The role of carbohydrates in vectorial exocytosis. The secretion of the gp 80 glycoprotein complex in a ricin-resistant mutant of MDCK cells.

In the polarized epithelial Madin-Darby canine kidney (MDCK) cell line an 80 kDa glycoprotein complex (gp 80) is sorted into the apical pathway of exocytosis and is secreted constitutively at the apical cell surface. The unglycosylated form of the protein complex is secreted in a nonpolar fashion at both surface domains [(1987) J. Cell. Biol. 105, 2735-2743]. Using ricin-resistant MDCK cells the role of the terminal galactose and sialic acid residues in the sorting of the gp 80 complex was analysed. The results suggest that the carbohydrate cores, rather than the ultimate or penultimate sugar residues, play a critical role in the intracellular transport of this protein.