Proton-Peptide Co-Transport in Broad Bean Leaf Tissues

The transport of [14C]glycyl-glycine (Gly-Gly) has been characterized in leaf discs from mature exporting leaves of broad bean (Vicia faba L.). In terms of glycine (Gly) equivalents, the rate of transport of Gly-Gly was similar to that of Gly uptake. Uptake of Gly-Gly was localized mainly in the mesophyll cells, with little accumulation in the veins. It was optimal at pH 6.0, sensitive to thiol reagents and metabolic inhibitors, and exhibited a single saturable phase with an apparent Michaelis constant of 16 mM. Gly-Gly did not inhibit the uptake of labeled Gly. Addition of Gly-Gly induced a concentration-dependent pH rise in the medium, showing that peptide uptake is mediated with proton co-transport. Gly-Gly also induced a concentration-dependent transmembrane depolarization of mesophyll cells with an apparent Michaelis constant of 15 mM. This depolarization was followed by a transient hyperpolarization. When present at a 10-fold excess, various peptides and tripeptides were able to inhibit Gly-Gly uptake with the following decreasing order of efficiency: Gly-Gly-Gly = leucine-Gly > Gly-tyrosine > Gly-glutamine = Gly-glutamic acid > Gly-phenylalanine > Gly-threonine > Gly-aspartic acid = Gly-asparagine = aspartic acid-Gly. Gly inhibited the uptake of Gly-Gly only slightly, whereas tetraGly and the tripeptide glutathione were not inhibitory. The dipeptides inhibiting Gly-Gly uptake also induced changes in the transmembrane potential difference of mesophyll cells and were able to affect in a complex way the response normally induced by Gly-Gly. Altogether, the data demonstrate the existence of a low-affinity, broad-specificity H+/peptide co-transporter at the plasma membrane of mesophyll cells. The physiological importance of this transporter for the exchange of nitrogenous compounds in mature leaves remains to be determined, as do the details of the electrophysiological events induced by the dipeptides.