The crystal structure of AMP-PNP bound to Lck was used to made a model of the Abl kinase domain. (a) The general architecture of the Abl kinase domain. The NH₂-terminal domain has a 5-strand β-sheet (blue) and 1 α-helix (red). The COOH-terminal domain is made up of several α-helices. ATP is shown in the ATP-binding cleft. (b) Topology of the ATP-binding cleft of the Abl protein kinase. The ATP-binding cleft can be subdivided into 6 regions: adenine-binding region (yellow), hydrophobic region I (blue), hydrophobic region II (aqua), phosphate-binding region (red-orange), the ribose pocket (purple), and the linker region (pink). The ligated ATP is shown in green-blue. (c) Amino acid side chains lining the ATP-binding cleft.