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Bacillus subtilis possesses two glyceraldehyde-3-phosphate dehydrogenases with opposite roles, the glycolytic NAD-dependent GapA and the NADP-dependent GapB enzyme, which is exclusively required during gluconeogenesis but not active under conditions promoting glycolysis. We propose that proteins that are no longer needed will be recognized and proteolyzed by Clp proteases and thereby recycled. To test this postulation, we analyzed the stability of the glycolytic enzyme GapA and the gluconeogenetic enzyme GapB in the presence and absence of glucose. It turned out that GapA remained rather stable under both glycolytic and gluconeogenetic conditions. In contrast, the gluconeogenetic enzyme GapB was degraded after a shift from malate to glucose (i.e., from gluconeogenesis to glycolysis), displaying an estimated half-life of approximately 3 h. Comparative in vivo pulse-chase labeling and immunoprecipitation experiments of the wild-type strain and isogenic mutants identified the ATP-dependent ClpCP protease as the enzyme responsible for the degradation of GapB. However, arginine protein phosphorylation, which was recently described as a general tagging mechanism for protein degradation, did not seem to play a role in GapB proteolysis, because GapB was also degraded in a mcsB mutant, lacking arginine kinase, in the same manner as in the wild type. [ABSTRACT FROM AUTHOR]