DOC Enhancement of Shigella flexneri Invasion Linked to IpaD Helix Discontinuity

Shigellosis (bacillary dysentery) results from infection by Shigella spp and represents one of the leading bacterial causes of diarrheal disease worldwide. With an extremely low infectious dose (10-100 organisms) and the emergence of antibiotic resistant strains, the development of anti-infective and therapeutic treatments are of great importance. Study of the biochemical mechanisms of Shigella infection may lead towards desperately needed therapeutic targets. The Type III Secretion System (T3SS) is the primary virulence factor of Shigella flexneri, allowing for direct secretion of effector proteins into the host cytoplasm. This conduit is formed by the Type III Secretion Apparatus (T3SA) a molecular structure resembling a “needle and syringe”, comprised of a basal body, external needle and tip complex. The tip complex undergoes a maturation process which is sensitive to environmental triggers. The nascent tip contains a pentamer of Invasion plasmid antigen D (IpaD) that upon exposure to bile salts (e.g. deoxycholate (DOC)) undergoes conformational changes and promotes recruitment of IpaB to the tip complex. Previously conformational changes in IpaD have been identified most notably near a ᴨ-bulge in a central helix. Directed mutagenesis was used to mutate residues near the ᴨ-bulge to alanine, in an attempt to stabilize the helix and mitigate the DOC induced conformational changes. The mutants all exhibit wild type levels of DOC binding, regulation of secretion, lysis of red blood cells and secondary structure content. However, the IpaD mutants drastically altered the invasive capabilities of S. flexneri strains. Fluorescence polarization studies of the IpaD-IpaB binding interaction determined that for the IpaD mutants the affinity was altered such that binding to IpaB is observed in the absence of DOC. These data give us insight into a key step of S. flexneri infection and may lead to new therapeutic target