THE THYROID HORMONE INACTIVATING DEIODINASE FUNCTIONS AS HOMODIMER
Résumé
The type 3 deiodinase (D3) inactivates thyroid hormone action by catalyzing tissue-specific inner ring de- iodination, predominantly during embryonic development. D3 has gained much attention as a player in the euthyroid sick syndrome, given its robust reactivation during injury and/or illness. While much of the structure biology of the deiodinases is derived from studies with D2, a dimeric endoplasmic-reticulum obligatory activating deiodinase, little is known about the holostructure of the plasma membrane-resident D3, the deiodinase capable of thyroid hormone inactivation. Here we used fluorescence resonance energy transfer (FRET) in live cells to demonstrate that D3 exists as homodimer. While D3 homodimerized in its native state, minor heterodimerization was also observed between D3:D1 and D3:D2 in intact cells, the significance of which remains elusive. Incubation with 0.5-1.2 M urea resulted in loss of D3 homodimeri- zation as assessed by bioluminescence resonance energy transfer (BRET) and a proportional loss of en- zyme activity, to a maximum of ~50%. Protein modeling using a D2-based scaffold identified potential dimerization surfaces in the transmembrane and globular domains. Truncation of the transmembrane do- main (ΔD3) abrogated dimerization and deiodinase activity except when co-expressed with full-length catalytically inactive deiodinase, thus assembled as ΔD3:D3 dimer; thus the D3 globular domain also ex- hibits dimerization surfaces. In conclusion, the inactivating deiodinase D3 exists as homo- or heterodimer in living intact cells, a feature that is critical for their catalytic activities.