In eukaryotic cells, heat stress triggers aggregation of proteins and RNA into heat-shock granules (HSGs). However, the formation and composition of HSGs remain poorly understood. Drummond and co-workers now provide a comprehensive proteomic analysis of heat-triggered aggregation and subsequent disaggregation in budding yeast (Cell 162, 1286–1298, 2015).

Credit: Reproduced with permission from Elsevier

The authors monitored aggregation of proteins into high-molecular-weight particles by ultracentrifugation and fractionation into supernatant and pellet. Stable-isotope labeling and mass spectrometry enabled estimation of the proportion of each protein in the supernatant before and after heat shock. After a shift from 30 °C to 46 °C, 177 proteins moved from the supernatant to the pellet, and only four moved from the pellet to the supernatant. Molecular chaperones such as Hsp104, which colocalize with HSGs, remained largely soluble, thus suggesting a biochemical distinction between aggregation and recruitment to aggregates. Interestingly, individual proteins aggregated in distinct subcellular compartments. For example, the nuclear poly(A) RNA–binding protein Gbp2 formed subnuclear granules during heat shock (left), whereas the HSG marker Pab1 formed cytosolic foci (left middle).

Next, the authors asked how aggregation affects protein function. AME, a heterotrimeric aminoacyl-tRNA synthetase complex, aggregated upon heat shock but, surprisingly, it retained substantial activity and fidelity in vitro. The authors suggest that AME aggregation could focus its activity in the cytosol, where it is needed during stress. Finally, the authors globally profiled disaggregation after recovery. Unexpectedly, they did not observe degradation of disaggregated proteins; instead, proteins were restored to solubility without degradation.

These findings should spur reinterpretation of heat stress–induced events: aggregates of mature proteins probably represent an adaptive cellular strategy rather than simply being misfolded proteins destined for degradation.