What are GPCRs composed of?

GPCRs are formed from a single chain of amino acids that folds into a rounded shape and is inserted into the plasma membrane of a cell. GPCRs are made up of seven hydrophobic transmembrane (TM) segments, three extracellular loops (ECLs) along with an amino terminus, as well as three intracellular loops (ICLs) accompanied by a carboxyl terminus. Additionally, the G-protein comprises three distinct subunits: alpha (α), beta (β), and gamma (γ), forming a heterotrimeric structure. When inactive, the alpha subunit of the G-protein binds to GDP. The highest similarity among GPCRs lies within the transmembrane segments.