Structure and assembly mechanism of the type III secretion system needle tip complex
The type III secretion system (T3SS) is a protein superstructure, consisting of hundreds of subunits, which self-assemble into a molecular syringe that injects virulence factors directly into the host cell. The tip complex caps an extracellular needle filament and is involved in penetrating the host cell membrane. As a surface-exposed antigen, it is also an attractive target for vaccine development. High resolution crystal structures of tip complex subunits have been determined from several species. However, their monomeric conformations appear to be incompatible with EM micrographs. While these also provide information on stoichiometry, the resolution of EM micrographs are at present far too low to determine the arrangement of subunits in an in-tact tip complex. This is in part because the tip complex can only be visualised as part of the entire T3SS superstructure. Here we combine, information from high-resolution crystal structures, solution X-ray scattering and molecular dynamics simulations to obtain insight into the assembly mechanism and structure of an in-tact T3SS tip complex. These structural models are then used to rationally design and construct protein constructs and synthetic structural scaffolds from DNA to artificially stabilise isolated T3SS tip complexes for structural characterisation.