RAB39 proteins are involved in endolysosomal trafficking and are localized to the Golgi membrane where they interact with the multisubunit tethering complex COG (Chen et al, 2003; Giannandrea et al, 2010; Miller et al, 2013; reviewed in Willet et al, 2013). Although RAB proteins are known to play key roles in trafficking and Golgi structure and function, the significance of some of these interactions is not yet clear (reviewed in Liu and Storrie, 2015). Loss-of-function mutations of RAB39B cause X-linked metal retardation, likely as a result of altered trafficking during growth cone and synapse formation (Giannandrea et al, 2010). Activation of RAB39 at the Golgi is dependent on the GEF activity of DENND5A and DENND5B (Yoshimura et al, 2010). Interaction of RAB39:GDP with its GEFs promotes release of GDP, allowing GTP to bind, and precludes the interaction of RAB39 with GDI and CHM proteins.