In GPCRs, the extracellular domain is typically involved in what function?

In GPCRs, the extracellular domain is typically involved in what function?

• A. Coupling to G-proteins
• B. Forming a pore for ions
• C. Catalyzing phosphorylation of substrates
• D. Binding the ligand

Answer: (D)

Ligand recognition is carried out by the extracellular region of GPCRs. This domain provides the binding pocket that confers ligand specificity—hormones, neurotransmitters, or peptide ligands bind here, and their interaction induces a conformational change across the receptor. That change is transmitted to the intracellular side to recruit and activate G proteins. The actual coupling to G-proteins happens via intracellular loops and tails, not through the extracellular domain. GPCRs do not form ion pores, which is a feature of ion channels, and they do not catalyze phosphorylation themselves; phosphorylation is typically done by kinases, though GPCRs can be targets for phosphorylation to regulate signaling. Thus, the extracellular domain's typical function is binding the ligand.

Ligand recognition is carried out by the extracellular region of GPCRs. This domain provides the binding pocket that confers ligand specificity—hormones, neurotransmitters, or peptide ligands bind here, and their interaction induces a conformational change across the receptor. That change is transmitted to the intracellular side to recruit and activate G proteins. The actual coupling to G-proteins happens via intracellular loops and tails, not through the extracellular domain. GPCRs do not form ion pores, which is a feature of ion channels, and they do not catalyze phosphorylation themselves; phosphorylation is typically done by kinases, though GPCRs can be targets for phosphorylation to regulate signaling. Thus, the extracellular domain's typical function is binding the ligand.