IN POLARIZED MDCK CELLS BASOLATERAL VESICLES ARISE FROM CLATHRIN-GAMMA-ADAPTIN-COATED DOMAINS ON ENDOSOMAL TUBULES

Human transferrin receptors (TR) and receptors for polymeric immunoglo bulins (pIgR) expressed in polarized MDCK cells maintain steady-state, asymmetric distributions on the separate basolateral and apical surfa ces even though they are trafficking continuously into and across thes e cells. The intracellular mechanisms required to maintain these asymm etric distributions have not been located. Here we show that TR and pI gR internalize from both surfaces to a common interconnected endosome compartment that includes tubules with buds coated with clathrin latti ces. These buds generate vesicles that carry TR to the basolateral bor der. The lattices contain gamma-adaptin and are dispersed by treatment with brefeldin A (BFA). Since BFA treatment abrogates the vectorial t rafficking of TR in polarized MDCK cells, we propose that the clathrin -coated domains of the endosome tubules contain the polarized sorting mechanism responsible for their preferential basolateral distribution.