Regulates the GDP/GTP exchange reaction of SEC4 by inhibiting the dissociation of GDP from it, and the subsequent binding of GTP to SEC4. Plays an essential role in the yeast secretory pathway (PubMed:8157010). Extracts GDP-bound YPT7 from vacuolar membranes, antagonizing vacuolar membrane fusion (PubMed:11118206). Interacts with the GDP-bound form of Rab GTPase YPT1. Interacts with YPT10. Present with 7280 molecules/cell in log phase SD medium. Belongs to the Rab GDI family.