Early annotation suggested this family, SepSecS, of several eukaryotic and archaeal proteins, was involved in antigen-antibodies responses in the liver and pancreas [1,2]. Structural studies show that the family is O-phosphoseryl-tRNA(Sec) selenium transferase, an enzyme involved in the synthesis of the amino acid selenocysteine (Sec). Sec is the only amino acid whose biosynthesis occurs on its cognate transfer RNA (tRNA). SepSecS catalyses the final step in the formation of the amino acid [3,4,5,6]. The early observation that autoantibodies isolated from patients with type I autoimmune hepatitis targeted a ribonucleoprotein complex containing tRNASec led to the identification and characterisation of the archaeal and the human SepSecS [2]. SepSecS forms its own branch in the family of fold-type I pyridoxal phosphate (PLP) enzymes that goes back to the last universal common ancestor which explains why the archaeal sequences Swiss:Q8TXK0 and Swiss:Q8TYR3 are annotated as being pyridoxal phosphate-dependent enzymes.

1: Identification of target antigen for SLA/LP autoantibodies in autoimmune hepatitis. Wies I, Brunner S, Henninger J, Herkel J, Kanzler S, Meyer zum Buschenfelde KH, Lohse AW; Lancet 2000;355:1510-1515. PMID:10801173

2: A bioinformatical approach suggests the function of the autoimmune hepatitis target antigen soluble liver antigen/liver pancreas. Kernebeck T, Lohse AW, Grotzinger J; Hepatology 2001;34:230-233. PMID:11481605

3: Biosynthesis of selenocysteine on its tRNA in eukaryotes. Xu XM, Carlson BA, Mix H, Zhang Y, Saira K, Glass RS, Berry MJ, Gladyshev VN, Hatfield DL; PLoS Biol. 2007;5:e4. PMID:17194211

4: Structural insights into the second step of RNA-dependent cysteine biosynthesis in archaea: crystal structure of Sep-tRNA:Cys-tRNA synthase from Archaeoglobus fulgidus. Fukunaga R, Yokoyama S; J Mol Biol. 2007;370:128-141. PMID:17512006

5: Structure and catalytic mechanism of eukaryotic selenocysteine synthase. Ganichkin OM, Xu XM, Carlson BA, Mix H, Hatfield DL, Gladyshev VN, Wahl MC; J Biol Chem. 2008;283:5849-5865. PMID:18093968

6: Structural insights into RNA-dependent eukaryal and archaeal selenocysteine formation. Araiso Y, Palioura S, Ishitani R, Sherrer RL, O'Donoghue P, Yuan J, Oshikane H, Domae N, Defranco J, Soll D, Nureki O; Nucleic Acids Res. 2008;36:1187-1199. PMID:18158303

7: The human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation. Palioura S, Sherrer RL, Steitz TA, Soll D, Simonovic M; Science. 2009;325:321-325. PMID:19608919