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

We investigated the mechanism by which T-4 regulates its activation to T-3 by the type 2 iodothyronine deiodinase (D2). D2 is a short- lived (t(1/2) 50 min), 31-kDa endoplasmic reticulum (ER) integral membrane selenoenzyme that generates intracellular T-3. 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 El 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 T-4 to T-3 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 T-4 regulation of its own activation in the central nervous system and pituitary tissues in which D2-catalyzed T-4 to T-3 conversion is the major source of intracellular T-3.