Listeriolysin Pore Forming Ability in Planar Lipid Membranes at Different pH

Listeriolysin O (LLO) is a cholesterol-dependent cytolysin secreted by the intracellular pathogen Listeria monocytogenes. Its main task is to enable escape of bacteria from the phagosomal vacuole into the cytoplasm. LLO exhibits optimal cytolytic activity at low pH but it is still able to bind membranes at physiological or even slightly basic pH values in a cholesterol-dependent fashion. High cholesterol concentrations in the membrane restore the low activity of LLO at high pH values. Based on this broad pH activity we investigated the electrophysiological properties of pores formed by LLO at room temperature and at different pHs using Planar Lipid Bilayer technique. LLO is able to form pores both at pH 5.5 and 7.5 with a similar permeabilizing ability and similar heterogeneous conductances in the range of picosiemes to nanosiemens. Cholesterol content directly correlates with LLO activity but it does not change the pore characteristics. Collectively, our results demonstrate that LLO activity at physiological pH cannot be neglected and that its action at sites distal to cell entry may have important physiological consequences for Listeria pathogenesis.