Structure of an N-terminally truncated selenophosphate synthetase from Aquifex aeolicus

Selenophosphate synthetase (SPS) catalyzes the activation of selenide with ATP to synthesize selenophosphate, the reactive selenium donor for biosyntheses of both the 21st amino acid selenocysteine and 2-selenouridine nucleotides in tRNA anticodons. The crystal structure of an N-terminally (25 residues) truncated fragment of SPS (SPS-ΔN) from Aquifex aeolicus has been determined at 2.0 Å resolution. The structure revealed SPS to be a two-domain α/β protein, with domain folds that are homologous to those of PurM-superfamily proteins. In the crystal, six monomers of SPS-ΔN form a hexamer of 204 kDa, whereas the molecular weight estimated by ultracentrifugation was ∼63 kDa, which is comparable to the calculated weight of the dimer (68 kDa).