Functions as guanine nucleotide exchange factor (GEF) for Rab small GTPases. Activates specifically RAB5A protein (Probable). Functions cooperatively with RAB5A to regulate post-Golgi dense vesicle-mediated transport of storage proteins to the type II protein bodies (PBII) protein storage vacuoles in developing endosperm (PubMed:23723154). Interacts with RAB5A (PubMed:23723154, PubMed:24488962). Interacts with GPA3 (via C-terminus) (PubMed:24488962). Expressed in trans-Golgi network and prevacuolar compartment when recruited by GPA3. Accumulation of proglutelins in seed endosperm (PubMed:23723154). Abnormal organization of the protein storage vesicles (type II protein bodies), and formation of secretory vesicle-like structures (paramural bodies) charged with dense vesicles in seed endosperm (PubMed:23723154).