Transmembrane domains of type III-secreted proteins affect bacterial-host interactions.

The type III secretion system (T3SS) is a major bacterial virulence factor of numerous deadly Gram-negative pathogens. It extends from the bacteria and specializes in translocating effector proteins from the bacterial cytoplasm directly into the host cells. Enteropathogenic E. coli (EPEC) T3SS contacts the host using two proteins, EspB and EspD, which hetero-oligomerize to form a translocon pore within the host membrane to allow translocation of effectors into the host cytoplasm.

Both EspB and EspD are transmembrane domain (TMD)-containing secreted proteins, therefore they must have a special mechanism to avoid bacterial membrane targeting and alternatively be successfully secreted and integrated into the host cell membrane. We investigated whether the TMDs of EspB and EspD have a role in escaping bacterial integration, and whether these TMDs are involved in the proteins activity within the host cell membrane. Using TMD exchanged mutated versions of EspB and EspD, we found that the TMDs of these proteins encode important information needed for proper protein secretion. In addition, we found that the TMD of EspB is also important for its function within the host cell membrane and for effector translocation into host cell cytoplasm.