PLASMA-MEMBRANE CLUSTERING OF SYSTEM Y(-1) AMINO-ACID TRANSPORTER AS DETECTED BY IMMUNOHISTOCHEMISTRY() (CAT)

Transport of cationic amino acids in fully differentiated mammalian ce lls is mediated primarily by system y(1)(+) [cationic amino acid trans porter (CAT)-1 gene product]. Antibodies, prepared against synthetic p eptide sequences predicted to be extracellular loops of the CAT-1 tran sporter protein, detected the transporter on the surface of cultured c ells. In human fibroblasts, porcine pulmonary artery endothelial cells , and cultured rat hepatoma cells, the CAT-1 transporter protein was c lustered in an apparent random pattern throughout the plasma membrane. In contrast, labeling of the fibroblasts with antibodies against the epidermal growth factor receptor or the GLUT-1 glucose transporter dem onstrated a uniform staining pattern covering the entire cell surface. The CAT-1 antibody labeling was specific, as demonstrated by peptide inhibition and the lack of staining by preimmune serum. Furthermore, h epatocytes did not exhibit specific antibody binding consistent with t he lack of system y(1)(+) activity. Disruption of the microtubule asse mbly resulted in a reversible loss of the CAT-1 transporter clusters a nd a more generalized labeling of the cell body. The data demonstrate the existence of microdomains within the plasma membrane that contain the CAT-1 transporter protein.