Figure 3-30 Schematic drawing of a G protein-coupled receptor. A two-dimensional version of receptor structure is shown with seven transmembrane domains, an extracellular amino (NH₂ ) terminus (with associated glycosylation sites [Y]), an intracellular carboxyl (COOH) terminus, palmitoylated cysteine residue (crooked line extending into the membrane), three extracellular loops, and four intracellular loops. Major sites of G protein interactions with the receptor are speckled; potential sites of phosphorylation in the third intracellular loop and carboxyl terminus are enclosed in boxes.