New glycoprotein-associated amino acid transporters

The L-type amino acid transporter LAT1 has recently been identified as being a disulfide-linked "light chain" of the ubiquitously expressed glycoprotein 4F2hc/CD98. Several LAT1-related transporters have been identified, which share the same putative 12-transmembrane segment topology and also associate with the single transmembrane domain 4F2hc protein. They display differing amino acid substrate specificities, transport kinetics and localizations such as, for instance, y(+)LAT1 which is localized at the basolateral membrane of transporting epithelia, and the defect of which causes lysinuric protein intolerance. The b(0,+)AT transporter which associates with the 4F2hc-related rBAT protein to form the luminal high-affinity diamino acid transporter defective in cystinuria, belongs to the same family of glycoprotein-associated amino acid transporters (gpaATs). These glycoprotein-associated transporters function as amino acid exchangers. They extend the specificity range of vectorial amino acid transport when located in the same membrane as carriers that unidirectionally transport one of the exchanged substrates. gpaATs belong to a phylogenetic cluster within the amino acid/polyamine/choline (APC) superfamily of transporters. This cluster, which we designate the LAT family (named after its first vertebrate member), includes some members from nematodes, yeast and bacteria. The latter of these proteins presumably lack association with a second subunit. In this review, we focus on the animal members of the LAT cluster that form, together with some of the nematode members, the family of glycoprotein-associated amino acid transporters (gpaAT family).