Phosphorylation by MPK6: a conserved transcriptional modification mediates nitrate reductase activation and NO production?

Abstract

Nitrate reductase is a central enzyme of nitrogen assimilation in plants. In a recent work, we have revealed MPK6 could phosphorylate Arabidopsis NIA2 at the serine 627 in hinge 2 region, this phosporylation may represent a rapid activation mechnism when plant need excessive nitrate reduction. Interestingly, all eukaryotic NRs have conserved docking sequence in their FAD domains, and many plant NR proteins have the conserved MAPK phosphorylation site. Those results indicated the MAPK cascades, the conserved signaling pathway also involved in lateral root development, mediated of NR phosporylation and NO generation. We noticed that the phosphorylation of S627 residue by MPK6 have a specially influence on the NO generation activity of NIA2. Although no homology of mammalian NOS has been identified in plants, NR may still share a similar regulation mechanism with mammalian NOS.

Figure 1

MAPK phosphorylation site and docking sequence are highly conserved in NR proteins. (A) Sequence model of the Arabidopsis NIA2 protein, phosphorylation sites S534 and S627 were marked. (B) Amino acid sequence comparison around the putative MAPK phosphorylation site and docking sequence (high lighted) of NR in high plants.