Which of the following is not an example of a GPCR?

• A. Receptor tyrosine kinase
• B. Muscarinic acetylcholine receptor
• C. GABAb receptor
• D. Adrenergic receptor

Answer: (D)

GPCRs are seven-pass membrane receptors that activate intracellular G proteins when a ligand binds. The muscarinic acetylcholine receptor is a classic GPCR, signaling through G proteins to regulate second messengers. The GABAb receptor is another GPCR, a metabotropic receptor that influences signaling via G proteins. Adrenergic receptors, including alpha and beta subtypes, are also GPCRs that respond to norepinephrine or epinephrine and modulate pathways like adenylyl cyclase or phospholipase C. In contrast, a receptor tyrosine kinase is not a GPCR. RTKs have intrinsic tyrosine kinase activity, and ligand binding promotes dimerization and autophosphorylation, creating docking sites for downstream signaling proteins and activating cascades such as MAPK and PI3K-Akt. Therefore, the non-GPCR among these is the receptor tyrosine kinase.