What happens to the alpha subunit of protein G when a ligand binds to a G protein-coupled receptor?

• A. GTP is hydrolyzed to GDP, activating the protein.
• B. GDP is phosphorylated to GTP, activating the protein.
• C. GDP is replaced by GTP, activating the protein.
• D. GTP is replaced by GDP, deactivating the protein.

Answer: (C)