Forces Driving Chaperone Action

Abstract

SUMMARY It is still unclear what molecular forces drive chaperone-mediated protein folding. Here, we obtain a detailed mechanistic understanding of the forces that dictate the four key steps of chaperone-client interaction: initial binding, complex stabilization, folding, and release. Contrary to the common belief that chaperones recognize unfolding intermediates by their hydrophobic nature, we discover that the model chaperone Spy uses long-range electrostatic interactions to rapidly bind to its unfolded client protein Im7. Short-range hydrophobic interactions follow, which serve to stabilize the complex. Hydrophobic collapse of the client protein then drives its folding. By burying hydrophobic residues in its core, the client’s affinity to Spy decreases, which causes client release. By allowing the client to fold itself, Spy circumvents the need for client-specific folding instructions. This mechanism might help explain how chaperones can facilitate the folding of various unrelated proteins.

Chaperone-client interactions: Non-specificity engenders multifunctionality

“…Client folding results in hydrophobic core formation and thus reduces stabilizing hydrophobic contacts between chaperone and client (32,33). This destabilizes the complex, helping to trigger client release (32). Therefore, rather than being dictated by the chaperone, client folding regulates client binding and release.…”

“…This combination of flexibility and amphiphilic binding surface allows Spy to dynamically bind the many conformational states that occur along the folding trajectory of its client proteins and hence mediate folding while remaining continuously but loosely bound to its clients (31,33). The attraction of the aggregation-prone unfolded client to the chaperone is driven by electrostatic forces, which are then complemented by hydrophobic interactions in the complex (32). This mixture of transient hydrophobic and hydrophilic interactions (32,34) allows the client to explore its folding landscape while bound (33,35).…”

“…During folding, Spy stays bound to thermodynamically unstable areas, thereby helping the client avoid aggregation (34). Client folding results in hydrophobic core formation and thus reduces stabilizing hydrophobic contacts between chaperone and client (32,33). This destabilizes the complex, helping to trigger client release (32).…”

“…The attraction of the aggregation-prone unfolded client to the chaperone is driven by electrostatic forces, which are then complemented by hydrophobic interactions in the complex (32). This mixture of transient hydrophobic and hydrophilic interactions (32,34) allows the client to explore its folding landscape while bound (33,35). Thus, the client is bound to Spy as a conformationally heterogeneous ensemble, sampling conformations ranging from unfolded to intermediately and near-natively folded states ( Fig.…”

“…Spy was very recently used as a simple chaperone folding system to identify the kinetic, thermodynamic, and structural properties that allow chaperones to promote client folding and to determine how they affect the folding landscape of client proteins (31)(32)(33). The "folding-friendly" amphiphilic and flexible nature of Spy's client-binding site was found to be critical to its chaperone activity (33).…”