Plays an important role in membrane trafficking through the secretory apparatus. In complex with CCZ1, acts as a guanine exchange factor (GEF) for Rab7 protein family. Promotes the exchange of GDP to GTP, converting it from an inactive GDP-bound form into an active GTP-bound form. The 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. Belongs to the MON1/SAND family.