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.
