57 Two steroid hormone receptors and Y both contain a ligand binding domain recombinant DNA technology, a modified hybrid receptor H is prepared such that it contains the ligand binding domain of X and DNA binding domain of Y, three sets of cells overexpressing receptors X. Y and H were then treated separately ether With hormone X or with hormone Y- assuming that there is no cross— reactivity. which one of the following graphs best represent the receptor ligand binding in each case?

Steroid Receptor Hybrid Experiment: Understanding Ligand and DNA Binding Specificity

Introduction

Steroid hormone receptors function through distinct modular domains: a ligand-binding domain (LBD) that determines hormone specificity, and a DNA-binding domain (DBD) that determines gene regulatory targets. Recombinant DNA allows creation of hybrid receptors with domains swapped between receptors.

In this experiment, receptor X provides the LBD and receptor Y provides the DBD to form hybrid receptor H. Analysis of cells overexpressing X, Y, and H treated separately with hormones X or Y (which do not cross-react) reveals insights about receptor modularity and specificity.

Key Concepts

• • • Receptor X: Has LBD specific to hormone X and own DBD.

    • Receptor Y: Has LBD specific to hormone Y and own DBD.

    • Hybrid receptor H: Contains LBD from X and DBD from Y.

Because hormones do not cross-react:

• • • Hormone X activates receptors containing X’s LBD only (X and hybrid H).

    • Hormone Y activates receptors containing Y’s LBD only (Y).

Expected Binding and Activation Patterns

This predicts:

• • • Hormone X activates X and H only.

    • Hormone Y activates only Y.

Graph Representation

A graph plotting receptor-ligand binding activity (y-axis) versus treatments (x-axis: hormone X, hormone Y) would show:

• • • Receptor X: High activity with hormone X, near zero with hormone Y.

    • Receptor Y: High activity with hormone Y, near zero with hormone X.

    • Hybrid H: High activity only with hormone X.

Biological Implications

This experiment supports that the ligand specificity is conferred predominantly by the ligand-binding domain, while the DNA-binding domain determines transcriptional target specificity. The modular nature of nuclear receptors is fundamental for receptor engineering, drug design, and understanding gene regulation.