Functional Characterization of Betaine/Proline Transporters in Betaine-accumulating Mangrove

Betaine is an important osmoprotectant in many plants, but its transport activity has only been demonstrated using a proline transporter from tomato, a betaine-nonaccumulating plant. In this study, two full-length and one partial transporter genes were isolated from betaine-accumulating mangrove Avicennia marina. Their homologies to betaine transporters from bacteria and betaine/4-aminobutyrate transporters from mammalian cells were low but were high to proline transporters from Arabidopsis and tomato. Two full-length transporters could complement the Na+-sensitive phenotype of the Escherichia coli mutant deficient in betT, putPA, proP, and proU. Both transporters could efficiently take up betaine and proline with similar affinities (Km, 0.32–0.43 mm) and maximum velocities (1.9–3.6 nmol/min/mg of protein). The uptakes of betaine and proline were significantly inhibited by mono- and dimethylglycine but only partially inhibited by betaine aldehyde, choline, and 4-aminobutyrate. Sodium and potassium chloride markedly enhanced betaine uptake rates with optimum concentrations at 0.5 m, whereas sucrose showed only modest activation. The change of amino acids Thr290-Thr-Ser292 in a putative periplasmic loop to Arg290-Gly-Arg292yielded the active transporter independent of salts, suggesting the positive charge induced a conformational change to the active form. These data clearly indicate that the betaine-accumulating mangrove contains betaine/proline transporters whose properties are distinct from betaine transporters of bacteria and mammalian cells.