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 Valr-2 mutant of tobacco (Nicotiana tabacum L.). Initial influxes were determined after the imposition of a pH-gradient (ΔpH, inside alkaline) and/or an electrical gradient (Δψ, inside negative) across the vesicle membrane. The initial magnitudes of the imposed gradients were ΔpH=2 and Δψ=−68 mV. In vesicles from the wild-type, the ΔpH-dependent valine influx could be analysed into a high-affinity (Km ≈20 µM) and a low-affinity (Km ≈3 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 Δψ. This strong stimulation of the high-affinity system may indicate that it transports 2H+/amino acid. In the Valr-2 mutant the high-affinity component appeared to be completely absent.