Continuous exchange of an inner-membrane ring component is required for assembly and function of the type III secretion system

Major bacterial pathogens manipulate eukaryotic target cells by injecting effector proteins through type III secretion systems (T3SS). Recent in situ observations revealed that these large molecular machines, often referred to as injectisomes, are remarkably dynamic and adaptive entities, with the cytosolic T3SS components forming a mobile network that recruits effectors to the export machinery. In contrast to these soluble components, the transmembrane rings anchoring the injectisome are stably associated - with one exception. Using functional assays, live cell microscopy, and photobleaching experiments, we found that SctD, which constitutes the inner membrane ring of the T3SS, exchanges subunits in secreting injectisomes in Yersinia enterocolitica. To elucidate the biological significance of this unexpected dynamic behavior of a key structural component, we investigated its role in the assembly and function of the T3SS. Using engineered SctD variants whose exchange rate can be modulated, we found that exchange supports the integration of export apparatus components into assembled membrane rings and efficient secretion of effectors. Our findings uncover a new aspect of the molecular function and regulation of the T3SS, which may apply to other secretion systems and molecular machines.