Guanine nucleotide-binding proteins, commonly known as G-proteins, are a family of proteins that play a crucial role in cellular signaling. They act as molecular switches, toggling between an active and inactive state based on whether they are bound to GTP (guanosine triphosphate) or GDP (guanosine diphosphate). When activated by a receptor, G-proteins transmit signals from outside the cell to the inside, influencing various cellular processes. These proteins are essential for many physiological functions, including vision, taste, and the regulation of hormones. G-proteins are classified into two main types: heterotrimeric G-proteins, which consist of three subunits, and small G-proteins, which are single subunit proteins. Their ability to relay signals makes them vital for maintaining cellular communication and function.