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Plant and Cell Physiology, 2000, Vol. 41, No. 11 1210-1217
© 2000 Oxford University Press

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.

Adrianus C. Borstlap1 and Jolanda A.M.J. Schuurmans

Transport Physiology Research Group, Plant Sciences, Utrecht University, Sorbonnelaan 16, NL-3584 CA Utrecht, The Netherlands

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 ({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 (Km {approx}20 µM) and a low-affinity (Km {approx}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 {Delta}{psi}. 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.

1 Corresponding author: E-mail, a.c.borstlap@bio.uu.nl; Fax: +31-30-251-8366.


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