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

Abstract

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.

Effect of Betaine and Arginine on Interaction of αB-Crystallin with Glycogen Phosphorylase b

“…On the c ( s ) distribution for HspB5 + Ph b under given conditions, the peak (20.4 S) corresponding to free αB-Cr disappears, and peaks with lower sedimentation coefficients appear, corresponding to smaller oligomeric complexes of chaperone-Ph b . This is consistent with the findings of Schiedt et al [ 7 ]. Based on the thermodynamic and kinetic characteristics of the binding of αB-Cr to α-synuclein fibrils, the authors concluded that there was a step of chaperone activation through the disassembly of chaperone complexes [ 7 ].…”

“…This is consistent with the findings of Schiedt et al [ 7 ]. Based on the thermodynamic and kinetic characteristics of the binding of αB-Cr to α-synuclein fibrils, the authors concluded that there was a step of chaperone activation through the disassembly of chaperone complexes [ 7 ]. This mechanism is consistent with previous findings on substrate activation and disassembly of other sHsps with anti-aggregation activity [ 41 , 42 , 43 ].…”

“…However, McHaourab et al suggest [ 48 ], and subsequent work confirmed this [ 7 , 49 ], that αB-Cr has various binding sites with high and low affinity to protein substrate. Some of binding sites on αB-Cr are favored by phosphorylation or interaction with compounds such as Arg, whilst others are not affected [ 29 ].…”

“…Compared with other PPIs, molecular chaperone–target protein interactions are particularly weak and transient [ 3 ]. The relatively weak affinity and poor shape complementarity of chaperone–client interactions seem to allow the chaperone to recognize a wide range of different sequences [ 4 , 5 , 6 ] and adapt to a changing and adaptable proteome [ 7 ].…”

General theory of specific binding: insights from a genetic-mechano-chemical protein model

“…75 One example of how this issue is sidestepped is by using intrinsically-disordered regions, either through coupled folding and binding, 77,78 or through a disordered region far from the binding site affected allosterically. 79 Other routes to decoupling specificity and affinity relate to the entropy of the ligand, oligomeric complex, 80 or solvent. 81,82…”