The axonal gamma-aminobutyric acid transporter GAT-1 is sorted to the apical membranes of polarized epithelial cells.

Recent studies suggest that epithelial cells and neurons employ similar mechanisms to target proteins to the distinct subdomains of their polarized cell surface membranes. We have examined the sorting behavior of the neuronal gamma-aminobutyric acid (GABA) transporter GAT-1 expressed by transfection in the polarized epithelial Madin-Darby canine kidney (MDCK) cell line. We find that the GABA transporters endogenously expressed by polarized hippocampal neurons in culture are restricted to axonal plasma membranes. In transfected MDCK cells, the GABA transporter is found to be localized primarily to the apical cell surface when examined by immunocytochemistry, cell surface biotinylation, and transport assay. MDCK cells exposed to hyperosmotic stress express a close relative of GAT-1, the betaine transporter (BGT-1). We find that BGT-1 expressed by transfection in MDCK cells accumulates predominantly at the basolateral cell surface. These observations suggest that the 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 and GAT-1 transporters manifest sorting signals which specify their targeting to distinct cell surface domains.