Final answer:
GPCRs are known as 7-TM because they feature seven transmembrane domains, which are integral for their role in cell signaling and interaction with G-proteins.
Step-by-step explanation:
GPCRs, or G-protein-coupled receptors, are called 7-TM because they have seven transmembrane domains. These domains are regions that span the cellular membrane, connecting the interior and exterior of the cell. GPCRs play a crucial role in cell communication, as they bind to an external ligand and activate an associated G-protein. The activated G-protein then interacts with other membrane proteins, which may be ion channels or enzymes.
All GPCRs share this common structure of seven transmembrane domains, but they also have specific extracellular domains and G-protein-binding sites tailored to their unique functions. GPCRs are involved in various cellular processes and they affect a vast array of physiological responses, making them vital components in the signaling pathways of cells.