Proton-symport of L-valine in plasma membrane vesicles isolated from leaves of the wild-type and the Valr-2 mutant of Nicotiana tabacum L.

Transport of amino acids across the plasma membranes of various cell types is a key process in controlling the nitrogen balance of leaves. We studied the transport of the neutral amino acid L-valine into plasma membrane vesicles obtained by aqueous polymer two-phase partitioning of a microsomal fraction isolated from leaves of the wild-type and the Val(r)-2 mutant of tobacco (Nicotiana tabacum L.). Initial influxes were determined after the imposition of a pH-gradient (Delta pH, inside alkaline) and/or an electrical gradient (Delta psi, inside negative) across the vesicle membrane. The initial magnitudes of the imposed gradients were Delta pH=2 and Delta psi=-68 mV. In vesicles from the wild-type, the Delta pH-dependent valine influx could be analysed into a high-affinity (K-m approximate to 20 muM) and a low-affinity (K-m approximate to3 mM) component. The influx of valine by the low-affinity system was stimulated about twofold, and that by the high-affinity system more than sixfold by the imposition of Delta psi. This strong stimulation of the high-affinity system may indicate that it transports 2H(+)/amino acid. In the Val(r)-2 mutant the high-affinity component appeared to be completely absent.