Integrative structural analysis of the type III secretion system needle complex from Shigella flexneri

Area of Science:

• Microbiology
• Structural Biology
• Bacterial Pathogenesis

Background:

• The type III secretion system (T3SS) is a critical virulence factor for pathogenic Gram-negative bacteria.
• Understanding T3SS structure is key for developing targeted therapeutics.
• Limited structural data exists for T3SSs beyond Salmonella, hindering cross-species comparisons.

Purpose of the Study:

• To determine the high-resolution structure of the T3SS needle complex from Shigella flexneri.
• To identify conserved and unique structural features of the Shigella T3SS compared to other species.
• To provide insights for the development of novel anti-virulence strategies.

Main Methods:

• Cryo-electron microscopy (cryo-EM) for high-resolution imaging.
• Cross-linking mass spectrometry (XL-MS) to map protein-protein interactions.
• Integrative modeling to assemble the complete T3SS needle complex structure.

Main Results:

• The Shigella T3SS secretin pore complex displays a novel fold in its S domain.
• The pilotin MxiM[SctG] is localized to the outer surface of the secretin pore.
• The export apparatus includes the N-terminal domain of SpaS[SctU], a feature not seen in prior virulence-related T3SS structures.
• A conserved pseudohelical arrangement of the export apparatus is maintained, anchored by flexible linkers.

Conclusions:

• The Shigella T3SS possesses unique structural characteristics differentiating it from other known T3SSs.
• Conserved and unique features of the T3SS necessitate species-specific structural analysis.
• The determined structure offers a foundation for designing targeted therapeutics against Shigella infections.