Subunit Interaction of Adenylylated Glutamine Synthetase

Abstract

The activity of Escherichia coli glutamine synthetase (GS) is regulated by the cyclic adenylylation and deadenylylation of a unique tyrosyl group in each subunit (for review, see Stadtman and Ginsburg 1974). As illustrated in Fig. 1, the adenylylation reaction is catalyzed by an adenylyltransferase (AT) and involves transfer of adenylyl groups from ATP to the enzyme with the concomitant formation of PPi. Since GS is composed of twelve identical subunits, and adenylylated subunits are catalytically inactive under most physiological conditions, the catalytic potential of the enzyme is nearly inversely proportional to the average number, n̅, of covalently bound adenylyl groups per enzyme molecule (Kingdon et al. 1967). Although the adenylylation reaction is reversible in the presence of PPi (Mantel and Holzer 1970), removal of the adenylyl group from GS under physiological conditions is achieved by phosphorolysis of the adenylyl-O-tyrosyl bond to yield ADP and unmodified GS (Anderson and Stadtman 1970). Because the adenylylation and deadenylylation reactions are catalyzed at separate, noninteracting catalytic sites on a single adenylyltransferase (Anderson et al. 1970; Rhee et al. 1978; Hennig and Ginsburg 1971), it is evident that in the absence of appropriate regulation the two reactions will be tightly coupled resulting simply in senseless phosphorolysis of ATP to ADP and PPi. As shown if Fig. 1, indiscriminate coupling of the adenylylation and deadenylylation reaction is prevented by the action of Shapiro’s regulatory protein, P_II, (Shapiro 1969) which also exists in an unmodified form, P_IIA, and a modified (uridylylated) form, P_IID (Brown et al. 1971; Mangum et al. 1973; Adler et al. 1975).