Binds unprenylated Rab proteins, presents it to the catalytic component B, and remains bound to it after the geranylgeranyl transfer reaction. The component A is thought to be regenerated by transferring its prenylated Rab to a protein acceptor (By similarity). Coordinated with the cell cycle, located around nuclei during prophase and telophase. Located to spindle poles during metaphase. Expressed in the syncytial embryo. Belongs to the Rab GDI family.