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. Primarily expressed in liver, pancreas, kidney and lung. Overexpressed in PHA-stimulated T-cells. The disease is caused by variants affecting the gene represented in this entry. Possible diagnostic marker for autoimmune hepatitis (AIH). May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the SepSecS family. Wrong choice of CDS. Wrong choice of CDS. Extended N-terminus.