MUC1 traverses apical recycling endosomes along the biosynthetic pathway in polarized MDCK cells.

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. Apically and basolaterally destined cargo are sorted at the trans-Golgi network into distinct vesicles, and proteins with lipid raft-dependent apical targeting signals and glycan-dependent apical targeting signals appear to specifically transit apical early endosomes (AEEs) and apical recycling endosomes (AREs), 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 AEEs by treatment with hydrogen peroxide and diaminobenzidine treatment after apical loading of endosomes with horseradish peroxidase-conjugated wheat germ agglutinin. However, expression of a GFP-tagged myosin Vb tail fragment (GFP-MyoVbT) that disrupts export from the ARE 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 AREs in polarized renal epithelial cells.