2. Which statement is correct for membrane receptors for signal transduction?
(1) Contain single or multiple membranes spanning domain
(2) Always coupled with trimeric G protein
(3) Always results in production of secondary messenger
(4) Recognized non-polar signaling molecules

Introduction

Membrane receptors are integral proteins embedded in the cellular plasma membrane that facilitate signal transduction — the conversion of extracellular signals into intracellular actions. These receptors are crucial for cells to respond to hormones, neurotransmitters, growth factors, and other signaling molecules. Their ability to detect and transmit signals regulates numerous physiological processes such as growth, metabolism, and neural communication.

Membrane-Spanning Domains: Single vs. Multiple

Most membrane receptors contain one or multiple membrane-spanning domains that anchor them in the lipid bilayer. For example, receptor tyrosine kinases (RTKs) generally have a single transmembrane helix that spans the membrane once. In contrast, G protein-coupled receptors (GPCRs) possess seven transmembrane helices, forming a complex folded structure that enables ligand binding and intracellular interaction.

This transmembrane architecture is fundamental, allowing receptors to transmit conformational changes from the extracellular ligand-binding domain to intracellular signaling machinery.

Main Classes of Membrane Receptors

1. G Protein-Coupled Receptors (GPCRs):

   • Characteristically have seven transmembrane domains.

   • Couple with heterotrimeric G proteins upon ligand binding, activating signaling cascades via second messengers like cAMP, IP3, or DAG.

   • Example ligands: hormones, neurotransmitters.

2. Receptor Tyrosine Kinases (RTKs):

   • Single transmembrane domain with intrinsic kinase activity.

   • Ligand binding induces receptor dimerization and autophosphorylation, triggering downstream phosphorylation cascades.

   • Often involved in growth and differentiation signaling.

3. Ion Channel-Linked Receptors:

   • Multi-pass transmembrane proteins forming ion channels.

   • Ligand binding directly opens or closes the channel,
     leading to changes in ion permeability and membrane potential.

Clarifying Common Misconceptions

• Not all membrane receptors are coupled with trimeric G proteins. Only GPCRs use G proteins to transmit signals inside the cell. RTKs and ion-channel receptors use different mechanisms.

• Signal transduction does not always produce secondary messengers. GPCR signaling often involves secondary messengers, but RTKs mainly initiate protein phosphorylation cascades, and ion channel receptors alter ion flux directly.

• Membrane receptors do not primarily recognize non-polar signaling molecules. Most ligands for membrane receptors are polar or charged molecules that cannot freely cross the membrane; non-polar signaling molecules often enter cells to interact with intracellular receptors instead.

Why Statement (1) is Correct

The statement “Contain single or multiple membranes spanning domain” accurately reflects the diverse structural organization of signaling membrane receptors. This feature is universal among membrane receptors enabling their key functional role in signal transduction.

Summary

Membrane receptors are central to signal transduction and exhibit diverse structural designs:

• They may contain either single or multiple transmembrane domains.

• Only certain receptors (GPCRs) couple with trimeric G proteins.

• Secondary messenger production is not a universal outcome of receptor activation.

• Membrane receptors typically bind polar ligands outside the cell.

Hence, the correct choice about membrane receptors in the given question is:

(1) Contain single or multiple membranes spanning domain

Understanding these receptor features helps clarify distinct signal transduction pathways governing cellular responses.