The type III secretion system (T3SS) is a multi-protein complex that plays a central role in the virulence of many Gram-negative bacterial pathogens, including species of Salmonella, Shigella, Yersinia, E. coli and Pseudomonas. This apparatus spans both the bacterial membranes and the host cell plasma membrane to transport virulence factors from the bacterial cytoplasm into eukaryotic host cells. These virulence factors target and manipulate key intracellular pathways to promote bacterial replication and transmission. The type III secretion apparatus (T3SA) secretes three distinct sets of substrates: early, middle and late. The early and the middle substrates are proteins that eventually assemble as part of the T3SA while the late substrates are virulence factors that are injected into the host cells. While the first switch, between early and middle substrates, is well characterized, the mechanism that controls the second switch, between middle and late substrates, is unknown. In our study, we found that two central protein-regulators of the E. coli T3SS, EscP and SepL, can interact under specific environmental conditions. Moreover, we found that these conditions affect the secretion profile of the T3SS. Overall, our data provide a model to describe how intimate association of a pathogen with its host cell alters the local conditions surrounding the bacteria, which affects protein interactions and promotes the second substrate switch of the T3SS.
