THE AXONAL GAMMA-AMINOBUTYRIC-ACID TRANSPORTER GAT-1 IS SORTED TO THEAPICAL MEMBRANES OF POLARIZED EPITHELIAL-CELLS

Recent studies suggest that epithelial cells and neu neurons employ si milar mechanisms to target proteins to the distinct subdomains of thei r polarized cell surface membranes. We have examined the sorting behav ior of the neuronal gamma-aminobutyric acid (GABA) transporter CAT-1 e xpressed by transfection in the polarized epithelial Madin-Darby canin e kidney (MDCK) cell line. We find that the GABA transporters endogeno usly expressed by polarized hippocampal neurons in culture are restric ted to axonal plasma membranes. In transfected MDCK cells, the GABA tr ansporter is found to be localized primarily to the apical cell surfac e when examined by immunocytochemistry, cell surface biotinylation, an d transport assay. MDCK cells exposed to hyperosmotic stress express a close relative of GAT-1, the betaine transporter (BGT-1). We find tha t BGT-1 expressed by transfection in MDCK cells accumulates predominan tly at the basolateral cell surface. These observations suggest that t he sorting information required for axonal targeting may be similar to that which mediates apical localization in epithelia. Furthermore, it would appear that despite their high degree of homology, the BGT-1 an d GAT-1 transporters manifest sorting signals which specify their targ eting to distinct cell surface domains.