Bile-mediated ToxS homodimerization provides a model for ToxRS periplasmic interactions

Area of Science:

• Microbiology
• Structural Biology
• Molecular Biology

Background:

• The ToxRS system is a transmembrane transcription regulator essential for virulence in bacterial pathogens like Vibrio.
• ToxS, a partner of the DNA-binding regulator ToxR, has a poorly understood function in modulating ToxR activity.
• Bile salts are known to influence ToxRS activity, but the molecular mechanism remains unclear.

Purpose of the Study:

• To elucidate the molecular mechanism of bile salt-mediated regulation of the ToxRS system.
• To determine the structure of the ToxS periplasmic domain and its interaction with bile salts.
• To propose a model for how bile salts activate ToxRS-mediated virulence gene expression.

Main Methods:

• X-ray crystallography was used to determine the structure of the ToxS periplasmic domain (ToxSp) from *Vibrio parahaemolyticus* with and without glycocholate.
• Biochemical assays were performed to analyze the binding of glycocholate to ToxSp.
• Structural modeling was employed to visualize the interaction between ToxR and glycocholate-bound ToxSp.

Main Results:

• ToxSp forms an 8-stranded broken β-barrel structure, homologous to chaperone proteins.
• ToxSp binds three molecules of the bile salt glycocholate.
• Glycocholate binding induces the formation of a strand-swapped ToxS homodimer.
• A structural model suggests bile salt-induced ToxS dimerization facilitates ToxR dimerization, forming an active heterotetramer.

Conclusions:

• Bile salts act as molecular switches, inducing ToxS dimerization and subsequent ToxR-ToxS heterotetramer formation.
• This mechanism provides a structural basis for bile salt-mediated virulence regulation in *Vibrio* species.
• The findings may extend to other pathogens utilizing similar membrane-bound transcriptional regulators.