Distinct role of TGN-resident clathrin adaptors for Rab5 activation in the TGN-endosome trafficking pathway

Clathrin-mediated vesicle trafficking plays central roles in the post-Golgi transport pathways from the trans-Golgi network (TGN) to endosomes. In yeast, two clathrin adaptors - AP-1 complex and GGA proteins (GGAs) - are predicted to generate distinct transport vesicles at the TGN, and epsin-related Ent3p/Ent5p act as accessories for these adaptors. Recently, we showed that vesicle transport from the TGN, rather than from the plasma membrane, is crucial for Rab5-mediated endosome formation, and that Ent3p/5p are crucial for this process, whereas AP-1 and GGAs are dispensable. However, these observations were incompatible with previous studies showing that these adaptors are required for Ent3p/5p recruitment to the TGN, and thus the overall mechanism responsible for regulation of Rab5 activity remains ambiguous. Here we investigated the functional relationships between clathrin adaptors in post-Golgi-mediated Rab5 activation. We were able to show that AP-1 disruption in ent3{Delta}/5{Delta} mutant impairs Rab5-GEF Vps9p transport to the Rab5 compartment, and severely reduces Rab5 activity. Additionally, GGAs, Golgi-resident PI4 kinase Pik1p and Rab11 GTPases Ypt31p/32p were found to have partially overlapping functions for recruitment of AP-1 and Ent3p/5p to the TGN. These findings suggest a distinct role of clathrin adaptors for Rab5 activation in the TGN-endosome trafficking pathway.