G protein-coupled receptors, introduced in section 9.3, are a very large family of cell-surface molecules. They are found in virtually every eukaryotic organism. In humans, for example, approximately 800 different G protein-coupled receptors have been found. They all have two characteristics in common. First, they have a similar structure, consisting of a single polypeptide chain that has seven transmembrane spanning regions, with the ligand-binding site on the outside of the cell and the portion that binds to the G protein on the inside of the cell. Second, when activated, they associate with a G protein. In this way, they are able to transmit the signal from the outside to the inside of the cell. These characteristics result from their shared evolutionary history.

In spite of their similarity, different G protein-coupled receptors are able to respond to a diverse set of different signaling molecules, including hormones, neurotransmitters, and small molecules. In addition, their effects are quite diverse. For example, signaling through these receptors is responsible for our senses of sight, smell, and taste (Chapter 36). In fact, it is thought that G protein-coupled receptors used for cell communication in multicellular organisms evolved from sensory receptors in unicellular eukaryotes.