Na+ -dependent amino acid transporters: their structure and function

Na+ -dependent amino acid transporters play essential roles in the uphill transport of amino acids across plasma membrane. Recent molecular cloning studies have established two families of Na+ -dependent transporters which include amino acid transporters: Na+/Cl- -dependent transporter family and Na+/K+ -dependent transporter family. Although each family includes transporters for various substrates, their amino acid binding sites appear to be similar in their spacial configuration within the family. Na+/K+ -dependent transporter family comprises glutamate transporters and system ASC-like neutral amino acid transporters. The newly isolated ASC transporter ASCT2 is distinct from the other members of the family in its substrate selectivity and ion coupling stoichiometry. The comparison of its primary structure and functional characteristics with the other members of the family, combined with results from detailed kinetic analyses, will provide clues to the dynamic structures of the transporters.