NA-INDEPENDENT TRANSPORT (UNIPORT) OF AMINO-ACIDS AND GLUCOSE IN MAMMALIAN-CELLS()

Recent advances have made possible the isolation of the genes and thei r cDNAs encoding Na+-independent amino acid transporters. Two classes of amino acid 'uniporters' have been isolated. One class contains the mCAT (murine cationic amino acid transporter) gene family that encodes proteins predicted to span the membrane 12-14 times and exhibits stru ctural properties similar to the GLUT (glucose transporter) family and to other well-known transporters. The other class consists of two kno wn genes, rBAT (related to B system amino acid transporters) and 4F2hc , that share amino acid sequence similarity with alpha-amylases and al pha-glucosidases. They are type II glycoproteins predicted to span the membrane only once, yet they mediate the Na+-independent transport of cationic and zwitterionic amino acids in Xenopus oocytes. Mutations i n the human rBAT gene have been identified by Palacin and his co-worke rs in several families suffering from a heritable form of cystinuria. This important finding clearly establishes a key role for rBAT in cyst ine transport. The two classes of amino acid transporters are compared with the well-studied GLUT family of Na+-independent glucose transpor ters.