Degradation of l-Glutamate Dehydrogenase from Escherichia coli: Allosteric Regulation of Enzyme Stability

l-glutamate dehydrogenase (GDH) is stable in exponentially growing Escherichia coli cells but is degraded at a rate of 20–30% per hour in cells starved for either nitrogen or carbon. GDH degradation is energy-dependent, and mutations in ATP-dependent proteases, ClpAP or Lon lead to partial stabilization. Degradation is inhibited by chloramphenicol and is completely blocked in relA mutant cells, suggesting that ribosome-mediated signaling may facilitate GDH degradation. Purified GDH has a single tight site for NADPH binding. Binding of NADPH in the absence of other ligands leads to destabilization of the enzyme. NADPH-induced instability and sensitivity to proteolysis is reversed by tri- and dicarboxylic acids or nucleoside di- and triphosphates. GTP and ppGpp bind to GDH at an allosteric site and reverse the destabilizing effects of NADPH. Native GDH is resistant to degradation by several purified ATP-dependent proteases: ClpAP, ClpXP, Lon, and ClpYQ, but denatured GDH is degraded by ClpAP. Our results suggest that, in vivo, GDH is sensitized to proteases by loss of a stabilizing ligand or interaction with an destabilizing metabolite that accumulates in starving cells, and that any of several ATP-dependent proteases degrade the sensitized protein.