Identification and characterization of GABA, proline and quaternary ammonium compound transporters from Arabidopsis thaliana

Arabidopsis thaliana grows efficiently on GABA as the sole nitrogen source, thereby providing evidence for the existence of GABA transporters in plants. Heterologous complementation of a GABA uptake-deficient yeast mutant identified two previously known plant amino acid transporters, AAP3 and ProT2, as GABA transporters with Michaelis constants of 12.9±1.7 and 1.7±0.3 mM at pH 4, respectively. The simultaneous transport of 1-¹⁴C]GABA and 2,3-³H]proline by ProT2 as a function of pH, provided evidence that the zwitterionic state of GABA is an important parameter in substrate recognition. ProT2-mediated 1-¹⁴C]GABA transport was inhibited by proline and quaternary ammonium compounds.