Final answer:
When a signaling molecule binds to the G-protein-coupled receptor, the G protein a subunit exchanges GTP for GDP, activating the G protein and initiating downstream signaling cascades.
Step-by-step explanation:
When a signaling molecule binds to the G-protein-coupled receptor, the G protein a subunit exchanges GTP for GDP. This exchange activates the G protein, which then interacts with other proteins in the cell to transmit the signal. For example, in the presence of a ligand, the G protein activates adenylate cyclase, leading to the production of cAMP and the activation of protein kinase A (PKA). PKA then phosphorylates various enzymes, regulating their activity and initiating downstream signaling cascades.