Plays an important role in membrane trafficking through the secretory apparatus. In complex with CCZ1, acts as a guanine exchange factor (GEF) for RABG3F of the Rab7 protein family. Promotes the exchange of GDP to GTP, converting RABG3F from an inactive GDP-bound form into an active GTP-bound form. The RABG3F active form is involved in protein trafficking from prevacuolar compartments (PVCs) to vacuoles. May serve as a linker between Rab5 and Rab7 protein families in PVCs and mediate PVC maturation. Interacts with CCZ1A, CCZ1B and RABF2B. Widely expressed at stable levels. Severe plant growth defects, such as short root phenotype, retarded growth and dwarf phenotype. Due to the stability of its transcription during plant development, this gene is proposed as a reference gene for transcript normalization. Belongs to the MON1/SAND family.