Acts synergistically with RAB5A and VPS9A to regulate dense vesicle-mediated post-Golgi trafficking of major storage proteins in the seed endosperm during grain filling and maturation. Interacts (via C-terminus) with VPS9A. Highly expressed in leaves. During endosperm development, the expression of GPA3 is low at an early stage, peaks at 18 days after flowering (DAF), and slowly decreases after 21 DAF. Accumulation of proglutelins in seed endosperm (PubMed:24488962). Mistargeting of dense vesicles (DVs) to the type II protein bodies (PBII) protein storage vacuoles and reduction of PBII size in endosperm. Formation of paramural bodies (PMBs) secretory vesicle-like structures charged with DVs in endosperm (PubMed:24488962).