Rab proteins, a family of Ras-related small GTP-binding proteins, play a key role in regulating intracellular vesicle trafficking (1?). Rab GDP dissociation inhibitor (Rab GDI or GDI2) forms a soluble complex with Rab proteins and thereby prevents the exchange of GDP for GTP (1?). In mammals, there exist two major isoforms, Rab GDI alpha (also known as XAP-4) and Rab GDI beta (1,4). While the mammalian Rab GDI beta-genes are ubiquitously expressed, the Rab GDI ?genes are predominantly brain-specific (1). Since it is expressed predominantly in neural and sensory tissues, Rab GDI a may serve a specific function in neural signal transmission (5). The gene sequences for the Rab GDI proteins are extremely conserved in evolution, with substantial homology preserved across three eukaryotic kingdoms (5).