Supermolecular structure of the enteropathogenic Escherichia coli type IIIsecretion system and its direct interaction with the EspA-sheath-like structure

Enteropathogenic Escherichia coli (EPEC) secretes several Esp proteins via the type III secretion system (secreton), EspA, EspB, and EspD are required for translocation of the effector proteins into host cells, in which EspB and EspD are thought to form a pore in the host membrane. Recent study has shown that EspA forms a filamentous structure that assembles as a physical bridge between bacteria and host cell surfaces, which then functions as a c onduit for the translocation of bacterial effectors into host cells. To inv estigate the supermolecular structure of the type III secreton in EPEC, we partially purified it from the bacteria membrane and observed it via transm ission electron microscopy. The EPEC type III secreton was composed of a ba sal body and a needle part and was similar to those of Salmonella and Shige lla, except for a sheath-like structure at the tip of the needle. The lengt h of sheath-like structures varied; it extended more than 600 nm and was 10 times longer than the Shigella needle part. The putative major needle comp onent, EscF, was required for both secretion of Esp proteins and needle com plex formation. Interestingly, elongation of the sheath-like structure was observed under constitutive expression of EspA but not of EscF. Furthermore , the transmission electron microscopy view with immunogold labeled anti-Es pA antibodies clearly showed that EspA is a component of the sheath-like st ructure. This study revealed, to our knowledge for the first time, the supe rmolecular structure of the EPEC type III secreton and its direct associati on with the EspA-sheath-like structure.