9. Which one of the following combinations is the correct pairing of ligands with their receptors?

(1) i-c, ii-a, iii-b
(2) i-a, ii-c, iii-b
(3) i-b, ii-c, iii-a
(4) i-c, ii-b, iii-a

Introduction

Understanding the correct pairing between signaling ligands and their receptors is foundational in cell biology, especially for key pathways that govern development, growth, and disease. The Fibroblast Growth Factor (FGF), Hedgehog, and Wnt families are critical signaling molecules whose specific receptor interactions mediate important biological processes.

Ligand-Receptor Pairings

1. Fibroblast Growth Factor (FGF): FGFs bind to and activate receptor tyrosine kinases (RTKs) called fibroblast growth factor receptors (FGFRs). FGFRs have an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular tyrosine kinase domain. Binding of FGF induces FGFR dimerization and activation of kinase activity, triggering downstream signaling cascades.

2. Hedgehog (Hh): Hedgehog proteins bind to the Patched receptor. Patched is a 12-pass transmembrane protein that acts as a receptor and inhibitor of the transmembrane protein Smoothened. Binding of Hedgehog to Patched alleviates this inhibition, permitting pathway activation.

3. Wnt: Wnt ligands bind to the Frizzled receptor family, which are 7-pass transmembrane proteins related to G protein-coupled receptors. Wnt-Frizzled interactions activate diverse signaling pathways including the canonical β-catenin pathway.

Correct Matching from Options

Based on molecular and biochemical evidence:

• FGF corresponds to receptor tyrosine kinase (c).

• Hedgehog corresponds to Patched receptor (a).

• Wnt corresponds to Frizzled receptor (b).

Thus, the correct pairing is:

(1) i-c, ii-a, iii-b

Importance of Correct Pairing

This classification reflects well-studied, fundamental signal transduction pathways:

• FGF/FGFR: Regulates cell proliferation, differentiation, and angiogenesis.

• Hedgehog/Patched: Controls embryonic patterning and stem cell regulation.

• Wnt/Frizzled: Modulates cell fate, polarity, and migration.

Summary

Ligands and their receptors function together to ensure precise communication within the cell environment. For FGF, Hedgehog, and Wnt, the pairings with receptor tyrosine kinase, Patched, and Frizzled receptors respectively are well established.