Structures of chaperone-substrate complexes docked onto the export gate in a type III secretion system

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• The flagellum and injectisome are bacterial nanomachines crucial for motility and pathogenesis.
• These systems utilize a type III secretion system (T3SS) for assembly and function.
• The mechanism of protein export via T3SS, involving cytoplasmic chaperones and an export apparatus, remains poorly understood.

Purpose of the Study:

• To elucidate the structural and mechanistic basis of protein transport through the T3SS.
• To investigate the interactions between flagellar chaperones, protein substrates, and the T3SS export gate component FlhA.

Main Methods:

• Determined the structures of two ternary complexes involving flagellar chaperones (FliT, FliS), protein substrates (FliD, FliC), and the export gate protein FlhA.
• Utilized structural biology techniques to analyze protein-protein interactions and conformational changes.

Main Results:

• Protein substrates do not directly bind FlhA.
• Substrates induce a binding-competent conformation in chaperones.
• This chaperone conformation exposes a conserved recognition motif for FlhA.
• Identified the recognition signal for a class of T3SS proteins.

Conclusions:

• The study reveals the mechanism by which T3SS chaperones present protein substrates to the export gate.
• Provides structural insights into the recognition signal and assembly of protein complexes at the T3SS export gate.
• Advances understanding of bacterial nanomachine assembly and function.