ZEITLUPE Contributes to a Thermoresponsive Protein Quality Control System in Arabidopsis

Cellular proteins undergo denaturation and oxidative damage under heat stress, forming insoluble aggregates that are toxic to cells. Plants possess versatile mechanisms to deal with insoluble protein aggregates. Denatured proteins are either renatured to their native conformations or removed from cellular compartments; these processes are often referred to as protein quality control. Heat shock proteins (HSPs) act as molecular chaperones that assist in the renaturation-degradation process. However, how protein aggregates are cleared from cells in plants is largely unknown. Here, we demonstrate that heat-induced protein aggregates are removed by a protein quality control system that includes the ZEITLUPE (ZTL) E3 ubiquitin ligase, a central circadian clock component in Arabidopsis thaliana. ZTL mediates the polyubiquitination of aggregated proteins, which leads to proteasomal degradation and enhances the thermotolerance of plants growing at high temperatures. The ZTL-defective ztl-105 mutant exhibited reduced thermotolerance, which was accompanied by a decline in polyubiquitination but an increase in protein aggregate formation. ZTL and its interacting partner HSP90 were cofractionated with insoluble aggregates under heat stress, indicating that ZTL contributes to the thermoresponsive protein quality control machinery. Notably, the circadian clock was hypersensitive to heat in ztl-105. We propose that ZTL-mediated protein quality control contributes to the thermal stability of the circadian clock.