InterPro domain: IPR011322

This superfamily represents a structural domain found in the nitrogen regulatory protein PII, in ATP phosphribosyltransferases (C-terminal domain), in the divalent ion tolerance protein CutA1, and in some bacterial hypothetical proteins. This domain consists of a ferredoxin-like alpha/beta sandwich, which forms trimeric structures with orthogonally packed beta-sheets around a three-fold axis.

PII is a tetrameric protein encoded by glnB that functions as a component of the adenylation cascade involved in the regulation of GS activity [ 1 ]. PII helps regulate the level of glutamine synthetase in response to nitrogen source availability. In nitrogen-limiting conditions, PII is uridylylated to form PII-UMP, which allows the deadenylation of glutamine synthetase, thus activating the enzyme. Conversely, in nitrogen excess, PI-UMP is deuridylated to PII, promoting the adenylation and deactivation of glutamine synthetase [ 2 ].

ATP phosphoribosyltransferase is the first enzyme of the histidine pathway. It is allosterically regulated, controlling the flow of intermediates through the pathway. The C-terminal domain is the regulatory region of the protein, which binds the allosteric inhibitor histidine [ 3 ].

CutA1 functions in divalent ion tolerance in bacteria, plants and animals [ 4 , 5 ]. Divalent metal ions play key roles in all living organisms, serving as cofactors for many proteins involved in a variety of electron-transfer activities. In Escherichia coli it is thought to be involved in copper ion tolerance, excessive copper ions being toxic [ 6 ].

1. Characterization of three different nitrogen-regulated promoter regions for the expression of glnB and glnA in Azospirillum brasilense. Mol. Gen. Genet. 224, 421-30
2. Characterization of pII family (GlnK1, GlnK2, and GlnB) protein uridylylation in response to nitrogen availability for Rhodopseudomonas palustris. Anal. Biochem. 357, 93-104
3. The structure of Escherichia coli ATP-phosphoribosyltransferase: identification of substrate binding sites and mode of AMP inhibition. J. Mol. Biol. 336, 131-44
4. The evolutionarily conserved trimeric structure of CutA1 proteins suggests a role in signal transduction. J. Biol. Chem. 278, 45999-6006
5. Molecular genetics of a chromosomal locus involved in copper tolerance in Escherichia coli K-12. Mol. Microbiol. 15, 1127-37
6. Cloning, crystallization and preliminary X-ray studies of XC2981 from Xanthomonas campestris, a putative CutA1 protein involved in copper-ion homeostasis. Acta Crystallogr. Sect. F Struct. Biol. Cryst. Commun. 62, 1113-5