The thyroid hormone-inactivating deiodinase functions as a homodimer

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Type 3 deiodinase (D3) is a key enzyme that inactivates thyroid hormones.
• D3 plays a role in embryonic development and euthyroid sick syndrome.
• The structure and function of D3, particularly its oligomeric state, are poorly understood compared to other deiodinases.

Purpose of the Study:

• To investigate the oligomeric state and structural characteristics of type 3 deiodinase (D3) in live cells.
• To determine the relationship between D3 dimerization and its catalytic activity.

Main Methods:

• Fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) in live cells.
• Urea denaturation assays to assess dimer stability.
• Protein modeling using a D2-based scaffold.
• Site-directed mutagenesis (transmembrane domain truncation).

Main Results:

• D3 exists as a homodimer in its native state within intact cells.
• Minor heterodimerization of D3 with D1 and D2 deiodinases was observed.
• Loss of D3 homodimerization, induced by urea, correlated with a significant reduction in enzyme activity (up to 50%).
• Protein modeling and truncation studies identified dimerization surfaces in both transmembrane and globular domains of D3.

Conclusions:

• The inactivating deiodinase D3 functions as a homo- or heterodimer in living cells.
• D3 dimerization is critical for its catalytic activity in thyroid hormone inactivation.