Purification of a protease from Escherichia coli with specificity for oxidized glutamine synthetase

Abstract

A soluble Escherichia coli protease has been identified and purified to homogeneity. The protease cleaves glutamine synthetase which has been modified by mixed function oxidation; native glutamine synthetase is not a substrate. Using [14C]glutamine synthetase as a substrate (prepared by growing E. coli on 14C-labeled amino acids), protease activity was assayed by determining the release of trichloroacetic acid-soluble material. The pure protease cleaves glutamine synthetase near the carboxyl terminus yielding 4,500 and 47,000 Mr products. The characteristics of this enzyme distinguish it from proteases previously purified from E. coli. These characteristics include a molecular weight of 75,000, alkaline pH optimum, lack of inhibition by serine protease inhibitors, and the ability to degrade insulin and casein. Oxidation of glutamine synthetase and other enzymes can be catalyzed by a variety of mixed function oxidase systems from bacterial and mammalian sources. Mixed function oxidation may be a "signal" or "marker" which consigns a protein for proteolytic degradation. Susceptibility to oxidation is subject to metabolic regulation, thereby providing control of proteolytic turnover. Isolation of a protease specific for modified glutamine synthetase provides the enzymatic basis for the specificity of this scheme.