Converts O-phosphoseryl-tRNA(Sec) to selenocysteinyl-tRNA(Sec) required for selenoprotein biosynthesis. H2O + O-phospho-L-seryl-tRNA(Sec) + selenophosphate = L-selenocysteinyl-tRNA(Sec) + 2 phosphate Aminoacyl-tRNA biosynthesis; selenocysteinyl-tRNA(Sec) biosynthesis; selenocysteinyl-tRNA(Sec) from L-seryl-tRNA(Sec) (archaeal/eukaryal route): step 2/2. Homotetramer formed by a catalytic dimer and a non-catalytic dimer serving as a binding platform that orients tRNASec for catalysis. Each tetramer binds the CCA ends of two tRNAs which point to the active sites of the catalytic dimer. Belongs to the SepSecS family.