CLONING AND EXPRESSION OF A CDNA FOR A MAMMALIAN TYPE-III IODOTHYRONINE DEIODINASE

The type III iodothyronine deiodinase metabolizes the active thyroid h ormones thyroxine and 3,5,3'-triiodothyronine to inactive compounds. R ecently, we have characterized a Xenopus laevis cDNA (XL-15) that enco des a selenoprotein with type III deiodinase activity (St. Germain, D. L., Schwartzman, R., Croteau, W., Kanamori, A., Wang, Z., Brown, D. D ., and Galton, V. A. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 7767-7 771). Using the XL-15 as a probe, we screened a rat neonatal skin cDNA library. Among the clones isolated was one (rNS43-1) which contained a 2.1-kilobase pair cDNA insert that manifested significant homology t o both the XL-15 and the G21 rat type I deiodinase cDNAs, including th e presence of an in-frame TGA codon. Expression studies demonstrated t hat the rNS43-1 cDNA encodes a protein with 5-, but not 5'-, deiodinas e activity that is resistant to inhibition by propylthiouracil and aur othioglucose. Northern analysis demonstrated a pattern of tissue expre ssion in the rat consistent with that of the type III deiodinase and s ite directed mutagenesis confirmed that the TGA triplet codes for sele nocysteine. We conclude that the rNS43-1 cDNA encodes the rat type III deiodinase and that the types I and III deiodinases present in amphib ians and mammals constitute a family of conserved selenoproteins impor tant in the metabolism of thyroid hormones.