MUC1 transits apical endosomes along the biosynthetic pathway

MUC1 is a heavily glycosylated transmembrane protein localized at the apical surface of polarized epithelial cells. Here we examined the biosynthetic route of newly synthesized MUC1 in polarized Madin-Darby canine kidney (MDCK) cells. Apicallyand basolaterallydestined cargo are sorted at the TGN into distinct vesicles, and proteins with raft-dependent apical targeting signals and glycan-dependent apical targeting signals appear to specifically transit apical early endosomes (AEE) and apical recycling endosomes (ARE), respectively. Using metabolic labeling we found that MUC1 is efficiently targeted to the apical surface of polarized MDCK cells with a t 1⁄2 of 45 min. Apical delivery was not altered by inactivation of apical early endosomes by treatment with hydrogen peroxide and diaminobenzidine treatment after apical loading of endosomes with horse-radish peroxidase-conjugated wheat germ agglutinin. However, expression of a GFP-tagged myosin Vb tail fragment (GFP-MyoVbT) that disrupts export from the apical recycling endosome significantly reduced MUC1 apical expression. Moreover, MUC1 expressed for brief periods in MDCK cells co-localized with GFP-MyoVbT. We conclude that MUC1 traffics to the apical surface via apical recycling endosomes in polarized renal epithelial cells.