Required for optimal acid stress protection. Exhibits a chaperone-like activity only at pH below 3 by suppressing non-specifically the aggregation of denaturated periplasmic proteins. Important for survival of enteric bacteria in the acidic environment of the host stomach. Also promotes the solubilization at neutral pH of proteins that had aggregated in their presence at acidic pHs. May cooperate with other periplasmic chaperones such as DegP and SurA. Homodimer at neutral pH. Dissociates into monomer at pH 4. Changes from a highly ordered form at pH above 3.1 to a highly disordered form at pH below 2.5 that is essential for its chaperone activity. Activated by a low pH-induced dimer-to-monomer transition. In vitro, HdeA is more efficient than HdeB at pH 2 and HdeB is more efficient than HdeA at pH 3. In vivo, both are required for optimal protection against acid stress at either pH 3 or pH 2 (PubMed:17085547). Belongs to the HdeA family.