25. Which one of the following is NOT a receptor tyrosine kinase?
(A) Platelet derived growth factor receptor
(B) Insulin like growth factor -1 receptor
(C) Macrophage colony stimulating factor receptor
(D) Transforming growth factor 𝛽 receptor

Transforming growth factor β receptor (TGF-β receptor) stands out as the option that is not a receptor tyrosine kinase (RTK), distinguishing it from the others in this biology MCQ. Receptor tyrosine kinases feature extracellular ligand-binding domains and intracellular tyrosine kinase domains that autophosphorylate upon activation, driving cell signaling in growth and proliferation. This article breaks down each option with scientific clarity for students and researchers tackling exams in molecular biology or genetics.​

Correct Answer

(D) Transforming growth factor β receptor is NOT a receptor tyrosine kinase. TGF-β receptors (types I and II) are serine/threonine kinases, where type II phosphorylates type I to initiate signaling cascades like SMAD activation, without tyrosine kinase activity.​

Option Breakdowns

Platelet-Derived Growth Factor Receptor (PDGFR)

Platelet-derived growth factor receptor belongs to class III RTKs with immunoglobulin-like extracellular domains and a split intracellular tyrosine kinase domain. Ligand binding induces dimerization (αα, ββ, or αβ), leading to autophosphorylation and pathways like PI3K/Akt and MAPK for cell proliferation.​

Insulin-Like Growth Factor-1 Receptor (IGF-1R)

IGF-1 receptor is a class II RTK, a heterotetramer (α2β2) where β subunits possess tyrosine kinase activity. IGF-1 or IGF-2 binding triggers autophosphorylation, activating insulin receptor substrates (IRS) and downstream signals for growth, survival, and metabolism—closely related to the insulin receptor.​

Macrophage Colony Stimulating Factor Receptor (CSF-1R)

Macrophage colony stimulating factor receptor, or CSF-1R (also FMS), is another class III RTK like PDGFR, with five Ig-like domains extracellularly. CSF-1 binding causes dimerization and tyrosine autophosphorylation, essential for monocyte/macrophage differentiation, survival, and function.​

Why TGF-β Receptor Differs

Unlike RTKs that phosphorylate tyrosines, TGF-β signaling relies on serine/threonine phosphorylation: TGF-β binds TβRII (serine/threonine kinase), recruiting and activating TβRI, which phosphorylates SMADs for gene regulation in processes like fibrosis and immunity. This mechanistic distinction makes option (D) the clear outlier.​