The binding of the small heat-shock protein αB-crystallin to fibrils of α-synuclein is driven by entropic forces

Abstract

Significance The formation of amyloid fibrils and toxic oligomeric species has been shown to be inhibited by their interactions with molecular chaperones, thus modulating monomer sequestration and toxicity in the context of neurodegenerative diseases. Understanding the physical and chemical properties underlying chaperone binding processes is essential to explore new therapeutic strategies to target toxic amyloid species. Here, we determine that the binding of the small heat-shock protein αB-crystallin to α-synculein fibrils, a protein which is related to the progression of Parkinson’s disease, is driven by entropic forces. By applying a microfluidic platform, we accurately quantified the thermodynamics and the kinetics of this intermolecular interaction in the condensed phase and hypothesize that αB-crystallin oligomers work as an entropic buffer system. Molecular chaperones are key components of the cellular proteostasis network whose role includes the suppression of the formation and proliferation of pathogenic aggregates associated with neurodegenerative diseases. The molecular principles that allow chaperones to recognize misfolded and aggregated proteins remain, however, incompletely understood. To address this challenge, here we probe the thermodynamics and kinetics of the interactions between chaperones and protein aggregates under native solution conditions using a microfluidic platform. We focus on the binding between amyloid fibrils of α-synuclein, associated with Parkinson’s disease, to the small heat-shock protein αB-crystallin, a chaperone widely involved in the cellular stress response. We find that αB-crystallin binds to α-synuclein fibrils with high nanomolar affinity and that the binding is driven by entropy rather than enthalpy. Measurements of the change in heat capacity indicate significant entropic gain originates from the disassembly of the oligomeric chaperones that function as an entropic buffer system. These results shed light on the functional roles of chaperone oligomerization and show that chaperones are stored as inactive complexes which are capable of releasing active subunits to target aberrant misfolded species.