Activates the ARF proteins by exchanging bound GDP for free GTP. Plays a role in vesicular protein sorting. Acts as the major regulator of retrograde Golgi to endoplasmic reticulum trafficking but is also involved in the endocytosis process. Could function redundantly with GNOM. Regulates vesicle trafficking required for the coordinated polar localization of auxin efflux carriers which in turn determines the direction of auxin flow. Mediates the endocytosis of PIN2 from plasma membrane to endosomal compartments. Required for maintenance of endoplasmic reticulum morphology. Homodimer. Soluble and partially membrane-bound. Slightly abnormal Golgi stacks with laterally expanded cisternae. Abnormal formation of spherical bodies in the endoplasmic reticulum.