Rab23 (Ras-related protein Rab 23) belongs to the Rab family of monomeric small guanosine triphosphatases (GTPases) involved in the regulation of membrane traffic. Rab GTPases are conserved from yeast to humans and coordinate the delivery of cargo to its correct destination within eukaryotic cells. Rab proteins regulate many membrane trafficking steps, including vesicle formation, budding, motility along the cytoskeleton, docking, and membrane fusion (Zerial and McBride 2001; Stenmark 2009). More than 60 members of the Rab family have been identified in humans to date (Zerial and McBride 2001). Rab proteins function as molecular switches cycling from an active GTP (guanosine triphosphate)-bound form to an inactive GDP (guanosine diphosphate)-bound form. The GDP/GTP exchange factors (GEFs) catalyze the conversion from GDP to GTP-bound forms, whereas GTP hydrolysis to GDP is catalyzed by...