Formation of intracellular glutamine synthetase bodies depends strongly upon cellular age and glucose availability

The enzyme glutamine synthetase serves key roles in central nitrogen metabolism, catalyzing the biosynthesis of glutamine, as well as regulating ammonia assimilation and integrating metabolic signals to balance nitrogen use. The budding yeast enzyme was recently found to form intracellular bodies (GS bodies) composed of glutamine synthetase and Hsp90 chaperones following various types of nutrient depletion or chemical stress. In order to better quantify and characterize the in vivo formation of GS bodies, we developed an assay for their formation in single yeast cells using imaging flow cytometry, which enables the quantitative measurement of rates of GS body formation and their population penetrance. Either reduction of supplied glucose, or addition of the competitive inhibitor of glycolysis, 2-deoxyglucose, markedly enhanced the formation of GS bodies. The occurrence of GS bodies increased with increasing cell size, a proxy for cell age, while treatment with rapamycin antagonized their formation. Direct measurement of GS body formation as a function of replicative age showed that mother cells exhibited a significantly higher incidence of GS bodies than daughter cells, and the frequency of GS body formation increased with increasing replicative cell age. Thus, we find that yeast glutamine synthetase bodies form in a manner strongly dependent on available glucose and increase markedly with cell age.