A cDNA for the rabbit low Mr polymeric immunoglobulin (poly-Ig) receptor was expressed in an immortalized rabbit mammary cell line. The intracellular routing of the receptor and its cell surface expression was analyzed in stably transfected cells grown on permeable supports. Initially the cells formed a monolayer with no transmural electrical resistance. All monolayer cells expressed the poly-Ig receptor and cytokeratin 7 filaments characteristic of luminal mammary cells but absent in myoepithelial cells. Within 7 d in culture, the cells underwent cytodifferentiation and formed a bilayer with a transepithelial electrical resistance of approximately 500 omega x cm2. Upper layer cells formed tight junctions with adjacent cells and gap junctions with basal cells. Expression of the poly-Ig receptor and cytokeratin 7 was restricted to the cells from the upper layer. The kinetics of receptor biosynthesis and processing was similar to that reported for rabbit mammary gland and rat liver. The receptor was cleaved at the apical cell surface and release of secretory component into the apical medium occurred with a half-time of approximately 2 h. Selective cell surface trypsinization combined with pulse-chase experiments served to determine at which cell surface domain newly synthesized receptor appeared first. The receptor was digested with a half-time of approximately 60 min with trypsin present in the basolateral medium and 90 min with apical trypsin. These data are consistent with selective targeting of newly synthesized receptor to the basolateral surface. The results indicate that transcytosis of the receptor from basolateral to apical membrane in the presence or the absence of its ligand requires approximately 30 min. Cleavage of the receptor by endogenous protease is not concomitant with its appearance at the apical surface, but requires additional time, thus explaining the presence of intact receptor on the apical membrane.

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