Lysine-372-dependent SUMOylation inhibits the enzymatic activity of glutamine synthases

Glutamine synthetase (GS) is a crucial enzyme involved in de novo synthesis of glutamine and participates in several biological processes, including nitrogen metabolism, nucleotide synthesis, and amino acid synthesis. Post-translational modification makes GS more adaptable to the needs of cells, and acetylation modification of GS at double sites has attracted considerable attention. Despite very intensive research, how SUMOylation affects GS activity at a molecular level remains unclear. Here, we report that previously undiscovered GS SUMOylation which is deficient mutant K372R of GS exhibits more bluntness under glutamine starvation. Mechanistically, glutamine deprivation triggers the GS SUMOylation, and this SUMOylation impaired the protein stability of GS, within a concomitant decrease in enzymatic activity. In addition, we identified SAE1, Ubc9, and PIAS1 as the assembly enzymes of GS SUMOylation respectively. Furthermore, Senp1/2 functions as a SUMO-specific protease to reverse the SUMOylation of GS. This study provides the first evidence that SUMOylation serves as a regulatory mechanism for determining the GS enzymatic activity, contributing to understanding the GS regulation roles in various cellular and pathophysiological processes.