In order to survive through the high acidity of their hosts’ gastro environment before infecting the intestinal mucosa, all enteric bacteria are in need of an acid resistance system that could protect the bacterial proteins from being denatured. The Eshcherichia coli genome encodes two periplasmic proteins HdeA and HdeB, both of which are suggested to act as acid-resistant chaperones by binding to target proteins to prevent them from aggregation, and assistance in their refolding process1 . Evidence revealed that under neutral pH conditions, the HdeA/B proteins form well-folded homo-dimers and do not display chaperone activities2,3 . When the environmental pH decreases, the dimers dissociate and unfolding processes take place, activating the chaperone activities. Although much biochemical and structural studies have been performed, the detailed mechanisms of HdeA/B activation and function remain elusive.
Herein, we present the investigation of the pH-dependent structures and dynamics of E. coli HdeA and HdeB proteins by solution NMR spectroscopy. pH titration experiments indicated that the two proteins have different pH responses. The HdeA protein undergoes gradual unfolding when the pH changes from 7 to 2. However, the HdeB protein appears to maintain much of the folded structure even at acidic pH, while several new sets of peaks appear for a subset of amino acids, indicating multiple conformations. We have solved the solution structures of both HdeA and HdeB homo-dimers in their inactive states, and characterized their backbone dynamics. Relaxation measurements were also carried out for HdeA/B proteins under low pH conditions, and the results suggested pH-dependent changes of oligomeric states and internal dynamics correlating to their activation processes.