Mildly acidic pH activates the extracellular molecular chaperone clusterin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Clusterin functions as a molecular chaperone, sharing similarities with intracellular small heat shock proteins (sHSPs).
• Both clusterin and sHSPs exist as heterogeneous aggregates in solution.
• sHSPs' chaperone activity is enhanced by temperature-induced aggregate dissociation, suggesting the dissociated form is the active species.

Purpose of the Study:

• To investigate the effects of temperature and pH on clusterin structure and chaperone activity.
• To explore similarities and differences in activation mechanisms between clusterin and sHSPs.
• To understand the structural basis for clusterin's pH-dependent chaperone activation.

Main Methods:

• Analysis of clusterin aggregate dissociation and structural changes under varying temperature and pH conditions.
• Assay of clusterin's chaperone action across different temperatures and pH levels.
• Characterization of structural changes, including solvent-exposed hydrophobicity, secondary, and tertiary structure.

Main Results:

• Increased temperature did not significantly alter clusterin aggregate structure or chaperone activity.
• Mildly acidic pH induced enhanced chaperone action of clusterin, a novel activation mechanism.
• pH-dependent activation was attributed to increased solvent-exposed hydrophobicity without major structural changes.

Conclusions:

• Clusterin's chaperone activity is uniquely activated by reduced pH, distinguishing it from sHSPs.
• Low pH promotes clusterin aggregate dissociation, increasing the concentration of the active heterodimeric species.
• The findings propose a model where pH modulates clusterin's chaperone function through structural rearrangements exposing hydrophobic regions.