EEA1, a tethering protein of the early sorting endosome, shows a polarizeddistribution in hippocampal neurons, epithelial cells, and fibroblasts

EEA1 is an early endosomal Rab5 effector protein that has been implicated i n the docking of incoming endocytic vesicles before fusion with early endos omes. Because of the presence of complex endosomal pathways in polarized an d nonpolarized cells, we have examined the distribution of EEA1 in diverse cell types. Ultrastructural analysis demonstrates that EEA1 is present on a subdomain of the early sorting endosome but not on clathrin-coated vesicle s, consistent with a role in providing directionality to early endosomal fu sion. Furthermore, EEA1 is associated with filamentous material that extend s from the cytoplasmic surface of the endosomal domain, which is also consi stent with a tethering/docking role for EEA1. In polarized cells (Madin-Dar by canine kidney cells and hippocampal neurons), EEA1 is present on a subse t of "basolateral-type" endosomal compartments, suggesting that EEA1 regula tes specific endocytic pathways. In both epithelial cells and fibroblastic cells, EEA1 and a transfected apical endosomal marker, endotubin, label dis tinct endosomal populations. Hence, there are at least two distinct sets of early endosomes in polarized and nonpolarized mammalian cells. EEA1 could provide specificity and directionality to fusion events occurring in a subs et of these endosomes in polarized and nonpolarized cells.