3.A.6.1.2
Type III secretion system, SpaLMNOPQRS.  Required for surface presentation of invasion plasmid antigens. Required for invasion and for secretion of the three IPA proteins. A cryo-EM structure of the isolated Shigella T3SS needle complex has been pulished (Lunelli et al. 2020). The inner membrane (IM) region of the basal body adopts 24-fold rotational symmetry and forms a channel system that connects the bacterial periplasm with the export apparatus cage. The secretin oligomer adopts a heterogeneous architecture with 16- and 15-fold cyclic symmetry in the periplasmic N-terminal connector and C-terminal outer membrane ring, respectively. Two out of three IM subunits bind the secretin connector via a β-sheet augmentation. The cryo-EM map also reveals the helical architecture of the export apparatus core, the inner rod, the needle and their intervening interfaces (Lunelli et al. 2020).