Molecular Mechanism Underlying the Plant NRT1.1 Dual-Affinity Nitrate Transporter.

Ji Sun,Ning Zheng

doi:10.3389/fphys.2015.00386

Abstract

Nitrate () is one of the most important sources of mineral nitrogen, which also serves as a key signaling molecule for plant growth and development. To cope with nitrate fluctuation in soil that varies by up to four orders of magnitude, plants have evolved high- and low-affinity nitrate transporter systems, consisting of distinct families of transporters. Interestingly, the first cloned nitrate transporter in Arabidopsis, NRT1.1 functions as a dual-affinity transporter, which can change its affinity for nitrate in response to substrate availability. Phosphorylation of a threonine residue, Thr101, switches NRT1.1 from low- to high-affinity state. Recent structural studies have unveiled that the unmodified NRT1.1 transporter works as homodimers with Thr101 located in close proximity to the dimer interface. Modification on the Thr101 residue is shown to not only decouple the dimer configuration, but also increase structural flexibility, thereby, altering the substrate affinity of NRT1.1. The structure of NRT1.1 helps establish a novel paradigm in which protein oligomerzation and posttranslational modification can synergistically expand the functional capacity of the major facilitator superfamily (MFS) transporters.

Highlights

Nitrate (NO−3 ) is critical for plants, both as a primary nutrient and as an important signaling molecule (Crawford, 1995; Krouk et al, 2010a)
In Arabidopsis thaliana, one of the most well studied nitrate transporters is NRT1.1 (CHL1 or NPF6.3), which is a multifunctional protein with a crucial role in both nitrate acquisition and signaling
NRT1.1 has recently been shown to serve as a nitrate sensor, regulating the gene expression of other nitrate transporters such as NRT2.1 (Krouk et al, 2006; Ho et al, 2009)

Summary

INTRODUCTION

Nitrate (NO−3 ) is critical for plants, both as a primary nutrient and as an important signaling molecule (Crawford, 1995; Krouk et al, 2010a). Nitrate functions as a critical signaling ion and regulates many aspects of plant growth and development (Castaings et al, 2011; Bouguyon et al, 2012), including nitrate-related gene expression (Wang et al, 2003), root architecture (Forde, 2014), seed dormancy (Alboresi et al, 2005), and flowering time (Castro Marín et al, 2011). Active nitrate uptake through membrane transporters by plant roots represents the key first step of nitrogen acquisition (Dechorgnat et al, 2011). Review on NRT1.1 Structure to the nitrate-regulated auxin translocation besides nitrate transport and sensing (Krouk et al, 2010b). We will mainly focus on the transporter functions of NRT1.1 and summarize recent structure-function studies, which have shed light on the molecular mechanism underlying its dual-affinity activity

Nitrate Transporters in Arabidopsis

Substrate Recognition and Proton

MFS oligomerization