HSP70-4 and farnesylated AtJ3 constitute a specific HSP70/HSP40-based chaperone machinery essential for prolonged heat stress tolerance in Arabidopsis

AtJ3 (J3)-a member of the Arabidopsis cytosolic HSP40 family-harbors a C-terminal CaaX motif for farnesylation, which is exclusively catalyzed by protein farnesyltransferase (PFT). Previously, prolonged incubation at 37 °C for 4 d was found to be lethal to the heat-intolerant 5 (hit5) mutant lacking PFT and transgenic j3 plants expressing a CaaX-abolishing J3^C417S construct, indicating that farnesylated J3 is essential for heat tolerance in plants. Given the role of HSP40s as cochaperones of HSP70s, the thermal sensitivity of five individual cytosolic HSP70 (HSP70-1 to HSP70-5) knockout mutants was tested in this study. Only hsp70-4 was sensitive to the prolonged heat treatment like hit5 and j3. The bimolecular fluorescence complementation (BiFC) assay revealed that HSP70-4 interacted with J3 and J3^C417Sin vivo at normal (23 °C) and high (37 °C) temperatures. At 23 °C, both HSP70-4-J3 and HSP70-4-J3^C417S BiFC signals were uniformly distributed across the cell. However, following treatment at 37 °C, HSP70-4-J3, but not HSP70-4-J3^C417S, BiFC signals were detected as discernable foci. These heat-induced HSP70-4-J3 BiFC foci were localized in heat stress granules (HSGs). In addition, hsp70-4 and J3^C417S accumulated more insoluble proteins than the wild type. Thus, farnesylated J3 dictates the chaperone function of HSP70-4 in HSGs. Collectively, this study identified the first HSP70/HSP40-type chaperone machinery playing a crucial role in protecting plants against prolonged heat stress, and demonstrated the significance of protein farnesylation in its protective function.

AtJ3, a specific HSP40 protein, mediates protein farnesylation-dependent response to heat stress in Arabidopsis

Despite AtJ3 and AtJ2 sharing a high protein-sequence identity and both being substrates of protein farnesyltransferase (PFT), AtJ3 but not AtJ2 mediates in Arabidopsis the heat-dependent phenotypes derived from farnesylation modification. Arabidopsis HEAT-INTOERANT 5 (HIT5)/ENHANCED RESPONSE TO ABA 1 (ERA1) encodes the β-subunit of the protein farnesyltransferase (PFT), and the hit5/era1 mutant is better able to tolerate heat-shock stress than the wild type. Given that Arabidopsis AtJ2 (J2) and AtJ3 (J3) are heat-shock protein 40 (HSP40) homologs, sharing 90% protein-sequence identity, and each contains a CaaX box for farnesylation; atj2 (j2) and atj3 (j3) mutants were subjected to heat-shock treatment. Results showed that j3 but not j2 manifested the heat-shock tolerant phenotype. In addition, transgenic j3 plants that expressed a CaaX- abolishing J3^C417S construct maintained the same capacity to tolerate heat shock as j3. The basal transcript levels of HEAT-SHOCK PROTEIN 101 (HSP101) in hit5/era1 and j3 were higher than those in the wild type. Although the capacities of j3/hsp101 and hit5/hsp101 double mutants to tolerate heat-shock stress declined compared to those of j3 and hit5/era1, they were still greater than that of the wild type. These results show that a lack of farnesylated J3 contributes to the heat-dependent phenotypes of hit5/era1, in part by the modulation of HSP101 activity, and also indicates that (a) mediator(s) other than J3 is (are) involved in the PFT-regulated heat-stress response. In addition, because HSP40s are known to function in dimer formation, bimolecular fluorescence complementation experiments were performed, and results show that J3 could dimerize regardless of farnesylation. In sum, in this study, a specific PFT substrate was identified, and its roles in the farnesylation-regulated heat-stress responses were clarified, which could be of use in future agricultural applications.