Human cytosolic 3,5,3′-triiodo-L-thyronine-binding protein, also called μ-crystallin or CRYM, plays important physiological roles in transporting 3,5,3′-triiodo-L-thyronine (T3) into nuclei and regulating thyroid-hormone-related gene expression. The crystal structure of human CRYM's bacterial homolog Pseudomonas putida ornithine cyclodeaminase and Archaeoglobus fulgidus alanine dehydrogenase have been available, but no CRYM structure has been reported. Here, we report the crystal structure of human CRYM bound with NADPH refined to 2.6 Å, and there is one dimer in the asymmetric unit. The structure contains two domains: a Rossmann fold–like NADPH-binding domain and a dimerization domain. Different conformations of the loop Arg83–His92 have been observed in two monomers of human CRYM in the same asymmetric unit. The peptide bond of Val89–Pro90 is a trans-configuration in one monomer but a cis-configuration in the other. A detailed comparison of the human μ-crystallin structure with its structurally characterized homologs including the overall comparison and superposition of active sites was conducted. Finally, a putative T3-binding site in human CRYM is proposed based on comparison with structural homologs
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