MOLECULAR CHARACTERIZATION OF GP40, A MUCIN-TYPE GLYCOPROTEIN FROM THE APICAL PLASMA-MEMBRANE OF MADIN-DARBY CANINE KIDNEY-CELLS (TYPE-I)

gp40 has been recently identified as a major apical cell-surface sialo glycoprotein of type-I Madin-Darby canine kidney cells, a cell line wi dely used for the study of polarized transport. The determination of t wo internal amino acid sequences of the purified glycoprotein by Edman degradation enabled us to isolate the cDNA encoding the 18.6 kDa prot ein backbone of gp40. Sequence analysis revealed that gp40 is a type-I membrane protein which has several characteristics in common with gly cophorin A and other mucin-type glycoproteins. At least 14 serine/thre onine residues were found to be used for O-glycosylation. No potential sites for N-glycosylation were detected. gp40 turned out to represent the canine homologue of a cell-surface antigen expressed by various e pithelial and nonepithelial cells in rat and mouse. Potential O-glycos ylation sites, transmembrane and cytoplasmic domains were found to be highly conserved in the three species. gp40 was detected in canine lun g, intestine, kidney, brain and heart but not in liver and spleen. The subline II of Madin-Darby canine kidney cells was found not to expres s gp40. Stable expression of gp40 in transfected type-II cells reveale d that gp40 is predominantly delivered to the apical plasma membrane. N-Glycans and a glycosylphosphatidylinositol anchor, both proposed api cal targeting signals, are absent from gp40, indicating that other det erminants are responsible for its polarized transport.