When Cdk2 is inactive, the small L12 helix pushes the large PSTAIRE helix outwards. The PSTAIRE helix contains a residue, glutamate 51, that is important for positioning the ATP phosphates.
When Cdk2 is not active, the small L12 helix moves the large PSTAIRE helix outward.  The PSTAIRE helix has a key residue, glutamate 51, that helps position the ATP phosphate groups.