Protein kinases are characterized by a bi-lobe architecture consisting of an amino-terminal β-sheet lobe and a larger α-helical carboxy-terminal domain, with a canonical fold that contains the nucleotide-binding site located between the two domains. Other conserved protein kinase features include the P-loop (shown in magenta), which contains a glycine-rich motif (GXGXXG), and the activation loop (shown in purple), which is characterized by an initial DFG and terminal APE motif. Upon ATP binding, a reorientation of the C-helix (shown in light blue) and the glycine-rich loop is often observed. This structure shows SYK tyrosine kinase in complex with the therapeutic kinase inhibitor imatinib (Protein Data Bank identification number: 1XBB).