pH-Driven Two-Stage Conformational Switching Activates an Acid-Responsive Chaperone

Acidification rapidly denatures bacterial periplasmic proteins, yet this compartment lacks ATP-dependent chaperones. We identify AchC as an acid-responsive, ATP-independent chaperone crucial for acid tolerance in Pseudomonas aeruginosa. As proton influx across the outer membrane leaves insufficient time for transcriptional or translational adaptation, AchC is activated post-translationally. We reveal that AchC is activated by precisely tuned, conformational transitions driven by pH changes. At neutral pH, AchC is maintained as an inactive dimer. Upon acidification, it undergoes two structural transitions: C-terminal conformational loosening under mildly acidic conditions (pH 4.0-5.5), followed by complete monomerization under severe acid stress (pH ≤ 3.0) to fully activate its substrate-holding function. Once activated, AchC exhibits potent anti-aggregation activity across a broad spectrum of substrates. Upon neutralization, AchC releases its substrates in refolding-competent state in ATP-independent manner. In vivo, the deletion of achC increases acid susceptibility, whereas its overexpression confers acid tolerance. Furthermore, this chaperone is important for biofilm formation, motility, and pathogenicity. Our findings demonstrate how this dynamic molecular remodeling directly couples environmental sensing to bacterial survival.