Visualization of the Assembly of a Two-Component Pore-Forming Toxin on a Target Membrane

Mushrooms from <i>Pleurotus</i> genus produce unique two-component pore-forming toxins consisting of a lipid-binding aegerolysin protein and a membrane-attack complex/perforin (MACPF) domain-containing protein, pleurotolysin B (PlyB). Here, we investigated the interaction between a well-characterized aegerolysin, ostreolysin A6 (OlyA6), and PlyB on membranes composed of ceramide phosphoethanolamine (CPE) and cholesterol (Chol). The binding of PlyB to OlyA6 induced its oligomerization and the formation of porous structures on the membrane. However, the efficiency of oligomerization depended strongly on the density of membrane-bound OlyA6. High OlyA6 density hindered the oligomerization process, resulting predominantly in arc-shaped oligomers rather than ring-shaped structures. This crowding also impaired the pore formation process. Our results suggest that the association of PlyB with OlyA6 occurs on the membrane surface, and the subsequent oligomerization and pore formation are highly regulated by the OlyA6/PlyB molar ratio, consistent with previous observations of inefficient pore formation at high OlyA6/PlyB molar ratios.