Each of the following activates a receptor tyrosine kinase except
A. PDGF.
B. EGF.
C. Insulin.
D. FGF.
E. TGF \beta
The correct answer and explanation is:
The correct answer is E. TGF β (Transforming Growth Factor Beta).
Explanation:
Receptor tyrosine kinases (RTKs) are a large family of cell surface receptors that, upon binding to their specific ligands, activate intracellular signaling pathways by phosphorylating tyrosine residues on themselves and other proteins. This phosphorylation triggers a cascade of downstream effects involved in cell growth, differentiation, metabolism, and survival.
Among the options given:
- PDGF (Platelet-Derived Growth Factor) binds to PDGF receptors, which are classic RTKs.
- EGF (Epidermal Growth Factor) binds to the EGF receptor (EGFR), an RTK.
- Insulin binds to the insulin receptor, another well-characterized RTK.
- FGF (Fibroblast Growth Factor) binds to FGF receptors, which are RTKs.
In contrast, TGF β does not activate receptor tyrosine kinases. Instead, it binds to a different class of receptors known as serine/threonine kinase receptors. The TGF β receptor complex phosphorylates serine and threonine residues on intracellular proteins called Smads, which then regulate gene expression in the nucleus. This signaling pathway is distinct from RTK pathways and plays key roles in cell growth inhibition, differentiation, immune regulation, and extracellular matrix production.
To summarize, while PDGF, EGF, Insulin, and FGF activate receptor tyrosine kinases, TGF β signals through serine/threonine kinase receptors, making option E the correct choice.