Differential targeting of an epithelial plasma membrane glycoprotein in polarized Madin-Darby canine kidney cells

Using monoclonal antibodies directed against the plasma membrane of Madin-Darby canine kidney (MDCK) cells, we demonstrated previously that a glycoprotein with an Mr = 23,000 (gp23) had a non-polarized cell surface distribution and was observed on both the apical and basolateral membranes (Ojakian, G. K., Romain, R. E., and Herz, R. E. (1987) Am. J. Physiol. 253, C433-C443). However, in parallel studies on MDCK clonal lines (D11, D18) with high transepithelial electrical resistances and in kidney cells in vivo it was determined that gp23 had a polarized cell surface distribution, being localized only to the basolateral membrane. The cell surface distribution of other glycoproteins was identical in both MDCK and MDCK clonal lines, indicating that MDCK cells were not deficient in the ability to properly sort membrane glycoproteins. Metabolic labeling with radioactive substrates followed by immunopurification and gel electrophoresis demonstrated that gp23 from both MDCK and MDCK clone D11 had many biochemical similarities including electrophoretic mobility, glycosylation, and palmitate incorporation. However, proteolytic digestion of gp23 from MDCK and clone D11 cells produced unique peptide maps suggesting that these closely related glycoproteins may have different primary sequences. In this report, we present evidence that the differential targeting of gp23 may be due to differences between the primary sequences of the basolateral and non-targeted proteins. The possibility that the observed differences in gp23 targeting are due to the presence of a basolateral recognition signal in gp23 from clone D11 cells is discussed.