The Common Structural Architecture of Shigella flexneri and Salmonella typhimurium Type Three Secretion Needles
Figure 5
Structural model of the T3SS Shigella needle.
A, Top and B, side views of the T3SS needle assembly of Shigella flexneri. The homology model of the 29-subunit MxiH needle assembly generated by Rosetta modeling (cartoon representation) shows a correlation of 0.66 with the 7.7-Å cryoEM density map (grey surface). Using the nomenclature of Fujii et al., the EM density regions H1, H2 and H3 are indicated for the central subunit (i), as well as the kink in region H3. The first 11 N-terminal residues are not depicted due to the poor sequence homology of the rigid-extended segment (Fig. 2); however the protrusion in the EM map at subunit (i), labeled prot. and highlighted by a purple dashed contour trace, may be well explained by the N-terminus of subunit (i+5). The central subunit (i), colored as in Fig. 1B, forms a lateral interface with subunits (i±5), in red, and subunits (i±6), in dark blue. The axial interface is formed with subunits (i±11) shown in pink.