A metabolite binding protein moonlights as a bile‐responsive chaperone

Bile salts are secreted into the gastrointestinal tract to aid in the absorption of lipids. In addition, bile salts show potent antimicrobial activity in part by mediating bacterial protein unfolding and aggregation. Here, using a protein folding sensor, we made the surprising discovery that the Escherichia coli periplasmic glycerol‐3‐phosphate (G3P)‐binding protein UgpB can serve, in the absence of its substrate, as a potent molecular chaperone that exhibits anti‐aggregation activity against bile salt‐induced protein aggregation. The substrate G3P, which is known to accumulate in the later compartments of the digestive system, triggers a functional switch between UgpB's activity as a molecular chaperone and its activity as a G3P transporter. A UgpB mutant unable to bind G3P is constitutively active as a chaperone, and its crystal structure shows that it contains a deep surface groove absent in the G3P‐bound wild‐type UgpB. Our work illustrates how evolution may be able to convert threats into signals that first activate and then inactivate a chaperone at the protein level in a manner that bypasses the need for ATP. Synopsis The periplasmic glycerol‐3‐phosphate binding protein, UgpB, was found to have dual functions, as a metabolite binding protein and as a bile‐responsive molecular chaperone. Stomach‐acid induced stripping of its glycerol‐3‐phosphate substrate functions as a switch that activates the chaperone activity of UgpB. A tripartite periplasmic protein folding sensor and Tn‐Seq uncover UgpB as a new chaperone. UgpB prevents bile‐induced protein aggregation when in its G3P‐free form. Stomach acid‐induced G3P stripping activates UgpB chaperone function. Crystal structure of a G3P‐nonbinding variant of UgpB reveals opening of a deep surface groove when compared to the structure of G3P‐bound wild‐type UgpB. Graphical Abstract A periplasmic folding sensor reveals a mechanism by which stomach acid‐induced G3P stripping remodels UgpB into a chaperone that prevents bile‐induced bacterial protein aggregation.