Recycling endosomes attach to the trans-side of Golgi stacks in Drosophila and mammalian cells

Historically, the trans-Golgi network (TGN) has been recognized as a sorting center of newly synthesized proteins, whereas the recycling endosome (RE) is a compartment where endocytosed materials transit before being recycled to the plasma membrane. However, recent findings revealed that both the TGN and RE connect endocytosis and exocytosis and, thus, are functionally overlapping. Here we report, in both Drosophila and microtubule-disrupted HeLa cells, that REs are interconvertible between two distinct states, namely Golgi-associated REs and free REs. Detachment and reattachment of REs and Golgi stacks are often observed, and newly synthesized glycosylphosphatidylinositol-anchored cargo protein but not vesicular stomatitis virus G protein is transported through these two types of RE. In plants, there are two types of TGN - Golgi-associated TGN and Golgi-independent TGN. We show that dynamics of REs in both Drosophila and mammalian cells are very similar compared with those of plant TGNs. And, together with the similarity on the molecular level, our results indicate that fly and mammalian REs are organelles that are equivalent to TGNs in plants. This suggests that the identities and functional relationships between REs and TGNs should be reconsidered.