New enzymic assays for glutamine synthetase adenylytransferase and its regulatory protein PIIA

Purification of P(II) and P(II)-UMP and in vitro studies of regulation of glutamine synthetase in Rhodospirillum rubrum

The P(II) protein from Rhodospirillum rubrum was fused with a histidine tag, overexpressed in Escherichia coli, and purified by Ni(2+)-chelating chromatography. The uridylylated form of the P(II) protein could be generated in E. coli. The effects on the regulation of glutamine synthetase by P(II), P(II)-UMP, glutamine, and alpha-ketoglutarate were studied in extracts from R. rubrum grown under different conditions. P(II) and glutamine were shown to stimulate the ATP-dependent inactivation (adenylylation) of glutamine synthetase, which could be totally inhibited by alpha-ketoglutarate. Deadenylylation (activation) of glutamine synthetase required phosphate, but none of the effectors studied had any major effect, which is different from their role in the E. coli system. In addition, deadenylylation was found to be much slower than adenylylation under the conditions investigated.

Distance changes at the regulatory and catalytic sites on Escherichia coli glutamine synthetase: a spin label study on the effect of substrate(s) binding

A spin-labeled ATP analogue, 2,2,6,6-tetramethylpiperidine-1-oxyl adenosine triphosphatase (Tempo-ATP) is used to adenylate Escherichia coli glutamine synthetase (L-glutamine: ammonia ligase (ADP-forming), EC 6.3.1.2). The Tempo adenylylated glutamine synthetase (Tempo-GS) exhibits similar catalytic properties, pH profile and inhibitor susceptibility as those of glutamine synthetase adenylylated with normal ATP. Using the spin label on the enzyme as a probe and employing the spin-spin interactions between the label probe and paramagnetic Mn2+, the distances from the nitroxyl moiety of the covalently bound Tempo-AMP to the two Mn2+ binding sites, n1 and n2 were determined. The n1 site is the structural site and n2 is located at the catalytic site. The distances from Mn2+ at n1 and n2 sites to the nitroxyl radical are 19 and 16 A, respectively. Binding of the substrate, L-Glu, causes a protein conformational change which is reflected by the reduction of approximately 2 A for the n1 to Tempo-AMP distance and lengthening of approximately 2 A for the n2 to the Tempo-AMP distance. Addition of ATP to the Tempo-GS/L-Glu complex increases the distance between n1 and Tempo-AMP, and n2 and Tempo-AMP by 4 and 3 A, respectively.