FIG. 6.

Architecture of the S. flexneri Mxi-Spa T3SS. The S. flexneri Mxi-Spa T3SS consists of four main parts. The seven-ringed basal body spans the bacterial IM, the periplasm, and the OM. The hollow needle is attached to a socket and protrudes from the basal body to the bacterial surface. Contact with host cell membranes (HM) triggers the IpaD-guided membrane insertion of the IpaB-IpaC translocon at the needle tip. The T3SS is completed by the cytoplasmic C ring, which is comprised of proteins that energize the transport process and mediate the recognition of substrates, chaperone release, and substrate unfolding.