Molecular Determinants for Differential Activation of the Vibrio parahaemolyticus Bile Acid Receptor

Bile acids are important for digestion of food and for antimicrobial activity. Pathogenic Vibrio parahaemolyticus senses bile acids via the co-component signal transduction system receptor VtrA/VtrC, an obligate membrane heterodimer. Intestinal bile acids bind to the periplasmic domain of the VtrA/VtrC complex, activating a DNA-binding domain in VtrA that induces expression of another membrane protein, VtrB. VtrB induces expression of the pathogenic Type III Secretion System 2. The bile acid taurodeoxycholate (TDC) activates VtrA/VtrC-induced VtrB expression, while others such as chenodeoxycholate (CDC) do not. This study demonstrates both CDC and TDC bind to the VtrA/VtrC periplasmic heterodimer using isothermal titration calorimetry (ITC). The crystal structure of the VtrA/VtrC heterodimer bound to CDC revealed it binds in the same hydrophobic pocket as TDC, but differently. Mutation of the binding pocket caused a decrease in bile acid binding affinity with exception of the S123A mutant, which bound with a similar affinity as the wild-type protein. The S123A mutant decreased TDC-induced T3SS2 activation, providing a molecular explanation for the selective activation of the T3SS2 by bile acids.