Structural analysis of the Saf pilus by electron microscopy and image processing.

Area of Science:

• Microbiology
• Structural Biology
• Biophysics

Background:

• Bacterial pili are crucial virulence factors for host attachment and biofilm formation.
• Salmonella atypical fimbriae (Saf pili) are assembled via the chaperone/usher pathway, but their supramolecular organization is poorly understood.
• Unlike typical pili, atypical pili exhibit distinct structural characteristics.

Purpose of the Study:

• To determine the three-dimensional structure of Salmonella typhimurium Saf pilus.
• To elucidate the supramolecular organization and flexibility of atypical fimbriae.
• To analyze subunit arrangement and intersubunit interactions in Saf pili.

Main Methods:

• Negative stain electron microscopy
• Single-particle image analysis
• Quantitative fitting of atomic structures
• Linker modeling and energy minimization

Main Results:

• Saf pili exhibit a "beads on a string" appearance, characterized by linear, flexible multi-subunit fibers.
• Globular subunits are connected by short linkers, providing a high degree of flexibility.
• Analysis revealed the molecular basis for Saf pilus flexibility lies in these short intersubunit linker regions.

Conclusions:

• Atypical fimbriae, like Saf pili, possess a unique flexible structure distinct from typical pili.
• The flexibility of Saf pili, mediated by linker regions, is a key feature for bacterial virulence.
• Understanding Saf pilus structure provides insights into bacterial adhesion and infection mechanisms.