Substrate specificity classes and the recognition signal for Salmonella type III flagellar export

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Pathogenesis

Background:

• Flagellar proteins in Salmonella are exported via a type III secretion system.
• Two export specificity classes, rod/hook and filament, are proposed based on studies of FlgE, FlgD, and FliC.
• Export properties of basal body rod proteins (FlgB, FlgC, FlgF, FlgG), FliE, and FlgJ were not well-characterized.

Purpose of the Study:

• To experimentally determine the export specificity of basal body rod proteins, FliE, and FlgJ.
• To investigate the effect of overproducing these proteins on flagellar assembly and export.
• To assess the relationship between protein synthesis and export for flagellar substrates.

Main Methods:

• Quantifying exported proteins in flgE and flgK mutants to determine export class.
• Analyzing the impact of overproducing FliE, FlgB, FlgC, FlgF, FlgG, or FlgJ on wild-type cell motility and FlgD export.
• Testing flagellar protein export in the presence of translation inhibitors (spectinomycin, chloramphenicol).

Main Results:

• Basal body rod proteins (FlgB, FlgC, FlgF, FlgG), FliE, and FlgJ were classified as hook-type export substrates.
• Overproduction of these proteins led to inhibited motility and impaired export of the hook-capping protein FlgD.
• All tested flagellar proteins could be exported even after protein synthesis was halted, indicating export is post-translational.

Conclusions:

• The export of basal body rod proteins, FliE, and FlgJ follows the hook-type specificity.
• These proteins play a role in regulating flagellar assembly and export, with overproduction causing cellular defects.
• The amino acid sequence of these flagellar proteins is sufficient for their recognition and export by the type III secretion system.