The Small GTPase RABA2a Regulates the Secretory Pathway in Parallel with the Exocyst Complex in Arabidopsis

Delivery of proteins to the plasma membrane occurs via secretion, which requires tethering, docking, priming, and fusion of vesicles. In yeast and mammalian cells, an evolutionarily conserved RAB GTPase activation cascade functions together with the exocyst and SNARE proteins to coordinate vesicle transport with fusion at the plasma membrane. However, whether this is the case in plants is unclear. In this study, we show that the small GTPase RABA2a recruits and interacts with the VAMP721/722-SYP121-SNAP33 SNARE ternary complex for membrane fusion. We also demonstrate that both RABA2a and the downstream SNARE proteins do not interact with the exocyst subunits. RABA2a inactivation does not affect the subcellular localization or assembly of the exocyst, and the mutation of exocyst subunit does not disrupt the RABA2a-SNARE association or SNARE assembly. Therefore, the RABA2a-SNARE- and exocyst-mediated secretory pathways are largely independent. Live videos reveal that the two sets of proteins follow non-overlapping trafficking routes. Genetic evidence support that the two pathways select different cargos. Finally, we demonstrate that the plant-specific RABA2a-SNARE pathway is essential to maintain potassium homeostasis. Our findings imply that highly conserved endomembrane proteins develop plant-specific trafficking mechanisms to accommodate the changing environment.