Crystal structure of Rab9 complexed to GDP reveals a dimer with an active conformation of switch II

The small GTPase Rab9 is an essential regulator of vesicular transport from the late endosome to the trans-Golgi network, as monitored by the redirection of the mannose-6-phosphate receptors. The crystal structure of Rab9 complexed to GDP, Mg 2+, and Sr 2+ reveals a unique dimer formed by an intermolecular β-sheet that buries the switch I regions. Surface area and shape complementarity calculations suggest that Rab9 dimers can form an inactive, membrane-bound pool of Rab9 · GDP that is independent of GDI. Mg 2+-bound Rab9 represents an inactive state, but Sr 2+-bound Rab9 · GDP displays activated switch region conformations, mimicking those of the GTP state. A hydrophobic tetrad is formed resembling an effector-discriminating epitope found only in GTP-bound Rab proteins.