38. The cytoplasmic domain of the receptor of which of the following proteins does NOT function as tyrosine kinase?
(1) Epidermal growth factor
(2) Platelet derived growth factor
(3) Insulin
(4) Asialoglycoprotein

Introduction

Receptor tyrosine kinases (RTKs) are a major class of cell surface receptors that play critical roles in cellular signal transduction. They possess intrinsic enzymatic activity in their cytoplasmic domains, which phosphorylate tyrosine residues to facilitate downstream signaling. This activity is essential for regulating processes such as cell growth, differentiation, metabolism, and survival.

However, not all receptors with signaling functions contain tyrosine kinase activity in their cytoplasmic domains. Understanding which receptors possess or lack this function is crucial for grasping their distinct mechanisms of action.

Receptors with Tyrosine Kinase Activity

1. Epidermal Growth Factor Receptor (EGF receptor): A classical RTK, the EGF receptor has intrinsic tyrosine kinase activity within its cytoplasmic domain, which is activated upon ligand binding and receptor dimerization.

2. Platelet-Derived Growth Factor Receptor (PDGF receptor): Similarly, the PDGF receptor is an RTK with cytoplasmic tyrosine kinase activity, critical for regulating cell proliferation and migration.

3. Insulin Receptor: A member of the RTK class II family, the insulin receptor’s cytoplasmic domain functions as a tyrosine kinase that autophosphorylates upon insulin binding and initiates signaling cascades regulating glucose metabolism.

The Asialoglycoprotein Receptor

The asialoglycoprotein receptor (ASGPR) differs fundamentally from the above receptors. It is a C-type lectin receptor that mediates endocytosis of glycoproteins lacking terminal sialic acid residues. Its cytoplasmic domain does not possess intrinsic tyrosine kinase activity. Instead, its primary function involves binding and internalizing ligands for recycling or degradation.

Summary

Hence, among the options:

• (1) Epidermal growth factor receptor: Has tyrosine kinase activity.

• (2) Platelet-derived growth factor receptor: Has tyrosine kinase activity.

• (3) Insulin receptor: Has tyrosine kinase activity.

• (4) Asialoglycoprotein receptor: Does NOT have cytoplasmic tyrosine kinase activity.

The receptor whose cytoplasmic domain does NOT function as a tyrosine kinase is the Asialoglycoprotein receptor.