Type III secretion systems and bacterial flagella: insights into their function from structural similarities

Abstract

Type III secretion systems and bacterial flagella are broadly compared at the level of their genetic structure, morphology, regulation, and function, integrating structural information, to provide an overview of how they might function at a molecular level.

Figure 1

Diagrams of known positions of major flagellar components (A) and established and hypothetical TTSS functional homologs (B). Functions of proteins conserved in both systems are marked by similar position, shading, and coloring whether they share sequence homologies or not. Those TTSS components for which no sequence homology or experimental evidence exists to establish their relation to the similarly positioned and colored flagellar components in A are shown transparently in B.

Figure 2

Model for switching from posttranslational to cotranslational secretion. (A and B) Brief TTSS activation. (C and D) Contact-independent secretion by the Shigella TTSS.

Figure 3

Docking of a Spa47 ATPase F1-like model (green) to the outline of the Shigella NC (purple). Spa47 was aligned to the sequences of α- and β- subunits of the F1-ATPase. Six different Spa47 monomer 3D models were generated and assembled by using whatif and the F1-coordinates (70). An outline of our NC reconstruction (10) was made by using spider and empirically scaled to the ATPase model by using the molecular dimensions of the complex measured in electron microscopy images (±15% accuracy). The ATPase was docked to the NC outline with ATP-binding sites facing the bacterial cytoplasm by using whatif.

Figure 4

Model of inserted putative translocation pore after activation of the S. flexneri TTS apparatus.