Plants pocket glutamine

Cell 159, 1188–1199 (2014)

The amino acid glutamine is a hub for nitrogen metabolism, accepting reduced nitrogen in nitrogen assimilation pathways and acting as an amino group donor throughout central and secondary metabolism. P_II signaling proteins, which are found in bacteria, plants and archaea, are conserved sensors of cellular nitrogen metabolism. Earlier studies have shown that P_II proteins in the chloroplasts of plants and cyanobacteria activate N-acetyl-L-glutamate kinase (NAGK)—a gateway enzyme in the arginine biosynthesis pathway—by formation of a specific P_II–NAGK complex. Unlike in prokaryotes, P_II proteins in plants have a C-terminal extension that has an unknown function despite being highly conserved. Biochemical and structural studies by Chellamuthu et al. now show that this newly termed 'Q-loop' creates a glutamine binding pocket on P_II proteins that couples glutamine sensing directly to NAGK regulation. Recombinant P_II proteins from the green alga Chlamydomonas reinhardtii and from Arabidopsis thaliana, which has a small deletion in the C-terminal extension, have almost identical biochemical properties. However, enzyme assays and surface plasmon resonance experiments on the C. reinhardtii system revealed that free glutamine and the C-terminal Q-loop of C. reinhardtii P_II (CrP_II) are required for it to bind and efficiently activate CrNAGK. To gain more molecular insight, the authors solved the X-ray crystal structure of a CrP_II–AtNAGK complex and found it to be virtually identical to the known complex of Arabidopsis proteins, featuring a hexameric toroid of NAGK capped on both sides by P_II trimers. However, differing from the Arabidopsis case, the Q-loop of CrP_II is organized into a helix-turn-helix motif that specifically binds glutamine and positions the T-loop of CrP_II, the P_II domain responsible for NAGK activation, for efficient engagement with NAGK. Comparative sequence analysis and characterization of the biochemical properties of P_II proteins from rice and a moss demonstrated that Q-loop–dependent glutamine sensing occurs in all plants except those in the Brassicaceae family, to which A. thaliana belongs.