Pre-N and C-terminal extension regions of Arabidopsis HSP90.7 regulate the chaperone activity and ER stress response.

The ER-localized molecular chaperone HSP90.7 plays a critical role in maintaining protein homeostasis in plants, particularly under stress conditions. However, the functional roles of its pre-N and C-terminal extension (CTE) regions remain poorly understood. In this study, we integrated molecular dynamics simulations, in vitro biochemical assays, and in vivo mutant analysis to investigate the roles of these regions. Deletion of either region did not affect normal seedling development but conferred pronounced hypersensitivity to endoplasmic reticulum (ER) stress. Molecular dynamics simulations revealed that both the pre-N and CTE form regulatory contacts with HSP90.7's N-terminal, middle, and C-terminal domains, likely modulating the chaperone's global stability and interdomain communication. Consistent with these findings, removing the pre-N region increased ATPase activity and altered ATP-binding kinetics, consistent with prior reports for mammalian GRP94, whereas deleting the CTE diminished ATP-independent holdase function. Thus, our findings highlight a conserved regulatory role of the pre-N across ER-localized HSP90s. Together, our results underscore the significance of the pre-N and CTE regions for HSP90.7's functional cycle and establish their specialized roles in ER homeostasis and plant stress resilience.