Heterotrimeric G-proteins are molecular switches that play a crucial role in transmitting signals from outside the cell to its interior. They are composed of three different subunits: alpha, beta, and gamma. When a signal, such as a hormone or neurotransmitter, binds to a receptor on the cell surface, the alpha subunit exchanges GDP for GTP, activating the G-protein and allowing it to interact with other proteins in the signaling pathway. These G-proteins are involved in various cellular processes, including metabolism, cell growth, and neurotransmission. Once the signal is relayed, the alpha subunit hydrolyzes GTP back to GDP, returning the G-protein to its inactive state. This cycle of activation and deactivation is essential for maintaining proper cellular function and responding to environmental changes.