pH-dependent structural modulation is conserved in the human small heat shock protein HSBP1

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Human small heat shock proteins (sHSPs) are crucial for cellular proteostasis and their function is modulated by environmental factors.
• Understanding the atomic-level mechanisms of sHSP regulation by stressors is essential for comprehending cellular stress responses.
• Previous work identified a conserved histidine in HSPB5 as a key regulator of its holdase activity and oligomeric state within a physiological pH range.

Purpose of the Study:

• To investigate the pH-dependent regulatory mechanism of human heat shock protein B1 (HSPB1).
• To elucidate how pH-induced structural changes in HSPB1 affect its stability, oligomeric state, and chaperone activity.
• To determine if a conserved histidine-mediated mechanism, similar to HSPB5, regulates HSPB1 function.

Main Methods:

• Site-directed mutagenesis to substitute histidine with lysine (His124Lys) in HSPB1.
• Analysis of HSPB1 structural changes and dimer interface stability using biophysical techniques.
• Assessment of HSPB1 oligomeric size and chaperone activity under varying pH conditions.

Main Results:

• HSPB1 responds to pH changes through structural alterations mediated by the protonation of a conserved histidine (His124).
• Protonation or substitution of His124 with lysine destabilizes the dimer interface within the alpha-crystallin domain.
• These structural changes lead to an increase in oligomeric size and a modest enhancement of chaperone activity.

Conclusions:

• A conserved mechanism of pH-dependent structural regulation exists among human sHSPs containing a specific conserved histidine.
• Protonation of this histidine residue influences the stability of the dimer interface, oligomeric state, and chaperone function.
• While the mechanism is conserved, the specific functional outcomes of these structural modulations can differ across various sHSPs.