Pore-forming toxins (PFTs) belong to a class of proteins expressed by bacteria to initiate infections by unregulated pore formation on the plasma membrane of host cells. Although cholesterol is a key sterol motif that promotes toxin activity, the influence of oxysterols, upregulated in senescent cells or in other inflammatory disorders, on lytic activity has not received much attention. Using all-atom molecular dynamics simulations, we study the changes to the sterol binding landscape of membrane-inserted cytolysin A (ClyA), an \(\alpha\) -PFT expressed by E. coli, in the presence of tail-oxidized 25-hydroxycholesterol (25-HC) in a palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC):cholesterol:25-HC (70:20:10) membrane. 25-HC was found to entirely replace previously identified cholesterol binding hotspots [PNAS,115 7323-7330] between the membrane-inserted \(\beta\) -tongue motifs with binding lifetimes on the order of microseconds. Although the overall sterol occupancy is lower for the N-terminal helix motif that forms the lining of the water channel, 25-HC binding is less when compared with cholesterol. The presence of the additional OH group on the 25th carbon enhances interactions with polar residues of the \(\beta\) -tongue, increasing 25-HC binding times by several fold when compared with cholesterol. We discuss the implications of this enhanced oxysterol interaction on pore formation of the \(\alpha\) family of toxins such as ClyA, in contrast with the cholesterol-dependent cytolysins, where oxysterols have been shown to be detrimental to pore formation.
