We have identified a new amino acid transporter from the Arabidopsis thaliana expressed sequence tag cDNA collection by functional complementation of a yeast amino acid transport mutant. Transport analysis of the expressed protein in yeast shows that it is a high-affinity transporter for both lysine (Lys) and histidine with Michaelis constant values of 175 and 400 (mu)M, respectively. This transporter (LHT1, lysine histidine transporter) has little affinity for arginine when measured directly in uptake experiments or indirectly with substrate competition. The cDNA is 1.7 kb with an open reading frame that codes for a protein with 446 amino acids and a calculated molecular mass of 50.5 kD. Hydropathy analysis shows that LHT1 is an integral membrane protein with 9 to 10 putative membrane-spanning domains. Southern-blot analysis suggests that LHT1 is a single-copy gene in the Arabidopsis genome. RNA gel-blot analysis shows that this transporter is present in all tissues, with the strongest expression in young leaves, flowers, and siliques. Wholemount, in situ hybridization revealed that expression is further localized on the surface of roots in young seedlings and in pollen. Overall, LHT1 belongs to a new class of amino acid transporter that is specific for Lys and histidine, and, given its substrate specificity, it has significant promise as a tool for improving the Lys content of Lys-deficient grains.
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