Revisiting the functional properties of NPF6.3/NRT1.1/CHL1 in xenopus oocytes

Within the Arabidopsis NPF proteins, most of the characterized nitrate transporters are low-affinity transporters, whereas the functional characterization of NPF6.3/NRT1.1 has revealed interesting transport properties: the transport of nitrate and auxin, the eletrogenicity of the nitrate transport and a dual-affinity transport behavior for nitrate depending on external nitrate concentration. However, some of these properties remained controversial and were challenged here. We functionally express WT NPF6.3/NRT1.1 and some of its mutant in Xenopus oocytes and used a combination of uptake experiments using 15N-labelled nitrate and two-electrode voltage-clamp. In our experimental conditions in xenopus oocytes, in the presence or in the absence of external chloride, NPF6.3/NRT1.1 behaves as a non-electrogenic and pure low-affinity transporter. Moreover, further functional characterization of a NPF6.3/NRT1.1 point mutant, P492L, allowed us to hypothesize that NPF6.3/NRT1.1 is regulated by internal nitrate concentration and that the internal perception site involves the P492 residue.