Bacterial pore-forming proteins induce non-monotonic dynamics due to lipid ejection and crowding

Abstract

Developing alternate strategies against pore forming toxin (PFT) mediated bacterial virulence factors require an understanding of the target cellular response to combat rising antimicrobial resistance. Membrane-bound protein complexes involving PFTs, released by virulent bacteria are known to form pores leading to host cell lysis. However, membrane disruption and related lipid mediated active repair processes during attack by PFTs remain largely unexplored. We report counter intuitive and non-monotonic variations in lipid diffusion, measured using confocal fluorescence correlation spectroscopy, due to interplay of lipid ejection and crowding by membrane bound oligomers of a prototypical cholesterol dependent cytolysin, Listeriolysin O (LLO). The observed protein concentration dependent dynamical cross-over is correlated with transitions of LLO oligomeric state populations from rings to arc-like pore complexes, predicted using a proposed two-state free area based diffusion model. At low PFT concentrations, a hitherto unexplored regime of increased lipid diffusivity is attributed to lipid ejection events due to a preponderance of ring-like pore states. At higher protein concentrations where membrane inserted arc-like pores dominate, lipid ejection is less efficient and the ensuing crowding results in a lowering of lipid diffusion. These variations in lipid dynamics are corroborated by macroscopic rheological response measurements of PFT bound vesicles. Our study correlates PFT oligomeric state transitions, membrane remodelling and mechanical property variations, providing unique insights into developing strategies to combat virulent bacterial pathogens responsible for several infectious diseases. SIGNIFICANCEDeveloping alternate strategies against pore forming toxin (PFT) mediated bacterial virulence factors requires understanding target cellular responses and cellular defence strategies to combat rising antimicrobial resistant strains. While it is well understood that PFTs exist in a wide variety of oligomeric states, the underlying membrane response to these states is unexplored. Using confocal fluorescence correlation spectroscopy and a membrane free area based model we relate non-monotonic variations in the lipid diffusivity arising from an interplay of lipid ejection events and membrane crowding due to variations in concentration of membrane bound listeriolysin O. Our observations have a direct bearing on understanding cellular defense and repair mechanisms effective during initial stages of bacterial infection and intrinsically connected to the underlying membrane fluidity.