A chaperone network that includes HSP70 and HSP90 is essential for maintaining proteins in their functional conformation and preventing their aggregation. Plants also harbour various co-chaperones, such as J-proteins, which stimulate HSP70 activity, but the localisation and functions of many of these factors is unclear. Now, Serena Schwenkert and colleagues (Dittmer et al., 2021) characterise DJC31 and DJC62, two J-proteins that carry clamp-type tetratricopeptide repeat domains, in Arabidopsis thaliana. Using GFP fusion constructs as well as cell fractionation, they determine that DJC31 and DJC62 localise to the cytosolic side of the ER membrane, and not to chloroplasts as previously suggested. The two proteins interact with HSP70 and HSP90 as shown by bimolecular fluorescence complementation and the authors also demonstrate that the interaction with HSP70 is essential; constructs in which the domain required to activate HSP70 is mutated fail to complement the growth retardation and curled leaf mutant phenotype of djc31 djc62 double mutants. Finally, the authors assess the function of DJC31 and DJC62 in abiotic stress responses and find that double mutants are more sensitive to osmotic stress but, surprisingly, more tolerant towards drought stress compared to wild-type plants. Double mutants were also hypersensitive to abscisic acid (ABA). Taken together, this study identifies DJC31 and DJC62 as novel plant co-chaperones that are important for osmotic and drought stress responses, likely through ABA-mediated signalling.
