In vitro characterization of iron-phytosiderophore interaction with maize root plasma membranes: evidences for slow association kinetics.

As an attempt to characterize iron(III)-phytosiderophore transport across plant membranes in vitro, a rapid filtration approach was set up in which plasma membrane vesicles from maize roots were incubated with 55Fe-labelled deoxymugineic acid (DMA). Fe-DMA, and not Fe-EDTA, could associate with plasma membrane vesicles. The rate of Fe-DMA association decreased with a half time of 15 min. The initial Fe-DMA association rate, estimated from the amount of Fe-DMA associated after 10 min incubation, exhibited a saturation curve as a function of Fe-DMA concentration. This curve could be satisfactorily fitted to the Michaelis-Menten model (KM=600 nM, Vmax=2 nmol min-1 mg-1 protein). The association rate of Fe-DMA with control liposomes remained negligible and constant in a pH range from 4 to 8, whereas it strongly increased at acidic pH with plasma membrane vesicles. However, the specific association of Fe-DMA to root plasma membrane could not be explained by a vesicle-filling process because: (i) lowering the vesicle volume by decreasing the osmotic potential of the assay medium with sorbitol did not decrease 55(Fe) labelling of the vesicles, (ii) creating inside-out vesicles by a Brij-58 treatment had almost no effect on Fe-DMA association to vesicles, (iii) 55(Fe) labelling is reversible by EDTA and excess free DMA, and (iv) 55(Fe) labelling was the same using plasmalemma vesicles prepared either from wild type maize or from the ys1 maize mutant deficient in iron-phytosiderophore transport. A model is proposed to account for the observed Fe-DMA association as the result of very slow binding kinetics onto membrane proteins. This model was validated by its ability to describe quantitatively both Fe-DMA association as a function of time and of substrate concentration. A prediction of the model was that association of Fe-DMA to plasma membranes might overcome a high activation energy barrier. Indeed, the Arrhenius plot for the association rate constant was linear with an activation energy of 64 kJ mol-1. Copyright 1998 Elsevier Science B.V.