Protease So from Escherichia coli preferentially degrades oxidatively damaged glutamine synthetase

After oxidative damage (e.g. induced with iron, ascorbate, and oxygen), the inactivated glutamine synthetase is selectively hydrolyzed in extracts of Escherichia coli. We therefore tested if glutamine synthetase treated with this system is hydrolyzed preferentially by any of the known E. coli proteases. Protease So, a cytoplasmic serine protease, was found to degrade the oxidized form of glutamine synthetase to acid-soluble peptides 5-10 times faster than the native glutamine synthetase. Degradation of the oxidized glutamine synthetase was inhibited by EDTA and stimulated 5-10-fold by Mg2+, Ca2+, or Mn2+, even though casein hydrolysis by protease So is not affected by divalent cations. Apparently, these cations affect the conformation of this substrate, making it more susceptible to proteolytic attack. Protease Re, another cytoplasmic protease, also degrades preferentially the oxidized form of glutamine synthetase and seems to correspond to the glutamine synthetase-degrading activity recently described by Roseman and Levine [1987) J. Biol. Chem. 262, 2101-2110). However, it is much less active in this reaction than protease So. No other soluble E. coli protease, including Do, Ci, Mi, Fa, Pi, or the ATP-dependent proteases Ti and La (the lon product), appears to degrade this oxidized protein. These results suggest that protease So participates in the hydrolysis of oxidatively damaged proteins and that E. coli has multiple systems for degrading different types of aberrant proteins.