Cryo-EM structure of the Shigella type III needle complex
Fig 5
Structure of the connector and its interface with the IM ring.
(A) Tilted top view of the connector composed of the N-terminal N0 and N1 domains of MxiD (green) and the C-terminal domain of MxiG (blue) in cartoon representation. The ring is formed by 16 subunits of each protein. (B) Close-up of two neighboring couples of MxiD and MxiG subunits. The MxiG subunits interact with the N-terminus of MxiD via β-sheet augmentation, forming a continuous circular β-sheet at the connector base. (C) N0 and N1 domains of MxiD between the C-terminal domain two MxiG subunits (left panel); residues represented as sticks are involved in intermolecular hydrophobic interactions, the position of the amino acid 347 in MxiG is indicated in red. Detailed view of the MxiG-MxiD hydrophobic interface colored according to the Eisenberg scale (white being the most hydrophobic) (right panel); black-labeled residues are at the interface and belong to MxiD (Y38, A40, I71, F75, L77 and A36, which is not visible in this view), MxiG residues are labeled in blue (L350, Y356, M358, W364 and F366). (D) Representative images of immunofluorescence stained Shigella M90T ΔMxiG complemented with truncated Strep-MxiG comprising residues 1 to 346 (MxiG-347-Stop). Green, lipophilic membrane stain; red, anti-Strep-tag antibody. strep-mxiH and strep-mxiG genes are expressed from a plasmid in non-permeabilized bacteria while permeabilized bacteria expressed only strep-mxiG. No needle complex is visible on the surface of the bacteria, while the MxiG mutant is localized in the bacterial membrane. The scale bar corresponds to 2 μm.