Abstract 2002 Mechanism of activation of a pathogenic bile acid receptor

Vibrio parahaemolyticus is a leading cause of seafood-borne gastroenteritis around the world. The occurrence of gastroenteritis is associated with the V. para Type III Secretion System 2 (T3SS2). Expression of the T3SS2 is controlled by a bile acid receptor, VtrA/VtrC. When certain bile acids like taurodeoxycholate (TDC) bind to the periplasmic domains of VtrA/VtrC, the cytoplasmic DNA binding domain in VtrA induces expression of the master regulator of the T3SS2, VtrB. Interestingly, other bile acids like chenodeoxycholate (CDC) do not induce expression of VtrB. Using isothermal titration calorimetry (ITC) and X-ray crystallography, we discovered CDC binds in the same pocket of the periplasmic domains as TDC but makes different interactions with binding pocket residues. Measuring the binding energetics of VtrA/VtrC mutants to bile using ITC and the mutant activities using a gfp reporter assay revealed the residues His50 and Ser123 dictate which bile acids activate the T3SS2. However, the mechanism of signal transduction from bile acid binding the periplasmic domains to activating the cytosolic DNA binding domain remains unknown. Based on the homology of the DNA binding domain to winged helix-turn-helix DNA binding proteins, we predict VtrA/VtrC forms a dimer of dimers upon bile acid binding, leading to the activation of VtrB transcription. We are currently testing the dimerization of VtrA/VtrC using biochemical, biophysical, and structural methods. This work was funded by the Welch Foundation, Once Upon a Time…Foundation, National Institutes of Health, and Howard Hughes Medical Institute.