Structural and Functional Characterization of the Bacterial Type III Secretion Export Apparatus

Area of Science:

• Microbiology
• Molecular Biology
• Structural Biology

Background:

• Bacterial type III secretion systems (T3SS) are crucial for pathogen virulence and symbiont interactions.
• The injectisome, a complex nanomachine, facilitates effector protein translocation across host cell membranes.
• The precise in vivo assembly and function of export apparatus components within the injectisome remain incompletely understood.

Purpose of the Study:

• To elucidate the structural organization and functional role of export apparatus components in the Salmonella Typhimurium T3SS.
• To define the interactions between SpaP, SpaR, SpaQ, and SpaS, and their contribution to the injectisome structure.
• To clarify the connection between the export apparatus and the needle filament for substrate translocation.

Main Methods:

• Cryo-electron microscopy to visualize component interactions.
• Biochemical assays to map protein-protein interactions.
• Transmembrane topology modeling.

Main Results:

• A SpaP pentamer forms a 15 Å pore, crucial for translocation.
• Detailed interaction maps of SpaP with SpaQ, SpaR, and SpaS were generated.
• Transmembrane topology models for SpaP and SpaR were refined.
• Interactions between SpaP/SpaR periplasmic domains and PrgJ establish a link between the export apparatus and needle filament.

Conclusions:

• The study provides a refined model for export apparatus assembly and function within the injectisome.
• SpaP and SpaR play key roles in forming the translocation pore and connecting the secretion apparatus to the needle.
• This work clarifies the structural basis for substrate translocation through the bacterial type III secretion system.