Cholesterol-dependent cytolysin
[1] Through multiple conformational changes, the β-barrel transmembrane structure (~250 Å in diameter depending on the toxin) is formed and inserted into the target cell membrane.
This is mainly reflected in the conserved core of about 471 amino acids shared by all CDCs, which essentially corresponds to the sequence of pneumolysin, the shortest member of the family.
[5] The extreme case is lectinolysin (LLY; UniProt: B3UZR3) from some strains of Streptococcus mitis and S. pseudopneumoniae contain a functional fucose-binding lectin at the amino terminus.
The core section of amino acids, which are required for pore formation, are more conserved between CDCs, which is illustrated by similar three-dimensional structures [9] and pore-forming mechanisms.
[10] Single amino acid modifications in these loops prevented perfringolysin O (PFO; TC# 1.C.12.1.1), which is a CDC secreted by Clostridium perfringens from binding to cholesterol rich liposomes.
[11] More recently, Farrand et al. has shown that two amino acids, a threonine-leucine pair in loop L1, comprise the cholesterol binding motif and is conserved in all known CDCs.
The mechanism of pore formation of perfringolysin O (PFO; TC# 1.C.12.1.1), which is secreted by Clostridium perfringens, begins with encountering and binding to cholesterol on the target membrane.
The binding of D4 to the membrane surface triggers a conformational change in domain 3, which rotates β5 away from β4, exposing β4 allowing it to interact with the β1 strand of another PFO molecule, initiating oligomerization.
[11] Pore formation begins once two amphipathic transmembrane β-hairpins from ~35 PFO monomers are inserted in a concerted fashion,[20] which then create a large β-barrel that perforates the membrane.
In the water-soluble monomeric form of CDC, the transmembrane β-hairpins that are located on both sides of the central β-sheet on domain 3 are each folded as three short α-helices to minimize the exposure of hydrophobic residues.
A concerted mechanism of insertion is required so that the hydrophilic surfaces of the β-hairpins remain exposed to the aqueous medium, and not the hydrophobic membrane core.
Since the orientation of the hydroxyl group has such an effect on the bind/pore-formation of CDC, the equatorial conformation may be required for docking of the sterol to the binding pocket in domain 4, or to be properly exposed at the surface of lipid structures.
[12] It has since been shown that the conserved undecapeptide is a key element in the allosteric pathway that couples membrane binding to the initiation of structural changes in domain 3 of the CDC monomer that allows it to begin the process of oligomerization into the prepore complex.
[26] Lipids having a conical molecular shape alter the energetic state of membrane cholesterol, augmenting the interaction of the sterol with the cholesterol-specific cytolysin.
