Rab5-mediated endosome formation is regulated at the trans-Golgi network

Early endosomes, also called sorting endosomes, are known to mature into late endosomesvia the Rab5-mediated endolysosomal trafficking pathway. Thus, early endosome existence isthought to be maintained by the continual fusion of transport vesicles from the plasmamembrane and thetrans-Golgi network (TGN). Here we show instead that endocytosis isdispensable and post-Golgi vesicle transport is crucial for the formation of endosomes andthe subsequent endolysosomal traffic regulated by yeast Rab5 Vps21p. Fittingly, all threeproteins required for endosomal nucleotide exchange on Vps21p arefirst recruited to theTGN before transport to the endosome, namely the GEF Vps9p and the epsin-relatedadaptors Ent3/5p. The TGN recruitment of these components is distinctly controlled, withVps9p appearing to require the Arf1p GTPase, and the Rab11s, Ypt31p/32p. These resultsprovide a different view of endosome formation and identify the TGN as a critical location forregulating progress through the endolysosomal trafficking pathway.