Reciprocal Interactions between Membrane Bilayers and Staphylococcus aureus PSMα3 Cross-α Amyloid-like Fibrils Provide Mechanistic Insight into Species-specific Cytotoxicity

Phenol-soluble modulin α3 (PSMα3) is a functional amyloid secreted by the pathogenic bacterium Staphylococcus aureus. This 22-residue peptide serves as a key virulence determinant, toxic to human cells via the formation of unique cross-α amyloid-like fibrils. We demonstrate that bilayer vesicles accelerated PSMα3 fibril formation, and the fibrils, in turn, inserted deeply into bilayers mimicking mammalian cell membranes, accounting for PSMα3 cellular toxicity. Importantly, a mere amphipathic helical conformation was not a sufficient determinant for membrane-activity of PSMα3. In contrast to its insertion into mammalian membrane bilayers, PSMα3, which lacks antibacterial activity, only interacted with the surface of bilayers mimicking bacterial membranes. Interestingly, bacterial membrane mimetics promoted aggregation of an otherwise non-fibrillating, single-residue mutant F3A-PSMα3, which exhibits antibacterial activity. F3A-PSMα3 nevertheless shows a less pronounced and altered pattern of interactions with membrane mimetics compared to PSMα3. Together, our data provide mechanistic insight into species-specific toxicity of a key bacterial amyloid virulence factor via reciprocal interactions with membranes, and open new perspectives into amyloid-related cytotoxicity mediated by helical fibril structures.