Poster: Membrane Transport
Abs #
1210: Two action modes of dual-affinity nitrate transporter CHL1 (AtNRT1) is switched by phosphorylation/dephosphorylation
For a long time, nitrate transporter CHL1 (AtNRT1) was thought to be a low-affinity nitrate transporter. However, recent studies indicated that CHL1 is a dual-affinity nitrate transporter involved in both high- and low-affinity nitrate uptakes. Given the presence of other high-affinity nitrate transporter AtNRT2, why should CHL1 have evolved dual-uptake activities? In addition to CHL1, AtKUP1 was also shown to be a dual-affinity potassium transporter. Therefore, presence of dual-affinity transporter might be a general phenomenon for nutrient uptake. It is interesting to find out how two action modes of a dual-affinity transporter operate, and what kind of benefit plant gain by having dual-affinity transporters. Using Xenopus oocyte expression study, we demonstrated that, in the case of CHL1, switching between the two modes of action is regulated by phosphorylation at threonine residue 101; when phosphorylated, CHL1 function as a high-affinity nitrate transporter, whereas, when dephosphorylated, it function as a low-affinity nitrate transporter. This regulatory pathway was confirmed by in vivo study. Kinetic analysis indicated that expressing the CHL1 T101A mutant rescued the high-, but not the low-, affinity nitrate uptake defect of chl1 plant. In vivo phosphorylation assay and western analysis using phosphorylation-specific antibodies indicated that CHL1 phosphorylation in plants is regulated by external nitrogen conditions. This novel regulatory mechanism allows plants to change rapidly between high- and low-affinity nitrate uptake which may be critical when competing for limited nitrogen. Further studies were undertaken to elucidate how phosphorylation of CHL1 is regulated and what kind of kinase/phosphotase is involved in this process.
