Use of AMP specific antibodies to differentiate between adenylylated and unadenylylated E. coli glutamine synthetase

The Pleiotropic Effects of Glutamine Metabolism in Cancer

Metabolic programs are known to be altered in cancers arising from various tissues. Malignant transformation can alter signaling pathways related to metabolism and increase the demand for both energy and biomass for the proliferating cancerous cells. This scenario is further complexed by the crosstalk between transformed cells and the microenvironment. One of the most common metabolic alterations, which occurs in many tissues and in the context of multiple oncogenic drivers, is the increased demand for the amino acid glutamine. Many studies have attributed this increased demand for glutamine to the carbon backbone and its role in the tricarboxylic acid (TCA) cycle anaplerosis. However, an increasing number of studies are now emphasizing the importance of glutamine functioning as a proteogenic building block, a nitrogen donor and carrier, an exchanger for import of other amino acids, and a signaling molecule. Herein, we highlight the recent literature on glutamine’s versatile role in cancer, with a focus on nitrogen metabolism, and therapeutic implications of glutamine metabolism in cancer.

Refolding a glutamine synthetase truncation mutant in vitro: identifying superior conditions using a combination of chaperonins and osmolytes

A new method that uses a combination of bacterial GroE chaperonins and cellular osmolytes for in vitro protein folding is described. With this method, one can form stable chaperonin-protein folding intermediate complexes to prevent deleterious protein aggregation and, using these complexes, screen a large array of osmolyte solutions to rapidly identify the superior folding conditions. As a test substrate, we used GSDelta468, a truncation mutant of bacterial glutamine synthetase (GS) that cannot be refolded to significant yields in vitro with either chaperones or osmolytes alone. When our chaperonin/osmolyte method was employed to identify and optimize GSDelta468 refolding conditions, 67% of enzyme activity was recovered, comparable with refolding yields of wild type GS. This method can potentially be applied to the refolding of a broad spectrum of proteins.

Immunoelectron microscopy of enzymes, multienzyme complexes, and selected other oligomeric proteins

The collective term "immunoelectron microscopy" subsumes a number of techniques in which the biological material is decorated with specific antibodies, prior to being visualized in the electron microscope. In this article, we have reviewed literature on immunoelectron microscopy that focusses on the analysis of the molecular architecture of proteins, in particular of enzymes and of multienzyme complexes. Molecular immunoelectron microscopy has been remarkably successful with multi-subunit enzymes of complex quaternary structures, and in many cases the data have been the basis for the eventual development of detailed three-dimensional molecular models. The elucidation of subunit composition and juxtaposition of a given enzyme, an important accomplishment in itself, has in turn stimulated and guided discussions on the catalytic mechanism; illustrative examples include F1 ATPase and citrate lyase, among others. Here we have chosen a variety of enzymes, multienzyme complexes, and non-enzymatic proteins to demonstrate the versatility of immunoelectron microscopy, to illustrate methodological prerequisites and limitations, and to discuss significance and implications of individual immunoelectron microscopy studies.

Detection of ligand-induced perturbations affecting the biotinyl group of mammalian acetyl-coenzyme A carboxylase by using biotin-binding antibodies

Biotin-binding antibodies were raised in rabbits by injecting biotin-bovine serum albumin conjugate. Neither the protomer nor the polymer of rat mammary-gland acetyl-CoA carboxylase formed precipitin bands with the anti-biotin. By virtue of its ability to bind biotin (apparent binding constant for free biotin about 1mum), the anti-biotin inhibited the carboxylase activity under certain conditions. This property of the antibody was employed to detect the ligand-induced changes affecting the biotinyl group in different conformational states of mammalian carboxylase. Depending on the ligand present, the biotinyl group in the protomeric form was either accessible or inaccessible to the antibody. The biotinyl group of the protomer generated by a relatively high concentration of NaCl (0.5m) reacted with the antibody, and the antibody-carboxylase complex could not be converted into active enzyme by citrate. Further experiments showed that citrate failed to induce polymerization in this protomer-antibody complex and that anti-biotin could be displaced rapidly from this complex with excess of biotin. The resulting protomer was converted into the polymeric state on citrate addition, with parallel regain of enzyme activity. In the presence of ADP+Mg(2+), ATP+Mg(2+) or ATP+Mg(2+)+HCO(3) (-), however, the enzyme remained as a protomer, but its configuration was such that the biotinyl group was essentially inaccessible to the antibody. Likewise, the biotinyl group of the different polymeric forms of the carboxylase (s approximately 30-45S) engendered by phosphate, malonyl-CoA, acetyl-CoA or citrate remained essentially inaccessible, since their activity was minimally affected by the anti-biotin. In the presence of 0.15m-NaCl, the phosphate-induced polymer reverted to a approximately 19S form with concomitant appearance of anti-biotin-sensitivity, whereas the other polymeric forms remained unaffected under similar experimental conditions.

Anti-AMP antibody precipitation of multiply adenylylated forms of glutamine synthetase from Escherichia coli: a model relating both concentration and density of antigenic sites with the antibody-antigen interaction

Sheep antibodies directed against an AMP-bovine serum albumin conjugate exhibit highly specific binding toward AMP. These antibodies bind to the AMP moiety of adenylylated glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] from Escherichia coli and to no other antigenic determinant on the protein. E coli glutamine synthetase can exist in variously modified (isomeric) forms that differ with respect to the number (0-12) and the distribution of identically adenylylated subunits [Ciardi, J. E., Cimino, F. & Stadtman, E. R. (1973) biochemistry 12, 4321-4330]. Using this enzyme, together with the AMP-specific antibodies, we have investigated the effects of the total concentration, population density, and topographical distribution of multiple identical antigenic determinants on the antigen-antibody interaction. Stopped flow fluorescence measurements show that the rate and extent of initial binding of the antibodies to the antigen are a function of the total concentration of AMP groups and are independent of the number of AMP groups der dodecamer. However, the rate of lattice formation increases with increasing epitope density, and the maximal amount of glutamine synthetase precipitated is directly proportional to the average number of adenylylated subunits per dodecamer. The data suggest that partially adenylylated enzyme preparations are composed of subpopulations of glutamine synthetase molecules that differ in their tendency to form precipitable aggregates, due presumably to differences in the topographical distribution of antigenic determinants on the surface of the enzyme. The enzyme species that form soluble immune complexes do so possibly due to intramolecular crosslinkage of the bivalent antibodies with adenylylated subunits to the exclusion of intermolecular crosslinkage.

Subunit interaction of adenylylated glutamine synthetase

The glutamine synthetase (GS) activity in Escherichia coli is regulated by a bicyclic interconvertible enzyme cascade which involves the cyclic adenylylation (inactivation) and deadenylylation (activation) of GS on the one hand, and the modulation of these processes by the uridylylation and deuridylylation of Shapiro's regulatory protein on the other. The specific activity of GS in a given metabolic state is determined by the fraction of its subunits that are adenylylated, and this fraction is determined by the concentration of over 40 metabolites. Through allosteric and substrate interactions with one or more of the cascade enzymes, these metabolites alter the rates of the covalent modification and demodification reactions. By means of immunoprecipitation studies with anti-AMP specific antibodies, it has been established that the partially adenylylated glutamine synthetase, which is present in a given steady state, is a mixture of hybrid molecules containing different numbers and possibly distributions of adenylylated subunits. Partial separation of these hybrid mixtures has been achieved by affinity chromatography on Affi-Blue Sepharose columns. From immunochemical studies it is evident that anti-AMP antibodies can react with adenylylated subunits of all molecular species of GS, but that the capacities of these primary antigen-antibody reactions to yield precipitable aggregates is very dependent of the number of adenylylated subunits per molecule, and much less so upon the total concentration of adenylylated subunits present. the studies suggest that precipitability is a function either of the distribution of adenylylated subunits within hybrid species, or of the kinetics of intra- vs intermolecular bivalent interactions.