Three-dimensional model of Salmonella's needle complex at subnanometer resolution.

Area of Science:

• Microbiology
• Structural Biology
• Bacterial Pathogenesis

Background:

• Type III secretion systems (T3SSs) are critical virulence factors in Gram-negative bacteria.
• T3SSs facilitate the injection of effector proteins into host cells, enabling bacterial invasion.
• The needle complex (NC) is the core structural component of the T3SS.

Purpose of the Study:

• To determine the high-resolution structure of the Salmonella typhimurium needle complex (NC).
• To elucidate the symmetry and organization of the NC for insights into its assembly and function.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to image top views of NCs and substructures.
• Image analysis involved local refinement and averaging techniques.
• Atomic structures were docked into the reconstructed composite structure.

Main Results:

• The NC exhibits distinct symmetries: 24-fold for inner rings and 15-fold for outer rings, resulting in an overall C3 symmetry.
• A subnanometer resolution composite structure of the NC was reconstructed.
• The organization of the central core and its structural requirements during assembly were revealed.

Conclusions:

• The determined structure provides detailed insights into the overall organization of the T3SS needle complex.
• Understanding the NC structure is key to comprehending bacterial virulence mechanisms and T3SS assembly.
• This structural information can inform the development of novel anti-virulence strategies.