Which statement best summarizes the GPCR signaling pathway?

Which statement best summarizes the GPCR signaling pathway?

• A. A first messenger directly activates the second messenger without involvement of G-proteins
• B. A first messenger activates the G-protein, which activates an effector, which then activates a second messenger
• C. A second messenger activates the G-protein, which then opens an ion channel
• D. The receptor activates a kinase cascade that bypasses G-proteins entirely

Answer: (B)

The signaling flow starts when a ligand binds the receptor on the cell surface, causing the receptor to activate a heterotrimeric G-protein. This activation prompts the G-protein to regulate an effector enzyme (such as adenylyl cyclase or phospholipase C), which then generates a second messenger like cAMP or IP3/DAG. The second messenger goes on to trigger downstream kinases and cellular responses. This sequence—first messenger → GPCR → G-protein → effector → second messenger—best captures how GPCR signaling fundamentally operates. The other descriptions omit or rearrange a key step (the involvement of G-proteins) or imply a bypass of the G-protein stage, which doesn’t reflect the canonical GPCR pathway.

The signaling flow starts when a ligand binds the receptor on the cell surface, causing the receptor to activate a heterotrimeric G-protein. This activation prompts the G-protein to regulate an effector enzyme (such as adenylyl cyclase or phospholipase C), which then generates a second messenger like cAMP or IP3/DAG. The second messenger goes on to trigger downstream kinases and cellular responses. This sequence—first messenger → GPCR → G-protein → effector → second messenger—best captures how GPCR signaling fundamentally operates. The other descriptions omit or rearrange a key step (the involvement of G-proteins) or imply a bypass of the G-protein stage, which doesn’t reflect the canonical GPCR pathway.