Unavoidable Disasters: the Hsp70 Chaperone Can Only Prevent Aggregation Over a Limited Time Span

Abstract

Cells must attain and maintain proper protein folding and avoid aggregation: this is a pervasive challenge across all domains of life. Molecular chaperones are central to this process, acting through mechanisms shaped by the free-energy landscapes of their client proteins. The impact of chaperones on client-specific kinetic and thermodynamic characteristics, however, remains poorly characterized. Here, we develop the CHAMPION70 computational model to perform kinetic simulations on the E. Coli Hsp70 chaperone system in the presence of client-protein folding and aggregation. Experimentally determined folding/unfolding/aggregation rate constants are taken into account. In the case of weakly aggregation-prone clients, CHAMPION70 predicts that Hsp70 prevents aggregation only over timescales approaching the average human lifespan (79 years). For strongly aggregation-prone clients, however, aggregation inevitably occurs starting from a few minutes, even at higher-than-physiological Hsp70 concentrations. Thus, the Hsp70 chaperone system merely delays, yet surprisingly does not eliminate, the onset of aggregation. Further, CHAMPION70 predicts that Hsp70 most effectively retards the aggregation of slow-folding, thermodynamically stable clients. In contrast, thermodynamically unstable clients remain engaged with Hsp70 for prolonged periods even when aggregation propensity is low, thereby consuming chaperone capacity and reducing the extent of productive folding. This outcome suggests that condensate formation by proteins containing disordered regions may serve to mitigate unnecessary Hsp70 engagement. In all, our results provide a quantitative framework for understanding how client-specific kinetic and thermodynamic properties shape chaperone activity and show that the Hsp70 chaperone system does not altogether prevent aggregation: it merely retards it.