Characterization of the hormone-binding domain of the chicken c-erbA/thyroid hormone receptor protein.

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Abstract
To identify and characterize the hormone‐binding domain of the thyroid hormone receptor, we analyzed the ligand‐binding capacities of proteins representing chimeras between the normal receptor and P75gag‐v‐erbA, the retrovirus‐encoded form deficient in binding ligand. Our results show that several mutations present in the carboxy‐terminal half of P75gag‐v‐erbA co‐operate in abolishing hormone binding, and that the ligand‐binding domain resides in a position analogous to that of steroid receptors. Furthermore, a point mutation that is located between the putative DNA and ligand‐binding domains of P75gag‐v‐erbA and that renders it biologically inactive fails to affect hormone binding by the c‐erbA protein. These results suggest that the mutation changed the ability of P75gag‐v‐erbA to affect transcription since it also had no effect on DNA binding. Our data also suggest that hormone‐independent activity of P75gag‐v‐erbA provided a selective advantage to the avian erythroblastosis virus during the original selection for a highly oncogenic strain of the virus.

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