50. Proteins with cytoplasmic domains having tyrosine kinase activity do NOT act as receptors for
(1) Epidermal growth factor (EGF)
(2) Platelet-derived growth factor (PDGF)
(3) Insulin
(4) Transferrin

---

Introduction

Receptor tyrosine kinases (RTKs) are transmembrane proteins crucial for signal transduction of many growth factors and hormones. These receptors typically have an extracellular ligand-binding domain, a transmembrane region, and an intracellular domain with intrinsic tyrosine kinase activity. Ligand binding leads to receptor dimerization and autophosphorylation, triggering downstream signaling pathways that modulate cellular processes such as growth, differentiation, and metabolism.

RTKs Recognizing EGF, PDGF, and Insulin

• Epidermal growth factor (EGF) receptor: A classical RTK that binds EGF, triggering tyrosine kinase activation.

• Platelet-derived growth factor (PDGF) receptor: Also an RTK that binds PDGF and activates intracellular kinase activity.

• Insulin receptor: A distinct RTK family member with intrinsic tyrosine kinase activity, modulating metabolism and growth.

All three receptors have cytoplasmic domains with intrinsic tyrosine kinase catalytic activity.

Transferrin Receptor Does Not Have Tyrosine Kinase Activity

• The transferrin receptor mediates uptake of transferrin-bound iron but does not possess intrinsic tyrosine kinase activity.

• It is a membrane receptor involved mainly in receptor-mediated endocytosis.

• Signaling through the transferrin receptor does not involve autophosphorylation of tyrosine residues or kinase activation, unlike RTKs.

Summary

Correct answer: (4) Transferrin

Proteins with cytoplasmic domains having tyrosine kinase activity do NOT act as receptors for transferrin, even though they act as receptors for EGF, PDGF, and insulin.

---