OsKish Coordinates with Lunapark and RHD3 to Maintain the Structural Stability of Endoplasmic Reticulum Network in Rice.

The endoplasmic reticulum (ER) is the largest intracellular membrane-bound organelle in eukaryotic cells, comprising an interconnected network of tubules and sheets. Lunapark (LNP) functions as an E3 ubiquitin ligase that targets RHD3, a dynamin-like GTPase essential for homotypic fusion of ER tubules, for proteasomal degradation, thereby modulating ER tubule stability. However, the regulatory mechanism governing LNP activity remains largely unknown. Here, we report the characterization of the rice oskish mutant, which exhibits aberrant aggregation of ER tubules and defective ER exit of seed storage proteins. OsKish is an ER-localized small protein that physically interacts with both OsLNPs and RHD3-like (RHD3L) protein. OsKish stabilizes OsLNPs by suppressing their auto-ubiquitination; concomitantly, OsKish attenuates the membrane fusion activity of RHD3L possibly by inhibiting its oligomerization. Altogether, our studies propose an OsKish-OsLNPs-RHD3L framework for fine-tuning homotypic ER tubule fusion in rice, providing mechanistic insights into the maintenance of ER architecture in plants.