Chaperone-Facilitated Aggregation of Thermo-Sensitive Proteins Shields Them from Degradation during Heat Stress

Abstract

Summary: Cells have developed protein quality-control strategies to manage the accumulation of misfolded substrates during heat stress. Using a soluble reporter of misfolding in fission yeast, Rho1.C17R-GFP, we demonstrate that upon mild heat shock, the reporter collapses in protein aggregate centers (PACs). They contain and/or require several chaperones, such as Hsp104, Hsp16, and the Hsp40/70 couple Mas5/Ssa2. Stress granules do not assemble at mild temperatures and, therefore, are not required for PAC formation; on the contrary, PACs may serve as nucleation centers for the assembly of stress granules. In contrast to the general belief, the dominant fate of these PACs is not degradation, and the aggregated reporter can be disassembled by chaperones and recovers native structure and activity. Using mass spectrometry, we show that thermo-unstable endogenous proteins form PACs as well. In conclusion, formation of PACs during heat shock is a chaperone-mediated adaptation strategy. : The formation of aggregate foci upon heat shock is often considered a hallmark of toxicity. Cabrera et al. show that misfolded substrates are sequestered into protein aggregate centers (PACs) in a chaperone-mediated adaptation strategy. Proteins at PACs are protected from degradation, and they can refold after heat stress. Keywords: heat stress, PAC, protein aggregates, J-protein, Mas5, stress granules, Hsp104, protein refolding, PQC, UPS