Functional diversification of Sec13 isoforms for storage protein trafficking in rice endosperm cells

Coat protein complex II (COPII) vesicles play crucial roles in mediating the endoplasmic reticulum (ER) exit of newly synthesized proteins to the Golgi in eukaryotic cells. However, the molecular functions of COPII components and their functional diversifications in plant seeds remain obscure. Here, we showed that the rice (Oryza sativa) glutelin precursor accumulation12 (gpa12) mutant is defective in storage protein export from the ER, resulting in the formation of aggregated protein bodies. Map-based cloning revealed that GPA12 encodes a COPII outer layer protein, Sec13a, that mainly localizes to endoplasmic reticulum exit sites (ERES) and partially localizes to the Golgi. Biochemical experiments verified that Sec13a physically interacts with Sec31 and Sec16, and mutation in Sec13 compromises its interaction with Sec31 and Sec16, thereby affecting the membrane association of the inner complex components Sar1b and Sec23c. Apart from Sec13a, the rice genome encodes 2 other Sec13 isoforms, Sec13b and Sec13c. Notably, we observed an abnormal accumulation of globular ER structures in the sec13bc double mutant but not in the single mutants, suggesting a functional redundancy of Sec13b and Sec13c in modulating ER morphology. Taken together, our results substantiated that Sec13a plays an important role in regulating storage protein export from the ER, while Sec13b and Sec13c are required for maintaining ER morphology in rice endosperm cells. Our findings provide insights into the functional diversification of COPII components in plants.