Fig. 29.5 Crystal structure of rhodopsin (purple) with arrestin (turquoise) bound. Rhodopsin has been phosphorylated three times at its C-terminus, with the phosphate groups being attached to a threonine and a serine in the structure. Arrestin, which is composed of β-sheets, contacts the C-terminus of the receptor via electrostatic interactions using the phosphate groups. For this purpose, arrestin uses several arginine and lysine residues (see detailed view on the left).