane insertion, each of these two bundles unfolds to prolong the β-sheet of domain 3 as illustrated on the right. This extension of the β-sheet would penetrate the membrane (from reference 3 with permission).