The activation cycle of Rab GTPase Ypt32 reveals structural determinants of effector recruitment and GDI binding.

Rab GTPases localize to distinct sub-cellular compartments and regulate vesicle trafficking in eukaryotic cells. Yeast Rabs Ypt31/32 and Sec4 have 68% homology and bind to common interactors, yet play distinct roles in the transport of exocytic vesicles. The structures of Ypt31/32 have not previously been reported in the uncomplexed state. We describe the crystal structures of GTP and GDP forms of Ypt32 to understand the molecular basis for Rab function. The structure of Ypt32(GTP) reveals that the switch II conformation is distinct from Sec4(GTP) in spite of a highly conserved amino acid sequence. Also, Ypt32(GDP) reveals a remarkable change in conformation of the switch II helix induced by binding to GDI, which has not been described previously.