Structural insights into the mechanisms of membrane binding and oligomerization of a bacterial pore-forming toxin

Abstract

Perfringolysin O (PFO), a cytolytic toxin from by the pathogenic bacterium Clostridium perfringens, perforates mammalian cell membranes by forming large aqueous pores. Secreted as water-soluble monomers, the toxin molecules bind to cholesterol-containing membranes, form large, circular oligomeric complexes on the membrane surface and then insert into the bilayer to create pores with diameters near 300 à . Using multiple independent fluorescence techniques as primary tools, the mechanisms of PFO membrane binding and oligomerization have been identified. Domain 4 (D4) of the protein interacts first with the membrane and is responsible for cholesterol recognition. Remarkably, only the short hydrophobic loops at the tip of the D4 β-sandwich are exposed to the bilayer interior, while the remainder of D4 projects from the membrane surface. Thus, a very limited interaction of D4 with the bilayer core appears to be sufficient to accomplish cholesterol recognition and initial PFO binding to the membrane. Upon PFO membrane binding, a structural element in domain 3 (D3) of the molecule moves to expose the edge of a previously-hidden β-strand that forms the monomer-monomer interface. The β-strands that form the interface each contain a single aromatic residue, and these aromatics appear to stack to align the transmembrane β-hairpins of adjacent monomers in the proper register for insertion. Membrane-dependent structural rearrangements are thus required to initiate and regulate PFO oligomerization. Fluorescence resonance energy transfer measurements reveal that the elongated toxin monomer arrives at the membrane in an ‘end-on’ orientation, with its long axis oriented nearly perpendicular to the plane of the membrane bilayer. This orientation is largely retained even after monomer association to form a prepore complex. In particular, the D3 polypeptide segments that form the transmembrane β-hairpins remain far above the membrane surface both at the membrane-bound monomer and prepore stages of pore formation. However, upon pore formation, the height of the oligomeric complex above the membrane surface is significantly reduced. The major topographical changes that occur during the prepore-to-pore transition of the PFO oligomer, therefore appears to result primarily from a collapsing of the extended domain 2 (D2) conformation in the monomer.