Rab GDI protein Rab proteins, a family of small Ras-related GTP-binding proteins, are involved in regulation of intracellular vesicle trafficking [<cite idref="PUB00000785"/>]. Rab GDP dissociation inhibitor (GDI) forms a soluble complex with Rab proteins, thereby preventing exchange of GDP for GTP. Rab GDI exists in several isoforms, and belongs to the TCD/MRS6 family of GDP dissociation inhibitors. <p>The crystal structure of the bovine alpha-isoform of Rab GDI has beendetermined to a resolution of 1.81A [<cite idref="PUB00004235"/>]. The protein is composed of twomain structural units: a large complex multi-sheet domain I, and a smalleralpha-helical domain II.</p><p>The structural organisation of domain I is closely related to FAD-containingmonooxygenases and oxidases [<cite idref="PUB00004235"/>]. Conserved regions common to GDI and thechoroideraemia gene product, which delivers Rab to catalytic subunits ofRab geranylgeranyltransferase II, are clustered on one face of the domain[<cite idref="PUB00000785"/>]. The two most conserved regions form a compact structure at the apex ofthe molecule; site-directed mutagenesis has shown these regions to play acritical role in the binding of Rab proteins [<cite idref="PUB00004235"/>].</p>