Q.33 Which of the following statements is/are CORRECT for G protein–coupled receptor (GPCR)
mediated signaling?
(A) GPCRs contain seven membrane spanning regions.
(B) GPCRs are linked to heterotrimeric G protein consisting of α, β and γ subunits.
(C) In the absence of GPCR interacting ligand, α subunit of G protein is bound to GTP and
complexed with βγ subunits.
(D) In the presence of GPCR interacting ligand, GTP is displaced from α subunit of G protein
by GDP, GDP bound α subunit dissociates from βγ dimer and activates the effector.

G Protein-Coupled Receptor (GPCR) Signaling: CSIR NET MCQ Solved with Detailed Analysis

G Protein-Coupled Receptors (GPCRs) mediate vital cellular signaling through interaction with heterotrimeric G proteins. Options (A) and (B) are correct, while (C) and (D) contain errors in nucleotide binding and activation sequence.​

Option Analysis

GPCRs represent the largest receptor family, crucial for CSIR NET cell signaling topics.

• (A) Correct: GPCRs feature seven α-helical transmembrane domains (7-TM), connected by extracellular and intracellular loops, forming their signature serpentine structure.​

• (B) Correct: GPCRs couple to heterotrimeric G proteins (Gα, Gβ, Gγ subunits); Gα binds nucleotides, while Gβγ modulates effectors.​

• (C) Incorrect: In the ligand-free state, inactive Gα binds GDP (not GTP) and remains complexed with Gβγ; GTP binding activates dissociation.​

• (D) Incorrect: Ligand binding induces GDP release from Gα (not GTP displacement), allowing GTP binding; GTP-Gα then dissociates from Gβγ to activate effectors like adenylyl cyclase.​​

Activation Mechanism

Ligand binding to GPCR extracellular domain triggers conformational change, exposing intracellular loops that act as GEF for Gα.​

GPCR signaling cycle:

• Inactive: GDP-Gαβγ trimer associates with GPCR.

• Activation: Ligand-bound GPCR catalyzes GDP → GTP exchange on Gα.

• Dissociation: GTP-Gα and Gβγ separately activate effectors (e.g., Gsα stimulates cAMP production).

• Termination: Intrinsic GTPase of Gα hydrolyzes GTP → GDP, reforming trimer.​