8.A.20 The Plant/Algal/Chlorella Nitrate Transporter Accessory Protein (NAR2.1) Family
AtNRT2.1 is a component of a plasma membrane, Arabidopsis thaliana, two-component, inducible, high-affinity, nitrate transport system. Yong et al. (2010) used immunological and transient protoplast expression methods to demonstrate that an intact two-component complex of AtNRT2.1 and AtNAR2.1 (AtNRT3.1) is localized in the plasma membrane. A. thaliana mutants lacking AtNAR2.1 exhibit extremely poor growth on low nitrate as the sole source of nitrogen. Using anti-NRT2.1 antibodies, a purified oligomeric polypeptide (approximate molecular mass 150kDa) was identified, but monomeric AtNRT2.1 was absent. This oligomer was also observed in the wild-type, and was resolved, using SDS-PAGE for the second dimension, into two polypeptides with molecular masses of approximately 48 and 26kDa, corresponding to AtNRT2.1 and myc-tagged AtNAR2.1, respectively. The oligomer was absent from NRT2.1 or NAR2.1 mutants, suggesting that this complex is the active nitrate transporter. The molecular mass of the intact oligomer suggests that the functional unit for high-affinity nitrate influx may be a tetramer consisting of two subunits each of AtNRT2.1 and AtNAR2.1. NAR2 has two putative TMSs at its N- and C- termini. The Chlorella protein is shorter and lacks the N-terminal TMS.