SUBSTRATE-SPECIFICITY AND EXPRESSION PROFILE OF AMINO-ACID TRANSPORTERS (AAPS) IN ARABIDOPSIS

Three amino acid transporter genes (AAP3-5) were isolated from Arabido psis by complementation of a yeast mutant defective in histidine uptak e. Transport is driven against a concentration gradient and sensitive to protonophores. Analysis of the substrate specificity demonstrates t hat the carriers have a broad substrate specificity covering the major transport forms of reduced nitrogen, i.e. glutamine and glutamate. Th e transporters have similar affinities for glutamate, glutamine, and a lanine but differ with respect to valine, phenylalanine, histidine, ar ginine, and lysine. AAP3 and AAP5 efficiently transport arginine and l ysine and are involved in basic amino acid transport. The predicted po lypeptides of 53 kDa are highly hydrophobic with 12 putative membrane- spanning regions and show significant homologies to Arabidopsis amino acid transporters AAP1 and AAP2. Each of the genes has a different org an-specific expression in the plant, AAP3 is exclusively expressed in roots and AAP4 mainly in source leaves, stems, and flowers, whereas AA P5 is found in all tissues. The specific distribution in the plant and the different substrate specificities of AAP transporters may indicat e that tissues differ both qualitatively and quantitatively regarding import or export of amino acids.