Abstract
The flagellum and the injectisome enable bacterial locomotion and pathogenesis, respectively. These nanomachines assemble and function using a type III secretion system (T3SS). Exported proteins are delivered to the export apparatus by dedicated cytoplasmic chaperones for their transport through the membrane. The structural and mechanistic basis of this process is poorly understood. Here we report the structures of two ternary complexes among flagellar chaperones (FliT and FliS), protein substrates (the filament-capping FliD and flagellin FliC), and the export gate platform protein FlhA. The substrates do not interact directly with FlhA; however, they are required to induce a binding-competent conformation to the chaperone that exposes the recognition motif featuring a highly conserved sequence recognized by FlhA. The structural data reveal the recognition signal in a class of T3SS proteins and provide new insight into the assembly of key protein complexes at the export gate.
Highlights
The flagellum and the injectisome enable bacterial locomotion and pathogenesis, respectively
T3SSs share the same morphology and overall structure and can be functionally classified into two classes[7,8]: the flagellar T3SS, which promotes bacterial locomotion and motility enabled by the flagellum, and the pathogenic T3SS, which uses the injectisome to transport virulence proteins into human or animal host cells[9,10]
The export apparatus is formed by six integral membrane proteins that are highly conserved in the flagellar and the pathogenic T3SSs5,6
Summary
The flagellum and the injectisome enable bacterial locomotion and pathogenesis, respectively. T3SSs share the same morphology and overall structure and can be functionally classified into two classes[7,8]: the flagellar T3SS, which promotes bacterial locomotion and motility enabled by the flagellum, and the pathogenic (or non-flagellar) T3SS, which uses the injectisome to transport virulence proteins into human or animal host cells[9,10]. Both the flagellum[11,12,13] and the injectisome[5,6] are supramolecular complexes that are assembled by several different proteins. The findings reveal how the export gate recognizes cognate exported proteins and suggest mechanisms of operation of these protein complexes within the T3S nanomachinery
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