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

Abstract

SummaryThe NRT1/PTR family of proton-coupled transporters are responsible for nitrogen assimilation in eukaryotes and bacteria through the uptake of peptides. However, in the majority of 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 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 how the NRT1/PTR family has evolved to recognize diverse nitrogenous ligands, whilst maintaining elements of a conserved coupling mechanism within this superfamily of nutrient transporters.

Scaling Laws in Phytoplankton Nutrient Uptake Affinity

“…In the estimation of the p, which is expressed as a fraction rather than a true density, we assumed a transporter opening (s) of 1 nm. This value appears to be close to the opening of a phosphate transporter (Huang et al, 2003;Pedersen et al, 2013), while a value closer to 10 nm was reported for a nitrate transporter (Parker and Newstead, 2014). We note, that the p estimates are affected by the actual choice of s. For example, the p estimates (x-axis in Figure 1) becomes 10 times higher if s is set equal to 10 instead of 1 nm.…”

“…where s is the radius of the effective catch area of an uptake site (Aksnes and Egge, 1991) which can be taken as the radius of the opening of a transporter (Parker and Newstead, 2014). The transporter density (p), is the total catch area of all uptake sites divided by the cell surface, i.e., p = nπs 2 /(4πr 2 ).…”

NRT1.5/NPF7.3 Functions as a Proton-Coupled H+/K+ Antiporter for K+ Loading into the Xylem in Arabidopsis

“…The substrate specificity of NRT1/PTR transporters is mainly determined by the specific side chains that line the aqueous pocket at the center of the membrane (Parker and Newstead, 2014;Sun et al, 2014;Yan, 2015). However, how plant NRT1/PTR transporters are able to recognize and transport diverse substrates remains unknown.…”

“…Since the cytoplasmic K + concentrations in root parenchyma cells are over dozens of millimolars, NRT1.5 should be a low-affinity H + /K + antiporter. According to the crystal structure of NRT1.1, the "EXXER" motif (containing three conserved residues) on TMH1 together with the conserved residue Lys-164 on TMH4 is responsible for proton coupling (Doki et al, 2013;Parker and Newstead, 2014;Sun et al, 2014). These four charged residues are highly conserved in most Arabidopsis NRT1 members.…”

Reducing the Genetic Redundancy of Arabidopsis PHOSPHATE TRANSPORTER1 Transporters to Study Phosphate Uptake and Signaling

“…Alternatively, some PHT1 members could present a dual affinity, a view put forth in this study for Arabidopsis. A characteristic already identified for other transporters such as the K Cup1/Arabidopsis K + Transporter1 (Fu and Luan, 1998) and the nitrate NRT1;1 (Parker and Newstead, 2014;Sun et al, 2014). The posttranslational modifications for PHT1 involved in such mechanism remain to be identified.…”