Heat Shock Triggers Assembly of tRNA Synthetases into an Active Supercomplex

Abstract

Heat shock stimulates cellular production of protein folding chaperones. This response is a reaction to, and likely triggered by, protein misfolding and aggregation. Using a proteome-scale mass spectrometry screen for protein aggregation upon heat shock in vivo, we found that many components of the translational apparatus associate into high-molecular weight species. One such component is the ternary yeast multi-tRNA synthetase complex (glutamyl- and methionyl-tRNA synthetases Gus1 and Mes1 with their aminoacylation cofactor Arc1). In vitro, the purified complex self-assembles in response to the same temperature shift. Remarkably, the self-assembled aminoacylation complex retains full activity toward tRNAMet. We found that assembly is directed by the N-terminal glutathione-S-transferase (GST)-like domains that mediate complex formation under non-shock conditions. Furthermore, the isolated N-terminal GST-like domain of Gus1 suffices to induce the assembly, and can be used to precipitate covalently-linked proteins. The phenomenon is conserved in the N-terminal of Gus1 from a thermophilic fungus that self-assembles near its heat shock temperature. Biophysical studies reveal only modest structural secondary structure changes between the monomeric and self-assembled states. These results indicate that this heat-triggered protein assembly is distinguishable from large-scale misfolding and nonspecific aggregation, and suggest a novel mechanism by which cells sense temperature changes.