Selective proteolysis of human type 2 deiodinase: a novel ubiquitin-proteasomal mediated mechanism for regulation of hormone activation

• Published in: Molecular endocrinology (Baltimore, Md.) Md.), 2000-11, Vol.14 (11), p.1697-1708
• Main Authors: Gereben, B, Goncalves, C, Harney, J W, Larsen, P R, Bianco, A C
• Format: Article
• Language: English
• Subjects: Animals; Base Sequence; Cell Line; Cysteine Endopeptidases - metabolism; Endoplasmic Reticulum - metabolism; Epitopes - genetics; Hormones - metabolism; Humans; Iodide Peroxidase - genetics; Iodide Peroxidase - metabolism; Iodothyronine Deiodinase Type II; Ligases - metabolism; Molecular Sequence Data; Multienzyme Complexes - metabolism; Oligopeptides; Peptides - genetics; Peptides - immunology; Peptides - metabolism; Proteasome Endopeptidase Complex; Protein Processing, Post-Translational; Recombinant Proteins - genetics; Recombinant Proteins - metabolism; Thyroxine - metabolism; Triiodothyronine - metabolism; Ubiquitin-Protein Ligases; Ubiquitins - metabolism
• ISSN: 0888-8809
• DOI: 10.1210/me.14.11.1697

We investigated the mechanism by which T4 regulates its activation to T3 by the type 2 iodothyronine deiodinase (D2). D2 is a short- lived (t1/2 50 min), 31-kDa endoplasmic reticulum (ER) integral membrane selenoenzyme that generates intracellular T3. Inhibition of the ubiquitin (Ub) activating enzyme, E1, or MG132, a proteasome blocker, inhibits both the basal and substrate-induced acceleration of D2 degradation. Using a catalytically active transiently expressed FLAG-tagged-NH2-D2, we found rapid synthesis of high molecular mass (100-300 kDa) Ub-D2 conjugates that are catalytically inactive. Ub-D2 increases when cells are exposed to D2 substrate or MG132 and disappears rapidly after E1 inactivation. Fusion of FLAG epitope to the COOH terminus of D2 prolongs its half-life approximately 2.5-fold and increases the levels of active and, especially, Ub-D2. This indicates that COOH-terminal modification interferes with proteasomal uptake of Ub-D2 that can then be deubiquitinated. Interestingly, the type 1 deiodinase, a related selenoenzyme that also converts T4 to T3 but with a half-life of >12 h, is inactivated but not ubiquitinated or degraded after exposure to substrate. Thus, ubiquitination of the ER-resident enzyme D2 constitutes a specific posttranslational mechanism for T4 regulation of its own activation in the central nervous system and pituitary tissues in which D2-catalyzed T4 to T3 conversion is the major source of intracellular T3.