In their recently published study, Joanne L. Parker and Simon Newstead investigate the structure and function of the plant nitrate transporter NRT1.1 which belongs to the NRT1/PTR family of proton-coupled transporters. These are responsible for nitrogen assimilation in eukaryotes and bacteria through the uptake of peptides. Nitrogen is a key element in biology since it is required for the synthesis of amino and nucleic acids and therefore is a fundamental nutrient for cellular metabolism. In most plant species, however, members of the NRT1/PTR family have evolved to transport nitrate as well as additional secondary metabolites and hormones. Interestingly, only minor changes to a previously characterized peptide transporter binding site are required to accommodate nitrate.
MicroScale Thermophoresis (MST) has been applied as the key technique to decipher the mechanism of nitrate uptake as well as to explore substrate specificity of the nitrate transporter NRT1.1. This study underlines the unmatched sensitivity of the MST technology to investigate binding of a single nitrate molecule to a large 12 transmembrane helices containing protein solubilized in detergent.
The NRT1/PTR family of proton-coupled transporters are responsible for nitrogen assimilation in eukaryotes and bacteria through the uptake of peptides. However, in most plant species members of this family have evolved to transport nitrate as well as additional secondary metabolites and hormones. In response to falling nitrate levels, NRT1.1 is phosphorylated on an intracellular threonine that switches the transporter from a low-affinity to high-affinity state. Here we present both the apo and nitrate-bound crystal structures of Arabidopsis thaliana NRT1.1, which together with in vitro binding and transport data identify a key role for His356 in nitrate binding. Our data support a model whereby phosphorylation increases structural flexibility and in turn the rate of transport. Comparison with peptide transporters further reveals show the NRT1/PTR family has evolved to recognize diverse nitrogenous ligands while maintaining elements of a conserved coupling mechanism within this superfamily of nutrient transporters.