Protein quality control is essential for the circadian clock in plants.

Extreme environmental conditions, such as heat and cold, often disturb cellular proteostasis, resulting in protein denaturation and oxidative damage that threaten cell viability. Therefore, living organisms have evolved versatile protein quality control mechanisms that clear damaged proteins from cellular compartments. It has been shown that a repertoire of molecular chaperones, including heat shock proteins (HSPs), works together with ubiquitin-proteasome systems in this biochemical process in animals and yeast. However, the protein quality control systems have not been well-characterized in plants. We have recently reported that the E3 ubiquitin ligase ZEITLUPE (ZTL), a central component of the plant circadian clock, constitutes a protein quality control system in conjunction with HSP90, which is responsible for clearing denatured protein aggregates at high temperatures. The ZTL-HSP90 protein complexes are colocalized in insoluble fractions in heat-exposed plants. Notably, lack of ZTL reduces protein polyubiquitination and disrupts the robustness of circadian rhythms under heat stress conditions, providing a novel role of ZTL: it mediates a heat-responsive protein quality control to sustain the clock function. We summarize the potential roles of ZTL in thermal responses and stability of the circadian clock in plants.