Differences in the regulation mechanisms of the glutamine synthetase from methanogenic archaea unveiled by structural investigations

Abstract

Glutamine synthetases catalyze the ATP-dependent ammonium assimilation, the initial step of nitrogen 18 acquisition that must be tightly regulated to fit cellular needs. While their catalytic mechanisms and 19 regulation are well-characterized in bacteria and eukaryotes, only limited knowledge exists about the 20 archaeal representatives. Here, we natively purified the glutamine synthetases type I-α from 21 Methanothermococcus thermolithotrophicus and Methermicoccus shengliensis, two thermophilic 22 methanogens belonging to different orders. Biochemical investigations combined with X-ray 23 crystallography unveiled the first structures of archaeal glutamine synthetases and highlighted 24 differences in their regulation. The enzyme from M. thermolithotrophicus is inactive in its resting state 25 and employs 2-oxoglutarate as an on-switch. The 2-oxoglutarate acts as a sensor of cellular nitrogen 26 deficiency, and its reported cellular concentration remarkably overlays with that required for the enzyme 27 activation. Its binding to an allosteric pocket leads to the reconfiguration of the active site and promotes 28 a catalytically competent state. The homolog from M. shengliensis does not harbor the 2-oxoglutarate