The plant RABC1 GTPase coordinates with exocyst component SEC5A in regulating ER-phagy under endoplasmic reticulum stress.

Small GTPase proteins regulate intracellular transport between endomembrane compartments, yet their roles in endoplasmic reticulum (ER) stress responses and selective autophagy remain poorly understood. Here, we characterize the plant RAB GTPase RABC1 as a regulator of ER-phagy during ER stress. Our results demonstrate that rabc1 mutants are hypersensitive to heat shock and ER stress inducers dithiothreitol (DTT) and tunicamycin (TM). RABC1 localizes primarily to the ER and Golgi with partial trans-Golgi network association. Upon ER stress, RABC1 is recruited to autophagosomes and subsequently delivered into the vacuole. Autophagic turnover of the ER chaperone Calnexin (CNX1)-GFP is impaired in the rabc1 mutant after DTT and TM treatments. Additionally, RABC1 interacts with the exocyst subunit SEC5A in planta, and this interaction is required for SEC5A recruitment to autophagosomes, promoting autophagosome formation. Double-mutant analysis indicates an additive genetic interaction between RABC1 and SEC5A in ER stress sensitivity. Taken together, our results suggest that RABC1 coordinates with SEC5A to promote autophagosome formation and ER-phagy during ER stress, revealing a novel mechanism by which a plant RAB GTPase regulate ER-phagy.