Characterization of rOrf8/EscI of the enteropathogenic Escherichia coli as an inner rod protein

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. This apparatus spans both bacterial membranes and transports virulence factors from the bacterial cytoplasm into eukaryotic host cells. While the inner rod and the needle structures are being assembled, translocators and effectors cannot be secreted. In this study, we characterized a small protein, designated rOrf8, that functions as the inner rod protein of enteropathogenic E. coli (EPEC). We demonstrated that rOrf8, is essential for type III secretion and is secreted by the T3SS, as previously reported for inner rod proteins of other T3SS. Furthermore, we showed that EscI self-associates and interacts with the outer membrane secretin, EscC, further supporting its function as an inner rod protein. Overall, our data suggest that EscI is the YscI/PrgJ/MxiI homolog in the T3SS of attaching and effacing pathogens.