Assembly and architecture of the type III secretion sorting platform

SUMMARY Type III secretion systems are bacterial nanomachines specialized in protein delivery into target eukaryotic cells. The structural and functional complexity of these machines demand highly coordinated mechanisms for their assembly and operation. The sorting platform is a critical component of type III secretion machines that ensures the timely engagement and secretion of proteins destined to travel this export pathway. However, the mechanisms that lead to the assembly of this multi-component structure have not been elucidated. Herein, employing structure modeling and an extensive in vivo crosslinking strategy, we provide a detailed inter-subunit-contact survey of the entire sorting platform complex. Using the identified crosslinks as signatures for pairwise inter-subunit interactions in combination with systematic genetic deletions, we mapped the assembly process of this unique bacterial structure. Insights generated by this study could serve as the bases for the development of anti-virulence strategies to combat several medically important bacterial pathogens.