59. Which one of the following state about the nuclear receptor super family is NOT true?
(1) The receptors are always cytosol where they remain associated with heat-shock proteins and have variable ligand binding domains in the N-terminal region.
(2) The receptors have characteristic repeat of the C4 zinc finger motif
(3) The receptors are either homodimer or heterodimeric and in the absence their hormone ligand, the hetero-dimeric receptors repress transcription, when bound to their response elements
(4) The receptors have a unique N-terminal region of variable length and may contain a nuclear localization signal between the DNA-and ligand- binding domains

Nuclear Receptor Superfamily: Identifying the Incorrect Statement

Introduction

The nuclear receptor (NR) superfamily consists of transcription factors that regulate gene expression in response to ligands such as steroid hormones, thyroid hormones, and other metabolites. Despite functional diversity, they share a common modular structure and conserved mechanisms of action.

Analysis of Given Statements

1. Statement (1): “The receptors are always cytosol where they remain associated with heat-shock proteins and have variable ligand binding domains in the N-terminal region.”

   • Incorrect aspects: While some nuclear receptors (e.g., steroid receptors) reside in the cytosol complexed with heat-shock proteins before ligand binding, many NRs are constitutively nuclear. Also, the ligand-binding domain (LBD) is located at the C-terminal region, not the N-terminal.

2. Statement (2): “The receptors have characteristic repeat of the C4 zinc finger motif.”

   • Correct: The DNA-binding domain (DBD) of NRs contains two C4-type zinc fingers that mediate DNA recognition.

3. Statement (3): “The receptors are either homodimer or heterodimeric and in the absence of their hormone ligand, the hetero-dimeric receptors repress transcription when bound to their response elements.”

   • Correct: Many NRs function as homodimers or heterodimers (e.g., RXR heterodimers) and can repress transcription in the absence of ligand by recruiting corepressors.

4. Statement (4): “The receptors have a unique N-terminal region of variable length and may contain a nuclear localization signal between the DNA- and ligand-binding domains.”

   • Partially true: The N-terminal domain is highly variable and important for transcription activation. However, nuclear localization signals (NLS) are commonly found in the hinge region between the DBD and LBD, not exclusively in the N-terminal region.

Conclusion

The incorrect statement is (1) because:

• The ligand-binding domain is at the C-terminal, not the N-terminal.

• Not all nuclear receptors reside in the cytosol; many are constitutively nuclear.

Summary

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