SelD homolog from Drosophila lacking selenide-dependent monoselenophosphate synthetase activity

The isolation and molecular characterization of an invertebrate gene that encodes a homolog of the human selenophosphate synthetase 1 is described. This Drosophila gene, termed selD-like, is located in the cytogenetic interval 50 D/E on the right arm of chromosome 2. It is expressed ubiquitously throughout embryogenesis and found to be highly enriched in the developing gut and in the nervous system of the embryo. The SelD-like from Drosophila was purified after expression in Escherichia coli. The purified protein does not catalyze the selenide-dependent ATP hydrolysis reaction and its gene does not complement a selD lesion in E. coli. These results and the fact that selD-like possesses an arginine residue at the position of the essential Cys17 (E. coli nomenclature) indicate that the Drosophila gene exerts a function different from that of the classical selenophosphate synthetases. Two classes of SelD proteins can therefore be differentiated. The class I proteins contain a cysteine or selenocysteine residue in the active site and display selenide-dependent selenophosphate synthetase activity. Class II proteins, including Drosophila selD-like and human selenophosphate synthetase 1 are devoid of this activity and they possess other amino acids in position 17.