Sphingomyelin−Cholesterol Synergy as a Lipid Chaperone Driving Cytolysin A Pore Formation

Bacterial pathogens rely on pore-forming toxins (PFTs) to breach host barriers, a feat that requires water-soluble monomers to spontaneously metamorphose into membrane-inserted pores. In the extracellular milieu, these toxins must navigate complex folding landscapes without ATP-dependent chaperones. Here, we show that the synergistic interplay of host sphingomyelin (SM) and cholesterol (CHOL) acts as an intrinsic “lipid chaperone” that drives the conformational maturation of the bacterial toxin Cytolysin A (ClyA). Using leakage assays on vesicles and suspended lipid bilayer (SuLBs) arrays, atomistic molecular dynamics (MD) simulations, cell membrane permeabilization assays and biophysical measurements, we demonstrate that SM-CHOL synergy accelerates the rate-limiting unfolding of the toxin’s membrane bound β-tongue motif into a reactive “molten globule” intermediate and guides its subsequent refolding into functional α-helical conformation crucial for pore formation. We identify conserved lysine residues (K175 and K206) as critical molecular sensors that detect this specific lipid signature to drive productive transformation. This assembly process reciprocally remodels the host membrane and dismantling liquid-ordered domains, suggesting a mechanical coupling between toxin folding and the disruption of host lipid homeostasis. Our findings establish a paradigm of lipid-mediated chaperoning, revealing how pathogens co-evolve to exploit host lipid complexity to overcome the energetic barriers of membrane insertion.