Identification and characterization of chloroplast amino acid transporters in Arabidopsis thaliana

Nitrogen (N) is an essential nutrient for plant growth and required in relatively high quantities. Plants generally take up N in the form of nitrate and ammonium that are assimilated into amino acids in root plastids or in chloroplasts of mature leaves. Most of the 20 proteinogenic amino acids are produced in the chloroplasts. Therefore, various transport systems are required in the plastidial envelope membrane to move amino acids from the stroma to the cytosol for cellular metabolism or for export from the leaf to supply growing sinks with N. However, despite their importance, transporters functioning in the plastidial export of amino acids remained mostly unknown. In this graduate project, three putative chloroplast transporters were identified in Arabidopsis based on the presence of chloroplast targeting peptides, chloroplast proteome data, and transporter co-expression with genes associated with N or carbon metabolism and transport in subcellular compartments. These candidates were USUALLY MULTIPLE ACID MOVE IN AND OUT TRANSPORTER 44 (UMAMIT44), L-type amino acid transporter 5 (LAT5), and tyrosine specific transporter 1 (TyrP1). The transporters were fused to green fluorescence proteins (GFP) and their localization to chloroplasts was demonstrated. This dissertation work then characterized the three transporters in more detail and resolved their function in plastidial amino acid export and importance for cellular metabolism, long-distance transport of N and overall plant physiology.