51. Receptor for which one of the following proteins spans the plasma membrane of target cells but DOES NOT contain intrinsic protein kinase activity?
(1) Epidermal growth factor
(2) Insulin
(3) Insulin tike growth factor
(4) Growth hormone

Receptor Without Intrinsic Protein Kinase Activity: Focus on Growth Hormone Receptor (GHR)

Introduction

Many receptors for protein hormones are transmembrane proteins that initiate intracellular signaling via intrinsic protein kinase activity. However, some receptors, while spanning the plasma membrane and being essential for signal transduction, lack such intrinsic enzymatic activity and depend on associated kinases.

Receptor Details

1. Epidermal growth factor (EGF) receptor: An intrinsic receptor tyrosine kinase. The receptor’s cytoplasmic domain contains the kinase that autophosphorylates upon ligand binding.

2. Insulin receptor: Also an intrinsic tyrosine kinase receptor, activating its kinase domain upon ligand binding.

3. Insulin-like growth factor (IGF) receptor: Similar to insulin receptor, possessing intrinsic tyrosine kinase activity.

4. Growth hormone receptor (GHR): Although it traverses the plasma membrane and mediates growth hormone effects, GHR does not have intrinsic tyrosine kinase activity in its cytoplasmic domain. Instead, it signals by associating with and activating an external tyrosine kinase — Janus kinase 2 (JAK2).

How Growth Hormone Receptor Transduces Signals

• GHR exists as a pre-formed dimer.

• Binding of growth hormone induces conformational changes.

• These changes activate the tightly associated JAK2 kinase.

• Activated JAK2 phosphorylates the receptor and downstream targets, initiating signaling cascades.

Conclusion

Thus, the receptor for growth hormone is the plasma membrane-spanning protein that does not contain intrinsic protein kinase activity but signals via associated kinases.

Correct Answer

(4) Growth hormone