DIFFERENTIAL EXPRESSION OF 2 RELATED AMINO-ACID TRANSPORTERS WITH DIFFERING SUBSTRATE-SPECIFICITY IN ARABIDOPSIS-THALIANA

A general amino acid permease cDNA (AAP2) was isolated from Arabidopsi s by complementation of a yeast mutant defective in citrulline uptake. Direct transport measurements in yeast show that the protein mediates uptake of L-[C-14]-citrulline and L-[C-14]-proline. Detailed analyses of the substrate specificity by competition studies demonstrate that all proteogenic amino acids are recognized by the carrier, including t hose that represent the major transport forms of reduced nitrogen in m any species, i.e. glutamine, glutamate and asparagine. Thus, AAP2 is l ess selective as compared with AAP1 and transports basic amino acids s uch as histidine as shown by expression in a histidine transport-defic ient yeast strain. The predicted polypeptide of 53 kDa is highly hydro phobic with 12 putative membrane-spanning regions and shows significan t homologies to the Arabidopsis broad specificity permease AAP1, and a limited homology to bacterial branched chain amino acid transporters, but not to any other known proteins. Alterations in the charged resid ues as compared with AAP1 in four regions might be involved in the dif ference in selectivity towards basic amino acids. Both genes are highl y expressed in developing pods indicating a role in supplying the deve loping seeds with reduced nitrogen. AAP2 is selectively expressed in t he stem and might therefore play a role in xylem-to-phloem transfer of amino acids during seed filling. Furthermore in situ hybridization sh ows that both genes are expressed in the vascular system of cotyledons in developing seedlings.