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, binds to a signaling molecule, such as a hormone. Once activated, G proteins can influence various cellular processes by interacting with other proteins and enzymes. These proteins exist in two main forms: G protein alpha and G protein beta-gamma subunits. When a G protein is activated, the alpha subunit exchanges GDP for GTP, leading to a conformational change. This change allows the G protein to interact with target proteins, ultimately resulting in a cellular response, such as changes in metabolism or gene expression.