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, often a G-protein-coupled receptor (GPCR), binds to a signaling molecule, such as a hormone or neurotransmitter. Once activated, G-proteins can influence various cellular processes by interacting with other proteins and enzymes. These proteins are composed of three subunits: alpha, beta, and gamma. The alpha subunit binds to GDP (guanosine diphosphate) in its inactive state and exchanges it for GTP (guanosine triphosphate) when activated. This exchange triggers a conformational change, allowing the G-protein to interact with target proteins, thereby initiating a cascade of cellular responses that regulate functions like metabolism, growth, and sensory perception.