Identification of a basolateral membrane targeting signal within the cytoplasmic domain of myelin/oligodendrocyte glycoprotein

Oligodendrocytes possess two distinct membrane compartments--uncompacted plasma membrane (cell body, processes) and compact myelin. Specific targeting mechanisms must exist to establish and maintain these membrane domains. Polarized epithelial cells have the best characterized system for targeting components to apical and basolateral compartments. Since oligodendrocytes arise from neuroepithelial cells, we investigated whether they might utilize targeting paradigms similar to polarized epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is a transmembrane Ig-like molecule restricted to uncompacted oligodendroglial plasma membrane. We stably expressed MOG in Madin-Darby canine kidney (MDCK) Type II epithelial cells, which have been extensively used in protein-targeting studies. Data from surface biotinylation assays and confocal microscopy revealed that MOG sorts exclusively to the basolateral membrane of MDCK cells. Expression vectors containing progressive truncations of MOG from the cytoplasmic C-terminus were expressed in MDCK cells to localize basolateral sorting signals. A loss of only four C-terminal residues results in some MOG expression at the apical surface. More strikingly, removal of the C-terminal membrane associated hydrophobic domain from MOG results in complete loss of basolateral sorting and specific targeting to the apical membrane. These data suggest that myelinating oligodendrocytes may utilize a sorting mechanism similar to that of polarized epithelia.