MDCK cells display fluid-phase transcytosis in both directions across the cell. Transcytosis of cell surface molecules was estimated by electron microscopic analysis of streptavidin-gold-labeled frozen sections of biotinylated cells. Within 3 h, approximately 10% of the surface molecules, biotinylated on the starting membrane domain, were detected on the opposite surface domain irrespective of the direction of transcytosis. This suggests that the transcytosis rates for surface molecules are equal in both directions across the cell as shown previously for fluid-phase markers. A biochemical assay was established to identify transcytosing glycoproteins in MDCKII-RCAr cells, a ricin-resistant mutant of MDCK. Due to a galactosylation defect, surface glycoproteins of these cells can be labeled efficiently with [3H]galactose. Transcytosis of [3H]galactose-labeled glycoproteins to the opposite membrane domain was detected by surface biotinylation. Detergent-solubilized glycoproteins derivatized with biotin were adsorbed onto streptavidin-agarose and separated by SDS-PAGE. A subset of the cell surface glycoproteins was shown to undergo transcytosis. Transport of these glycoproteins across the cell was time and temperature dependent. By comparative two-dimensional gel analysis, three classes of glycoproteins were defined. Two groups of glycoproteins were found to be transported unidirectionally by transcytosis, one from the apical to the basolateral surface and another from the basolateral to the apical surface. A third group of glycoproteins which has not been described previously, was found to be transported bidirectionally across the cell.

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