Crystal structure of mouse mu-crystallin complexed with NADPH and the T3 thyroid hormone
Franck Borel1, Isma Hachi, Andres Palencia
1Institut de Biologie Structurale, Université de Grenoble Alpes, CEA, CNRS, France.
Abstract:
Mu-crystallin (CRYM), first described as a structural component of the eye lens in marsupials, has been characterized as an NADPH-dependent cytosolic T3 thyroid hormone (triiodothyronine) binding protein. More recently, CRYM has also been associated with ketimine reductase activity. Here, we report three crystal structures: mouse CRYM (mCRYM) in its apo form, in a form complexed with NADPH, and in a form with both NADPH and triiodothyronine bound. Comparison of the apo and NADPH forms reveals a rearrangement of the protein upon NADPH binding that reduces the degrees of freedom of several residues and traps the conformation of the binding pocket in a more T3 competent state. These findings are in agreement with the cooperative mechanism identified using isothermal titration calorimetry. Our structure with T3 reveals for the first time the location of the hormone binding site and shows its detailed interactions. T3 binding involves mainly hydrophobic interactions. Only five residues, either directly or through bridging water molecules, are hydrogen bonded to the hormone. Using in silico docking analysis, a series of ring-containing hydrophobic molecules were identified as potential mCRYM ligands, suggesting that the specificity for the recognition of the hydrophobic part of the hormone might be low. This is in agreement with the ketimine reductase activity that has been identified for ovine CRYM, as it demonstrates how a protein known as a thyroid hormone transporter can accommodate the ringed molecules required for its ketimine reductase activity. In the light of our results, a putative role of CRYM in thyroid hormone metabolism is also discussed.
Structured Digital Abstract:
CRYM and CRYM bind by x-ray crystallography (View interaction).
Insights
Mu-crystallin (CRYM) is a thyroid hormone binding protein that also acts as a ketimine reductase. Structural analysis reveals how CRYM binds both NADPH and triiodothyronine (T3), explaining its dual function.
Area of Science:
- Biochemistry
- Structural Biology
- Molecular Endocrinology
Background:
- Mu-crystallin (CRYM) is known as an NADPH-dependent cytosolic T3 thyroid hormone binding protein.
- Recent studies also associate CRYM with ketimine reductase activity, suggesting a dual role.
Purpose of the Study:
- To elucidate the structural basis of CRYM's dual function by determining crystal structures.
- To understand the binding mechanism of NADPH and triiodothyronine (T3) to mouse CRYM (mCRYM).
Main Methods:
- X-ray crystallography was used to obtain structures of mCRYM in apo, NADPH-bound, and NADPH/T3-bound states.
- Isothermal titration calorimetry (ITC) was employed to study the binding mechanism.
- In silico docking analysis was performed to identify potential ligands.
Main Results:
- NADPH binding induces a conformational change in mCRYM, optimizing the T3 binding pocket.
- The crystal structure reveals the detailed interactions of T3 within the hormone binding site, primarily through hydrophobic interactions.
- In silico analysis suggests low specificity for the hydrophobic portion of T3, consistent with its ketimine reductase activity.
Conclusions:
- CRYM's structure supports a cooperative binding mechanism for NADPH and T3.
- The findings reconcile CRYM's roles as a thyroid hormone binder and a ketimine reductase.
- A potential role for CRYM in thyroid hormone metabolism is proposed.
