X-ray structures of human cyclin-dependent kinase 2 (Cdk2). (a) Cdk2 in complex with ATP. The protein is initially shown in the "standard" protein kinase orientation with its N-terminal lobe pink, its C-terminal lobe cyan, its PSTAIRE helix (residues 45–56) magenta, and its T loop (residues 152–170) orange. The ATP is shown in space-filling form and the phosphorylatable side chains of Thr 14, Tyr 15, and Thr 160 are shown in stick form, all colored according to atom type (C green, N blue, O red, and P yellow). [Based on an X-ray structure by Sung-Hou Kim, University of California at Berkeley. PDBid 1HCK.] (b) The complex of T160-phosphorylated Cdk2 with cyclin A and ATP. The Cdk2 and ATP are represented as in Part a and viewed similarly. The cyclin A is colored light green with its cyclin box (residues 206–306) dark green. The Cdk2 phosphoThr 160 phosphoryl group is drawn in space-filling form. Check the boxes below both Jmols to see how the binding of cyclin A together with the phosphorylation of Cdc2 Thr 160 causes a major structural reorganization of the T loop together with significant conformational adjustments of the Cdk2 N-terminal lobe, including its PSTAIRE helix. Also note the different conformations of the ATP triphosphate group in the two structures. [Based on an X-ray structure by Nikola Pavletich, Memorial Sloan-Kettering Cancer Center, New York, New York. PDBid 1JST.]