Understanding the interaction mechanism of bacterial virulence factors and effector proteins with host lipid membrane using single particle cryo-electron microscopy

Abstract

Pathogenic bacteria employ an array of biological macromolecules and cellular structures that help in the initiation and progression of disease in host organism. These ‘virulence factors’ can either be secreted by the bacterium, or membrane- associated, or a component of the cytosol. Virulence factors are critical for host invasion, manifestation of the diseased condition and for evasion of host immune response. Prominent examples of bacterial virulence factors are motility factors such as flagella, adherence factors like pili, capsules that mask bacteria from phagocytosis, siderophores to enable iron uptake, virulence plasmids, endotoxins such as the lipopolysaccharides (LPS) in the cell wall of Gram-negative bacteria, exotoxins such as neurotoxins, pore-forming toxins, and host invasion factors such as effector proteins secreted by the membrane-bound secretion systems. For successful colonization and concomitant pathogenesis, virulence factors must first breach the protective barriers of the host – skin or the mucous membrane and gain entry within tissues or the bloodstream. Bacterial pathogens thus adopt various strategies to enable their uptake by modulating host membranes. The work presented in this thesis aims to obtain structural insights into how certain classes of membrane-binding virulence- associated proteins interact with host membrane lipids, with the help of single-particle cryo-electron microscopy. Overall, our study shows that EspB is a membrane-binding effector protein of the T7SS, which has affinity for anionic phospholipids such as PA, PS and PIPs, in host cell membrane. In summary, this thesis outlines the interaction of bacterial pore-forming toxin VCC and T7SS secreted substrate EspB, with lipids of the host membrane. Structural information on how virulence-associated proteins bind to the host membrane and their lipid specificity can help develop alternative therapeutic measures to combat resistance of pathogenic bacteria to existing treatment regimens