Molecular basis of nitrate uptake by the plant nitrate transporter NRT1.1

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 His 356 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 how 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. In Arabidopsis thaliana the phosphorylation state of the ‘dual affinity’ transporter, NRT1.1, allows the uptake of nitrate over a wide concentration range; the crystal structure and molecular basis for this is described in this study. Soil levels of nitrate, a primary nutrient for plant growth, can vary dramatically. Plants therefore need a versatile mechanism for obtaining nitrate from the environment. In the model plant Arabidopsis thaliana, the dual-affinity transporter NRT1.1 can take up nitrate across a broad range of concentrations, switching from low- to high-affinity mode according to the phosphorylation status of a key threonine residue. Two studies published in this issue of Nature describe the crystal structures of full-length NRT1.1, providing insights into how this post-translational modification switches the transporter between the low-affinity and high-affinity states.