Structural plasticity of an acid-activated chaperone allows promiscuous substrate binding

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Folding

Background:

• HdeA is a small molecular chaperone (9.7 kDa) that prevents acid-induced protein aggregation.
• Unlike typical chaperones, HdeA functions as a disordered monomer at low pH without ATP.
• HdeA undergoes an acid-induced dimer to monomer transition for activation.

Purpose of the Study:

• To investigate the conformational changes of HdeA during acid-induced activation and substrate binding.
• To understand the mechanism by which HdeA binds substrates in a small, energy-independent manner.

Main Methods:

• Studied conformational changes using intramolecular Förster Resonance Energy Transfer (FRET).
• Analyzed the kinetics of acid-induced unfolding and monomerization.
• Investigated the role of the dimer interface in substrate binding.

Main Results:

• HdeA activation (unfolding and monomerization) occurs rapidly (k >3.5 s(-1)) upon exposure to low pH.
• The hydrophobic dimer interface is exposed upon activation and is critical for substrate binding.
• Active HdeA adopts diverse conformations, enabling recognition and high-affinity binding of various substrate proteins.

Conclusions:

• HdeA's rapid, pH-regulated activation allows it to function as an efficient, small, and energy-independent chaperone.
• The chaperone's ability to adopt different conformations is key to its broad substrate specificity.
• HdeA provides a model for understanding minimalist chaperone mechanisms in cellular stress responses.