TRANSPORT THROUGH THE YEAST ENDOCYTIC PATHWAY OCCURS THROUGH MORPHOLOGICALLY DISTINCT COMPARTMENTS AND REQUIRES AN ACTIVE SECRETORY PATHWAYAND SEC18P N-ETHYLMALEIMIDE-SENSITIVE FUSION PROTEIN/

Molecules travel through the yeast endocytic pathway from the cell sur face to the lysosome-like vacuole by passing through two sequential in termediates. Immunofluorescent detection of an endocytosed pheromone r eceptor was used to morphologically identify these intermediates, the early and late endosomes. The early endosome is a peripheral organelle that is heterogeneous in appearance, whereas the late endosome is a l arge perivacuolar compartment that corresponds to the prevacuolar comp artment previously shown to be an endocytic intermediate. We demonstra te that inhibiting transport through the early secretory pathway in se c mutants quickly impedes transport from the early endosome. Treatment of sensitive cells with brefeldin A also blocks transport from this c ompartment. We provide evidence that Sec18p/N-ethylmaleimide-sensitive fusion protein, a protein required for membrane fusion, is directly r equired in vivo for forward transport early in the endocytic pathway. Inhibiting protein synthesis does not affect transport from the early endosome but causes endocytosed proteins to accumulate in the late end osome. As newly synthesized proteins and the late steps of secretion a re not required for early to late endosome transport, but endoplasmic reticulum through Golgi traffic is, we propose that efficient forward transport in the early endocytic pathway requires delivery of lipid fr om secretory organelles to endosomes.