Selenium donor protein. Catalyzes the magnesium-dependent synthesis of a labile selenium donor compound from selenide and ATP (formation of mono-selenophosphate). This compound is required both for the incorporation of selenium into the wobble uridine-34 of several mnm5s2U-containing tRNAs and for the biosynthesis of selenocysteine from an L-serine residue ester bonded to unique tRNA-Ser (anticodon UCA). Exchange of sulfur in mnm5s2U-tRNA into mnm5se2U-tRNA is catalyzed by SelU. In certain Eukarya SelD-like protein lacks the selenide-dependent monophosphate synthetase activity and catalyses only the biosynthesis of selenocysteine.