The HSFA1a is stabilized by the co-chaperone HSP70-HSP90 organizing protein HOP in Arabidopsis

Living organisms have the capacity to sense and respond to environmental stimuli, including warm conditions. Upon the increase of temperature, eukaryotic cells promote the onset of a universal transcriptional response called the heat stress response (HSR). HSR is triggered by the activation of the heat shock factor 1 (HSF1), whose activity is highly regulated in all eukaryotes. A common process of HSFA1 regulation involves its binding to HSP70 and HSP90 chaperones. However, the effect of this binding differs among eukaryotes and is not fully elucidated in plants. Moreover, the role of chaperone auxiliary proteins, such as HOP, in HSF1 regulation remains unknown. Here, we show that HOP is involved in the folding and stabilization of the HSFA1a and is required for the onset of the HSR during thermomorphogenesis in Arabidopsis. Our results demonstrate that the members of the AtHOP family bind in vivo to the HSFA1a and that the expression of multiple genes involved in the HSR, including numerous HSFA1a-responsive genes, is altered in the hop1 hop2 hop3 mutant under warm temperature. HOP is not involved in the subcellular localization of the AtHSFA1a. Instead, HSFA1a is accumulated at lower levels in the hop1 hop2 hop3 mutant while control levels are recovered in the presence of the proteasome inhibitor MG132 or the synthetic chaperone TUDCA. This uncovers the HSFA1a as a new client of HOP in plants and reveals the participation of HOP in HSFA1a stability, expanding the notorious function of the co-chaperone HOP in thermomorphogenesis.