Structure of a bacterial type III secretion system in contact with a host membrane in situ

Many bacterial pathogens of animals and plants use a conserved type III secretion system (T3SS) to inject virulence effector proteins directly into eukaryotic cells to subvert host functions. Contact with host membranes is critical for T3SS activation, yet little is known about T3SS architecture in this state or the conformational changes that drive effector translocation. Here we use cryo-electron tomography and sub-tomogram averaging to derive the intact structure of the primordial Chlamydia trachomatis T3SS in the presence and absence of host membrane contact. Comparison of the averaged structures demonstrates a marked compaction of the basal body (4 nm) occurs when the needle tip contacts the host cell membrane. This compaction is coupled to a stabilization of the cytosolic sorting platform– ATPase. Our findings reveal the first structure of a bacterial T3SS from a major human pathogen engaged with a eukaryotic host, and reveal striking ‘pump-action’ conformational changes that underpin effector injection

Structural studies of T4S systems by electron microscopy

Abstract: Type IV secretion (T4S) systems are large dynamic nanomachines that transport DNA and/or proteins through the membranes of bacteria. Analysis of T4S system architecture is an extremely challenging task taking into account their multi protein organisation and lack of overall global symmetry. Nonetheless the last decade demonstrated an amazing progress achieved by X-ray crystallography and cryo-electron microscopy. In this review we present a structural analysis of this dynamic complex based on recent advances in biochemical, biophysical and structural studies

Structure of a pathogenic type 3 secretion system in action

Type 3 secretion systems use 3.5-megadalton syringe-like, membrane-embedded 'injectisomes', each containing an ~800-Å-long needle complex to connect intracellular compartments of infectious bacteria and hosts. Here we identify requirements for substrate association with, transport through and exit from the injectisome of Salmonella enterica serovar Typhimurium. This guided the design of substrates that become trapped within the secretion path and enabled visualization of injectisomes in action in situ. We used cryo-EM to define the secretion path, providing a structural explanation as to why effector proteins must be unfolded during transport. Furthermore, trapping of a heterologous substrate in the needle prevents secretion of natural bacterial effectors. Together, the data reveal the path of protein secretion across multiple membranes and show that mechanisms rejecting unacceptable substrates can be undermined, and transport of bacterial effectors across an already assembled type 3 secretion system can be inhibited