Zn⁺² induces reversible cross-linking of human placental thyroid hormone nuclear receptor with no effect on hormone binding

Zn⁺² is required for specific binding of c-erbA proteins to the hormone response elements of target genes. It is unclear whether Zn⁺² is important for the binding of ligand to c-erbA proteins. The present study evaluated the effect of Zn⁺² and other divalent cations on the binding of 3,3′,5-triiodo-L-thyronine(T₃) to the purified human placental c-erbA protein (h-TRβ1). Zn⁺² induced cross-linking of h-TRβ1 to form aggregates in a dose-dependent manner with an apparent half-maximal concentration of ∼ 200 μM at 22°C. Cross-linking was reversible by the addition of 5 μM EDTA or 10 mM dithiothreitol. The cross-linked h-TRβ1 bound T₃. These results indicated Zn⁺² had no effect on T₃ binding and suggested that the cysteines and histidines involved in crosslinking are not essential for T₃ binding.