Immunochemical studies of the inactivation of aspartate transcarbamylase by stationary phase Bacillus subtilis cells. Evidence for selective, energy-dependent degradation.

The aspartate transcarbamylase of Bacillus subtilis is stable in exponentially growing cells, but undergoes rapid, energy-dependent inactivation when growth is inhibited by nutrient depletion or addition of antibiotics or other inhibitors of metabolism. This inactivation has been analyzed by a variety of immunochemical techniques, including direct and indirect immunoprecipitation of extracts of cells labeled with 3H-amino-acids, microcomplement fixation, and neutralization of enzymatic activity. The ability of the antibody preparation to react with various denatured, chemically modified, and proteolytically degraded forms of aspartate transcarbamylase was demonstrated. All of the techniques showed that cross-reactive protein disappeared from the cells at the same rate as enzymatic activity, and that little or no immunoprecipitable material of lower than native molecular weight was detectable during inactivation. The disappearance of material cross-reactive with aspartate transcarbamylase occurred prior to the increase in protein degradation that normally occurs in stationary B. subtilis cells and proceeded at a rate at least 20 times greater than general protein degradation. The rate of disappearance was unaffected in mutant strains deficient in intracellular protease activity or in cells treated with inhibitors of protein turnover. Aspartate transcarbamylase was shown to be stable in growing cells. We conclude that the inactivation of aspartate transcarbamylase in vivo involves, or is rapidly followed by, selective, energy-dependent degradation of the protein by a system that appears to involve a previously undescribed protease of B. subtilis.