Toxicity of the model protein 3Ã?GFP arises from degradation overload, not from aggregate formation

While protein aggregation can cause cytotoxicity, it also forms to mitigate cytotoxicity from misfolded proteins, though the nature of these contrasting aggregates remains unclear. We previously found that overproduction (op) of a three green fluorescent protein linked protein (3Ã?GFP) in yeast cells induces giant aggregates, and is detrimental to growth. Here, we investigated the mechanism of growth inhibition by 3Ã?GFP-op using non-aggregative 3Ã?MOX-op as a control. The 3Ã?GFP aggregates were induced by misfolding, and 3Ã?GFP-op had higher cytotoxicity than 3Ã?MOX-op because it perturbs the ubiquitin-proteasome system. Static aggregates formed by 3Ã?GFP-op dynamically trapped Hsp70, causing the heat shock response. Systematic analysis of mutants deficient in the protein quality control suggested that 3Ã?GFP-op did not cause critical Hsp70 depletion and that the formation of aggregates functioned in the direction of mitigating toxicity. Artificial trapping of essential cell cycle regulators into 3Ã?GFP aggregates caused abnormalities in the cell cycle. In conclusion, the formation of the giant 3Ã?GFP aggregates itself is not cytotoxic, as it does not entrap and deplete essential proteins. Rather, it is productive, inducing the heat shock response while preventing an overload to the degradation system.