In the above signaling cascade, which of the following molecules is denote by ‘B’?
(1) single-pass.      (2) three-pass
(3) five-pass.         (4) Seven-pass.

Understanding the Structure of Cellular Receptors: Identifying ‘B’ in Signaling Cascades

---

Introduction

Cellular receptors are crucial proteins embedded in the plasma membrane that detect external signals and initiate intracellular responses. Many such receptors have characteristic numbers of transmembrane domains, which define how many times their polypeptide chain crosses the lipid bilayer.

Understanding the number of transmembrane passes helps identify receptor families and predict their signaling mechanisms.

Types of Membrane-Spanning Receptors

• Single-Pass Receptors: Cross the membrane only once. Examples include receptor tyrosine kinases (e.g., EGF receptor).

• Multi-Pass Receptors: Cross multiple times. Based on the exact number of passes, receptors are grouped (three-pass, five-pass, seven-pass, etc.).

The Seven-Pass Receptors: G Protein-Coupled Receptors (GPCRs)

Receptor ‘B’ in many signaling cascades is typically a seven-pass transmembrane receptor, known as a GPCR.

• GPCRs have seven α-helical segments crossing the membrane.

• They translate extracellular signals into intracellular actions via coupling to heterotrimeric G proteins.

• GPCRs regulate diverse physiological processes and represent the largest family of cell-surface receptors.

Why Seven-Pass is Correct for ‘B’ in Signaling Cascades

According to current cellular physiology knowledge:

• Ligand binding to GPCR activates G proteins inside the cell.

• The seven transmembrane passes form a pocket for ligand binding and a site for G protein interaction.

• Common examples: adrenergic receptors, muscarinic receptors, and many hormone receptors.

Summary

Given the options:

1. Single-pass

2. Three-pass

3. Five-pass

4. Seven-pass

The correct answer is (4) Seven-pass, signifying receptor ‘B’ is a GPCR type receptor involved in the signaling cascade.

---