The endocytic recycling pathway is controlled by the ADP-ribosylated GTPase Rab14
The GTPase Rab14 is localized at the trans-Golgi network and at the intermediate compartment associated to sorting/recycling endosomes-like structures of the transferrin-recycling pathway: as other Rab family members, it is involved in the regulation of intracellular vesicle trafficking, though its role and functional relationship with effector/endosomal proteins is still incomplete. We have analysed whether post-translational modifications could affect Rab14 activity: the results obtained define mono-ADP-ribosylation (MARylation) as the yet-unknown Rab14 modification, catalysed by the ADP-ribosyltransferase PARP12, which specifically modifies glutamic acid residues in position 159/162. This modification is essential for the Rab14-dependent endosome progression. Accordingly, recycling of the transferrin receptor is inhibited when MARylation of Rab14 is prevented by PARP12 knocking-down or inhibition, or by overexpression of Rab14 ADP-ribosylation-defective mutant. Under these conditions, Rab14 and transferrin receptors are withheld at the cell periphery at the level of the Rab4-RUFY1-positive sorting endosomes, indicating that the interaction of Rab14 with the dual effectors RUFY and then FIP1c (which specifically binds both Rab11 and Rab14) determines the progression between the Rab4-RUFY- and Rab11-FIP1c-specific vesicles. Therefore Rab14-MARylation determines the sequential binding of this GTPase to RUFY and FIP1c, thus controlling endosome progression (i.e., transferrin receptors recycling) through the Rab4-, Rab14- and Rab11-specific vesicles. This identifies a Rab14-specific compartment of the recycling pathway and a crucial enzymatic reaction amenable to pharmacological control.