Intracellular vesicle trafficking and protein transport. Functions in autophagy. Involved in xylem and tracheary element differentiation. Interacts with VPS39. Expressed in xylem cells of inflorescence stems. Plants silencing RABG3B display a significant decrease in the number of both protoxylem and metaxylem cells in the basal region of inflorescence stems. Belongs to the small GTPase superfamily. Rab family.