Substrate-engaged type III secretion system structures reveal gating mechanism for unfolded protein translocation

Many bacterial pathogens rely on virulent type III secretion systems (T3SSs) or injectisomes to translocate effector proteins in order to establish infection. The central component of the injectisome is the needle complex which assembles a continuous conduit crossing the bacterial envelope and the host cell membrane to mediate effector protein translocation. However, the molecular principles underlying type III secretion remain elusive. Here, we report a structure of an active Salmonella enterica serovar Typhimurium needle complex engaged with the effector protein SptP in two functional states, revealing the complete 800 angstrom-long secretion conduit and unraveling the critical role of the export apparatus (EA) subcomplex in type III secretion. Unfolded substrates enter the EA through a hydrophilic constriction formed by SpaQ proteins, which enables side chain-independent substrate transport. Above, a methionine gasket formed by SpaP proteins functions as a gate that dilates to accommodate substrates while preventing leaky pore formation. Following gate penetration, a moveable SpaR loop first folds up to then support substrate transport. Together, these findings establish the molecular basis for substrate translocation through T3SSs and improve our understanding of bacterial pathogenicity and motility. Virulent type III secretion systems (T3SSs) or injectisomes enable pathogenic bacteria to inject effector proteins directly into the host cell cytoplasm. Structures of a needle complex engaged with the effector protein reveal the complete secretion channel and provide insights into the mechanism of substrate translocation through T3SSs.

Automated summary

The study suggests that pathogenic bacteria can inject effector proteins directly into host cell cytoplasm through virulent type III secretion systems or injectisomes. Structures of a needle complex engaged with the effector protein reveal the complete secretion channel and provide insights into the mechanism of substrate translocation through T3SSs.