G-proteins, or guanine nucleotide-binding proteins, are molecular switches that play a crucial role in transmitting signals from outside the cell to its interior. They are activated when a receptor on the cell surface binds to a ligand, such as a hormone or neurotransmitter. Once activated, G-proteins can influence various cellular processes by interacting with other proteins, enzymes, or ion channels. These proteins are composed of three subunits: alpha, beta, and gamma. The alpha subunit binds to GDP or GTP, which determines whether the G-protein is in an inactive or active state. When GTP is bound, the G-protein can initiate a cascade of signaling events, ultimately leading to a specific cellular response.