The type III secretion system (T3SS) is a macromolecular ‘injectisome’, that allows bacterial pathogens to transport virulence proteins into the eukaryotic host cell. This macromolecular complex is constituted by connected ring-like structures that span both bacterial membranes. The crystal structures of the periplasmic domain of the outer membrane (OM) secretin EscC and the inner membrane (IM) protein PrgH reveal the conservation of a modular fold among the three proteins which form the OM and IM rings of the T3SS. This leads to the hypothesis that this conserved fold provides a common ring-building motif that allows for the assembly of the variably sized OM and IM rings characteristic of the T3SS. Utilizing an integrated structural and experimental approach, ring-models for the periplasmic domain of EscC were generated and placed in the context of the assembled T3SS, providing evidence for direct interaction between the OM and IM ring components and an unprecedented span of the OM secretin.

The prgHIJK operon encodes components of the Salmonella typhimurium pathogenicity island 1 type III secretion system (TTSS). Previously, prgH and prgK were shown to be required for formation of the supramolecular type III secretion needle complex (