G-protein-linked receptor. Illustration of G-protein-mediated intracellular signalling. A chemical signal (ligand, orange, upper left) attaches to a receptor (red) in the cell membrane (blue). This causes the receptor to bind to a G-protein (yellow). A series of reactions is triggered and the G-protein activates a membrane enzyme called adelynate cyclase (red, centre right). Further reactions involving cAMP (cyclic adenosine monophosphate), various enzymes, and ATP (adenosine triphosphate), result in the target protein (bottom right, yellow) being phosphorylated, controlling its activity. For this artwork with labels, see C023/8845.